xnu-4570.51.1.tar.gz

[apple/xnu.git] / bsd / vfs / vfs_cluster.c

/*
 * Copyright (c) 2000-2014 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 * 
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/* Copyright (c) 1995 NeXT Computer, Inc. All Rights Reserved */
/*
 * Copyright (c) 1993
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)vfs_cluster.c       8.10 (Berkeley) 3/28/95
 */

#include <sys/param.h>
#include <sys/proc_internal.h>
#include <sys/buf_internal.h>
#include <sys/mount_internal.h>
#include <sys/vnode_internal.h>
#include <sys/trace.h>
#include <sys/malloc.h>
#include <sys/time.h>
#include <sys/kernel.h>
#include <sys/resourcevar.h>
#include <miscfs/specfs/specdev.h>
#include <sys/uio_internal.h>
#include <libkern/libkern.h>
#include <machine/machine_routines.h>

#include <sys/ubc_internal.h>
#include <vm/vnode_pager.h>

#include <mach/mach_types.h>
#include <mach/memory_object_types.h>
#include <mach/vm_map.h>
#include <mach/upl.h>
#include <kern/task.h>
#include <kern/policy_internal.h>

#include <vm/vm_kern.h>
#include <vm/vm_map.h>
#include <vm/vm_pageout.h>
#include <vm/vm_fault.h>

#include <sys/kdebug.h>
#include <libkern/OSAtomic.h>  

#include <sys/sdt.h>

#include <stdbool.h>

#include <vfs/vfs_disk_conditioner.h>

#if 0
#undef KERNEL_DEBUG
#define KERNEL_DEBUG KERNEL_DEBUG_CONSTANT
#endif


#define CL_READ         0x01
#define CL_WRITE        0x02
#define CL_ASYNC        0x04
#define CL_COMMIT       0x08
#define CL_PAGEOUT      0x10
#define CL_AGE          0x20
#define CL_NOZERO       0x40
#define CL_PAGEIN       0x80
#define CL_DEV_MEMORY   0x100
#define CL_PRESERVE     0x200
#define CL_THROTTLE     0x400
#define CL_KEEPCACHED   0x800
#define CL_DIRECT_IO    0x1000
#define CL_PASSIVE      0x2000
#define CL_IOSTREAMING  0x4000
#define CL_CLOSE        0x8000
#define CL_ENCRYPTED    0x10000
#define CL_RAW_ENCRYPTED        0x20000
#define CL_NOCACHE      0x40000

#define MAX_VECTOR_UPL_ELEMENTS 8
#define MAX_VECTOR_UPL_SIZE     (2 * MAX_UPL_SIZE_BYTES)

#define CLUSTER_IO_WAITING              ((buf_t)1)

extern upl_t vector_upl_create(vm_offset_t);
extern boolean_t vector_upl_is_valid(upl_t);
extern boolean_t vector_upl_set_subupl(upl_t,upl_t, u_int32_t);
extern void vector_upl_set_pagelist(upl_t);
extern void vector_upl_set_iostate(upl_t, upl_t, vm_offset_t, u_int32_t);

struct clios {
        lck_mtx_t io_mtxp;
        u_int  io_completed;       /* amount of io that has currently completed */
        u_int  io_issued;          /* amount of io that was successfully issued */
        int    io_error;           /* error code of first error encountered */
        int    io_wanted;          /* someone is sleeping waiting for a change in state */
};

struct cl_direct_read_lock {
        LIST_ENTRY(cl_direct_read_lock)         chain;
        int32_t                                                         ref_count;
        vnode_t                                                         vp;
        lck_rw_t                                                        rw_lock;
};

#define CL_DIRECT_READ_LOCK_BUCKETS 61

static LIST_HEAD(cl_direct_read_locks, cl_direct_read_lock)
        cl_direct_read_locks[CL_DIRECT_READ_LOCK_BUCKETS];

static lck_spin_t cl_direct_read_spin_lock;

static lck_grp_t        *cl_mtx_grp;
static lck_attr_t       *cl_mtx_attr;
static lck_grp_attr_t   *cl_mtx_grp_attr;
static lck_mtx_t        *cl_transaction_mtxp;

#define IO_UNKNOWN      0
#define IO_DIRECT       1
#define IO_CONTIG       2
#define IO_COPY         3

#define PUSH_DELAY      0x01
#define PUSH_ALL        0x02
#define PUSH_SYNC       0x04


static void cluster_EOT(buf_t cbp_head, buf_t cbp_tail, int zero_offset);
static void cluster_wait_IO(buf_t cbp_head, int async);
static void cluster_complete_transaction(buf_t *cbp_head, void *callback_arg, int *retval, int flags, int needwait);

static int cluster_io_type(struct uio *uio, int *io_type, u_int32_t *io_length, u_int32_t min_length);

static int cluster_io(vnode_t vp, upl_t upl, vm_offset_t upl_offset, off_t f_offset, int non_rounded_size,
                      int flags, buf_t real_bp, struct clios *iostate, int (*)(buf_t, void *), void *callback_arg);
static int cluster_iodone(buf_t bp, void *callback_arg);
static int cluster_ioerror(upl_t upl, int upl_offset, int abort_size, int error, int io_flags, vnode_t vp);
static int cluster_is_throttled(vnode_t vp);

static void cluster_iostate_wait(struct clios *iostate, u_int target, const char *wait_name);

static void cluster_syncup(vnode_t vp, off_t newEOF, int (*)(buf_t, void *), void *callback_arg, int flags);

static void cluster_read_upl_release(upl_t upl, int start_pg, int last_pg, int take_reference);
static int cluster_copy_ubc_data_internal(vnode_t vp, struct uio *uio, int *io_resid, int mark_dirty, int take_reference);

static int cluster_read_copy(vnode_t vp, struct uio *uio, u_int32_t io_req_size,  off_t filesize, int flags,
                             int (*)(buf_t, void *), void *callback_arg);
static int cluster_read_direct(vnode_t vp, struct uio *uio, off_t filesize, int *read_type, u_int32_t *read_length,
                               int flags, int (*)(buf_t, void *), void *callback_arg);
static int cluster_read_contig(vnode_t vp, struct uio *uio, off_t filesize, int *read_type, u_int32_t *read_length,
                               int (*)(buf_t, void *), void *callback_arg, int flags);

static int cluster_write_copy(vnode_t vp, struct uio *uio, u_int32_t io_req_size, off_t oldEOF, off_t newEOF,
                              off_t headOff, off_t tailOff, int flags, int (*)(buf_t, void *), void *callback_arg);
static int cluster_write_direct(vnode_t vp, struct uio *uio, off_t oldEOF, off_t newEOF,
                                int *write_type, u_int32_t *write_length, int flags, int (*)(buf_t, void *), void *callback_arg);
static int cluster_write_contig(vnode_t vp, struct uio *uio, off_t newEOF,
                                int *write_type, u_int32_t *write_length, int (*)(buf_t, void *), void *callback_arg, int bflag);

static int cluster_align_phys_io(vnode_t vp, struct uio *uio, addr64_t usr_paddr, u_int32_t xsize, int flags, int (*)(buf_t, void *), void *callback_arg);

static int      cluster_read_prefetch(vnode_t vp, off_t f_offset, u_int size, off_t filesize, int (*callback)(buf_t, void *), void *callback_arg, int bflag);
static void     cluster_read_ahead(vnode_t vp, struct cl_extent *extent, off_t filesize, struct cl_readahead *ra, int (*callback)(buf_t, void *), void *callback_arg, int bflag);

static int      cluster_push_now(vnode_t vp, struct cl_extent *, off_t EOF, int flags, int (*)(buf_t, void *), void *callback_arg);

static int      cluster_try_push(struct cl_writebehind *, vnode_t vp, off_t EOF, int push_flag, int flags, int (*)(buf_t, void *), void *callback_arg, int *err);

static void     sparse_cluster_switch(struct cl_writebehind *, vnode_t vp, off_t EOF, int (*)(buf_t, void *), void *callback_arg);
static int      sparse_cluster_push(void **cmapp, vnode_t vp, off_t EOF, int push_flag, int io_flags, int (*)(buf_t, void *), void *callback_arg);
static void     sparse_cluster_add(void **cmapp, vnode_t vp, struct cl_extent *, off_t EOF, int (*)(buf_t, void *), void *callback_arg);

static kern_return_t vfs_drt_mark_pages(void **cmapp, off_t offset, u_int length, u_int *setcountp);
static kern_return_t vfs_drt_get_cluster(void **cmapp, off_t *offsetp, u_int *lengthp);
static kern_return_t vfs_drt_control(void **cmapp, int op_type);


/*
 * For throttled IO to check whether
 * a block is cached by the boot cache
 * and thus it can avoid delaying the IO.
 *
 * bootcache_contains_block is initially
 * NULL. The BootCache will set it while
 * the cache is active and clear it when
 * the cache is jettisoned.
 *
 * Returns 0 if the block is not
 * contained in the cache, 1 if it is
 * contained.
 *
 * The function pointer remains valid
 * after the cache has been evicted even
 * if bootcache_contains_block has been
 * cleared.
 *
 * See rdar://9974130 The new throttling mechanism breaks the boot cache for throttled IOs
 */
int (*bootcache_contains_block)(dev_t device, u_int64_t blkno) = NULL;


/*
 * limit the internal I/O size so that we
 * can represent it in a 32 bit int
 */
#define MAX_IO_REQUEST_SIZE     (1024 * 1024 * 512)
#define MAX_IO_CONTIG_SIZE      MAX_UPL_SIZE_BYTES
#define MAX_VECTS               16
/*
 * The MIN_DIRECT_WRITE_SIZE governs how much I/O should be issued before we consider
 * allowing the caller to bypass the buffer cache.  For small I/Os (less than 16k), 
 * we have not historically allowed the write to bypass the UBC. 
 */
#define MIN_DIRECT_WRITE_SIZE   (16384)

#define WRITE_THROTTLE          6
#define WRITE_THROTTLE_SSD      2
#define WRITE_BEHIND            1
#define WRITE_BEHIND_SSD        1

#if CONFIG_EMBEDDED
#define PREFETCH                1
#define PREFETCH_SSD            1
uint32_t speculative_prefetch_max = (2048 * 1024);              /* maximum bytes in a specluative read-ahead */
uint32_t speculative_prefetch_max_iosize = (512 * 1024);        /* maximum I/O size to use in a specluative read-ahead */
#else
#define PREFETCH                3
#define PREFETCH_SSD            2
uint32_t speculative_prefetch_max = (MAX_UPL_SIZE_BYTES * 3);   /* maximum bytes in a specluative read-ahead */
uint32_t speculative_prefetch_max_iosize = (512 * 1024);        /* maximum I/O size to use in a specluative read-ahead on SSDs*/
#endif


#define IO_SCALE(vp, base)              (vp->v_mount->mnt_ioscale * (base))
#define MAX_CLUSTER_SIZE(vp)            (cluster_max_io_size(vp->v_mount, CL_WRITE))
#define MAX_PREFETCH(vp, size, is_ssd)  (size * IO_SCALE(vp, ((is_ssd) ? PREFETCH_SSD : PREFETCH)))

int     speculative_reads_disabled = 0;

/*
 * throttle the number of async writes that
 * can be outstanding on a single vnode
 * before we issue a synchronous write 
 */
#define THROTTLE_MAXCNT 0

uint32_t throttle_max_iosize = (128 * 1024);

#define THROTTLE_MAX_IOSIZE (throttle_max_iosize)

SYSCTL_INT(_debug, OID_AUTO, lowpri_throttle_max_iosize, CTLFLAG_RW | CTLFLAG_LOCKED, &throttle_max_iosize, 0, "");


void
cluster_init(void) {
        /*
         * allocate lock group attribute and group
         */
        cl_mtx_grp_attr = lck_grp_attr_alloc_init();
        cl_mtx_grp = lck_grp_alloc_init("cluster I/O", cl_mtx_grp_attr);
                
        /*
         * allocate the lock attribute
         */
        cl_mtx_attr = lck_attr_alloc_init();

        cl_transaction_mtxp = lck_mtx_alloc_init(cl_mtx_grp, cl_mtx_attr);

        if (cl_transaction_mtxp == NULL)
                panic("cluster_init: failed to allocate cl_transaction_mtxp");

        lck_spin_init(&cl_direct_read_spin_lock, cl_mtx_grp, cl_mtx_attr);

        for (int i = 0; i < CL_DIRECT_READ_LOCK_BUCKETS; ++i)
                LIST_INIT(&cl_direct_read_locks[i]);
}


uint32_t
cluster_max_io_size(mount_t mp, int type)
{
        uint32_t        max_io_size;
        uint32_t        segcnt;
        uint32_t        maxcnt;

        switch(type) {

        case CL_READ:
                segcnt = mp->mnt_segreadcnt;
                maxcnt = mp->mnt_maxreadcnt;
                break;
        case CL_WRITE:
                segcnt = mp->mnt_segwritecnt;
                maxcnt = mp->mnt_maxwritecnt;
                break;
        default:
                segcnt = min(mp->mnt_segreadcnt, mp->mnt_segwritecnt);
                maxcnt = min(mp->mnt_maxreadcnt, mp->mnt_maxwritecnt);
                break;
        }
        if (segcnt > (MAX_UPL_SIZE_BYTES >> PAGE_SHIFT)) {
               /*
                * don't allow a size beyond the max UPL size we can create
                */
               segcnt = MAX_UPL_SIZE_BYTES >> PAGE_SHIFT;
       }
       max_io_size = min((segcnt * PAGE_SIZE), maxcnt);

       if (max_io_size < MAX_UPL_TRANSFER_BYTES) {
               /*
                * don't allow a size smaller than the old fixed limit
                */
               max_io_size = MAX_UPL_TRANSFER_BYTES;
       } else {
               /*
                * make sure the size specified is a multiple of PAGE_SIZE
                */
               max_io_size &= ~PAGE_MASK;
       }
       return (max_io_size);
}




#define CLW_ALLOCATE            0x01
#define CLW_RETURNLOCKED        0x02
#define CLW_IONOCACHE           0x04
#define CLW_IOPASSIVE   0x08

/*
 * if the read ahead context doesn't yet exist,
 * allocate and initialize it...
 * the vnode lock serializes multiple callers
 * during the actual assignment... first one
 * to grab the lock wins... the other callers
 * will release the now unnecessary storage
 * 
 * once the context is present, try to grab (but don't block on)
 * the lock associated with it... if someone
 * else currently owns it, than the read
 * will run without read-ahead.  this allows
 * multiple readers to run in parallel and
 * since there's only 1 read ahead context,
 * there's no real loss in only allowing 1
 * reader to have read-ahead enabled.
 */
static struct cl_readahead *
cluster_get_rap(vnode_t vp)
{
        struct ubc_info         *ubc;
        struct cl_readahead     *rap;

        ubc = vp->v_ubcinfo;

        if ((rap = ubc->cl_rahead) == NULL) {
                MALLOC_ZONE(rap, struct cl_readahead *, sizeof *rap, M_CLRDAHEAD, M_WAITOK);

                bzero(rap, sizeof *rap);
                rap->cl_lastr = -1;
                lck_mtx_init(&rap->cl_lockr, cl_mtx_grp, cl_mtx_attr);

                vnode_lock(vp);
                
                if (ubc->cl_rahead == NULL)
                        ubc->cl_rahead = rap;
                else {
                        lck_mtx_destroy(&rap->cl_lockr, cl_mtx_grp);
                        FREE_ZONE((void *)rap, sizeof *rap, M_CLRDAHEAD);
                        rap = ubc->cl_rahead;
                }
                vnode_unlock(vp);
        }
        if (lck_mtx_try_lock(&rap->cl_lockr) == TRUE)
                return(rap);
        
        return ((struct cl_readahead *)NULL);
}


/*
 * if the write behind context doesn't yet exist,
 * and CLW_ALLOCATE is specified, allocate and initialize it...
 * the vnode lock serializes multiple callers
 * during the actual assignment... first one
 * to grab the lock wins... the other callers
 * will release the now unnecessary storage
 * 
 * if CLW_RETURNLOCKED is set, grab (blocking if necessary)
 * the lock associated with the write behind context before
 * returning
 */

static struct cl_writebehind *
cluster_get_wbp(vnode_t vp, int flags)
{
        struct ubc_info *ubc;
        struct cl_writebehind *wbp;

        ubc = vp->v_ubcinfo;

        if ((wbp = ubc->cl_wbehind) == NULL) {

                if ( !(flags & CLW_ALLOCATE))
                        return ((struct cl_writebehind *)NULL);
          
                MALLOC_ZONE(wbp, struct cl_writebehind *, sizeof *wbp, M_CLWRBEHIND, M_WAITOK);

                bzero(wbp, sizeof *wbp);
                lck_mtx_init(&wbp->cl_lockw, cl_mtx_grp, cl_mtx_attr);

                vnode_lock(vp);
                
                if (ubc->cl_wbehind == NULL)
                        ubc->cl_wbehind = wbp;
                else {
                        lck_mtx_destroy(&wbp->cl_lockw, cl_mtx_grp);
                        FREE_ZONE((void *)wbp, sizeof *wbp, M_CLWRBEHIND);
                        wbp = ubc->cl_wbehind;
                }
                vnode_unlock(vp);
        }
        if (flags & CLW_RETURNLOCKED)
                lck_mtx_lock(&wbp->cl_lockw);

        return (wbp);
}


static void
cluster_syncup(vnode_t vp, off_t newEOF, int (*callback)(buf_t, void *), void *callback_arg, int flags)
{
        struct cl_writebehind *wbp;

        if ((wbp = cluster_get_wbp(vp, 0)) != NULL) {
          
                if (wbp->cl_number) {
                        lck_mtx_lock(&wbp->cl_lockw);

                        cluster_try_push(wbp, vp, newEOF, PUSH_ALL | flags, 0, callback, callback_arg, NULL);

                        lck_mtx_unlock(&wbp->cl_lockw);
                }
        }
}


static int
cluster_io_present_in_BC(vnode_t vp, off_t f_offset)
{
        daddr64_t blkno;
        size_t    io_size;
        int (*bootcache_check_fn)(dev_t device, u_int64_t blkno) = bootcache_contains_block;
        
        if (bootcache_check_fn && vp->v_mount && vp->v_mount->mnt_devvp) {
                if (VNOP_BLOCKMAP(vp, f_offset, PAGE_SIZE, &blkno, &io_size, NULL, VNODE_READ | VNODE_BLOCKMAP_NO_TRACK, NULL))
                        return(0);

                if (io_size == 0)
                        return (0);

                if (bootcache_check_fn(vp->v_mount->mnt_devvp->v_rdev, blkno))
                        return(1);
        }
        return(0);
}


static int 
cluster_is_throttled(vnode_t vp)
{
        return (throttle_io_will_be_throttled(-1, vp->v_mount));
}


static void
cluster_iostate_wait(struct clios *iostate, u_int target, const char *wait_name)
{

        lck_mtx_lock(&iostate->io_mtxp);

        while ((iostate->io_issued - iostate->io_completed) > target) {

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 95)) | DBG_FUNC_START,
                             iostate->io_issued, iostate->io_completed, target, 0, 0);

                iostate->io_wanted = 1;
                msleep((caddr_t)&iostate->io_wanted, &iostate->io_mtxp, PRIBIO + 1, wait_name, NULL);

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 95)) | DBG_FUNC_END,
                             iostate->io_issued, iostate->io_completed, target, 0, 0);
        }       
        lck_mtx_unlock(&iostate->io_mtxp);
}

static void cluster_handle_associated_upl(struct clios *iostate, upl_t upl,
                                                                                  upl_offset_t upl_offset, upl_size_t size)
{
        if (!size)
                return;

        upl_t associated_upl = upl_associated_upl(upl);

        if (!associated_upl)
                return;

#if 0
        printf("1: %d %d\n", upl_offset, upl_offset + size);
#endif

        /*
         * The associated UPL is page aligned to file offsets whereas the
         * UPL it's attached to has different alignment requirements.  The
         * upl_offset that we have refers to @upl.  The code that follows
         * has to deal with the first and last pages in this transaction
         * which might straddle pages in the associated UPL.  To keep
         * track of these pages, we use the mark bits: if the mark bit is
         * set, we know another transaction has completed its part of that
         * page and so we can unlock that page here.
         *
         * The following illustrates what we have to deal with:
         *
         *    MEM u <------------ 1 PAGE ------------> e
         *        +-------------+----------------------+-----------------
         *        |             |######################|#################
         *        +-------------+----------------------+-----------------
         *   FILE | <--- a ---> o <------------ 1 PAGE ------------>
         *
         * So here we show a write to offset @o.  The data that is to be
         * written is in a buffer that is not page aligned; it has offset
         * @a in the page.  The upl that carries the data starts in memory
         * at @u.  The associated upl starts in the file at offset @o.  A
         * transaction will always end on a page boundary (like @e above)
         * except for the very last transaction in the group.  We cannot
         * unlock the page at @o in the associated upl until both the
         * transaction ending at @e and the following transaction (that
         * starts at @e) has completed.
         */

        /*
         * We record whether or not the two UPLs are aligned as the mark
         * bit in the first page of @upl.
         */
        upl_page_info_t *pl = UPL_GET_INTERNAL_PAGE_LIST(upl);
        bool is_unaligned = upl_page_get_mark(pl, 0);

        if (is_unaligned) {
                upl_page_info_t *assoc_pl = UPL_GET_INTERNAL_PAGE_LIST(associated_upl);

                upl_offset_t upl_end = upl_offset + size;
                assert(upl_end >= PAGE_SIZE);

                upl_size_t assoc_upl_size = upl_get_size(associated_upl);

                /*
                 * In the very first transaction in the group, upl_offset will
                 * not be page aligned, but after that it will be and in that
                 * case we want the preceding page in the associated UPL hence
                 * the minus one.
                 */
                assert(upl_offset);
                if (upl_offset)
                        upl_offset = trunc_page_32(upl_offset - 1);

                lck_mtx_lock_spin(&iostate->io_mtxp);

                // Look at the first page...
                if (upl_offset
                        && !upl_page_get_mark(assoc_pl, upl_offset >> PAGE_SHIFT)) {
                        /*
                         * The first page isn't marked so let another transaction
                         * completion handle it.
                         */
                        upl_page_set_mark(assoc_pl, upl_offset >> PAGE_SHIFT, true);
                        upl_offset += PAGE_SIZE;
                }

                // And now the last page...

                /*
                 * This needs to be > rather than >= because if it's equal, it
                 * means there's another transaction that is sharing the last
                 * page.
                 */
                if (upl_end > assoc_upl_size)
                        upl_end = assoc_upl_size;
                else {
                        upl_end = trunc_page_32(upl_end);
                        const int last_pg = (upl_end >> PAGE_SHIFT) - 1;

                        if (!upl_page_get_mark(assoc_pl, last_pg)) {
                                /*
                                 * The last page isn't marked so mark the page and let another
                                 * transaction completion handle it.
                                 */
                                upl_page_set_mark(assoc_pl, last_pg, true);
                                upl_end -= PAGE_SIZE;
                        }
                }

                lck_mtx_unlock(&iostate->io_mtxp);

#if 0
                printf("2: %d %d\n", upl_offset, upl_end);
#endif

                if (upl_end <= upl_offset)
                        return;

                size = upl_end - upl_offset;
        } else {
                assert(!(upl_offset & PAGE_MASK));
                assert(!(size & PAGE_MASK));
        }

        boolean_t empty;

        /*
         * We can unlock these pages now and as this is for a
         * direct/uncached write, we want to dump the pages too.
         */
        kern_return_t kr = upl_abort_range(associated_upl, upl_offset, size,
                                                                           UPL_ABORT_DUMP_PAGES, &empty);

        assert(!kr);

        if (!kr && empty) {
                upl_set_associated_upl(upl, NULL);
                upl_deallocate(associated_upl);
        }
}

static int
cluster_ioerror(upl_t upl, int upl_offset, int abort_size, int error, int io_flags, vnode_t vp)
{
        int upl_abort_code = 0;
        int page_in  = 0;
        int page_out = 0;

        if ((io_flags & (B_PHYS | B_CACHE)) == (B_PHYS | B_CACHE))
                /*
                 * direct write of any flavor, or a direct read that wasn't aligned
                 */
                ubc_upl_commit_range(upl, upl_offset, abort_size, UPL_COMMIT_FREE_ON_EMPTY);
        else {
                if (io_flags & B_PAGEIO) {
                        if (io_flags & B_READ)
                                page_in  = 1;
                        else
                                page_out = 1;
                }
                if (io_flags & B_CACHE)
                        /*
                         * leave pages in the cache unchanged on error
                         */
                        upl_abort_code = UPL_ABORT_FREE_ON_EMPTY;
                else if (page_out && ((error != ENXIO) || vnode_isswap(vp)))
                        /*
                         * transient error... leave pages unchanged
                         */
                        upl_abort_code = UPL_ABORT_FREE_ON_EMPTY;
                else if (page_in)
                        upl_abort_code = UPL_ABORT_FREE_ON_EMPTY | UPL_ABORT_ERROR;
                else
                        upl_abort_code = UPL_ABORT_FREE_ON_EMPTY | UPL_ABORT_DUMP_PAGES;

                ubc_upl_abort_range(upl, upl_offset, abort_size, upl_abort_code);
        }
        return (upl_abort_code);
}


static int
cluster_iodone(buf_t bp, void *callback_arg)
{
        int     b_flags;
        int     error;
        int     total_size;
        int     total_resid;
        int     upl_offset;
        int     zero_offset;
        int     pg_offset = 0;
        int     commit_size = 0;
        int     upl_flags = 0;
        int     transaction_size = 0;
        upl_t   upl;
        buf_t   cbp;
        buf_t   cbp_head;
        buf_t   cbp_next;
        buf_t   real_bp;
        vnode_t vp;
        struct  clios *iostate;
        boolean_t       transaction_complete = FALSE;

        __IGNORE_WCASTALIGN(cbp_head = (buf_t)(bp->b_trans_head));

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 20)) | DBG_FUNC_START,
                     cbp_head, bp->b_lblkno, bp->b_bcount, bp->b_flags, 0);

        if (cbp_head->b_trans_next || !(cbp_head->b_flags & B_EOT)) {
                lck_mtx_lock_spin(cl_transaction_mtxp);

                bp->b_flags |= B_TDONE;

                for (cbp = cbp_head; cbp; cbp = cbp->b_trans_next) {
                        /*
                         * all I/O requests that are part of this transaction
                         * have to complete before we can process it
                         */
                        if ( !(cbp->b_flags & B_TDONE)) {

                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 20)) | DBG_FUNC_END,
                                             cbp_head, cbp, cbp->b_bcount, cbp->b_flags, 0);

                                lck_mtx_unlock(cl_transaction_mtxp);

                                return 0;
                        }

                        if (cbp->b_trans_next == CLUSTER_IO_WAITING) {
                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 20)) | DBG_FUNC_END,
                                                         cbp_head, cbp, cbp->b_bcount, cbp->b_flags, 0);

                                lck_mtx_unlock(cl_transaction_mtxp);
                                wakeup(cbp);

                                return 0;
                        }

                        if (cbp->b_flags & B_EOT)
                                transaction_complete = TRUE;
                }
                lck_mtx_unlock(cl_transaction_mtxp);

                if (transaction_complete == FALSE) {
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 20)) | DBG_FUNC_END,
                                     cbp_head, 0, 0, 0, 0);
                        return 0;
                }
        }
        error       = 0;
        total_size  = 0;
        total_resid = 0;

        cbp        = cbp_head;
        vp         = cbp->b_vp;
        upl_offset = cbp->b_uploffset;
        upl        = cbp->b_upl;
        b_flags    = cbp->b_flags;
        real_bp    = cbp->b_real_bp;
        zero_offset= cbp->b_validend;
        iostate    = (struct clios *)cbp->b_iostate;

        if (real_bp)
                real_bp->b_dev = cbp->b_dev;

        while (cbp) {
                if ((cbp->b_flags & B_ERROR) && error == 0)
                        error = cbp->b_error;

                total_resid += cbp->b_resid;
                total_size  += cbp->b_bcount;

                cbp_next = cbp->b_trans_next;

                if (cbp_next == NULL)
                        /*
                         * compute the overall size of the transaction
                         * in case we created one that has 'holes' in it
                         * 'total_size' represents the amount of I/O we
                         * did, not the span of the transaction w/r to the UPL
                         */
                        transaction_size = cbp->b_uploffset + cbp->b_bcount - upl_offset;

                if (cbp != cbp_head)
                        free_io_buf(cbp);

                cbp = cbp_next;
        }

        if (ISSET(b_flags, B_COMMIT_UPL)) {
                cluster_handle_associated_upl(iostate,
                                                                          cbp_head->b_upl,
                                                                          upl_offset,
                                                                          transaction_size);
        }

        if (error == 0 && total_resid)
                error = EIO;

        if (error == 0) {
                int     (*cliodone_func)(buf_t, void *) = (int (*)(buf_t, void *))(cbp_head->b_cliodone);

                if (cliodone_func != NULL) {
                        cbp_head->b_bcount = transaction_size;

                        error = (*cliodone_func)(cbp_head, callback_arg);
                }
        }
        if (zero_offset)
                cluster_zero(upl, zero_offset, PAGE_SIZE - (zero_offset & PAGE_MASK), real_bp);

        free_io_buf(cbp_head);

        if (iostate) {
                int need_wakeup = 0;

                /*
                 * someone has issued multiple I/Os asynchrounsly
                 * and is waiting for them to complete (streaming)
                 */
                lck_mtx_lock_spin(&iostate->io_mtxp);

                if (error && iostate->io_error == 0)
                        iostate->io_error = error;

                iostate->io_completed += total_size;

                if (iostate->io_wanted) {
                        /*
                         * someone is waiting for the state of
                         * this io stream to change
                         */
                        iostate->io_wanted = 0;
                        need_wakeup = 1;
                }
                lck_mtx_unlock(&iostate->io_mtxp);

                if (need_wakeup)
                        wakeup((caddr_t)&iostate->io_wanted);
        }

        if (b_flags & B_COMMIT_UPL) {
                pg_offset   = upl_offset & PAGE_MASK;
                commit_size = (pg_offset + transaction_size + (PAGE_SIZE - 1)) & ~PAGE_MASK;

                if (error)
                        upl_flags = cluster_ioerror(upl, upl_offset - pg_offset, commit_size, error, b_flags, vp);
                else {
                        upl_flags = UPL_COMMIT_FREE_ON_EMPTY;

                        if ((b_flags & B_PHYS) && (b_flags & B_READ)) 
                                upl_flags |= UPL_COMMIT_SET_DIRTY;

                        if (b_flags & B_AGE)
                                upl_flags |= UPL_COMMIT_INACTIVATE;

                        ubc_upl_commit_range(upl, upl_offset - pg_offset, commit_size, upl_flags);
                }
        }
        if (real_bp) {
                if (error) {
                        real_bp->b_flags |= B_ERROR;
                        real_bp->b_error = error;
                }
                real_bp->b_resid = total_resid;

                buf_biodone(real_bp);
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 20)) | DBG_FUNC_END,
                     upl, upl_offset - pg_offset, commit_size, (error << 24) | upl_flags, 0);

        return (error);
}


uint32_t
cluster_throttle_io_limit(vnode_t vp, uint32_t *limit)
{
        if (cluster_is_throttled(vp)) {
                *limit = THROTTLE_MAX_IOSIZE;
                return 1;
        }
        return 0;   
}


void
cluster_zero(upl_t upl, upl_offset_t upl_offset, int size, buf_t bp)
{

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 23)) | DBG_FUNC_START,
                     upl_offset, size, bp, 0, 0);

        if (bp == NULL || bp->b_datap == 0) {
                upl_page_info_t *pl;
                addr64_t        zero_addr;

                pl = ubc_upl_pageinfo(upl);

                if (upl_device_page(pl) == TRUE) {
                        zero_addr = ((addr64_t)upl_phys_page(pl, 0) << PAGE_SHIFT) + upl_offset;

                        bzero_phys_nc(zero_addr, size);
                } else {
                        while (size) {
                                int     page_offset;
                                int     page_index;
                                int     zero_cnt;

                                page_index  = upl_offset / PAGE_SIZE;
                                page_offset = upl_offset & PAGE_MASK;

                                zero_addr = ((addr64_t)upl_phys_page(pl, page_index) << PAGE_SHIFT) + page_offset;
                                zero_cnt  = min(PAGE_SIZE - page_offset, size);

                                bzero_phys(zero_addr, zero_cnt);

                                size       -= zero_cnt;
                                upl_offset += zero_cnt;
                        }
                }
        } else
                bzero((caddr_t)((vm_offset_t)bp->b_datap + upl_offset), size);

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 23)) | DBG_FUNC_END,
                     upl_offset, size, 0, 0, 0);
}


static void
cluster_EOT(buf_t cbp_head, buf_t cbp_tail, int zero_offset)
{
        cbp_head->b_validend = zero_offset;
        cbp_tail->b_flags |= B_EOT;
}

static void
cluster_wait_IO(buf_t cbp_head, int async)
{
        buf_t   cbp;

        if (async) {
                /*
                 * Async callback completion will not normally generate a
                 * wakeup upon I/O completion.  To get woken up, we set
                 * b_trans_next (which is safe for us to modify) on the last
                 * buffer to CLUSTER_IO_WAITING so that cluster_iodone knows
                 * to wake us up when all buffers as part of this transaction
                 * are completed.  This is done under the umbrella of
                 * cl_transaction_mtxp which is also taken in cluster_iodone.
                 */
                bool done = true;
                buf_t last = NULL;

                lck_mtx_lock_spin(cl_transaction_mtxp);

                for (cbp = cbp_head; cbp; last = cbp, cbp = cbp->b_trans_next) {
                        if (!ISSET(cbp->b_flags, B_TDONE))
                                done = false;
                }

                if (!done) {
                        last->b_trans_next = CLUSTER_IO_WAITING;

                        DTRACE_IO1(wait__start, buf_t, last);
                        do {
                                msleep(last, cl_transaction_mtxp, PSPIN | (PRIBIO+1), "cluster_wait_IO", NULL);

                                /*
                                 * We should only have been woken up if all the
                                 * buffers are completed, but just in case...
                                 */
                                done = true;
                                for (cbp = cbp_head; cbp != CLUSTER_IO_WAITING; cbp = cbp->b_trans_next) {
                                        if (!ISSET(cbp->b_flags, B_TDONE)) {
                                                done = false;
                                                break;
                                        }
                                }
                        } while (!done);
                        DTRACE_IO1(wait__done, buf_t, last);

                        last->b_trans_next = NULL;
                }

                lck_mtx_unlock(cl_transaction_mtxp);
        } else { // !async
                for (cbp = cbp_head; cbp; cbp = cbp->b_trans_next)
                        buf_biowait(cbp);
        }
}

static void
cluster_complete_transaction(buf_t *cbp_head, void *callback_arg, int *retval, int flags, int needwait)
{
        buf_t   cbp;
        int     error;
        boolean_t isswapout = FALSE;

        /*
         * cluster_complete_transaction will
         * only be called if we've issued a complete chain in synchronous mode
         * or, we've already done a cluster_wait_IO on an incomplete chain
         */
        if (needwait) {
                for (cbp = *cbp_head; cbp; cbp = cbp->b_trans_next)
                        buf_biowait(cbp);
        }
        /*
         * we've already waited on all of the I/Os in this transaction,
         * so mark all of the buf_t's in this transaction as B_TDONE
         * so that cluster_iodone sees the transaction as completed
         */
        for (cbp = *cbp_head; cbp; cbp = cbp->b_trans_next)
                cbp->b_flags |= B_TDONE;
        cbp = *cbp_head;

        if ((flags & (CL_ASYNC | CL_PAGEOUT)) == CL_PAGEOUT && vnode_isswap(cbp->b_vp))
                isswapout = TRUE;

        error = cluster_iodone(cbp, callback_arg);

        if ( !(flags & CL_ASYNC) && error && *retval == 0) {
                if (((flags & (CL_PAGEOUT | CL_KEEPCACHED)) != CL_PAGEOUT) || (error != ENXIO))
                        *retval = error;
                else if (isswapout == TRUE)
                        *retval = error;
        }
        *cbp_head = (buf_t)NULL;
}


static int
cluster_io(vnode_t vp, upl_t upl, vm_offset_t upl_offset, off_t f_offset, int non_rounded_size,
           int flags, buf_t real_bp, struct clios *iostate, int (*callback)(buf_t, void *), void *callback_arg)
{
        buf_t   cbp;
        u_int   size;
        u_int   io_size;
        int     io_flags;
        int     bmap_flags;
        int     error = 0;
        int     retval = 0;
        buf_t   cbp_head = NULL;
        buf_t   cbp_tail = NULL;
        int     trans_count = 0;
        int     max_trans_count;
        u_int   pg_count;
        int     pg_offset;
        u_int   max_iosize;
        u_int   max_vectors;
        int     priv;
        int     zero_offset = 0;
        int     async_throttle = 0;
        mount_t mp;
        vm_offset_t upl_end_offset;
        boolean_t   need_EOT = FALSE;

        /*
         * we currently don't support buffers larger than a page
         */
        if (real_bp && non_rounded_size > PAGE_SIZE)
                panic("%s(): Called with real buffer of size %d bytes which "
                                "is greater than the maximum allowed size of "
                                "%d bytes (the system PAGE_SIZE).\n",
                                __FUNCTION__, non_rounded_size, PAGE_SIZE);

        mp = vp->v_mount;

        /*
         * we don't want to do any funny rounding of the size for IO requests
         * coming through the DIRECT or CONTIGUOUS paths...  those pages don't
         * belong to us... we can't extend (nor do we need to) the I/O to fill
         * out a page
         */
        if (mp->mnt_devblocksize > 1 && !(flags & (CL_DEV_MEMORY | CL_DIRECT_IO))) {
                /*
                 * round the requested size up so that this I/O ends on a
                 * page boundary in case this is a 'write'... if the filesystem
                 * has blocks allocated to back the page beyond the EOF, we want to
                 * make sure to write out the zero's that are sitting beyond the EOF
                 * so that in case the filesystem doesn't explicitly zero this area
                 * if a hole is created via a lseek/write beyond the current EOF,
                 * it will return zeros when it's read back from the disk.  If the
                 * physical allocation doesn't extend for the whole page, we'll
                 * only write/read from the disk up to the end of this allocation
                 * via the extent info returned from the VNOP_BLOCKMAP call.
                 */
                pg_offset = upl_offset & PAGE_MASK;

                size = (((non_rounded_size + pg_offset) + (PAGE_SIZE - 1)) & ~PAGE_MASK) - pg_offset;
        } else {
                /*
                 * anyone advertising a blocksize of 1 byte probably
                 * can't deal with us rounding up the request size
                 * AFP is one such filesystem/device
                 */
                size = non_rounded_size;
        }
        upl_end_offset = upl_offset + size;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 22)) | DBG_FUNC_START, (int)f_offset, size, upl_offset, flags, 0);

        /*
         * Set the maximum transaction size to the maximum desired number of
         * buffers.
         */
        max_trans_count = 8;
        if (flags & CL_DEV_MEMORY)
                max_trans_count = 16;

        if (flags & CL_READ) {
                io_flags = B_READ;
                bmap_flags = VNODE_READ;

                max_iosize  = mp->mnt_maxreadcnt;
                max_vectors = mp->mnt_segreadcnt;
        } else {
                io_flags = B_WRITE;
                bmap_flags = VNODE_WRITE;

                max_iosize  = mp->mnt_maxwritecnt;
                max_vectors = mp->mnt_segwritecnt;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 22)) | DBG_FUNC_NONE, max_iosize, max_vectors, mp->mnt_devblocksize, 0, 0);

        /*
         * make sure the maximum iosize is a
         * multiple of the page size
         */
        max_iosize  &= ~PAGE_MASK;

        /*
         * Ensure the maximum iosize is sensible.
         */
        if (!max_iosize)
                max_iosize = PAGE_SIZE;

        if (flags & CL_THROTTLE) {
                if ( !(flags & CL_PAGEOUT) && cluster_is_throttled(vp)) {
                        if (max_iosize > THROTTLE_MAX_IOSIZE)
                                max_iosize = THROTTLE_MAX_IOSIZE;
                        async_throttle = THROTTLE_MAXCNT;
                } else {
                        if ( (flags & CL_DEV_MEMORY) )
                                async_throttle = IO_SCALE(vp, VNODE_ASYNC_THROTTLE);
                        else {
                                u_int max_cluster;
                                u_int max_cluster_size;
                                u_int scale;

                                if (vp->v_mount->mnt_minsaturationbytecount) {
                                        max_cluster_size = vp->v_mount->mnt_minsaturationbytecount;

                                        scale = 1;
                                } else {
                                        max_cluster_size = MAX_CLUSTER_SIZE(vp);

                                        if (disk_conditioner_mount_is_ssd(vp->v_mount))
                                                scale = WRITE_THROTTLE_SSD;
                                        else
                                                scale = WRITE_THROTTLE;
                                }
                                if (max_iosize > max_cluster_size)
                                        max_cluster = max_cluster_size;
                                else
                                        max_cluster = max_iosize;

                                if (size < max_cluster)
                                        max_cluster = size;
                                
                                if (flags & CL_CLOSE)
                                        scale += MAX_CLUSTERS;
                                
                                async_throttle = min(IO_SCALE(vp, VNODE_ASYNC_THROTTLE), ((scale * max_cluster_size) / max_cluster) - 1);
                        }
                }
        }
        if (flags & CL_AGE)
                io_flags |= B_AGE;
        if (flags & (CL_PAGEIN | CL_PAGEOUT))
                io_flags |= B_PAGEIO;
        if (flags & (CL_IOSTREAMING))
                io_flags |= B_IOSTREAMING;
        if (flags & CL_COMMIT)
                io_flags |= B_COMMIT_UPL;
        if (flags & CL_DIRECT_IO)
                io_flags |= B_PHYS;
        if (flags & (CL_PRESERVE | CL_KEEPCACHED))
                io_flags |= B_CACHE;
        if (flags & CL_PASSIVE)
                io_flags |= B_PASSIVE;
        if (flags & CL_ENCRYPTED)
                io_flags |= B_ENCRYPTED_IO;     

        if (vp->v_flag & VSYSTEM)
                io_flags |= B_META;

        if ((flags & CL_READ) && ((upl_offset + non_rounded_size) & PAGE_MASK) && (!(flags & CL_NOZERO))) {
                /*
                 * then we are going to end up
                 * with a page that we can't complete (the file size wasn't a multiple
                 * of PAGE_SIZE and we're trying to read to the end of the file
                 * so we'll go ahead and zero out the portion of the page we can't
                 * read in from the file
                 */
                zero_offset = upl_offset + non_rounded_size;
        } else if (!ISSET(flags, CL_READ) && ISSET(flags, CL_DIRECT_IO)) {
                assert(ISSET(flags, CL_COMMIT));

                // For a direct/uncached write, we need to lock pages...

                upl_t cached_upl;

                /*
                 * Create a UPL to lock the pages in the cache whilst the
                 * write is in progress.
                 */
                ubc_create_upl_kernel(vp, f_offset, non_rounded_size, &cached_upl,
                                           NULL, UPL_SET_LITE, VM_KERN_MEMORY_FILE);

                /*
                 * Attach this UPL to the other UPL so that we can find it
                 * later.
                 */
                upl_set_associated_upl(upl, cached_upl);

                if (upl_offset & PAGE_MASK) {
                        /*
                         * The two UPLs are not aligned, so mark the first page in
                         * @upl so that cluster_handle_associated_upl can handle
                         * it accordingly.
                         */
                        upl_page_info_t *pl = UPL_GET_INTERNAL_PAGE_LIST(upl);
                        upl_page_set_mark(pl, 0, true);
                }
        }

        while (size) {
                daddr64_t blkno;
                daddr64_t lblkno;
                u_int   io_size_wanted;
                size_t  io_size_tmp;

                if (size > max_iosize)
                        io_size = max_iosize;
                else
                        io_size = size;

                io_size_wanted = io_size;
                io_size_tmp = (size_t)io_size;
                
                if ((error = VNOP_BLOCKMAP(vp, f_offset, io_size, &blkno, &io_size_tmp, NULL, bmap_flags, NULL)))
                        break;

                if (io_size_tmp > io_size_wanted)
                        io_size = io_size_wanted;
                else
                        io_size = (u_int)io_size_tmp;

                if (real_bp && (real_bp->b_blkno == real_bp->b_lblkno))
                        real_bp->b_blkno = blkno;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 24)) | DBG_FUNC_NONE,
                             (int)f_offset, (int)(blkno>>32), (int)blkno, io_size, 0);

                if (io_size == 0) {
                        /*
                         * vnop_blockmap didn't return an error... however, it did
                         * return an extent size of 0 which means we can't
                         * make forward progress on this I/O... a hole in the
                         * file would be returned as a blkno of -1 with a non-zero io_size
                         * a real extent is returned with a blkno != -1 and a non-zero io_size
                         */
                        error = EINVAL;
                        break;
                }
                if ( !(flags & CL_READ) && blkno == -1) {
                        off_t   e_offset;
                        int     pageout_flags;

                        if (upl_get_internal_vectorupl(upl))
                                panic("Vector UPLs should not take this code-path\n");
                        /*
                         * we're writing into a 'hole'
                         */
                        if (flags & CL_PAGEOUT) {
                                /*
                                 * if we got here via cluster_pageout 
                                 * then just error the request and return
                                 * the 'hole' should already have been covered
                                 */
                                error = EINVAL;
                                break;
                        }
                        /*
                         * we can get here if the cluster code happens to 
                         * pick up a page that was dirtied via mmap vs
                         * a 'write' and the page targets a 'hole'...
                         * i.e. the writes to the cluster were sparse
                         * and the file was being written for the first time
                         *
                         * we can also get here if the filesystem supports
                         * 'holes' that are less than PAGE_SIZE.... because
                         * we can't know if the range in the page that covers
                         * the 'hole' has been dirtied via an mmap or not,
                         * we have to assume the worst and try to push the
                         * entire page to storage.
                         *
                         * Try paging out the page individually before
                         * giving up entirely and dumping it (the pageout
                         * path will insure that the zero extent accounting
                         * has been taken care of before we get back into cluster_io)
                         *
                         * go direct to vnode_pageout so that we don't have to
                         * unbusy the page from the UPL... we used to do this
                         * so that we could call ubc_msync, but that results
                         * in a potential deadlock if someone else races us to acquire
                         * that page and wins and in addition needs one of the pages
                         * we're continuing to hold in the UPL
                         */
                        pageout_flags = UPL_MSYNC | UPL_VNODE_PAGER | UPL_NESTED_PAGEOUT;

                        if ( !(flags & CL_ASYNC))
                                pageout_flags |= UPL_IOSYNC;
                        if ( !(flags & CL_COMMIT))
                                pageout_flags |= UPL_NOCOMMIT;

                        if (cbp_head) {
                                buf_t prev_cbp;
                                int   bytes_in_last_page;

                                /*
                                 * first we have to wait for the the current outstanding I/Os
                                 * to complete... EOT hasn't been set yet on this transaction
                                 * so the pages won't be released
                                 */
                                cluster_wait_IO(cbp_head, (flags & CL_ASYNC));

                                bytes_in_last_page = cbp_head->b_uploffset & PAGE_MASK;
                                for (cbp = cbp_head; cbp; cbp = cbp->b_trans_next)
                                        bytes_in_last_page += cbp->b_bcount;
                                bytes_in_last_page &= PAGE_MASK;
                                
                                while (bytes_in_last_page) {
                                        /*
                                         * we've got a transcation that
                                         * includes the page we're about to push out through vnode_pageout...
                                         * find the bp's in the list which intersect this page and either
                                         * remove them entirely from the transaction (there could be multiple bp's), or
                                         * round it's iosize down to the page boundary (there can only be one)...
                                         *
                                         * find the last bp in the list and act on it
                                         */
                                        for (prev_cbp = cbp = cbp_head; cbp->b_trans_next; cbp = cbp->b_trans_next)
                                                prev_cbp = cbp;

                                        if (bytes_in_last_page >= cbp->b_bcount) {
                                                /*
                                                 * this buf no longer has any I/O associated with it
                                                 */
                                                bytes_in_last_page -= cbp->b_bcount;
                                                cbp->b_bcount = 0;

                                                free_io_buf(cbp);

                                                if (cbp == cbp_head) {
                                                        assert(bytes_in_last_page == 0);
                                                        /*
                                                         * the buf we just freed was the only buf in
                                                         * this transaction... so there's no I/O to do
                                                         */
                                                        cbp_head = NULL;
                                                        cbp_tail = NULL;
                                                } else {
                                                        /*
                                                         * remove the buf we just freed from
                                                         * the transaction list
                                                         */
                                                        prev_cbp->b_trans_next = NULL;
                                                        cbp_tail = prev_cbp;
                                                }
                                        } else {
                                                /*
                                                 * this is the last bp that has I/O
                                                 * intersecting the page of interest
                                                 * only some of the I/O is in the intersection
                                                 * so clip the size but keep it in the transaction list
                                                 */
                                                cbp->b_bcount -= bytes_in_last_page;
                                                cbp_tail = cbp;
                                                bytes_in_last_page = 0;
                                        }
                                }
                                if (cbp_head) {
                                        /*
                                         * there was more to the current transaction
                                         * than just the page we are pushing out via vnode_pageout...
                                         * mark it as finished and complete it... we've already
                                         * waited for the I/Os to complete above in the call to cluster_wait_IO
                                         */
                                        cluster_EOT(cbp_head, cbp_tail, 0);

                                        cluster_complete_transaction(&cbp_head, callback_arg, &retval, flags, 0);

                                        trans_count = 0;
                                }
                        }
                        if (vnode_pageout(vp, upl, trunc_page(upl_offset), trunc_page_64(f_offset), PAGE_SIZE, pageout_flags, NULL) != PAGER_SUCCESS) {
                                error = EINVAL;
                        }
                        e_offset = round_page_64(f_offset + 1);
                        io_size = e_offset - f_offset;

                        f_offset   += io_size;
                        upl_offset += io_size;

                        if (size >= io_size)
                                size -= io_size;
                        else
                                size = 0;
                        /*
                         * keep track of how much of the original request
                         * that we've actually completed... non_rounded_size
                         * may go negative due to us rounding the request
                         * to a page size multiple (i.e.  size > non_rounded_size)
                         */
                        non_rounded_size -= io_size;

                        if (non_rounded_size <= 0) {
                                /*
                                 * we've transferred all of the data in the original
                                 * request, but we were unable to complete the tail
                                 * of the last page because the file didn't have
                                 * an allocation to back that portion... this is ok.
                                 */
                                size = 0;
                        }
                        if (error) {
                                if (size == 0)
                                        flags &= ~CL_COMMIT;
                                break;
                        }
                        continue;
                }
                lblkno = (daddr64_t)(f_offset / 0x1000);
                /*
                 * we have now figured out how much I/O we can do - this is in 'io_size'
                 * pg_offset is the starting point in the first page for the I/O
                 * pg_count is the number of full and partial pages that 'io_size' encompasses
                 */
                pg_offset = upl_offset & PAGE_MASK;

                if (flags & CL_DEV_MEMORY) {
                        /*
                         * treat physical requests as one 'giant' page
                         */
                        pg_count = 1;
                } else
                        pg_count  = (io_size + pg_offset + (PAGE_SIZE - 1)) / PAGE_SIZE;

                if ((flags & CL_READ) && blkno == -1) {
                        vm_offset_t  commit_offset;
                        int bytes_to_zero;
                        int complete_transaction_now = 0;

                        /*
                         * if we're reading and blkno == -1, then we've got a
                         * 'hole' in the file that we need to deal with by zeroing
                         * out the affected area in the upl
                         */
                        if (io_size >= (u_int)non_rounded_size) {
                                /*
                                 * if this upl contains the EOF and it is not a multiple of PAGE_SIZE
                                 * than 'zero_offset' will be non-zero
                                 * if the 'hole' returned by vnop_blockmap extends all the way to the eof
                                 * (indicated by the io_size finishing off the I/O request for this UPL)
                                 * than we're not going to issue an I/O for the
                                 * last page in this upl... we need to zero both the hole and the tail
                                 * of the page beyond the EOF, since the delayed zero-fill won't kick in 
                                 */
                                bytes_to_zero = non_rounded_size;
                                if (!(flags & CL_NOZERO))
                                        bytes_to_zero = (((upl_offset + io_size) + (PAGE_SIZE - 1)) & ~PAGE_MASK) - upl_offset;

                                zero_offset = 0;
                        } else
                                bytes_to_zero = io_size;

                        pg_count = 0;

                        cluster_zero(upl, upl_offset, bytes_to_zero, real_bp);
                          
                        if (cbp_head) {
                                int     pg_resid;

                                /*
                                 * if there is a current I/O chain pending
                                 * then the first page of the group we just zero'd
                                 * will be handled by the I/O completion if the zero
                                 * fill started in the middle of the page
                                 */
                                commit_offset = (upl_offset + (PAGE_SIZE - 1)) & ~PAGE_MASK;

                                pg_resid = commit_offset - upl_offset;
                                        
                                if (bytes_to_zero >= pg_resid) {
                                        /*
                                         * the last page of the current I/O 
                                         * has been completed...
                                         * compute the number of fully zero'd 
                                         * pages that are beyond it
                                         * plus the last page if its partial
                                         * and we have no more I/O to issue...
                                         * otherwise a partial page is left
                                         * to begin the next I/O
                                         */
                                        if ((int)io_size >= non_rounded_size)
                                                pg_count = (bytes_to_zero - pg_resid + (PAGE_SIZE - 1)) / PAGE_SIZE;
                                        else
                                                pg_count = (bytes_to_zero - pg_resid) / PAGE_SIZE;
                                        
                                        complete_transaction_now = 1;
                                }
                        } else {
                                /*
                                 * no pending I/O to deal with
                                 * so, commit all of the fully zero'd pages
                                 * plus the last page if its partial
                                 * and we have no more I/O to issue...
                                 * otherwise a partial page is left
                                 * to begin the next I/O
                                 */
                                if ((int)io_size >= non_rounded_size)
                                        pg_count = (pg_offset + bytes_to_zero + (PAGE_SIZE - 1)) / PAGE_SIZE;
                                else
                                        pg_count = (pg_offset + bytes_to_zero) / PAGE_SIZE;

                                commit_offset = upl_offset & ~PAGE_MASK;
                        }

                        // Associated UPL is currently only used in the direct write path
                        assert(!upl_associated_upl(upl));

                        if ( (flags & CL_COMMIT) && pg_count) {
                                ubc_upl_commit_range(upl, commit_offset, pg_count * PAGE_SIZE,
                                                     UPL_COMMIT_CLEAR_DIRTY | UPL_COMMIT_FREE_ON_EMPTY);
                        }
                        upl_offset += io_size;
                        f_offset   += io_size;
                        size       -= io_size;

                        /*
                         * keep track of how much of the original request
                         * that we've actually completed... non_rounded_size
                         * may go negative due to us rounding the request
                         * to a page size multiple (i.e.  size > non_rounded_size)
                         */
                        non_rounded_size -= io_size;

                        if (non_rounded_size <= 0) {
                                /*
                                 * we've transferred all of the data in the original
                                 * request, but we were unable to complete the tail
                                 * of the last page because the file didn't have
                                 * an allocation to back that portion... this is ok.
                                 */
                                size = 0;
                        }
                        if (cbp_head && (complete_transaction_now || size == 0))  {
                                cluster_wait_IO(cbp_head, (flags & CL_ASYNC));

                                cluster_EOT(cbp_head, cbp_tail, size == 0 ? zero_offset : 0);

                                cluster_complete_transaction(&cbp_head, callback_arg, &retval, flags, 0);

                                trans_count = 0;
                        }
                        continue;
                }
                if (pg_count > max_vectors) {
                        if (((pg_count - max_vectors) * PAGE_SIZE) > io_size) {
                                io_size = PAGE_SIZE - pg_offset;
                                pg_count = 1;
                        } else {
                                io_size -= (pg_count - max_vectors) * PAGE_SIZE;
                                pg_count = max_vectors;
                        }
                }
                /*
                 * If the transaction is going to reach the maximum number of
                 * desired elements, truncate the i/o to the nearest page so
                 * that the actual i/o is initiated after this buffer is
                 * created and added to the i/o chain.
                 *
                 * I/O directed to physically contiguous memory 
                 * doesn't have a requirement to make sure we 'fill' a page
                 */
                if ( !(flags & CL_DEV_MEMORY) && trans_count >= max_trans_count &&
                                ((upl_offset + io_size) & PAGE_MASK)) {
                        vm_offset_t aligned_ofs;

                        aligned_ofs = (upl_offset + io_size) & ~PAGE_MASK;
                        /*
                         * If the io_size does not actually finish off even a
                         * single page we have to keep adding buffers to the
                         * transaction despite having reached the desired limit.
                         *
                         * Eventually we get here with the page being finished
                         * off (and exceeded) and then we truncate the size of
                         * this i/o request so that it is page aligned so that
                         * we can finally issue the i/o on the transaction.
                         */
                        if (aligned_ofs > upl_offset) {
                                io_size = aligned_ofs - upl_offset;
                                pg_count--;
                        }
                }

                if ( !(mp->mnt_kern_flag & MNTK_VIRTUALDEV))
                        /*
                         * if we're not targeting a virtual device i.e. a disk image
                         * it's safe to dip into the reserve pool since real devices
                         * can complete this I/O request without requiring additional
                         * bufs from the alloc_io_buf pool
                         */
                        priv = 1;
                else if ((flags & CL_ASYNC) && !(flags & CL_PAGEOUT))
                        /*
                         * Throttle the speculative IO
                         */
                        priv = 0;
                else
                        priv = 1;

                cbp = alloc_io_buf(vp, priv);

                if (flags & CL_PAGEOUT) {
                        u_int i;

                        /*
                         * since blocks are in offsets of 0x1000, scale
                         * iteration to (PAGE_SIZE * pg_count) of blks.
                         */
                        for (i = 0; i < (PAGE_SIZE * pg_count)/0x1000; i++) {
                                if (buf_invalblkno(vp, lblkno + i, 0) == EBUSY)
                                        panic("BUSY bp found in cluster_io");
                        }
                }
                if (flags & CL_ASYNC) {
                        if (buf_setcallback(cbp, (void *)cluster_iodone, callback_arg))
                                panic("buf_setcallback failed\n");
                }
                cbp->b_cliodone = (void *)callback;
                cbp->b_flags |= io_flags;
                if (flags & CL_NOCACHE)
                        cbp->b_attr.ba_flags |= BA_NOCACHE;

                cbp->b_lblkno = lblkno;
                cbp->b_blkno  = blkno;
                cbp->b_bcount = io_size;

                if (buf_setupl(cbp, upl, upl_offset))
                        panic("buf_setupl failed\n");
#if CONFIG_IOSCHED
                upl_set_blkno(upl, upl_offset, io_size, blkno);
#endif
                cbp->b_trans_next = (buf_t)NULL;

                if ((cbp->b_iostate = (void *)iostate))
                        /*
                         * caller wants to track the state of this
                         * io... bump the amount issued against this stream
                         */
                        iostate->io_issued += io_size;

                if (flags & CL_READ) {
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 26)) | DBG_FUNC_NONE,
                                     (int)cbp->b_lblkno, (int)cbp->b_blkno, upl_offset, io_size, 0);
                }
                else {
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 27)) | DBG_FUNC_NONE,
                                     (int)cbp->b_lblkno, (int)cbp->b_blkno, upl_offset, io_size, 0);
                }

                if (cbp_head) {
                        cbp_tail->b_trans_next = cbp;
                        cbp_tail = cbp;
                } else {
                        cbp_head = cbp;
                        cbp_tail = cbp;

                        if ( (cbp_head->b_real_bp = real_bp) )
                                real_bp = (buf_t)NULL;
                }
                *(buf_t *)(&cbp->b_trans_head) = cbp_head;

                trans_count++;

                upl_offset += io_size;
                f_offset   += io_size;
                size       -= io_size;
                /*
                 * keep track of how much of the original request
                 * that we've actually completed... non_rounded_size
                 * may go negative due to us rounding the request
                 * to a page size multiple (i.e.  size > non_rounded_size)
                 */
                non_rounded_size -= io_size;

                if (non_rounded_size <= 0) {
                        /*
                         * we've transferred all of the data in the original
                         * request, but we were unable to complete the tail
                         * of the last page because the file didn't have
                         * an allocation to back that portion... this is ok.
                         */
                        size = 0;
                }
                if (size == 0) {
                        /*
                         * we have no more I/O to issue, so go
                         * finish the final transaction
                         */
                        need_EOT = TRUE;
                } else if ( ((flags & CL_DEV_MEMORY) || (upl_offset & PAGE_MASK) == 0) &&
                            ((flags & CL_ASYNC) || trans_count > max_trans_count) ) {
                        /*
                         * I/O directed to physically contiguous memory...
                         * which doesn't have a requirement to make sure we 'fill' a page
                         * or... 
                         * the current I/O we've prepared fully
                         * completes the last page in this request
                         * and ...
                         * it's either an ASYNC request or 
                         * we've already accumulated more than 8 I/O's into
                         * this transaction so mark it as complete so that
                         * it can finish asynchronously or via the cluster_complete_transaction
                         * below if the request is synchronous
                         */
                        need_EOT = TRUE;
                }
                if (need_EOT == TRUE)
                        cluster_EOT(cbp_head, cbp_tail, size == 0 ? zero_offset : 0);

                if (flags & CL_THROTTLE)
                        (void)vnode_waitforwrites(vp, async_throttle, 0, 0, "cluster_io");

                if ( !(io_flags & B_READ))
                        vnode_startwrite(vp);
                                
                if (flags & CL_RAW_ENCRYPTED) {
                        /* 
                         * User requested raw encrypted bytes.
                         * Twiddle the bit in the ba_flags for the buffer
                         */
                        cbp->b_attr.ba_flags |= BA_RAW_ENCRYPTED_IO;
                }
                
                (void) VNOP_STRATEGY(cbp);

                if (need_EOT == TRUE) {
                        if ( !(flags & CL_ASYNC))
                                cluster_complete_transaction(&cbp_head, callback_arg, &retval, flags, 1);

                        need_EOT = FALSE;
                        trans_count = 0;
                        cbp_head = NULL;
                }
        }
        if (error) {
                int abort_size;

                io_size = 0;

                if (cbp_head) {
                        /*
                         * Wait until all of the outstanding I/O
                         * for this partial transaction has completed
                         */
                        cluster_wait_IO(cbp_head, (flags & CL_ASYNC));

                        /*
                         * Rewind the upl offset to the beginning of the
                         * transaction.
                         */
                        upl_offset = cbp_head->b_uploffset;
                }

                if (ISSET(flags, CL_COMMIT)) {
                        cluster_handle_associated_upl(iostate, upl, upl_offset,
                                                                                  upl_end_offset - upl_offset);
                }

                // Free all the IO buffers in this transaction
                for (cbp = cbp_head; cbp;) {
                        buf_t   cbp_next;
 
                        size       += cbp->b_bcount;
                        io_size    += cbp->b_bcount;

                        cbp_next = cbp->b_trans_next;
                        free_io_buf(cbp);
                        cbp = cbp_next;
                }

                if (iostate) {
                        int need_wakeup = 0;

                        /*
                         * update the error condition for this stream
                         * since we never really issued the io
                         * just go ahead and adjust it back
                         */
                        lck_mtx_lock_spin(&iostate->io_mtxp);

                        if (iostate->io_error == 0)
                                iostate->io_error = error;
                        iostate->io_issued -= io_size;

                        if (iostate->io_wanted) {
                                /*
                                 * someone is waiting for the state of
                                 * this io stream to change
                                 */
                                iostate->io_wanted = 0;
                                need_wakeup = 1;
                        }
                        lck_mtx_unlock(&iostate->io_mtxp);

                        if (need_wakeup)
                                wakeup((caddr_t)&iostate->io_wanted);
                }

                if (flags & CL_COMMIT) {
                        int     upl_flags;

                        pg_offset  = upl_offset & PAGE_MASK;
                        abort_size = (upl_end_offset - upl_offset + PAGE_MASK) & ~PAGE_MASK;

                        upl_flags = cluster_ioerror(upl, upl_offset - pg_offset, abort_size, error, io_flags, vp);
                        
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 28)) | DBG_FUNC_NONE,
                                     upl, upl_offset - pg_offset, abort_size, (error << 24) | upl_flags, 0);
                }
                if (retval == 0)
                        retval = error;
        } else if (cbp_head)
                        panic("%s(): cbp_head is not NULL.\n", __FUNCTION__);

        if (real_bp) {
                /*
                 * can get here if we either encountered an error
                 * or we completely zero-filled the request and
                 * no I/O was issued
                 */
                if (error) {
                        real_bp->b_flags |= B_ERROR;
                        real_bp->b_error = error;
                }
                buf_biodone(real_bp);
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 22)) | DBG_FUNC_END, (int)f_offset, size, upl_offset, retval, 0);

        return (retval);
}

#define reset_vector_run_state()                                                                                \
        issueVectorUPL = vector_upl_offset = vector_upl_index = vector_upl_iosize = vector_upl_size = 0;        

static int
vector_cluster_io(vnode_t vp, upl_t vector_upl, vm_offset_t vector_upl_offset, off_t v_upl_uio_offset, int vector_upl_iosize,
           int io_flag, buf_t real_bp, struct clios *iostate, int (*callback)(buf_t, void *), void *callback_arg)
{
        vector_upl_set_pagelist(vector_upl);

        if(io_flag & CL_READ) { 
                if(vector_upl_offset == 0 && ((vector_upl_iosize & PAGE_MASK)==0))
                        io_flag &= ~CL_PRESERVE; /*don't zero fill*/
                else
                        io_flag |= CL_PRESERVE; /*zero fill*/
        }       
        return (cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize, io_flag, real_bp, iostate, callback, callback_arg));

}

static int
cluster_read_prefetch(vnode_t vp, off_t f_offset, u_int size, off_t filesize, int (*callback)(buf_t, void *), void *callback_arg, int bflag)
{
        int           pages_in_prefetch;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 49)) | DBG_FUNC_START,
                     (int)f_offset, size, (int)filesize, 0, 0);

        if (f_offset >= filesize) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 49)) | DBG_FUNC_END,
                             (int)f_offset, 0, 0, 0, 0);
                return(0);
        }
        if ((off_t)size > (filesize - f_offset))
                size = filesize - f_offset;
        pages_in_prefetch = (size + (PAGE_SIZE - 1)) / PAGE_SIZE;

        advisory_read_ext(vp, filesize, f_offset, size, callback, callback_arg, bflag);

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 49)) | DBG_FUNC_END,
                     (int)f_offset + size, pages_in_prefetch, 0, 1, 0);

        return (pages_in_prefetch);
}



static void
cluster_read_ahead(vnode_t vp, struct cl_extent *extent, off_t filesize, struct cl_readahead *rap, int (*callback)(buf_t, void *), void *callback_arg,
                   int bflag)
{
        daddr64_t       r_addr;
        off_t           f_offset;
        int             size_of_prefetch;
        u_int           max_prefetch;


        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_START,
                     (int)extent->b_addr, (int)extent->e_addr, (int)rap->cl_lastr, 0, 0);

        if (extent->b_addr == rap->cl_lastr && extent->b_addr == extent->e_addr) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_END,
                             rap->cl_ralen, (int)rap->cl_maxra, (int)rap->cl_lastr, 0, 0);
                return;
        }
        if (rap->cl_lastr == -1 || (extent->b_addr != rap->cl_lastr && extent->b_addr != (rap->cl_lastr + 1))) {
                rap->cl_ralen = 0;
                rap->cl_maxra = 0;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_END,
                             rap->cl_ralen, (int)rap->cl_maxra, (int)rap->cl_lastr, 1, 0);

                return;
        }
        max_prefetch = MAX_PREFETCH(vp, cluster_max_io_size(vp->v_mount, CL_READ), disk_conditioner_mount_is_ssd(vp->v_mount));

        if (max_prefetch > speculative_prefetch_max)
                max_prefetch = speculative_prefetch_max;

        if (max_prefetch <= PAGE_SIZE) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_END,
                             rap->cl_ralen, (int)rap->cl_maxra, (int)rap->cl_lastr, 6, 0);
                return;
        }
        if (extent->e_addr < rap->cl_maxra && rap->cl_ralen >= 4) {
                if ((rap->cl_maxra - extent->e_addr) > (rap->cl_ralen / 4)) {

                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_END,
                                     rap->cl_ralen, (int)rap->cl_maxra, (int)rap->cl_lastr, 2, 0);
                        return;
                }
        }
        r_addr = max(extent->e_addr, rap->cl_maxra) + 1;
        f_offset = (off_t)(r_addr * PAGE_SIZE_64);

        size_of_prefetch = 0;

        ubc_range_op(vp, f_offset, f_offset + PAGE_SIZE_64, UPL_ROP_PRESENT, &size_of_prefetch);

        if (size_of_prefetch) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_END,
                             rap->cl_ralen, (int)rap->cl_maxra, (int)rap->cl_lastr, 3, 0);
                return;
        }
        if (f_offset < filesize) {
                daddr64_t read_size;

                rap->cl_ralen = rap->cl_ralen ? min(max_prefetch / PAGE_SIZE, rap->cl_ralen << 1) : 1;

                read_size = (extent->e_addr + 1) - extent->b_addr;

                if (read_size > rap->cl_ralen) {
                        if (read_size > max_prefetch / PAGE_SIZE)
                                rap->cl_ralen = max_prefetch / PAGE_SIZE;
                        else
                                rap->cl_ralen = read_size;
                }
                size_of_prefetch = cluster_read_prefetch(vp, f_offset, rap->cl_ralen * PAGE_SIZE, filesize, callback, callback_arg, bflag);

                if (size_of_prefetch)
                        rap->cl_maxra = (r_addr + size_of_prefetch) - 1;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 48)) | DBG_FUNC_END,
                     rap->cl_ralen, (int)rap->cl_maxra, (int)rap->cl_lastr, 4, 0);
}


int
cluster_pageout(vnode_t vp, upl_t upl, upl_offset_t upl_offset, off_t f_offset,
                int size, off_t filesize, int flags)
{
        return cluster_pageout_ext(vp, upl, upl_offset, f_offset, size, filesize, flags, NULL, NULL);

}


int
cluster_pageout_ext(vnode_t vp, upl_t upl, upl_offset_t upl_offset, off_t f_offset,
                int size, off_t filesize, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        int           io_size;
        int           rounded_size;
        off_t         max_size;
        int           local_flags;

        local_flags = CL_PAGEOUT | CL_THROTTLE;

        if ((flags & UPL_IOSYNC) == 0) 
                local_flags |= CL_ASYNC;
        if ((flags & UPL_NOCOMMIT) == 0) 
                local_flags |= CL_COMMIT;
        if ((flags & UPL_KEEPCACHED))
                local_flags |= CL_KEEPCACHED;
        if (flags & UPL_PAGING_ENCRYPTED)
                local_flags |= CL_ENCRYPTED;


        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 52)) | DBG_FUNC_NONE,
                     (int)f_offset, size, (int)filesize, local_flags, 0);

        /*
         * If they didn't specify any I/O, then we are done...
         * we can't issue an abort because we don't know how
         * big the upl really is
         */
        if (size <= 0)
                return (EINVAL);

        if (vp->v_mount->mnt_flag & MNT_RDONLY) {
                if (local_flags & CL_COMMIT)
                        ubc_upl_abort_range(upl, upl_offset, size, UPL_ABORT_FREE_ON_EMPTY);
                return (EROFS);
        }
        /*
         * can't page-in from a negative offset
         * or if we're starting beyond the EOF
         * or if the file offset isn't page aligned
         * or the size requested isn't a multiple of PAGE_SIZE
         */
        if (f_offset < 0 || f_offset >= filesize ||
           (f_offset & PAGE_MASK_64) || (size & PAGE_MASK)) {
                if (local_flags & CL_COMMIT)
                        ubc_upl_abort_range(upl, upl_offset, size, UPL_ABORT_FREE_ON_EMPTY);
                return (EINVAL);
        }
        max_size = filesize - f_offset;

        if (size < max_size)
                io_size = size;
        else
                io_size = max_size;

        rounded_size = (io_size + (PAGE_SIZE - 1)) & ~PAGE_MASK;

        if (size > rounded_size) {
                if (local_flags & CL_COMMIT)
                        ubc_upl_abort_range(upl, upl_offset + rounded_size, size - rounded_size,
                                        UPL_ABORT_FREE_ON_EMPTY);
        }
        return (cluster_io(vp, upl, upl_offset, f_offset, io_size,
                           local_flags, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg));
}


int
cluster_pagein(vnode_t vp, upl_t upl, upl_offset_t upl_offset, off_t f_offset,
               int size, off_t filesize, int flags)
{
        return cluster_pagein_ext(vp, upl, upl_offset, f_offset, size, filesize, flags, NULL, NULL);
}


int
cluster_pagein_ext(vnode_t vp, upl_t upl, upl_offset_t upl_offset, off_t f_offset,
               int size, off_t filesize, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        u_int         io_size;
        int           rounded_size;
        off_t         max_size;
        int           retval;
        int           local_flags = 0;

        if (upl == NULL || size < 0)
                panic("cluster_pagein: NULL upl passed in");

        if ((flags & UPL_IOSYNC) == 0)
                local_flags |= CL_ASYNC;
        if ((flags & UPL_NOCOMMIT) == 0) 
                local_flags |= CL_COMMIT;
        if (flags & UPL_IOSTREAMING)
                local_flags |= CL_IOSTREAMING;
        if (flags & UPL_PAGING_ENCRYPTED)
                local_flags |= CL_ENCRYPTED;


        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 56)) | DBG_FUNC_NONE,
                     (int)f_offset, size, (int)filesize, local_flags, 0);

        /*
         * can't page-in from a negative offset
         * or if we're starting beyond the EOF
         * or if the file offset isn't page aligned
         * or the size requested isn't a multiple of PAGE_SIZE
         */
        if (f_offset < 0 || f_offset >= filesize ||
           (f_offset & PAGE_MASK_64) || (size & PAGE_MASK) || (upl_offset & PAGE_MASK)) {
                if (local_flags & CL_COMMIT)
                        ubc_upl_abort_range(upl, upl_offset, size, UPL_ABORT_FREE_ON_EMPTY | UPL_ABORT_ERROR);
                return (EINVAL);
        }
        max_size = filesize - f_offset;

        if (size < max_size)
                io_size = size;
        else
                io_size = max_size;

        rounded_size = (io_size + (PAGE_SIZE - 1)) & ~PAGE_MASK;

        if (size > rounded_size && (local_flags & CL_COMMIT))
                ubc_upl_abort_range(upl, upl_offset + rounded_size,
                                    size - rounded_size, UPL_ABORT_FREE_ON_EMPTY | UPL_ABORT_ERROR);
        
        retval = cluster_io(vp, upl, upl_offset, f_offset, io_size,
                            local_flags | CL_READ | CL_PAGEIN, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);

        return (retval);
}


int
cluster_bp(buf_t bp)
{
       return cluster_bp_ext(bp, NULL, NULL);
}


int
cluster_bp_ext(buf_t bp, int (*callback)(buf_t, void *), void *callback_arg)
{
        off_t  f_offset;
        int    flags;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 19)) | DBG_FUNC_START,
                     bp, (int)bp->b_lblkno, bp->b_bcount, bp->b_flags, 0);

        if (bp->b_flags & B_READ)
                flags = CL_ASYNC | CL_READ;
        else
                flags = CL_ASYNC;
        if (bp->b_flags & B_PASSIVE) 
                flags |= CL_PASSIVE;

        f_offset = ubc_blktooff(bp->b_vp, bp->b_lblkno);

        return (cluster_io(bp->b_vp, bp->b_upl, 0, f_offset, bp->b_bcount, flags, bp, (struct clios *)NULL, callback, callback_arg));
}



int
cluster_write(vnode_t vp, struct uio *uio, off_t oldEOF, off_t newEOF, off_t headOff, off_t tailOff, int xflags)
{
        return cluster_write_ext(vp, uio, oldEOF, newEOF, headOff, tailOff, xflags, NULL, NULL);
}


int
cluster_write_ext(vnode_t vp, struct uio *uio, off_t oldEOF, off_t newEOF, off_t headOff, off_t tailOff,
                  int xflags, int (*callback)(buf_t, void *), void *callback_arg)
{
        user_ssize_t    cur_resid;
        int             retval = 0;
        int             flags;
        int             zflags;
        int             bflag;
        int             write_type = IO_COPY;
        u_int32_t       write_length;

        flags = xflags;

        if (flags & IO_PASSIVE)
                bflag = CL_PASSIVE;
        else
                bflag = 0;

        if (vp->v_flag & VNOCACHE_DATA){
                flags |= IO_NOCACHE;
                bflag |= CL_NOCACHE;
        }
        if (uio == NULL) {
                /*
                 * no user data...
                 * this call is being made to zero-fill some range in the file
                 */
                retval = cluster_write_copy(vp, NULL, (u_int32_t)0, oldEOF, newEOF, headOff, tailOff, flags, callback, callback_arg);

                return(retval);
        }
        /*
         * do a write through the cache if one of the following is true....
         *   NOCACHE is not true or NODIRECT is true
         *   the uio request doesn't target USERSPACE
         * otherwise, find out if we want the direct or contig variant for
         * the first vector in the uio request
         */
        if ( ((flags & (IO_NOCACHE | IO_NODIRECT)) == IO_NOCACHE) && UIO_SEG_IS_USER_SPACE(uio->uio_segflg) )
                retval = cluster_io_type(uio, &write_type, &write_length, MIN_DIRECT_WRITE_SIZE);

        if ( (flags & (IO_TAILZEROFILL | IO_HEADZEROFILL)) && write_type == IO_DIRECT)
                /*
                 * must go through the cached variant in this case
                 */
                write_type = IO_COPY;

        while ((cur_resid = uio_resid(uio)) && uio->uio_offset < newEOF && retval == 0) {
          
                switch (write_type) {

                case IO_COPY:
                        /*
                         * make sure the uio_resid isn't too big...
                         * internally, we want to handle all of the I/O in
                         * chunk sizes that fit in a 32 bit int
                         */
                        if (cur_resid > (user_ssize_t)(MAX_IO_REQUEST_SIZE)) {
                                /*
                                 * we're going to have to call cluster_write_copy
                                 * more than once...
                                 *
                                 * only want the last call to cluster_write_copy to
                                 * have the IO_TAILZEROFILL flag set and only the
                                 * first call should have IO_HEADZEROFILL
                                 */
                                zflags = flags & ~IO_TAILZEROFILL;
                                flags &= ~IO_HEADZEROFILL;

                                write_length = MAX_IO_REQUEST_SIZE;
                        } else {
                                /*
                                 * last call to cluster_write_copy
                                 */
                                zflags = flags;
                          
                                write_length = (u_int32_t)cur_resid;
                        }
                        retval = cluster_write_copy(vp, uio, write_length, oldEOF, newEOF, headOff, tailOff, zflags, callback, callback_arg);
                        break;

                case IO_CONTIG:
                        zflags = flags & ~(IO_TAILZEROFILL | IO_HEADZEROFILL);

                        if (flags & IO_HEADZEROFILL) {
                                /*
                                 * only do this once per request
                                 */
                                flags &= ~IO_HEADZEROFILL;

                                retval = cluster_write_copy(vp, (struct uio *)0, (u_int32_t)0, (off_t)0, uio->uio_offset,
                                                            headOff, (off_t)0, zflags | IO_HEADZEROFILL | IO_SYNC, callback, callback_arg);
                                if (retval)
                                        break;
                        }
                        retval = cluster_write_contig(vp, uio, newEOF, &write_type, &write_length, callback, callback_arg, bflag);

                        if (retval == 0 && (flags & IO_TAILZEROFILL) && uio_resid(uio) == 0) {
                                /*
                                 * we're done with the data from the user specified buffer(s)
                                 * and we've been requested to zero fill at the tail
                                 * treat this as an IO_HEADZEROFILL which doesn't require a uio
                                 * by rearranging the args and passing in IO_HEADZEROFILL
                                 */
                                retval = cluster_write_copy(vp, (struct uio *)0, (u_int32_t)0, (off_t)0, tailOff, uio->uio_offset,
                                                            (off_t)0, zflags | IO_HEADZEROFILL | IO_SYNC, callback, callback_arg);
                        }
                        break;

                case IO_DIRECT:
                        /*
                         * cluster_write_direct is never called with IO_TAILZEROFILL || IO_HEADZEROFILL
                         */
                        retval = cluster_write_direct(vp, uio, oldEOF, newEOF, &write_type, &write_length, flags, callback, callback_arg);
                        break;

                case IO_UNKNOWN:
                        retval = cluster_io_type(uio, &write_type, &write_length, MIN_DIRECT_WRITE_SIZE);
                        break;
                }
                /*
                 * in case we end up calling cluster_write_copy (from cluster_write_direct)
                 * multiple times to service a multi-vector request that is not aligned properly
                 * we need to update the oldEOF so that we
                 * don't zero-fill the head of a page if we've successfully written
                 * data to that area... 'cluster_write_copy' will zero-fill the head of a
                 * page that is beyond the oldEOF if the write is unaligned... we only
                 * want that to happen for the very first page of the cluster_write, 
                 * NOT the first page of each vector making up a multi-vector write.
                 */
                if (uio->uio_offset > oldEOF)
                        oldEOF = uio->uio_offset;
        }
        return (retval);
}


static int
cluster_write_direct(vnode_t vp, struct uio *uio, off_t oldEOF, off_t newEOF, int *write_type, u_int32_t *write_length,
                     int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        upl_t            upl;
        upl_page_info_t  *pl;
        vm_offset_t      upl_offset;
        vm_offset_t      vector_upl_offset = 0;
        u_int32_t        io_req_size;
        u_int32_t        offset_in_file;
        u_int32_t        offset_in_iovbase;
        u_int32_t        io_size;
        int              io_flag = 0;
        upl_size_t       upl_size, vector_upl_size = 0;
        vm_size_t        upl_needed_size;
        mach_msg_type_number_t  pages_in_pl;
        upl_control_flags_t upl_flags;
        kern_return_t    kret;
        mach_msg_type_number_t  i;
        int              force_data_sync;
        int              retval = 0;
        int              first_IO = 1;
        struct clios     iostate;
        user_addr_t      iov_base;
        u_int32_t        mem_alignment_mask;
        u_int32_t        devblocksize;
        u_int32_t        max_io_size;
        u_int32_t        max_upl_size;
        u_int32_t        max_vector_size;
        u_int32_t        bytes_outstanding_limit;
        boolean_t        io_throttled = FALSE;

        u_int32_t        vector_upl_iosize = 0;
        int              issueVectorUPL = 0,useVectorUPL = (uio->uio_iovcnt > 1);
        off_t            v_upl_uio_offset = 0;
        int              vector_upl_index=0;
        upl_t            vector_upl = NULL;


        /*
         * When we enter this routine, we know
         *  -- the resid will not exceed iov_len
         */
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 75)) | DBG_FUNC_START,
                     (int)uio->uio_offset, *write_length, (int)newEOF, 0, 0);

        max_upl_size = cluster_max_io_size(vp->v_mount, CL_WRITE);

        io_flag = CL_ASYNC | CL_PRESERVE | CL_COMMIT | CL_THROTTLE | CL_DIRECT_IO;

        if (flags & IO_PASSIVE)
                io_flag |= CL_PASSIVE;
        
        if (flags & IO_NOCACHE)
                io_flag |= CL_NOCACHE;
        
        if (flags & IO_SKIP_ENCRYPTION)
                io_flag |= CL_ENCRYPTED;

        iostate.io_completed = 0;
        iostate.io_issued = 0;
        iostate.io_error = 0;
        iostate.io_wanted = 0;

        lck_mtx_init(&iostate.io_mtxp, cl_mtx_grp, cl_mtx_attr);

        mem_alignment_mask = (u_int32_t)vp->v_mount->mnt_alignmentmask;
        devblocksize = (u_int32_t)vp->v_mount->mnt_devblocksize;

        if (devblocksize == 1) {
               /*
                * the AFP client advertises a devblocksize of 1
                * however, its BLOCKMAP routine maps to physical
                * blocks that are PAGE_SIZE in size...
                * therefore we can't ask for I/Os that aren't page aligned
                * or aren't multiples of PAGE_SIZE in size
                * by setting devblocksize to PAGE_SIZE, we re-instate
                * the old behavior we had before the mem_alignment_mask
                * changes went in...
                */
               devblocksize = PAGE_SIZE;
        }

next_dwrite:
        io_req_size = *write_length;
        iov_base = uio_curriovbase(uio);

        offset_in_file = (u_int32_t)uio->uio_offset & PAGE_MASK;
        offset_in_iovbase = (u_int32_t)iov_base & mem_alignment_mask;

        if (offset_in_file || offset_in_iovbase) {
                /*
                 * one of the 2 important offsets is misaligned
                 * so fire an I/O through the cache for this entire vector
                 */
                goto wait_for_dwrites;
        }
        if (iov_base & (devblocksize - 1)) {
                /*
                 * the offset in memory must be on a device block boundary
                 * so that we can guarantee that we can generate an
                 * I/O that ends on a page boundary in cluster_io
                 */
                goto wait_for_dwrites;
        }

        task_update_logical_writes(current_task(), (io_req_size & ~PAGE_MASK), TASK_WRITE_IMMEDIATE, vp);
        while (io_req_size >= PAGE_SIZE && uio->uio_offset < newEOF && retval == 0) {
                int     throttle_type;

                if ( (throttle_type = cluster_is_throttled(vp)) ) {
                        /*
                         * we're in the throttle window, at the very least
                         * we want to limit the size of the I/O we're about
                         * to issue
                         */
                        if ( (flags & IO_RETURN_ON_THROTTLE) && throttle_type == THROTTLE_NOW) {
                                /*
                                 * we're in the throttle window and at least 1 I/O
                                 * has already been issued by a throttleable thread
                                 * in this window, so return with EAGAIN to indicate
                                 * to the FS issuing the cluster_write call that it
                                 * should now throttle after dropping any locks
                                 */
                                throttle_info_update_by_mount(vp->v_mount);

                                io_throttled = TRUE;
                                goto wait_for_dwrites;
                        }
                        max_vector_size = THROTTLE_MAX_IOSIZE;
                        max_io_size = THROTTLE_MAX_IOSIZE;
                } else {
                        max_vector_size = MAX_VECTOR_UPL_SIZE;
                        max_io_size = max_upl_size;
                }

                if (first_IO) {
                        cluster_syncup(vp, newEOF, callback, callback_arg, callback ? PUSH_SYNC : 0);
                        first_IO = 0;
                }
                io_size  = io_req_size & ~PAGE_MASK;
                iov_base = uio_curriovbase(uio);

                if (io_size > max_io_size)
                        io_size = max_io_size;

                if(useVectorUPL && (iov_base & PAGE_MASK)) {
                        /*
                         * We have an iov_base that's not page-aligned.
                         * Issue all I/O's that have been collected within 
                         * this Vectored UPL.
                         */
                        if(vector_upl_index) {
                                retval = vector_cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize, io_flag, (buf_t)NULL, &iostate, callback, callback_arg);
                                reset_vector_run_state();
                        }
                        
                       /*
                        * After this point, if we are using the Vector UPL path and the base is
                        * not page-aligned then the UPL with that base will be the first in the vector UPL.
                        */
                }

                upl_offset = (vm_offset_t)((u_int32_t)iov_base & PAGE_MASK);
                upl_needed_size = (upl_offset + io_size + (PAGE_SIZE -1)) & ~PAGE_MASK;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 76)) | DBG_FUNC_START,
                             (int)upl_offset, upl_needed_size, (int)iov_base, io_size, 0);

                vm_map_t map = UIO_SEG_IS_USER_SPACE(uio->uio_segflg) ? current_map() : kernel_map;
                for (force_data_sync = 0; force_data_sync < 3; force_data_sync++) {
                        pages_in_pl = 0;
                        upl_size = upl_needed_size;
                        upl_flags = UPL_FILE_IO | UPL_COPYOUT_FROM | UPL_NO_SYNC |
                                    UPL_CLEAN_IN_PLACE | UPL_SET_INTERNAL | UPL_SET_LITE | UPL_SET_IO_WIRE;

                        kret = vm_map_get_upl(map,
                                              (vm_map_offset_t)(iov_base & ~((user_addr_t)PAGE_MASK)),
                                              &upl_size,
                                              &upl, 
                                              NULL, 
                                              &pages_in_pl,
                                              &upl_flags,
                                              VM_KERN_MEMORY_FILE,
                                              force_data_sync);

                        if (kret != KERN_SUCCESS) {
                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 76)) | DBG_FUNC_END,
                                             0, 0, 0, kret, 0);
                                /*
                                 * failed to get pagelist
                                 *
                                 * we may have already spun some portion of this request
                                 * off as async requests... we need to wait for the I/O
                                 * to complete before returning
                                 */
                                goto wait_for_dwrites;
                        }
                        pl = UPL_GET_INTERNAL_PAGE_LIST(upl);
                        pages_in_pl = upl_size / PAGE_SIZE;

                        for (i = 0; i < pages_in_pl; i++) {
                                if (!upl_valid_page(pl, i))
                                        break;            
                        }
                        if (i == pages_in_pl)
                                break;

                        /*
                         * didn't get all the pages back that we
                         * needed... release this upl and try again
                         */
                        ubc_upl_abort(upl, 0);
                }
                if (force_data_sync >= 3) {
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 76)) | DBG_FUNC_END,
                                     i, pages_in_pl, upl_size, kret, 0);
                        /*
                         * for some reason, we couldn't acquire a hold on all
                         * the pages needed in the user's address space
                         *
                         * we may have already spun some portion of this request
                         * off as async requests... we need to wait for the I/O
                         * to complete before returning
                         */
                        goto wait_for_dwrites;
                }

                /*
                 * Consider the possibility that upl_size wasn't satisfied.
                 */
                if (upl_size < upl_needed_size) {
                        if (upl_size && upl_offset == 0)
                                io_size = upl_size;
                        else
                                io_size = 0;
                }
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 76)) | DBG_FUNC_END,
                             (int)upl_offset, upl_size, (int)iov_base, io_size, 0);                    

                if (io_size == 0) {
                        ubc_upl_abort(upl, 0);
                        /*
                         * we may have already spun some portion of this request
                         * off as async requests... we need to wait for the I/O
                         * to complete before returning
                         */
                        goto wait_for_dwrites;
                }
                
                if(useVectorUPL) {
                        vm_offset_t end_off = ((iov_base + io_size) & PAGE_MASK);
                        if(end_off)
                                issueVectorUPL = 1;
                        /*
                         * After this point, if we are using a vector UPL, then
                         * either all the UPL elements end on a page boundary OR
                         * this UPL is the last element because it does not end
                         * on a page boundary.
                         */
                }

                /*
                 * we want push out these writes asynchronously so that we can overlap
                 * the preparation of the next I/O
                 * if there are already too many outstanding writes
                 * wait until some complete before issuing the next
                 */
                if (vp->v_mount->mnt_minsaturationbytecount)
                        bytes_outstanding_limit = vp->v_mount->mnt_minsaturationbytecount;
                else
                        bytes_outstanding_limit = max_upl_size * IO_SCALE(vp, 2);

                cluster_iostate_wait(&iostate, bytes_outstanding_limit, "cluster_write_direct");

                if (iostate.io_error) {
                        /*
                         * one of the earlier writes we issued ran into a hard error
                         * don't issue any more writes, cleanup the UPL
                         * that was just created but not used, then
                         * go wait for all writes that are part of this stream
                         * to complete before returning the error to the caller
                         */
                        ubc_upl_abort(upl, 0);

                        goto wait_for_dwrites;
                }

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 77)) | DBG_FUNC_START,
                             (int)upl_offset, (int)uio->uio_offset, io_size, io_flag, 0);

                if(!useVectorUPL)
                        retval = cluster_io(vp, upl, upl_offset, uio->uio_offset,
                                   io_size, io_flag, (buf_t)NULL, &iostate, callback, callback_arg);

                else {
                        if(!vector_upl_index) {
                                vector_upl = vector_upl_create(upl_offset);
                                v_upl_uio_offset = uio->uio_offset;
                                vector_upl_offset = upl_offset;
                        }

                        vector_upl_set_subupl(vector_upl,upl,upl_size);
                        vector_upl_set_iostate(vector_upl, upl, vector_upl_size, upl_size);
                        vector_upl_index++;
                        vector_upl_iosize += io_size;
                        vector_upl_size += upl_size;

                        if(issueVectorUPL || vector_upl_index ==  MAX_VECTOR_UPL_ELEMENTS || vector_upl_size >= max_vector_size) {
                                retval = vector_cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize, io_flag, (buf_t)NULL, &iostate, callback, callback_arg);
                                reset_vector_run_state();
                        }
                }       

                /*
                 * update the uio structure to
                 * reflect the I/O that we just issued
                 */
                uio_update(uio, (user_size_t)io_size);

                /*
                 * in case we end up calling through to cluster_write_copy to finish
                 * the tail of this request, we need to update the oldEOF so that we
                 * don't zero-fill the head of a page if we've successfully written
                 * data to that area... 'cluster_write_copy' will zero-fill the head of a
                 * page that is beyond the oldEOF if the write is unaligned... we only
                 * want that to happen for the very first page of the cluster_write, 
                 * NOT the first page of each vector making up a multi-vector write.
                 */
                if (uio->uio_offset > oldEOF)
                        oldEOF = uio->uio_offset;

                io_req_size -= io_size;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 77)) | DBG_FUNC_END,
                             (int)upl_offset, (int)uio->uio_offset, io_req_size, retval, 0);

        } /* end while */

        if (retval == 0 && iostate.io_error == 0 && io_req_size == 0) {

                retval = cluster_io_type(uio, write_type, write_length, MIN_DIRECT_WRITE_SIZE);

                if (retval == 0 && *write_type == IO_DIRECT) {

                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 75)) | DBG_FUNC_NONE,
                                     (int)uio->uio_offset, *write_length, (int)newEOF, 0, 0);

                        goto next_dwrite;
                }
        }

wait_for_dwrites:

        if (retval == 0 && iostate.io_error == 0 && useVectorUPL && vector_upl_index) {
                retval = vector_cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize, io_flag, (buf_t)NULL, &iostate, callback, callback_arg);
                reset_vector_run_state();       
        }
        /*
         * make sure all async writes issued as part of this stream
         * have completed before we return
         */
        cluster_iostate_wait(&iostate, 0, "cluster_write_direct");

        if (iostate.io_error)
                retval = iostate.io_error;

        lck_mtx_destroy(&iostate.io_mtxp, cl_mtx_grp);

        if (io_throttled == TRUE && retval == 0)
                retval = EAGAIN;

        if (io_req_size && retval == 0) {
                /*
                 * we couldn't handle the tail of this request in DIRECT mode
                 * so fire it through the copy path
                 *
                 * note that flags will never have IO_HEADZEROFILL or IO_TAILZEROFILL set
                 * so we can just pass 0 in for the headOff and tailOff
                 */
                if (uio->uio_offset > oldEOF)
                        oldEOF = uio->uio_offset;

                retval = cluster_write_copy(vp, uio, io_req_size, oldEOF, newEOF, (off_t)0, (off_t)0, flags, callback, callback_arg);

                *write_type = IO_UNKNOWN;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 75)) | DBG_FUNC_END,
                     (int)uio->uio_offset, io_req_size, retval, 4, 0);

        return (retval);
}


static int
cluster_write_contig(vnode_t vp, struct uio *uio, off_t newEOF, int *write_type, u_int32_t *write_length,
                     int (*callback)(buf_t, void *), void *callback_arg, int bflag)
{
        upl_page_info_t *pl;
        addr64_t         src_paddr = 0;
        upl_t            upl[MAX_VECTS];
        vm_offset_t      upl_offset;
        u_int32_t        tail_size = 0;
        u_int32_t        io_size;
        u_int32_t        xsize;
        upl_size_t       upl_size;
        vm_size_t        upl_needed_size;
        mach_msg_type_number_t  pages_in_pl;
        upl_control_flags_t upl_flags;
        kern_return_t    kret;
        struct clios     iostate;
        int              error  = 0;
        int              cur_upl = 0;
        int              num_upl = 0;
        int              n;
        user_addr_t      iov_base;
        u_int32_t        devblocksize;
        u_int32_t        mem_alignment_mask;

        /*
         * When we enter this routine, we know
         *  -- the io_req_size will not exceed iov_len
         *  -- the target address is physically contiguous
         */
        cluster_syncup(vp, newEOF, callback, callback_arg, callback ? PUSH_SYNC : 0);

        devblocksize = (u_int32_t)vp->v_mount->mnt_devblocksize;
        mem_alignment_mask = (u_int32_t)vp->v_mount->mnt_alignmentmask;

        iostate.io_completed = 0;
        iostate.io_issued = 0;
        iostate.io_error = 0;
        iostate.io_wanted = 0;

        lck_mtx_init(&iostate.io_mtxp, cl_mtx_grp, cl_mtx_attr);

next_cwrite:
        io_size = *write_length;

        iov_base = uio_curriovbase(uio);

        upl_offset = (vm_offset_t)((u_int32_t)iov_base & PAGE_MASK);
        upl_needed_size = upl_offset + io_size;

        pages_in_pl = 0;
        upl_size = upl_needed_size;
        upl_flags = UPL_FILE_IO | UPL_COPYOUT_FROM | UPL_NO_SYNC | 
                    UPL_CLEAN_IN_PLACE | UPL_SET_INTERNAL | UPL_SET_LITE | UPL_SET_IO_WIRE;

        vm_map_t map = UIO_SEG_IS_USER_SPACE(uio->uio_segflg) ? current_map() : kernel_map;
        kret = vm_map_get_upl(map,
                              (vm_map_offset_t)(iov_base & ~((user_addr_t)PAGE_MASK)),
                              &upl_size, &upl[cur_upl], NULL, &pages_in_pl, &upl_flags, VM_KERN_MEMORY_FILE, 0);

        if (kret != KERN_SUCCESS) {
                /*
                 * failed to get pagelist
                 */
                error = EINVAL;
                goto wait_for_cwrites;
        }
        num_upl++;

        /*
         * Consider the possibility that upl_size wasn't satisfied.
         */
        if (upl_size < upl_needed_size) {
                /*
                 * This is a failure in the physical memory case.
                 */
                error = EINVAL;
                goto wait_for_cwrites;
        }
        pl = ubc_upl_pageinfo(upl[cur_upl]);

        src_paddr = ((addr64_t)upl_phys_page(pl, 0) << PAGE_SHIFT) + (addr64_t)upl_offset;

        while (((uio->uio_offset & (devblocksize - 1)) || io_size < devblocksize) && io_size) {
                u_int32_t   head_size;

                head_size = devblocksize - (u_int32_t)(uio->uio_offset & (devblocksize - 1));

                if (head_size > io_size)
                        head_size = io_size;

                error = cluster_align_phys_io(vp, uio, src_paddr, head_size, 0, callback, callback_arg);

                if (error)
                        goto wait_for_cwrites;

                upl_offset += head_size;
                src_paddr  += head_size;
                io_size    -= head_size;

                iov_base   += head_size;
        }
        if ((u_int32_t)iov_base & mem_alignment_mask) {
                /*
                 * request doesn't set up on a memory boundary
                 * the underlying DMA engine can handle...
                 * return an error instead of going through
                 * the slow copy path since the intent of this
                 * path is direct I/O from device memory
                 */
                error = EINVAL;
                goto wait_for_cwrites;
        }

        tail_size = io_size & (devblocksize - 1);
        io_size  -= tail_size;

        while (io_size && error == 0) {

                if (io_size > MAX_IO_CONTIG_SIZE)
                        xsize = MAX_IO_CONTIG_SIZE;
                else
                        xsize = io_size;
                /*
                 * request asynchronously so that we can overlap
                 * the preparation of the next I/O... we'll do
                 * the commit after all the I/O has completed
                 * since its all issued against the same UPL
                 * if there are already too many outstanding writes
                 * wait until some have completed before issuing the next
                 */
                cluster_iostate_wait(&iostate, MAX_IO_CONTIG_SIZE * IO_SCALE(vp, 2), "cluster_write_contig");

                if (iostate.io_error) {
                        /*
                         * one of the earlier writes we issued ran into a hard error
                         * don't issue any more writes...
                         * go wait for all writes that are part of this stream
                         * to complete before returning the error to the caller
                         */
                        goto wait_for_cwrites;
                }
                /*
                 * issue an asynchronous write to cluster_io
                 */
                error = cluster_io(vp, upl[cur_upl], upl_offset, uio->uio_offset,
                                   xsize, CL_DEV_MEMORY | CL_ASYNC | bflag, (buf_t)NULL, (struct clios *)&iostate, callback, callback_arg);

                if (error == 0) {
                        /*
                         * The cluster_io write completed successfully,
                         * update the uio structure
                         */
                        uio_update(uio, (user_size_t)xsize);

                        upl_offset += xsize;
                        src_paddr  += xsize;
                        io_size    -= xsize;
                }
        }
        if (error == 0 && iostate.io_error == 0 && tail_size == 0 && num_upl < MAX_VECTS) {

                error = cluster_io_type(uio, write_type, write_length, 0);

                if (error == 0 && *write_type == IO_CONTIG) {
                        cur_upl++;
                        goto next_cwrite;
                }
        } else
                *write_type = IO_UNKNOWN;

wait_for_cwrites:
        /*
         * make sure all async writes that are part of this stream
         * have completed before we proceed
         */
        cluster_iostate_wait(&iostate, 0, "cluster_write_contig");

        if (iostate.io_error)
                error = iostate.io_error;

        lck_mtx_destroy(&iostate.io_mtxp, cl_mtx_grp);

        if (error == 0 && tail_size)
                error = cluster_align_phys_io(vp, uio, src_paddr, tail_size, 0, callback, callback_arg);

        for (n = 0; n < num_upl; n++)
                /*
                 * just release our hold on each physically contiguous
                 * region without changing any state
                 */
                ubc_upl_abort(upl[n], 0);

        return (error);
}


/*
 * need to avoid a race between an msync of a range of pages dirtied via mmap
 * vs a filesystem such as HFS deciding to write a 'hole' to disk via cluster_write's
 * zerofill mechanism before it has seen the VNOP_PAGEOUTs for the pages being msync'd
 *
 * we should never force-zero-fill pages that are already valid in the cache...
 * the entire page contains valid data (either from disk, zero-filled or dirtied
 * via an mmap) so we can only do damage by trying to zero-fill
 *
 */
static int
cluster_zero_range(upl_t upl, upl_page_info_t *pl, int flags, int io_offset, off_t zero_off, off_t upl_f_offset, int bytes_to_zero)
{
        int zero_pg_index;
        boolean_t need_cluster_zero = TRUE;

        if ((flags & (IO_NOZEROVALID | IO_NOZERODIRTY))) {

                bytes_to_zero = min(bytes_to_zero, PAGE_SIZE - (int)(zero_off & PAGE_MASK_64));
                zero_pg_index = (int)((zero_off - upl_f_offset) / PAGE_SIZE_64);

                if (upl_valid_page(pl, zero_pg_index)) {
                        /*
                         * never force zero valid pages - dirty or clean
                         * we'll leave these in the UPL for cluster_write_copy to deal with
                         */
                        need_cluster_zero = FALSE;
                } 
        }
        if (need_cluster_zero == TRUE)
                cluster_zero(upl, io_offset, bytes_to_zero, NULL);

        return (bytes_to_zero);
}


static int
cluster_write_copy(vnode_t vp, struct uio *uio, u_int32_t io_req_size, off_t oldEOF, off_t newEOF, off_t headOff,
                   off_t tailOff, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        upl_page_info_t *pl;
        upl_t            upl;
        vm_offset_t      upl_offset = 0;
        vm_size_t        upl_size;
        off_t            upl_f_offset;
        int              pages_in_upl;
        int              start_offset;
        int              xfer_resid;
        int              io_size;
        int              io_offset;
        int              bytes_to_zero;
        int              bytes_to_move;
        kern_return_t    kret;
        int              retval = 0;
        int              io_resid;
        long long        total_size;
        long long        zero_cnt;
        off_t            zero_off;
        long long        zero_cnt1;
        off_t            zero_off1;
        off_t            write_off = 0;
        int              write_cnt = 0;
        boolean_t        first_pass = FALSE;
        struct cl_extent cl;
        struct cl_writebehind *wbp;
        int              bflag;
        u_int            max_cluster_pgcount;
        u_int            max_io_size;

        if (uio) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 40)) | DBG_FUNC_START,
                             (int)uio->uio_offset, io_req_size, (int)oldEOF, (int)newEOF, 0);

                io_resid = io_req_size;
        } else {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 40)) | DBG_FUNC_START,
                             0, 0, (int)oldEOF, (int)newEOF, 0);

                io_resid = 0;
        }
        if (flags & IO_PASSIVE)
                bflag = CL_PASSIVE;
        else
                bflag = 0;
        if (flags & IO_NOCACHE)
                bflag |= CL_NOCACHE;
        
        if (flags & IO_SKIP_ENCRYPTION)
                bflag |= CL_ENCRYPTED;

        zero_cnt  = 0;
        zero_cnt1 = 0;
        zero_off  = 0;
        zero_off1 = 0;

        max_cluster_pgcount = MAX_CLUSTER_SIZE(vp) / PAGE_SIZE;
        max_io_size = cluster_max_io_size(vp->v_mount, CL_WRITE);

        if (flags & IO_HEADZEROFILL) {
                /*
                 * some filesystems (HFS is one) don't support unallocated holes within a file...
                 * so we zero fill the intervening space between the old EOF and the offset
                 * where the next chunk of real data begins.... ftruncate will also use this
                 * routine to zero fill to the new EOF when growing a file... in this case, the
                 * uio structure will not be provided
                 */
                if (uio) {
                        if (headOff < uio->uio_offset) {
                                zero_cnt = uio->uio_offset - headOff;
                                zero_off = headOff;
                        }
                } else if (headOff < newEOF) {  
                        zero_cnt = newEOF - headOff;
                        zero_off = headOff;
                }
        } else {
                if (uio && uio->uio_offset > oldEOF) {
                        zero_off = uio->uio_offset & ~PAGE_MASK_64;

                        if (zero_off >= oldEOF) {
                                zero_cnt = uio->uio_offset - zero_off;

                                flags |= IO_HEADZEROFILL;
                        }
                }
        }
        if (flags & IO_TAILZEROFILL) {
                if (uio) {
                        zero_off1 = uio->uio_offset + io_req_size;

                        if (zero_off1 < tailOff)
                                zero_cnt1 = tailOff - zero_off1;
                }       
        } else {
                if (uio && newEOF > oldEOF) {
                        zero_off1 = uio->uio_offset + io_req_size;

                        if (zero_off1 == newEOF && (zero_off1 & PAGE_MASK_64)) {
                                zero_cnt1 = PAGE_SIZE_64 - (zero_off1 & PAGE_MASK_64);

                                flags |= IO_TAILZEROFILL;
                        }
                }
        }
        if (zero_cnt == 0 && uio == (struct uio *) 0) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 40)) | DBG_FUNC_END,
                             retval, 0, 0, 0, 0);
                return (0);
        }
        if (uio) {
                write_off = uio->uio_offset;
                write_cnt = uio_resid(uio);
                /*
                 * delay updating the sequential write info
                 * in the control block until we've obtained
                 * the lock for it
                 */
                first_pass = TRUE;
        }
        while ((total_size = (io_resid + zero_cnt + zero_cnt1)) && retval == 0) {
                /*
                 * for this iteration of the loop, figure out where our starting point is
                 */
                if (zero_cnt) {
                        start_offset = (int)(zero_off & PAGE_MASK_64);
                        upl_f_offset = zero_off - start_offset;
                } else if (io_resid) {
                        start_offset = (int)(uio->uio_offset & PAGE_MASK_64);
                        upl_f_offset = uio->uio_offset - start_offset;
                } else {
                        start_offset = (int)(zero_off1 & PAGE_MASK_64);
                        upl_f_offset = zero_off1 - start_offset;
                }
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 46)) | DBG_FUNC_NONE,
                             (int)zero_off, (int)zero_cnt, (int)zero_off1, (int)zero_cnt1, 0);

                if (total_size > max_io_size)
                        total_size = max_io_size;

                cl.b_addr = (daddr64_t)(upl_f_offset / PAGE_SIZE_64);
                
                if (uio && ((flags & (IO_SYNC | IO_HEADZEROFILL | IO_TAILZEROFILL)) == 0)) {
                        /*
                         * assumption... total_size <= io_resid
                         * because IO_HEADZEROFILL and IO_TAILZEROFILL not set
                         */
                        if ((start_offset + total_size) > max_io_size)
                                total_size = max_io_size - start_offset;
                        xfer_resid = total_size;

                        retval = cluster_copy_ubc_data_internal(vp, uio, &xfer_resid, 1, 1);

                        if (retval)
                                break;

                        io_resid    -= (total_size - xfer_resid);
                        total_size   = xfer_resid;
                        start_offset = (int)(uio->uio_offset & PAGE_MASK_64);
                        upl_f_offset = uio->uio_offset - start_offset;

                        if (total_size == 0) {
                                if (start_offset) {
                                        /*
                                         * the write did not finish on a page boundary
                                         * which will leave upl_f_offset pointing to the
                                         * beginning of the last page written instead of
                                         * the page beyond it... bump it in this case
                                         * so that the cluster code records the last page
                                         * written as dirty
                                         */
                                        upl_f_offset += PAGE_SIZE_64;
                                }
                                upl_size = 0;
                                
                                goto check_cluster;
                        }
                }
                /*
                 * compute the size of the upl needed to encompass
                 * the requested write... limit each call to cluster_io
                 * to the maximum UPL size... cluster_io will clip if
                 * this exceeds the maximum io_size for the device,
                 * make sure to account for 
                 * a starting offset that's not page aligned
                 */
                upl_size = (start_offset + total_size + (PAGE_SIZE - 1)) & ~PAGE_MASK;

                if (upl_size > max_io_size)
                        upl_size = max_io_size;

                pages_in_upl = upl_size / PAGE_SIZE;
                io_size      = upl_size - start_offset;
                
                if ((long long)io_size > total_size)
                        io_size = total_size;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 41)) | DBG_FUNC_START, upl_size, io_size, total_size, 0, 0);
                        

                /*
                 * Gather the pages from the buffer cache.
                 * The UPL_WILL_MODIFY flag lets the UPL subsystem know
                 * that we intend to modify these pages.
                 */
                kret = ubc_create_upl_kernel(vp,
                                      upl_f_offset,
                                      upl_size,
                                      &upl,
                                      &pl,
                                      UPL_SET_LITE | (( uio!=NULL && (uio->uio_flags & UIO_FLAGS_IS_COMPRESSED_FILE)) ? 0 : UPL_WILL_MODIFY),
                                      VM_KERN_MEMORY_FILE);
                if (kret != KERN_SUCCESS)
                        panic("cluster_write_copy: failed to get pagelist");

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 41)) | DBG_FUNC_END,
                        upl, (int)upl_f_offset, start_offset, 0, 0);

                if (start_offset && upl_f_offset < oldEOF && !upl_valid_page(pl, 0)) {
                        int   read_size;

                        /*
                         * we're starting in the middle of the first page of the upl
                         * and the page isn't currently valid, so we're going to have
                         * to read it in first... this is a synchronous operation
                         */
                        read_size = PAGE_SIZE;

                        if ((upl_f_offset + read_size) > oldEOF)
                                read_size = oldEOF - upl_f_offset;

                        retval = cluster_io(vp, upl, 0, upl_f_offset, read_size,
                                            CL_READ | bflag, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);
                        if (retval) {
                                /*
                                 * we had an error during the read which causes us to abort
                                 * the current cluster_write request... before we do, we need
                                 * to release the rest of the pages in the upl without modifying
                                 * there state and mark the failed page in error
                                 */
                                ubc_upl_abort_range(upl, 0, PAGE_SIZE, UPL_ABORT_DUMP_PAGES|UPL_ABORT_FREE_ON_EMPTY);

                                if (upl_size > PAGE_SIZE)
                                        ubc_upl_abort_range(upl, 0, upl_size, UPL_ABORT_FREE_ON_EMPTY);

                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 45)) | DBG_FUNC_NONE,
                                             upl, 0, 0, retval, 0);
                                break;
                        }
                }
                if ((start_offset == 0 || upl_size > PAGE_SIZE) && ((start_offset + io_size) & PAGE_MASK)) {
                        /* 
                         * the last offset we're writing to in this upl does not end on a page
                         * boundary... if it's not beyond the old EOF, then we'll also need to
                         * pre-read this page in if it isn't already valid
                         */
                        upl_offset = upl_size - PAGE_SIZE;

                        if ((upl_f_offset + start_offset + io_size) < oldEOF &&
                            !upl_valid_page(pl, upl_offset / PAGE_SIZE)) {
                                int   read_size;

                                read_size = PAGE_SIZE;

                                if ((off_t)(upl_f_offset + upl_offset + read_size) > oldEOF)
                                        read_size = oldEOF - (upl_f_offset + upl_offset);

                                retval = cluster_io(vp, upl, upl_offset, upl_f_offset + upl_offset, read_size,
                                                    CL_READ | bflag, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);
                                if (retval) {
                                        /*
                                         * we had an error during the read which causes us to abort
                                         * the current cluster_write request... before we do, we
                                         * need to release the rest of the pages in the upl without
                                         * modifying there state and mark the failed page in error
                                         */
                                        ubc_upl_abort_range(upl, upl_offset, PAGE_SIZE, UPL_ABORT_DUMP_PAGES|UPL_ABORT_FREE_ON_EMPTY);

                                        if (upl_size > PAGE_SIZE)
                                                ubc_upl_abort_range(upl, 0, upl_size, UPL_ABORT_FREE_ON_EMPTY);

                                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 45)) | DBG_FUNC_NONE,
                                                     upl, 0, 0, retval, 0);
                                        break;
                                }
                        }
                }
                xfer_resid = io_size;
                io_offset = start_offset;

                while (zero_cnt && xfer_resid) {

                        if (zero_cnt < (long long)xfer_resid)
                                bytes_to_zero = zero_cnt;
                        else
                                bytes_to_zero = xfer_resid;

                        bytes_to_zero = cluster_zero_range(upl, pl, flags, io_offset, zero_off, upl_f_offset, bytes_to_zero);

                        xfer_resid -= bytes_to_zero;
                        zero_cnt   -= bytes_to_zero;
                        zero_off   += bytes_to_zero;
                        io_offset  += bytes_to_zero;
                }
                if (xfer_resid && io_resid) {
                        u_int32_t  io_requested;

                        bytes_to_move = min(io_resid, xfer_resid);
                        io_requested = bytes_to_move;

                        retval = cluster_copy_upl_data(uio, upl, io_offset, (int *)&io_requested);

                        if (retval) {
                                ubc_upl_abort_range(upl, 0, upl_size, UPL_ABORT_DUMP_PAGES | UPL_ABORT_FREE_ON_EMPTY);

                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 45)) | DBG_FUNC_NONE,
                                             upl, 0, 0, retval, 0);
                        } else {
                                io_resid   -= bytes_to_move;
                                xfer_resid -= bytes_to_move;
                                io_offset  += bytes_to_move;
                        }
                }
                while (xfer_resid && zero_cnt1 && retval == 0) {

                        if (zero_cnt1 < (long long)xfer_resid)
                                bytes_to_zero = zero_cnt1;
                        else
                                bytes_to_zero = xfer_resid;

                        bytes_to_zero = cluster_zero_range(upl, pl, flags, io_offset, zero_off1, upl_f_offset, bytes_to_zero);

                        xfer_resid -= bytes_to_zero;
                        zero_cnt1  -= bytes_to_zero;
                        zero_off1  += bytes_to_zero;
                        io_offset  += bytes_to_zero;
                }
                if (retval == 0) {
                        int cl_index;
                        int ret_cluster_try_push;
                        int do_zeroing = 1;

                        
                        io_size += start_offset;
                        

                        /* Force more restrictive zeroing behavior only on APFS */
                        if ((vnode_tag(vp) == VT_APFS) && (newEOF < oldEOF)) {
                                do_zeroing = 0;
                        }


                        if (do_zeroing && (upl_f_offset + io_size) >= newEOF && (u_int)io_size < upl_size) {

                                /*
                                 * if we're extending the file with this write
                                 * we'll zero fill the rest of the page so that
                                 * if the file gets extended again in such a way as to leave a
                                 * hole starting at this EOF, we'll have zero's in the correct spot
                                 */
                                cluster_zero(upl, io_size, upl_size - io_size, NULL); 
                        }
                        /*
                         * release the upl now if we hold one since...
                         * 1) pages in it may be present in the sparse cluster map
                         *    and may span 2 separate buckets there... if they do and 
                         *    we happen to have to flush a bucket to make room and it intersects
                         *    this upl, a deadlock may result on page BUSY
                         * 2) we're delaying the I/O... from this point forward we're just updating
                         *    the cluster state... no need to hold the pages, so commit them
                         * 3) IO_SYNC is set...
                         *    because we had to ask for a UPL that provides currenty non-present pages, the
                         *    UPL has been automatically set to clear the dirty flags (both software and hardware)
                         *    upon committing it... this is not the behavior we want since it's possible for
                         *    pages currently present as part of a mapped file to be dirtied while the I/O is in flight.
                         *    we'll pick these pages back up later with the correct behavior specified.
                         * 4) we don't want to hold pages busy in a UPL and then block on the cluster lock... if a flush
                         *    of this vnode is in progress, we will deadlock if the pages being flushed intersect the pages
                         *    we hold since the flushing context is holding the cluster lock.
                         */
                        ubc_upl_commit_range(upl, 0, upl_size,
                                             UPL_COMMIT_SET_DIRTY | UPL_COMMIT_INACTIVATE | UPL_COMMIT_FREE_ON_EMPTY);
check_cluster:
                        /*
                         * calculate the last logical block number 
                         * that this delayed I/O encompassed
                         */
                        cl.e_addr = (daddr64_t)((upl_f_offset + (off_t)upl_size) / PAGE_SIZE_64);

                        if (flags & IO_SYNC) {
                                /*
                                 * if the IO_SYNC flag is set than we need to 
                                 * bypass any clusters and immediately issue
                                 * the I/O
                                 */
                                goto issue_io;
                        }
                        /*
                         * take the lock to protect our accesses
                         * of the writebehind and sparse cluster state
                         */
                        wbp = cluster_get_wbp(vp, CLW_ALLOCATE | CLW_RETURNLOCKED);

                        if (wbp->cl_scmap) {

                                if ( !(flags & IO_NOCACHE)) {
                                        /*
                                         * we've fallen into the sparse
                                         * cluster method of delaying dirty pages
                                         */
                                        sparse_cluster_add(&(wbp->cl_scmap), vp, &cl, newEOF, callback, callback_arg);

                                        lck_mtx_unlock(&wbp->cl_lockw);

                                        continue;
                                }
                                /*
                                 * must have done cached writes that fell into
                                 * the sparse cluster mechanism... we've switched
                                 * to uncached writes on the file, so go ahead
                                 * and push whatever's in the sparse map
                                 * and switch back to normal clustering
                                 */
                                wbp->cl_number = 0;

                                sparse_cluster_push(&(wbp->cl_scmap), vp, newEOF, PUSH_ALL, 0, callback, callback_arg);
                                /*
                                 * no clusters of either type present at this point
                                 * so just go directly to start_new_cluster since
                                 * we know we need to delay this I/O since we've
                                 * already released the pages back into the cache
                                 * to avoid the deadlock with sparse_cluster_push
                                 */
                                goto start_new_cluster;
                        }
                        if (first_pass) {
                                if (write_off == wbp->cl_last_write)
                                        wbp->cl_seq_written += write_cnt;
                                else
                                        wbp->cl_seq_written = write_cnt;

                                wbp->cl_last_write = write_off + write_cnt;

                                first_pass = FALSE;
                        }
                        if (wbp->cl_number == 0)
                                /*
                                 * no clusters currently present
                                 */
                                goto start_new_cluster;

                        for (cl_index = 0; cl_index < wbp->cl_number; cl_index++) {
                                /*
                                 * check each cluster that we currently hold
                                 * try to merge some or all of this write into
                                 * one or more of the existing clusters... if
                                 * any portion of the write remains, start a
                                 * new cluster
                                 */
                                if (cl.b_addr >= wbp->cl_clusters[cl_index].b_addr) {
                                        /*
                                         * the current write starts at or after the current cluster
                                         */
                                        if (cl.e_addr <= (wbp->cl_clusters[cl_index].b_addr + max_cluster_pgcount)) {
                                                /*
                                                 * we have a write that fits entirely
                                                 * within the existing cluster limits
                                                 */
                                                if (cl.e_addr > wbp->cl_clusters[cl_index].e_addr)
                                                        /*
                                                         * update our idea of where the cluster ends
                                                         */
                                                        wbp->cl_clusters[cl_index].e_addr = cl.e_addr;
                                                break;
                                        }
                                        if (cl.b_addr < (wbp->cl_clusters[cl_index].b_addr + max_cluster_pgcount)) {
                                                /*
                                                 * we have a write that starts in the middle of the current cluster
                                                 * but extends beyond the cluster's limit... we know this because
                                                 * of the previous checks
                                                 * we'll extend the current cluster to the max
                                                 * and update the b_addr for the current write to reflect that
                                                 * the head of it was absorbed into this cluster...
                                                 * note that we'll always have a leftover tail in this case since
                                                 * full absorbtion would have occurred in the clause above
                                                 */
                                                wbp->cl_clusters[cl_index].e_addr = wbp->cl_clusters[cl_index].b_addr + max_cluster_pgcount;

                                                cl.b_addr = wbp->cl_clusters[cl_index].e_addr;
                                        }
                                        /*
                                         * we come here for the case where the current write starts
                                         * beyond the limit of the existing cluster or we have a leftover
                                         * tail after a partial absorbtion
                                         *
                                         * in either case, we'll check the remaining clusters before 
                                         * starting a new one
                                         */
                                } else {
                                        /*
                                         * the current write starts in front of the cluster we're currently considering
                                         */
                                        if ((wbp->cl_clusters[cl_index].e_addr - cl.b_addr) <= max_cluster_pgcount) {
                                                /*
                                                 * we can just merge the new request into
                                                 * this cluster and leave it in the cache
                                                 * since the resulting cluster is still 
                                                 * less than the maximum allowable size
                                                 */
                                                wbp->cl_clusters[cl_index].b_addr = cl.b_addr;

                                                if (cl.e_addr > wbp->cl_clusters[cl_index].e_addr) {
                                                        /*
                                                         * the current write completely
                                                         * envelops the existing cluster and since
                                                         * each write is limited to at most max_cluster_pgcount pages
                                                         * we can just use the start and last blocknos of the write
                                                         * to generate the cluster limits
                                                         */
                                                        wbp->cl_clusters[cl_index].e_addr = cl.e_addr;
                                                }
                                                break;
                                        }

                                        /*
                                         * if we were to combine this write with the current cluster
                                         * we would exceed the cluster size limit.... so,
                                         * let's see if there's any overlap of the new I/O with
                                         * the cluster we're currently considering... in fact, we'll
                                         * stretch the cluster out to it's full limit and see if we
                                         * get an intersection with the current write
                                         * 
                                         */
                                        if (cl.e_addr > wbp->cl_clusters[cl_index].e_addr - max_cluster_pgcount) {
                                                /*
                                                 * the current write extends into the proposed cluster
                                                 * clip the length of the current write after first combining it's
                                                 * tail with the newly shaped cluster
                                                 */
                                                wbp->cl_clusters[cl_index].b_addr = wbp->cl_clusters[cl_index].e_addr - max_cluster_pgcount;

                                                cl.e_addr = wbp->cl_clusters[cl_index].b_addr;
                                        }
                                        /*
                                         * if we get here, there was no way to merge
                                         * any portion of this write with this cluster 
                                         * or we could only merge part of it which 
                                         * will leave a tail...
                                         * we'll check the remaining clusters before starting a new one
                                         */
                                }
                        }
                        if (cl_index < wbp->cl_number)
                                /*
                                 * we found an existing cluster(s) that we
                                 * could entirely merge this I/O into
                                 */
                                goto delay_io;

                        if (!((unsigned int)vfs_flags(vp->v_mount) & MNT_DEFWRITE) &&
                            wbp->cl_number == MAX_CLUSTERS &&
                            wbp->cl_seq_written >= (MAX_CLUSTERS * (max_cluster_pgcount * PAGE_SIZE))) {
                                uint32_t        n;

                                if (vp->v_mount->mnt_minsaturationbytecount) {
                                        n = vp->v_mount->mnt_minsaturationbytecount / MAX_CLUSTER_SIZE(vp);
                                        
                                        if (n > MAX_CLUSTERS)
                                                n = MAX_CLUSTERS;
                                } else
                                        n = 0;

                                if (n == 0) {
                                        if (disk_conditioner_mount_is_ssd(vp->v_mount))
                                                n = WRITE_BEHIND_SSD;
                                        else
                                                n = WRITE_BEHIND;
                                }
                                while (n--)
                                        cluster_try_push(wbp, vp, newEOF, 0, 0, callback, callback_arg, NULL);
                        }
                        if (wbp->cl_number < MAX_CLUSTERS) {
                                /*
                                 * we didn't find an existing cluster to
                                 * merge into, but there's room to start
                                 * a new one
                                 */
                                goto start_new_cluster;
                        }
                        /*
                         * no exisitng cluster to merge with and no
                         * room to start a new one... we'll try 
                         * pushing one of the existing ones... if none of
                         * them are able to be pushed, we'll switch
                         * to the sparse cluster mechanism
                         * cluster_try_push updates cl_number to the
                         * number of remaining clusters... and
                         * returns the number of currently unused clusters
                         */
                        ret_cluster_try_push = 0;

                        /*
                         * if writes are not deferred, call cluster push immediately
                         */
                        if (!((unsigned int)vfs_flags(vp->v_mount) & MNT_DEFWRITE)) {
                                
                                ret_cluster_try_push = cluster_try_push(wbp, vp, newEOF, (flags & IO_NOCACHE) ? 0 : PUSH_DELAY, 0, callback, callback_arg, NULL);
                        }

                        /*
                         * execute following regardless of writes being deferred or not
                         */
                        if (ret_cluster_try_push == 0) {
                                /*
                                 * no more room in the normal cluster mechanism
                                 * so let's switch to the more expansive but expensive
                                 * sparse mechanism....
                                 */
                                sparse_cluster_switch(wbp, vp, newEOF, callback, callback_arg);
                                sparse_cluster_add(&(wbp->cl_scmap), vp, &cl, newEOF, callback, callback_arg);

                                lck_mtx_unlock(&wbp->cl_lockw);

                                continue;
                        }
start_new_cluster:
                        wbp->cl_clusters[wbp->cl_number].b_addr = cl.b_addr;
                        wbp->cl_clusters[wbp->cl_number].e_addr = cl.e_addr;

                        wbp->cl_clusters[wbp->cl_number].io_flags = 0;

                        if (flags & IO_NOCACHE)
                                wbp->cl_clusters[wbp->cl_number].io_flags |= CLW_IONOCACHE;

                        if (bflag & CL_PASSIVE)
                                wbp->cl_clusters[wbp->cl_number].io_flags |= CLW_IOPASSIVE;

                        wbp->cl_number++;
delay_io:
                        lck_mtx_unlock(&wbp->cl_lockw);

                        continue;
issue_io:
                        /*
                         * we don't hold the lock at this point
                         *
                         * we've already dropped the current upl, so pick it back up with COPYOUT_FROM set
                         * so that we correctly deal with a change in state of the hardware modify bit...
                         * we do this via cluster_push_now... by passing along the IO_SYNC flag, we force
                         * cluster_push_now to wait until all the I/Os have completed... cluster_push_now is also
                         * responsible for generating the correct sized I/O(s)
                         */
                        retval = cluster_push_now(vp, &cl, newEOF, flags, callback, callback_arg);
                }
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 40)) | DBG_FUNC_END, retval, 0, io_resid, 0, 0);

        return (retval);
}



int
cluster_read(vnode_t vp, struct uio *uio, off_t filesize, int xflags)
{
        return cluster_read_ext(vp, uio, filesize, xflags, NULL, NULL);
}


int
cluster_read_ext(vnode_t vp, struct uio *uio, off_t filesize, int xflags, int (*callback)(buf_t, void *), void *callback_arg)
{
        int             retval = 0;
        int             flags;
        user_ssize_t    cur_resid;
        u_int32_t       io_size;
        u_int32_t       read_length = 0;
        int             read_type = IO_COPY;

        flags = xflags;

        if (vp->v_flag & VNOCACHE_DATA)
                flags |= IO_NOCACHE;
        if ((vp->v_flag & VRAOFF) || speculative_reads_disabled)
                flags |= IO_RAOFF;

        if (flags & IO_SKIP_ENCRYPTION)
                flags |= IO_ENCRYPTED;

        /*
         * do a read through the cache if one of the following is true....
         *   NOCACHE is not true
         *   the uio request doesn't target USERSPACE
         * Alternatively, if IO_ENCRYPTED is set, then we want to bypass the cache as well.
         * Reading encrypted data from a CP filesystem should never result in the data touching
         * the UBC.
         *
         * otherwise, find out if we want the direct or contig variant for
         * the first vector in the uio request
         */
        if ( ((flags & IO_NOCACHE) && UIO_SEG_IS_USER_SPACE(uio->uio_segflg)) || (flags & IO_ENCRYPTED) ) {

                retval = cluster_io_type(uio, &read_type, &read_length, 0);
        }

        while ((cur_resid = uio_resid(uio)) && uio->uio_offset < filesize && retval == 0) {

                switch (read_type) {
                
                case IO_COPY:
                        /*
                         * make sure the uio_resid isn't too big...
                         * internally, we want to handle all of the I/O in
                         * chunk sizes that fit in a 32 bit int
                         */
                        if (cur_resid > (user_ssize_t)(MAX_IO_REQUEST_SIZE))
                                io_size = MAX_IO_REQUEST_SIZE;
                        else
                                io_size = (u_int32_t)cur_resid;

                        retval = cluster_read_copy(vp, uio, io_size, filesize, flags, callback, callback_arg);
                        break;

                case IO_DIRECT:
                        retval = cluster_read_direct(vp, uio, filesize, &read_type, &read_length, flags, callback, callback_arg);
                        break;

                case IO_CONTIG:
                        retval = cluster_read_contig(vp, uio, filesize, &read_type, &read_length, callback, callback_arg, flags);
                        break;
                  
                case IO_UNKNOWN:
                        retval = cluster_io_type(uio, &read_type, &read_length, 0);
                        break;
                }
        }
        return (retval);
}



static void
cluster_read_upl_release(upl_t upl, int start_pg, int last_pg, int take_reference)
{
        int range;
        int abort_flags = UPL_ABORT_FREE_ON_EMPTY;

        if ((range = last_pg - start_pg)) {
                if (take_reference)
                        abort_flags |= UPL_ABORT_REFERENCE;

                ubc_upl_abort_range(upl, start_pg * PAGE_SIZE, range * PAGE_SIZE, abort_flags);
        }
}


static int
cluster_read_copy(vnode_t vp, struct uio *uio, u_int32_t io_req_size, off_t filesize, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        upl_page_info_t *pl;
        upl_t            upl;
        vm_offset_t      upl_offset;
        u_int32_t        upl_size;
        off_t            upl_f_offset;
        int              start_offset;
        int              start_pg;
        int              last_pg;
        int              uio_last = 0;
        int              pages_in_upl;
        off_t            max_size;
        off_t            last_ioread_offset;
        off_t            last_request_offset;
        kern_return_t    kret;
        int              error  = 0;
        int              retval = 0;
        u_int32_t        size_of_prefetch;
        u_int32_t        xsize;
        u_int32_t        io_size;
        u_int32_t        max_rd_size;
        u_int32_t        max_io_size;
        u_int32_t        max_prefetch;
        u_int            rd_ahead_enabled = 1;
        u_int            prefetch_enabled = 1;
        struct cl_readahead *   rap;
        struct clios            iostate;
        struct cl_extent        extent;
        int              bflag;
        int              take_reference = 1;
        int              policy = IOPOL_DEFAULT;
        boolean_t        iolock_inited = FALSE;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 32)) | DBG_FUNC_START,
                     (int)uio->uio_offset, io_req_size, (int)filesize, flags, 0);
        
        if (flags & IO_ENCRYPTED) {
                panic ("encrypted blocks will hit UBC!");
        }
                         
        policy = throttle_get_io_policy(NULL);

        if (policy == THROTTLE_LEVEL_TIER3 || policy == THROTTLE_LEVEL_TIER2 || (flags & IO_NOCACHE))
                take_reference = 0;

        if (flags & IO_PASSIVE)
                bflag = CL_PASSIVE;
        else
                bflag = 0;

        if (flags & IO_NOCACHE)
                bflag |= CL_NOCACHE;

        if (flags & IO_SKIP_ENCRYPTION)
                bflag |= CL_ENCRYPTED;

        max_io_size = cluster_max_io_size(vp->v_mount, CL_READ);
        max_prefetch = MAX_PREFETCH(vp, max_io_size, disk_conditioner_mount_is_ssd(vp->v_mount));
        max_rd_size = max_prefetch;

        last_request_offset = uio->uio_offset + io_req_size;

        if (last_request_offset > filesize)
                last_request_offset = filesize;

        if ((flags & (IO_RAOFF|IO_NOCACHE)) || ((last_request_offset & ~PAGE_MASK_64) == (uio->uio_offset & ~PAGE_MASK_64))) {
                rd_ahead_enabled = 0;
                rap = NULL;
        } else {
                if (cluster_is_throttled(vp)) {
                        /*
                         * we're in the throttle window, at the very least
                         * we want to limit the size of the I/O we're about
                         * to issue
                         */
                        rd_ahead_enabled = 0;
                        prefetch_enabled = 0;

                        max_rd_size = THROTTLE_MAX_IOSIZE;
                }
                if ((rap = cluster_get_rap(vp)) == NULL)
                        rd_ahead_enabled = 0;
                else {
                        extent.b_addr = uio->uio_offset / PAGE_SIZE_64;
                        extent.e_addr = (last_request_offset - 1) / PAGE_SIZE_64;
                }
        }
        if (rap != NULL && rap->cl_ralen && (rap->cl_lastr == extent.b_addr || (rap->cl_lastr + 1) == extent.b_addr)) {
                /*
                 * determine if we already have a read-ahead in the pipe courtesy of the
                 * last read systemcall that was issued...
                 * if so, pick up it's extent to determine where we should start
                 * with respect to any read-ahead that might be necessary to 
                 * garner all the data needed to complete this read systemcall
                 */
                last_ioread_offset = (rap->cl_maxra * PAGE_SIZE_64) + PAGE_SIZE_64;

                if (last_ioread_offset < uio->uio_offset)
                        last_ioread_offset = (off_t)0;
                else if (last_ioread_offset > last_request_offset)
                        last_ioread_offset = last_request_offset;
        } else
                last_ioread_offset = (off_t)0;

        while (io_req_size && uio->uio_offset < filesize && retval == 0) {

                max_size = filesize - uio->uio_offset;

                if ((off_t)(io_req_size) < max_size)
                        io_size = io_req_size;
                else
                        io_size = max_size;

                if (!(flags & IO_NOCACHE)) {

                        while (io_size) {
                                u_int32_t io_resid;
                                u_int32_t io_requested;

                                /*
                                 * if we keep finding the pages we need already in the cache, then
                                 * don't bother to call cluster_read_prefetch since it costs CPU cycles
                                 * to determine that we have all the pages we need... once we miss in
                                 * the cache and have issued an I/O, than we'll assume that we're likely
                                 * to continue to miss in the cache and it's to our advantage to try and prefetch
                                 */
                                if (last_request_offset && last_ioread_offset && (size_of_prefetch = (last_request_offset - last_ioread_offset))) {
                                        if ((last_ioread_offset - uio->uio_offset) <= max_rd_size && prefetch_enabled) {
                                                /*
                                                 * we've already issued I/O for this request and
                                                 * there's still work to do and
                                                 * our prefetch stream is running dry, so issue a
                                                 * pre-fetch I/O... the I/O latency will overlap
                                                 * with the copying of the data
                                                 */
                                                if (size_of_prefetch > max_rd_size)
                                                        size_of_prefetch = max_rd_size;

                                                size_of_prefetch = cluster_read_prefetch(vp, last_ioread_offset, size_of_prefetch, filesize, callback, callback_arg, bflag);

                                                last_ioread_offset += (off_t)(size_of_prefetch * PAGE_SIZE);
                                
                                                if (last_ioread_offset > last_request_offset)
                                                        last_ioread_offset = last_request_offset;
                                        }
                                }
                                /*
                                 * limit the size of the copy we're about to do so that 
                                 * we can notice that our I/O pipe is running dry and 
                                 * get the next I/O issued before it does go dry
                                 */
                                if (last_ioread_offset && io_size > (max_io_size / 4))
                                        io_resid = (max_io_size / 4);
                                else
                                        io_resid = io_size;

                                io_requested = io_resid;

                                retval = cluster_copy_ubc_data_internal(vp, uio, (int *)&io_resid, 0, take_reference);

                                xsize = io_requested - io_resid;

                                io_size -= xsize;
                                io_req_size -= xsize;

                                if (retval || io_resid)
                                        /*
                                         * if we run into a real error or
                                         * a page that is not in the cache
                                         * we need to leave streaming mode
                                         */
                                        break;
                                
                                if (rd_ahead_enabled && (io_size == 0 || last_ioread_offset == last_request_offset)) {
                                        /*
                                         * we're already finished the I/O for this read request
                                         * let's see if we should do a read-ahead
                                         */
                                        cluster_read_ahead(vp, &extent, filesize, rap, callback, callback_arg, bflag);
                                }
                        }
                        if (retval)
                                break;
                        if (io_size == 0) {
                                if (rap != NULL) {
                                        if (extent.e_addr < rap->cl_lastr)
                                                rap->cl_maxra = 0;
                                        rap->cl_lastr = extent.e_addr;
                                }
                                break;
                        }
                        /*
                         * recompute max_size since cluster_copy_ubc_data_internal
                         * may have advanced uio->uio_offset
                         */
                        max_size = filesize - uio->uio_offset;
                }

                iostate.io_completed = 0;
                iostate.io_issued = 0;
                iostate.io_error = 0;
                iostate.io_wanted = 0;

                if ( (flags & IO_RETURN_ON_THROTTLE) ) {
                        if (cluster_is_throttled(vp) == THROTTLE_NOW) {
                                if ( !cluster_io_present_in_BC(vp, uio->uio_offset)) {
                                        /*
                                         * we're in the throttle window and at least 1 I/O
                                         * has already been issued by a throttleable thread
                                         * in this window, so return with EAGAIN to indicate
                                         * to the FS issuing the cluster_read call that it
                                         * should now throttle after dropping any locks
                                         */
                                        throttle_info_update_by_mount(vp->v_mount);

                                        retval = EAGAIN;
                                        break;
                                }
                        }
                }

                /*
                 * compute the size of the upl needed to encompass
                 * the requested read... limit each call to cluster_io
                 * to the maximum UPL size... cluster_io will clip if
                 * this exceeds the maximum io_size for the device,
                 * make sure to account for 
                 * a starting offset that's not page aligned
                 */
                start_offset = (int)(uio->uio_offset & PAGE_MASK_64);
                upl_f_offset = uio->uio_offset - (off_t)start_offset;

                if (io_size > max_rd_size)
                        io_size = max_rd_size;

                upl_size = (start_offset + io_size + (PAGE_SIZE - 1)) & ~PAGE_MASK;

                if (flags & IO_NOCACHE) {
                        if (upl_size > max_io_size)
                                upl_size = max_io_size;
                } else {
                        if (upl_size > max_io_size / 4) {
                                upl_size = max_io_size / 4;
                                upl_size &= ~PAGE_MASK;
                                
                                if (upl_size == 0)
                                        upl_size = PAGE_SIZE;
                        }
                }
                pages_in_upl = upl_size / PAGE_SIZE;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 33)) | DBG_FUNC_START,
                             upl, (int)upl_f_offset, upl_size, start_offset, 0);

                kret = ubc_create_upl_kernel(vp,
                                      upl_f_offset,
                                      upl_size,
                                      &upl,
                                      &pl,
                                      UPL_FILE_IO | UPL_SET_LITE,
                                      VM_KERN_MEMORY_FILE);
                if (kret != KERN_SUCCESS)
                        panic("cluster_read_copy: failed to get pagelist");

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 33)) | DBG_FUNC_END,
                             upl, (int)upl_f_offset, upl_size, start_offset, 0);

                /*
                 * scan from the beginning of the upl looking for the first
                 * non-valid page.... this will become the first page in
                 * the request we're going to make to 'cluster_io'... if all
                 * of the pages are valid, we won't call through to 'cluster_io'
                 */
                for (start_pg = 0; start_pg < pages_in_upl; start_pg++) {
                        if (!upl_valid_page(pl, start_pg))
                                break;
                }

                /*
                 * scan from the starting invalid page looking for a valid
                 * page before the end of the upl is reached, if we 
                 * find one, then it will be the last page of the request to
                 * 'cluster_io'
                 */
                for (last_pg = start_pg; last_pg < pages_in_upl; last_pg++) {
                        if (upl_valid_page(pl, last_pg))
                                break;
                }

                if (start_pg < last_pg) {               
                        /*
                         * we found a range of 'invalid' pages that must be filled
                         * if the last page in this range is the last page of the file
                         * we may have to clip the size of it to keep from reading past
                         * the end of the last physical block associated with the file
                         */
                        if (iolock_inited == FALSE) {
                                lck_mtx_init(&iostate.io_mtxp, cl_mtx_grp, cl_mtx_attr);

                                iolock_inited = TRUE;
                        }
                        upl_offset = start_pg * PAGE_SIZE;
                        io_size    = (last_pg - start_pg) * PAGE_SIZE;

                        if ((off_t)(upl_f_offset + upl_offset + io_size) > filesize)
                                io_size = filesize - (upl_f_offset + upl_offset);

                        /*
                         * issue an asynchronous read to cluster_io
                         */

                        error = cluster_io(vp, upl, upl_offset, upl_f_offset + upl_offset,
                                           io_size, CL_READ | CL_ASYNC | bflag, (buf_t)NULL, &iostate, callback, callback_arg);

                        if (rap) {
                                if (extent.e_addr < rap->cl_maxra) {
                                       /*
                                        * we've just issued a read for a block that should have been
                                        * in the cache courtesy of the read-ahead engine... something
                                        * has gone wrong with the pipeline, so reset the read-ahead
                                        * logic which will cause us to restart from scratch
                                        */
                                        rap->cl_maxra = 0;
                               }
                        }
                }
                if (error == 0) {
                        /*
                         * if the read completed successfully, or there was no I/O request
                         * issued, than copy the data into user land via 'cluster_upl_copy_data'
                         * we'll first add on any 'valid'
                         * pages that were present in the upl when we acquired it.
                         */
                        u_int  val_size;

                        for (uio_last = last_pg; uio_last < pages_in_upl; uio_last++) {
                                if (!upl_valid_page(pl, uio_last))
                                        break;
                        }
                        if (uio_last < pages_in_upl) {
                                /*
                                 * there were some invalid pages beyond the valid pages
                                 * that we didn't issue an I/O for, just release them
                                 * unchanged now, so that any prefetch/readahed can
                                 * include them
                                 */
                                ubc_upl_abort_range(upl, uio_last * PAGE_SIZE,
                                                    (pages_in_upl - uio_last) * PAGE_SIZE, UPL_ABORT_FREE_ON_EMPTY);
                        }

                        /*
                         * compute size to transfer this round,  if io_req_size is
                         * still non-zero after this attempt, we'll loop around and
                         * set up for another I/O.
                         */
                        val_size = (uio_last * PAGE_SIZE) - start_offset;
                
                        if (val_size > max_size)
                                val_size = max_size;

                        if (val_size > io_req_size)
                                val_size = io_req_size;

                        if ((uio->uio_offset + val_size) > last_ioread_offset)
                                last_ioread_offset = uio->uio_offset + val_size;

                        if ((size_of_prefetch = (last_request_offset - last_ioread_offset)) && prefetch_enabled) {

                                if ((last_ioread_offset - (uio->uio_offset + val_size)) <= upl_size) {
                                        /*
                                         * if there's still I/O left to do for this request, and...
                                         * we're not in hard throttle mode, and...
                                         * we're close to using up the previous prefetch, then issue a
                                         * new pre-fetch I/O... the I/O latency will overlap
                                         * with the copying of the data
                                         */
                                        if (size_of_prefetch > max_rd_size)
                                                size_of_prefetch = max_rd_size;

                                        size_of_prefetch = cluster_read_prefetch(vp, last_ioread_offset, size_of_prefetch, filesize, callback, callback_arg, bflag);

                                        last_ioread_offset += (off_t)(size_of_prefetch * PAGE_SIZE);
                                
                                        if (last_ioread_offset > last_request_offset)
                                                last_ioread_offset = last_request_offset;
                                }

                        } else if ((uio->uio_offset + val_size) == last_request_offset) {
                                /*
                                 * this transfer will finish this request, so...
                                 * let's try to read ahead if we're in 
                                 * a sequential access pattern and we haven't
                                 * explicitly disabled it
                                 */
                                if (rd_ahead_enabled)
                                        cluster_read_ahead(vp, &extent, filesize, rap, callback, callback_arg, bflag);
                                        
                                if (rap != NULL) {
                                        if (extent.e_addr < rap->cl_lastr)
                                                rap->cl_maxra = 0;
                                        rap->cl_lastr = extent.e_addr;
                                }
                        }
                        if (iolock_inited == TRUE)
                                cluster_iostate_wait(&iostate, 0, "cluster_read_copy");

                        if (iostate.io_error)
                                error = iostate.io_error;
                        else {
                                u_int32_t io_requested;

                                io_requested = val_size;

                                retval = cluster_copy_upl_data(uio, upl, start_offset, (int *)&io_requested);
                                
                                io_req_size -= (val_size - io_requested);
                        }
                } else {
                        if (iolock_inited == TRUE)
                                cluster_iostate_wait(&iostate, 0, "cluster_read_copy");
                }
                if (start_pg < last_pg) {
                        /*
                         * compute the range of pages that we actually issued an I/O for
                         * and either commit them as valid if the I/O succeeded
                         * or abort them if the I/O failed or we're not supposed to 
                         * keep them in the cache
                         */
                        io_size = (last_pg - start_pg) * PAGE_SIZE;

                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 35)) | DBG_FUNC_START, upl, start_pg * PAGE_SIZE, io_size, error, 0);

                        if (error || (flags & IO_NOCACHE))
                                ubc_upl_abort_range(upl, start_pg * PAGE_SIZE, io_size,
                                                    UPL_ABORT_DUMP_PAGES | UPL_ABORT_FREE_ON_EMPTY);
                        else {
                                int     commit_flags = UPL_COMMIT_CLEAR_DIRTY | UPL_COMMIT_FREE_ON_EMPTY;

                                if (take_reference)
                                        commit_flags |= UPL_COMMIT_INACTIVATE;
                                else
                                        commit_flags |= UPL_COMMIT_SPECULATE;

                                ubc_upl_commit_range(upl, start_pg * PAGE_SIZE, io_size, commit_flags);
                        }
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 35)) | DBG_FUNC_END, upl, start_pg * PAGE_SIZE, io_size, error, 0);
                }
                if ((last_pg - start_pg) < pages_in_upl) {
                        /*
                         * the set of pages that we issued an I/O for did not encompass
                         * the entire upl... so just release these without modifying
                         * their state
                         */
                        if (error)
                                ubc_upl_abort_range(upl, 0, upl_size, UPL_ABORT_FREE_ON_EMPTY);
                        else {

                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 35)) | DBG_FUNC_START,
                                             upl, -1, pages_in_upl - (last_pg - start_pg), 0, 0);

                                /*
                                 * handle any valid pages at the beginning of
                                 * the upl... release these appropriately
                                 */
                                cluster_read_upl_release(upl, 0, start_pg, take_reference);

                                /*
                                 * handle any valid pages immediately after the
                                 * pages we issued I/O for... ... release these appropriately
                                 */
                                cluster_read_upl_release(upl, last_pg, uio_last, take_reference);

                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 35)) | DBG_FUNC_END, upl, -1, -1, 0, 0);
                        }
                }
                if (retval == 0)
                        retval = error;

                if (io_req_size) {
                        if (cluster_is_throttled(vp)) {
                                /*
                                 * we're in the throttle window, at the very least
                                 * we want to limit the size of the I/O we're about
                                 * to issue
                                 */
                                rd_ahead_enabled = 0;
                                prefetch_enabled = 0;
                                max_rd_size = THROTTLE_MAX_IOSIZE;
                        } else {
                                if (max_rd_size == THROTTLE_MAX_IOSIZE) {
                                        /*
                                         * coming out of throttled state
                                         */
                                        if (policy != THROTTLE_LEVEL_TIER3 && policy != THROTTLE_LEVEL_TIER2) {
                                                if (rap != NULL)
                                                        rd_ahead_enabled = 1;
                                                prefetch_enabled = 1;
                                        }
                                        max_rd_size = max_prefetch;
                                        last_ioread_offset = 0;
                                }
                        }
                }
        }
        if (iolock_inited == TRUE) {
                /*
                 * cluster_io returned an error after it
                 * had already issued some I/O.  we need
                 * to wait for that I/O to complete before
                 * we can destroy the iostate mutex...
                 * 'retval' already contains the early error
                 * so no need to pick it up from iostate.io_error
                 */
                cluster_iostate_wait(&iostate, 0, "cluster_read_copy");

                lck_mtx_destroy(&iostate.io_mtxp, cl_mtx_grp);
        }
        if (rap != NULL) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 32)) | DBG_FUNC_END,
                             (int)uio->uio_offset, io_req_size, rap->cl_lastr, retval, 0);

                lck_mtx_unlock(&rap->cl_lockr);
        } else {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 32)) | DBG_FUNC_END,
                             (int)uio->uio_offset, io_req_size, 0, retval, 0);
        }

        return (retval);
}

/*
 * We don't want another read/write lock for every vnode in the system
 * so we keep a hash of them here.  There should never be very many of
 * these around at any point in time.
 */
cl_direct_read_lock_t *cluster_lock_direct_read(vnode_t vp, lck_rw_type_t type)
{
        struct cl_direct_read_locks *head
                = &cl_direct_read_locks[(uintptr_t)vp / sizeof(*vp)
                                                                % CL_DIRECT_READ_LOCK_BUCKETS];

        struct cl_direct_read_lock *lck, *new_lck = NULL;

        for (;;) {
                lck_spin_lock(&cl_direct_read_spin_lock);

                LIST_FOREACH(lck, head, chain) {
                        if (lck->vp == vp) {
                                ++lck->ref_count;
                                lck_spin_unlock(&cl_direct_read_spin_lock);
                                if (new_lck) {
                                        // Someone beat us to it, ditch the allocation
                                        lck_rw_destroy(&new_lck->rw_lock, cl_mtx_grp);
                                        FREE(new_lck, M_TEMP);
                                }
                                lck_rw_lock(&lck->rw_lock, type);
                                return lck;
                        }
                }

                if (new_lck) {
                        // Use the lock we allocated
                        LIST_INSERT_HEAD(head, new_lck, chain);
                        lck_spin_unlock(&cl_direct_read_spin_lock);
                        lck_rw_lock(&new_lck->rw_lock, type);
                        return new_lck;
                }

                lck_spin_unlock(&cl_direct_read_spin_lock);

                // Allocate a new lock
                MALLOC(new_lck, cl_direct_read_lock_t *, sizeof(*new_lck),
                           M_TEMP, M_WAITOK);
                lck_rw_init(&new_lck->rw_lock, cl_mtx_grp, cl_mtx_attr);
                new_lck->vp = vp;
                new_lck->ref_count = 1;

                // Got to go round again
        }
}

void cluster_unlock_direct_read(cl_direct_read_lock_t *lck)
{
        lck_rw_done(&lck->rw_lock);

        lck_spin_lock(&cl_direct_read_spin_lock);
        if (lck->ref_count == 1) {
                LIST_REMOVE(lck, chain);
                lck_spin_unlock(&cl_direct_read_spin_lock);
                lck_rw_destroy(&lck->rw_lock, cl_mtx_grp);
                FREE(lck, M_TEMP);
        } else {
                --lck->ref_count;
                lck_spin_unlock(&cl_direct_read_spin_lock);
        }
}

static int
cluster_read_direct(vnode_t vp, struct uio *uio, off_t filesize, int *read_type, u_int32_t *read_length,
                    int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        upl_t            upl;
        upl_page_info_t  *pl;
        off_t            max_io_size;
        vm_offset_t      upl_offset, vector_upl_offset = 0;
        upl_size_t       upl_size, vector_upl_size = 0;
        vm_size_t        upl_needed_size;
        unsigned int     pages_in_pl;
        upl_control_flags_t upl_flags;
        kern_return_t    kret;
        unsigned int     i;
        int              force_data_sync;
        int              retval = 0;
        int              no_zero_fill = 0;
        int              io_flag = 0;
        int              misaligned = 0;
        struct clios     iostate;
        user_addr_t      iov_base;
        u_int32_t        io_req_size;
        u_int32_t        offset_in_file;
        u_int32_t        offset_in_iovbase;
        u_int32_t        io_size;
        u_int32_t        io_min;
        u_int32_t        xsize;
        u_int32_t        devblocksize;
        u_int32_t        mem_alignment_mask;
        u_int32_t        max_upl_size;
        u_int32_t        max_rd_size;
        u_int32_t        max_rd_ahead;
        u_int32_t        max_vector_size;
        boolean_t        strict_uncached_IO = FALSE;
        boolean_t        io_throttled = FALSE;

        u_int32_t        vector_upl_iosize = 0;
        int              issueVectorUPL = 0,useVectorUPL = (uio->uio_iovcnt > 1);
        off_t            v_upl_uio_offset = 0;
        int              vector_upl_index=0;
        upl_t            vector_upl = NULL;
        cl_direct_read_lock_t *lock = NULL;

        user_addr_t      orig_iov_base = 0;
        user_addr_t      last_iov_base = 0;
        user_addr_t      next_iov_base = 0;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 70)) | DBG_FUNC_START,
                     (int)uio->uio_offset, (int)filesize, *read_type, *read_length, 0);

        max_upl_size = cluster_max_io_size(vp->v_mount, CL_READ);

        max_rd_size = max_upl_size;
        max_rd_ahead = max_rd_size * IO_SCALE(vp, 2);

        io_flag = CL_COMMIT | CL_READ | CL_ASYNC | CL_NOZERO | CL_DIRECT_IO;

        if (flags & IO_PASSIVE)
                io_flag |= CL_PASSIVE;

        if (flags & IO_ENCRYPTED) {
                io_flag |= CL_RAW_ENCRYPTED;
        }

        if (flags & IO_NOCACHE) {
                io_flag |= CL_NOCACHE;
        }

        if (flags & IO_SKIP_ENCRYPTION)
                io_flag |= CL_ENCRYPTED;

        iostate.io_completed = 0;
        iostate.io_issued = 0;
        iostate.io_error = 0;
        iostate.io_wanted = 0;

        lck_mtx_init(&iostate.io_mtxp, cl_mtx_grp, cl_mtx_attr);

        devblocksize = (u_int32_t)vp->v_mount->mnt_devblocksize;
        mem_alignment_mask = (u_int32_t)vp->v_mount->mnt_alignmentmask;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 70)) | DBG_FUNC_NONE,
                     (int)devblocksize, (int)mem_alignment_mask, 0, 0, 0);

        if (devblocksize == 1) {
               /*
                * the AFP client advertises a devblocksize of 1
                * however, its BLOCKMAP routine maps to physical
                * blocks that are PAGE_SIZE in size...
                * therefore we can't ask for I/Os that aren't page aligned
                * or aren't multiples of PAGE_SIZE in size
                * by setting devblocksize to PAGE_SIZE, we re-instate
                * the old behavior we had before the mem_alignment_mask
                * changes went in...
                */
               devblocksize = PAGE_SIZE;
        }

        strict_uncached_IO = ubc_strict_uncached_IO(vp);

        orig_iov_base = uio_curriovbase(uio);
        last_iov_base = orig_iov_base;

next_dread:
        io_req_size = *read_length;
        iov_base = uio_curriovbase(uio);

        offset_in_file = (u_int32_t)uio->uio_offset & (devblocksize - 1);
        offset_in_iovbase = (u_int32_t)iov_base & mem_alignment_mask;

        if (offset_in_file || offset_in_iovbase) {
                /*
                 * one of the 2 important offsets is misaligned
                 * so fire an I/O through the cache for this entire vector
                 */
                misaligned = 1;
        }
        if (iov_base & (devblocksize - 1)) {
                /*
                 * the offset in memory must be on a device block boundary
                 * so that we can guarantee that we can generate an
                 * I/O that ends on a page boundary in cluster_io
                 */
                misaligned = 1;
    }

        max_io_size = filesize - uio->uio_offset;

        /* 
         * The user must request IO in aligned chunks.  If the 
         * offset into the file is bad, or the userland pointer 
         * is non-aligned, then we cannot service the encrypted IO request.
         */
        if (flags & IO_ENCRYPTED) {
                if (misaligned || (io_req_size & (devblocksize - 1)))
                        retval = EINVAL;

                max_io_size = roundup(max_io_size, devblocksize);
        }

        if ((off_t)io_req_size > max_io_size)
                io_req_size = max_io_size;

        /*
         * When we get to this point, we know...
         *  -- the offset into the file is on a devblocksize boundary
         */

        while (io_req_size && retval == 0) {
                u_int32_t io_start;

                if (cluster_is_throttled(vp)) {
                        /*
                         * we're in the throttle window, at the very least
                         * we want to limit the size of the I/O we're about
                         * to issue
                         */
                        max_rd_size  = THROTTLE_MAX_IOSIZE;
                        max_rd_ahead = THROTTLE_MAX_IOSIZE - 1;
                        max_vector_size = THROTTLE_MAX_IOSIZE;
                } else {
                        max_rd_size  = max_upl_size;
                        max_rd_ahead = max_rd_size * IO_SCALE(vp, 2);
                        max_vector_size = MAX_VECTOR_UPL_SIZE;
                }
                io_start = io_size = io_req_size;

                /*
                 * First look for pages already in the cache
                 * and move them to user space.  But only do this
                 * check if we are not retrieving encrypted data directly
                 * from the filesystem;  those blocks should never
                 * be in the UBC. 
                 *
                 * cluster_copy_ubc_data returns the resid
                 * in io_size
                 */
                if ((strict_uncached_IO == FALSE) && ((flags & IO_ENCRYPTED) == 0)) {
                        retval = cluster_copy_ubc_data_internal(vp, uio, (int *)&io_size, 0, 0);
                }
                /*
                 * calculate the number of bytes actually copied
                 * starting size - residual
                 */
                xsize = io_start - io_size;

                io_req_size -= xsize;

                if(useVectorUPL && (xsize || (iov_base & PAGE_MASK))) {
                        /*
                         * We found something in the cache or we have an iov_base that's not
                         * page-aligned.
                         * 
                         * Issue all I/O's that have been collected within this Vectored UPL.
                         */
                        if(vector_upl_index) {
                                retval = vector_cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize, io_flag, (buf_t)NULL, &iostate, callback, callback_arg);
                                reset_vector_run_state();
                        }
                        
                        if(xsize)
                                useVectorUPL = 0;

                       /*
                        * After this point, if we are using the Vector UPL path and the base is
                        * not page-aligned then the UPL with that base will be the first in the vector UPL.
                        */
                }

                /*
                 * check to see if we are finished with this request.
                 *
                 * If we satisfied this IO already, then io_req_size will be 0.
                 * Otherwise, see if the IO was mis-aligned and needs to go through 
                 * the UBC to deal with the 'tail'.
                 *
                 */
                if (io_req_size == 0 || (misaligned)) {
                        /*
                         * see if there's another uio vector to
                         * process that's of type IO_DIRECT
                         *
                         * break out of while loop to get there
                         */
                        break;
                }
                /*
                 * assume the request ends on a device block boundary
                 */
                io_min = devblocksize;

                /*
                 * we can handle I/O's in multiples of the device block size
                 * however, if io_size isn't a multiple of devblocksize we
                 * want to clip it back to the nearest page boundary since
                 * we are going to have to go through cluster_read_copy to
                 * deal with the 'overhang'... by clipping it to a PAGE_SIZE
                 * multiple, we avoid asking the drive for the same physical
                 * blocks twice.. once for the partial page at the end of the
                 * request and a 2nd time for the page we read into the cache
                 * (which overlaps the end of the direct read) in order to 
                 * get at the overhang bytes
                 */
                if (io_size & (devblocksize - 1)) {
                        assert(!(flags & IO_ENCRYPTED));
                        /*
                         * Clip the request to the previous page size boundary
                         * since request does NOT end on a device block boundary
                         */
                        io_size &= ~PAGE_MASK;
                        io_min = PAGE_SIZE;
                }
                if (retval || io_size < io_min) {
                        /*
                         * either an error or we only have the tail left to
                         * complete via the copy path...
                         * we may have already spun some portion of this request
                         * off as async requests... we need to wait for the I/O
                         * to complete before returning
                         */
                        goto wait_for_dreads;
                }

                /*
                 * Don't re-check the UBC data if we are looking for uncached IO
                 * or asking for encrypted blocks.
                 */
                if ((strict_uncached_IO == FALSE) && ((flags & IO_ENCRYPTED) == 0)) {

                        if ((xsize = io_size) > max_rd_size)
                                xsize = max_rd_size;

                        io_size = 0;

                        if (!lock) {
                                /*
                                 * We hold a lock here between the time we check the
                                 * cache and the time we issue I/O.  This saves us
                                 * from having to lock the pages in the cache.  Not
                                 * all clients will care about this lock but some
                                 * clients may want to guarantee stability between
                                 * here and when the I/O is issued in which case they
                                 * will take the lock exclusively.
                                 */
                                lock = cluster_lock_direct_read(vp, LCK_RW_TYPE_SHARED);
                        }

                        ubc_range_op(vp, uio->uio_offset, uio->uio_offset + xsize, UPL_ROP_ABSENT, (int *)&io_size);

                        if (io_size == 0) {
                                /*
                                 * a page must have just come into the cache
                                 * since the first page in this range is no
                                 * longer absent, go back and re-evaluate
                                 */
                                continue;
                        }
                }
                if ( (flags & IO_RETURN_ON_THROTTLE) ) {
                        if (cluster_is_throttled(vp) == THROTTLE_NOW) {
                                if ( !cluster_io_present_in_BC(vp, uio->uio_offset)) {
                                        /*
                                         * we're in the throttle window and at least 1 I/O
                                         * has already been issued by a throttleable thread
                                         * in this window, so return with EAGAIN to indicate
                                         * to the FS issuing the cluster_read call that it
                                         * should now throttle after dropping any locks
                                         */
                                        throttle_info_update_by_mount(vp->v_mount);

                                        io_throttled = TRUE;
                                        goto wait_for_dreads;
                                }
                        }
                }
                if (io_size > max_rd_size)
                        io_size = max_rd_size;

                iov_base = uio_curriovbase(uio);

                upl_offset = (vm_offset_t)((u_int32_t)iov_base & PAGE_MASK);
                upl_needed_size = (upl_offset + io_size + (PAGE_SIZE -1)) & ~PAGE_MASK;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 72)) | DBG_FUNC_START,
                             (int)upl_offset, upl_needed_size, (int)iov_base, io_size, 0);

                if (upl_offset == 0 && ((io_size & PAGE_MASK) == 0))
                        no_zero_fill = 1;
                else
                        no_zero_fill = 0;

                vm_map_t map = UIO_SEG_IS_USER_SPACE(uio->uio_segflg) ? current_map() : kernel_map;
                for (force_data_sync = 0; force_data_sync < 3; force_data_sync++) {
                        pages_in_pl = 0;
                        upl_size = upl_needed_size;
                        upl_flags = UPL_FILE_IO | UPL_NO_SYNC | UPL_SET_INTERNAL | UPL_SET_LITE | UPL_SET_IO_WIRE;
                        if (no_zero_fill)
                                upl_flags |= UPL_NOZEROFILL;
                        if (force_data_sync)
                                upl_flags |= UPL_FORCE_DATA_SYNC;

                        kret = vm_map_create_upl(map,
                                                 (vm_map_offset_t)(iov_base & ~((user_addr_t)PAGE_MASK)),
                                                 &upl_size, &upl, NULL, &pages_in_pl, &upl_flags, VM_KERN_MEMORY_FILE);

                        if (kret != KERN_SUCCESS) {
                                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 72)) | DBG_FUNC_END,
                                             (int)upl_offset, upl_size, io_size, kret, 0);
                                /*
                                 * failed to get pagelist
                                 *
                                 * we may have already spun some portion of this request
                                 * off as async requests... we need to wait for the I/O
                                 * to complete before returning
                                 */
                                goto wait_for_dreads;
                        }
                        pages_in_pl = upl_size / PAGE_SIZE;
                        pl = UPL_GET_INTERNAL_PAGE_LIST(upl);

                        for (i = 0; i < pages_in_pl; i++) {
                                if (!upl_page_present(pl, i))
                                        break;            
                        }
                        if (i == pages_in_pl)
                                break;

                        ubc_upl_abort(upl, 0);
                }
                if (force_data_sync >= 3) {
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 72)) | DBG_FUNC_END,
                                     (int)upl_offset, upl_size, io_size, kret, 0);
                  
                        goto wait_for_dreads;
                }
                /*
                 * Consider the possibility that upl_size wasn't satisfied.
                 */
                if (upl_size < upl_needed_size) {
                        if (upl_size && upl_offset == 0)
                                io_size = upl_size;
                        else
                                io_size = 0;
                }
                if (io_size == 0) {
                        ubc_upl_abort(upl, 0);
                        goto wait_for_dreads;
                }
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 72)) | DBG_FUNC_END,
                             (int)upl_offset, upl_size, io_size, kret, 0);

                if(useVectorUPL) {
                        vm_offset_t end_off = ((iov_base + io_size) & PAGE_MASK);
                        if(end_off) 
                                issueVectorUPL = 1;
                        /*
                         * After this point, if we are using a vector UPL, then
                         * either all the UPL elements end on a page boundary OR
                         * this UPL is the last element because it does not end
                         * on a page boundary.
                         */
                }

                /*
                 * request asynchronously so that we can overlap
                 * the preparation of the next I/O
                 * if there are already too many outstanding reads
                 * wait until some have completed before issuing the next read
                 */
                cluster_iostate_wait(&iostate, max_rd_ahead, "cluster_read_direct");

                if (iostate.io_error) {
                        /*
                         * one of the earlier reads we issued ran into a hard error
                         * don't issue any more reads, cleanup the UPL
                         * that was just created but not used, then
                         * go wait for any other reads to complete before
                         * returning the error to the caller
                         */
                        ubc_upl_abort(upl, 0);

                        goto wait_for_dreads;
                }
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 73)) | DBG_FUNC_START,
                             upl, (int)upl_offset, (int)uio->uio_offset, io_size, 0);

                if(!useVectorUPL) {
                        if (no_zero_fill)
                                io_flag &= ~CL_PRESERVE;
                        else
                                io_flag |= CL_PRESERVE;
                
                        retval = cluster_io(vp, upl, upl_offset, uio->uio_offset, io_size, io_flag, (buf_t)NULL, &iostate, callback, callback_arg);

                } else {

                        if(!vector_upl_index) {
                                vector_upl = vector_upl_create(upl_offset);
                                v_upl_uio_offset = uio->uio_offset;
                                vector_upl_offset = upl_offset;
                        }

                        vector_upl_set_subupl(vector_upl,upl, upl_size);
                        vector_upl_set_iostate(vector_upl, upl, vector_upl_size, upl_size);
                        vector_upl_index++;
                        vector_upl_size += upl_size;
                        vector_upl_iosize += io_size;
                        
                        if(issueVectorUPL || vector_upl_index ==  MAX_VECTOR_UPL_ELEMENTS || vector_upl_size >= max_vector_size) {
                                retval = vector_cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize,  io_flag, (buf_t)NULL, &iostate, callback, callback_arg);
                                reset_vector_run_state();       
                        }
                }
                last_iov_base = iov_base + io_size;

                if (lock) {
                        // We don't need to wait for the I/O to complete
                        cluster_unlock_direct_read(lock);
                        lock = NULL;
                }

                /*
                 * update the uio structure
                 */
                if ((flags & IO_ENCRYPTED) && (max_io_size < io_size)) {
                        uio_update(uio, (user_size_t)max_io_size);
                }
                else {
                        uio_update(uio, (user_size_t)io_size);
                }

                io_req_size -= io_size;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 73)) | DBG_FUNC_END,
                             upl, (int)uio->uio_offset, io_req_size, retval, 0);

        } /* end while */

        if (retval == 0 && iostate.io_error == 0 && io_req_size == 0 && uio->uio_offset < filesize) {

                retval = cluster_io_type(uio, read_type, read_length, 0);
          
                if (retval == 0 && *read_type == IO_DIRECT) {

                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 70)) | DBG_FUNC_NONE,
                                     (int)uio->uio_offset, (int)filesize, *read_type, *read_length, 0);

                        goto next_dread;
                }
        }

wait_for_dreads:

        if(retval == 0 && iostate.io_error == 0 && useVectorUPL && vector_upl_index) {
                retval = vector_cluster_io(vp, vector_upl, vector_upl_offset, v_upl_uio_offset, vector_upl_iosize,  io_flag, (buf_t)NULL, &iostate, callback, callback_arg);
                reset_vector_run_state();
        }

        // We don't need to wait for the I/O to complete
        if (lock)
                cluster_unlock_direct_read(lock);

        /*
         * make sure all async reads that are part of this stream
         * have completed before we return
         */
        cluster_iostate_wait(&iostate, 0, "cluster_read_direct");

        if (iostate.io_error)
                retval = iostate.io_error;

        lck_mtx_destroy(&iostate.io_mtxp, cl_mtx_grp);

        if (io_throttled == TRUE && retval == 0)
                retval = EAGAIN;

        for (next_iov_base = orig_iov_base; next_iov_base < last_iov_base; next_iov_base += PAGE_SIZE) {
                /*
                 * This is specifically done for pmap accounting purposes.
                 * vm_pre_fault() will call vm_fault() to enter the page into
                 * the pmap if there isn't _a_ physical page for that VA already.
                 */
                vm_pre_fault(vm_map_trunc_page(next_iov_base, PAGE_MASK));
        }

        if (io_req_size && retval == 0) {
                /*
                 * we couldn't handle the tail of this request in DIRECT mode
                 * so fire it through the copy path
                 */
                retval = cluster_read_copy(vp, uio, io_req_size, filesize, flags, callback, callback_arg);

                *read_type = IO_UNKNOWN;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 70)) | DBG_FUNC_END,
                     (int)uio->uio_offset, (int)uio_resid(uio), io_req_size, retval, 0);

        return (retval);
}


static int
cluster_read_contig(vnode_t vp, struct uio *uio, off_t filesize, int *read_type, u_int32_t *read_length,
                    int (*callback)(buf_t, void *), void *callback_arg, int flags)
{
        upl_page_info_t *pl;
        upl_t            upl[MAX_VECTS];
        vm_offset_t      upl_offset;
        addr64_t         dst_paddr = 0;
        user_addr_t      iov_base;
        off_t            max_size;
        upl_size_t       upl_size;
        vm_size_t        upl_needed_size;
        mach_msg_type_number_t  pages_in_pl;
        upl_control_flags_t upl_flags;
        kern_return_t    kret;
        struct clios     iostate;
        int              error= 0;
        int              cur_upl = 0;
        int              num_upl = 0;
        int              n;
        u_int32_t        xsize;
        u_int32_t        io_size;
        u_int32_t        devblocksize;
        u_int32_t        mem_alignment_mask;
        u_int32_t        tail_size = 0;
        int              bflag;

        if (flags & IO_PASSIVE)
                bflag = CL_PASSIVE;
        else
                bflag = 0;
        
        if (flags & IO_NOCACHE)
                bflag |= CL_NOCACHE;
        
        /*
         * When we enter this routine, we know
         *  -- the read_length will not exceed the current iov_len
         *  -- the target address is physically contiguous for read_length
         */
        cluster_syncup(vp, filesize, callback, callback_arg, PUSH_SYNC);

        devblocksize = (u_int32_t)vp->v_mount->mnt_devblocksize;
        mem_alignment_mask = (u_int32_t)vp->v_mount->mnt_alignmentmask;

        iostate.io_completed = 0;
        iostate.io_issued = 0;
        iostate.io_error = 0;
        iostate.io_wanted = 0;

        lck_mtx_init(&iostate.io_mtxp, cl_mtx_grp, cl_mtx_attr);

next_cread:
        io_size = *read_length;

        max_size = filesize - uio->uio_offset;

        if (io_size > max_size)
                io_size = max_size;

        iov_base = uio_curriovbase(uio);

        upl_offset = (vm_offset_t)((u_int32_t)iov_base & PAGE_MASK);
        upl_needed_size = upl_offset + io_size;

        pages_in_pl = 0;
        upl_size = upl_needed_size;
        upl_flags = UPL_FILE_IO | UPL_NO_SYNC | UPL_CLEAN_IN_PLACE | UPL_SET_INTERNAL | UPL_SET_LITE | UPL_SET_IO_WIRE;


        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 92)) | DBG_FUNC_START,
                     (int)upl_offset, (int)upl_size, (int)iov_base, io_size, 0);

        vm_map_t map = UIO_SEG_IS_USER_SPACE(uio->uio_segflg) ? current_map() : kernel_map;
        kret = vm_map_get_upl(map,
                              (vm_map_offset_t)(iov_base & ~((user_addr_t)PAGE_MASK)),
                              &upl_size, &upl[cur_upl], NULL, &pages_in_pl, &upl_flags, VM_KERN_MEMORY_FILE, 0);

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 92)) | DBG_FUNC_END,
                     (int)upl_offset, upl_size, io_size, kret, 0);

        if (kret != KERN_SUCCESS) {
                /*
                 * failed to get pagelist
                 */
                error = EINVAL;
                goto wait_for_creads;
        }
        num_upl++;

        if (upl_size < upl_needed_size) {
                /*
                 * The upl_size wasn't satisfied.
                 */
                error = EINVAL;
                goto wait_for_creads;
        }
        pl = ubc_upl_pageinfo(upl[cur_upl]);

        dst_paddr = ((addr64_t)upl_phys_page(pl, 0) << PAGE_SHIFT) + (addr64_t)upl_offset;

        while (((uio->uio_offset & (devblocksize - 1)) || io_size < devblocksize) && io_size) {
                u_int32_t   head_size;

                head_size = devblocksize - (u_int32_t)(uio->uio_offset & (devblocksize - 1));

                if (head_size > io_size)
                        head_size = io_size;

                error = cluster_align_phys_io(vp, uio, dst_paddr, head_size, CL_READ, callback, callback_arg);

                if (error)
                        goto wait_for_creads;

                upl_offset += head_size;
                dst_paddr  += head_size;
                io_size    -= head_size;

                iov_base   += head_size;
        }
        if ((u_int32_t)iov_base & mem_alignment_mask) {
                /*
                 * request doesn't set up on a memory boundary
                 * the underlying DMA engine can handle...
                 * return an error instead of going through
                 * the slow copy path since the intent of this
                 * path is direct I/O to device memory
                 */
                error = EINVAL;
                goto wait_for_creads;
        }

        tail_size = io_size & (devblocksize - 1);

        io_size  -= tail_size;

        while (io_size && error == 0) {

                if (io_size > MAX_IO_CONTIG_SIZE)
                        xsize = MAX_IO_CONTIG_SIZE;
                else
                        xsize = io_size;
                /*
                 * request asynchronously so that we can overlap
                 * the preparation of the next I/O... we'll do
                 * the commit after all the I/O has completed
                 * since its all issued against the same UPL
                 * if there are already too many outstanding reads
                 * wait until some have completed before issuing the next
                 */
                cluster_iostate_wait(&iostate, MAX_IO_CONTIG_SIZE * IO_SCALE(vp, 2), "cluster_read_contig");

                if (iostate.io_error) {
                        /*
                         * one of the earlier reads we issued ran into a hard error
                         * don't issue any more reads...
                         * go wait for any other reads to complete before
                         * returning the error to the caller
                         */
                        goto wait_for_creads;
                }
                error = cluster_io(vp, upl[cur_upl], upl_offset, uio->uio_offset, xsize, 
                                   CL_READ | CL_NOZERO | CL_DEV_MEMORY | CL_ASYNC | bflag,
                                   (buf_t)NULL, &iostate, callback, callback_arg);
                /*
                 * The cluster_io read was issued successfully,
                 * update the uio structure
                 */
                if (error == 0) {
                        uio_update(uio, (user_size_t)xsize);

                        dst_paddr  += xsize;
                        upl_offset += xsize;
                        io_size    -= xsize;
                }
        }
        if (error == 0 && iostate.io_error == 0 && tail_size == 0 && num_upl < MAX_VECTS && uio->uio_offset < filesize) {

                error = cluster_io_type(uio, read_type, read_length, 0);
          
                if (error == 0 && *read_type == IO_CONTIG) {
                        cur_upl++;
                        goto next_cread;
                }
        } else
                *read_type = IO_UNKNOWN;

wait_for_creads:
        /*
         * make sure all async reads that are part of this stream
         * have completed before we proceed
         */
        cluster_iostate_wait(&iostate, 0, "cluster_read_contig");

        if (iostate.io_error)
                error = iostate.io_error;

        lck_mtx_destroy(&iostate.io_mtxp, cl_mtx_grp);

        if (error == 0 && tail_size)
                error = cluster_align_phys_io(vp, uio, dst_paddr, tail_size, CL_READ, callback, callback_arg);

        for (n = 0; n < num_upl; n++)
                /*
                 * just release our hold on each physically contiguous
                 * region without changing any state
                 */
                ubc_upl_abort(upl[n], 0);
        
        return (error);
}


static int
cluster_io_type(struct uio *uio, int *io_type, u_int32_t *io_length, u_int32_t min_length)
{
        user_size_t      iov_len;
        user_addr_t      iov_base = 0;
        upl_t            upl;
        upl_size_t       upl_size;
        upl_control_flags_t upl_flags;
        int              retval = 0;

        /*
         * skip over any emtpy vectors
         */
        uio_update(uio, (user_size_t)0);

        iov_len = uio_curriovlen(uio);

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 94)) | DBG_FUNC_START, uio, (int)iov_len, 0, 0, 0);

        if (iov_len) {
                iov_base = uio_curriovbase(uio);
                /*
                 * make sure the size of the vector isn't too big...
                 * internally, we want to handle all of the I/O in
                 * chunk sizes that fit in a 32 bit int
                 */
                if (iov_len > (user_size_t)MAX_IO_REQUEST_SIZE)
                        upl_size = MAX_IO_REQUEST_SIZE;
                else
                        upl_size = (u_int32_t)iov_len;

                upl_flags = UPL_QUERY_OBJECT_TYPE;

                vm_map_t map = UIO_SEG_IS_USER_SPACE(uio->uio_segflg) ? current_map() : kernel_map;
                if ((vm_map_get_upl(map,
                                    (vm_map_offset_t)(iov_base & ~((user_addr_t)PAGE_MASK)),
                                    &upl_size, &upl, NULL, NULL, &upl_flags, VM_KERN_MEMORY_FILE, 0)) != KERN_SUCCESS) {
                        /*
                         * the user app must have passed in an invalid address
                         */
                        retval = EFAULT;
                }
                if (upl_size == 0)
                        retval = EFAULT;

                *io_length = upl_size;

                if (upl_flags & UPL_PHYS_CONTIG)
                        *io_type = IO_CONTIG;
                else if (iov_len >= min_length)
                        *io_type = IO_DIRECT;
                else
                        *io_type = IO_COPY;
        } else {
                /*
                 * nothing left to do for this uio
                 */
                *io_length = 0;
                *io_type   = IO_UNKNOWN;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 94)) | DBG_FUNC_END, iov_base, *io_type, *io_length, retval, 0);

        return (retval);
}


/*
 * generate advisory I/O's in the largest chunks possible
 * the completed pages will be released into the VM cache
 */
int
advisory_read(vnode_t vp, off_t filesize, off_t f_offset, int resid)
{
        return advisory_read_ext(vp, filesize, f_offset, resid, NULL, NULL, CL_PASSIVE);
}

int
advisory_read_ext(vnode_t vp, off_t filesize, off_t f_offset, int resid, int (*callback)(buf_t, void *), void *callback_arg, int bflag)
{
        upl_page_info_t *pl;
        upl_t            upl;
        vm_offset_t      upl_offset;
        int              upl_size;
        off_t            upl_f_offset;
        int              start_offset;
        int              start_pg;
        int              last_pg;
        int              pages_in_upl;
        off_t            max_size;
        int              io_size;
        kern_return_t    kret;
        int              retval = 0;
        int              issued_io;
        int              skip_range;
        uint32_t         max_io_size;


        if ( !UBCINFOEXISTS(vp))
                return(EINVAL);

        if (resid < 0)
                return(EINVAL);

        max_io_size = cluster_max_io_size(vp->v_mount, CL_READ);

#if CONFIG_EMBEDDED
        if (max_io_size > speculative_prefetch_max_iosize)
                max_io_size = speculative_prefetch_max_iosize;
#else
        if (disk_conditioner_mount_is_ssd(vp->v_mount)) {
                if (max_io_size > speculative_prefetch_max_iosize)
                        max_io_size = speculative_prefetch_max_iosize;
        }
#endif

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 60)) | DBG_FUNC_START,
                     (int)f_offset, resid, (int)filesize, 0, 0);

        while (resid && f_offset < filesize && retval == 0) {
                /*
                 * compute the size of the upl needed to encompass
                 * the requested read... limit each call to cluster_io
                 * to the maximum UPL size... cluster_io will clip if
                 * this exceeds the maximum io_size for the device,
                 * make sure to account for 
                 * a starting offset that's not page aligned
                 */
                start_offset = (int)(f_offset & PAGE_MASK_64);
                upl_f_offset = f_offset - (off_t)start_offset;
                max_size     = filesize - f_offset;

                if (resid < max_size)
                        io_size = resid;
                else
                        io_size = max_size;

                upl_size = (start_offset + io_size + (PAGE_SIZE - 1)) & ~PAGE_MASK;
                if ((uint32_t)upl_size > max_io_size)
                        upl_size = max_io_size;

                skip_range = 0;
                /*
                 * return the number of contiguously present pages in the cache
                 * starting at upl_f_offset within the file
                 */
                ubc_range_op(vp, upl_f_offset, upl_f_offset + upl_size, UPL_ROP_PRESENT, &skip_range);

                if (skip_range) {
                        /*
                         * skip over pages already present in the cache
                         */
                        io_size = skip_range - start_offset;

                        f_offset += io_size;
                        resid    -= io_size;

                        if (skip_range == upl_size)
                                continue;
                        /*
                         * have to issue some real I/O
                         * at this point, we know it's starting on a page boundary
                         * because we've skipped over at least the first page in the request
                         */
                        start_offset = 0;
                        upl_f_offset += skip_range;
                        upl_size     -= skip_range;
                }
                pages_in_upl = upl_size / PAGE_SIZE;

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 61)) | DBG_FUNC_START,
                             upl, (int)upl_f_offset, upl_size, start_offset, 0);

                kret = ubc_create_upl_kernel(vp,
                                      upl_f_offset,
                                      upl_size,
                                      &upl,
                                      &pl,
                                      UPL_RET_ONLY_ABSENT | UPL_SET_LITE,
                                      VM_KERN_MEMORY_FILE);
                if (kret != KERN_SUCCESS)
                        return(retval);
                issued_io = 0;

                /*
                 * before we start marching forward, we must make sure we end on 
                 * a present page, otherwise we will be working with a freed
                 * upl
                 */
                for (last_pg = pages_in_upl - 1; last_pg >= 0; last_pg--) {
                        if (upl_page_present(pl, last_pg))
                                break;
                }
                pages_in_upl = last_pg + 1;


                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 61)) | DBG_FUNC_END,
                             upl, (int)upl_f_offset, upl_size, start_offset, 0);


                for (last_pg = 0; last_pg < pages_in_upl; ) {
                        /*
                         * scan from the beginning of the upl looking for the first
                         * page that is present.... this will become the first page in
                         * the request we're going to make to 'cluster_io'... if all
                         * of the pages are absent, we won't call through to 'cluster_io'
                         */
                        for (start_pg = last_pg; start_pg < pages_in_upl; start_pg++) {
                                if (upl_page_present(pl, start_pg))
                                        break;
                        }

                        /*
                         * scan from the starting present page looking for an absent
                         * page before the end of the upl is reached, if we 
                         * find one, then it will terminate the range of pages being
                         * presented to 'cluster_io'
                         */
                        for (last_pg = start_pg; last_pg < pages_in_upl; last_pg++) {
                                if (!upl_page_present(pl, last_pg))
                                        break;
                        }

                        if (last_pg > start_pg) {               
                                /*
                                 * we found a range of pages that must be filled
                                 * if the last page in this range is the last page of the file
                                 * we may have to clip the size of it to keep from reading past
                                 * the end of the last physical block associated with the file
                                 */
                                upl_offset = start_pg * PAGE_SIZE;
                                io_size    = (last_pg - start_pg) * PAGE_SIZE;

                                if ((off_t)(upl_f_offset + upl_offset + io_size) > filesize)
                                        io_size = filesize - (upl_f_offset + upl_offset);

                                /*
                                 * issue an asynchronous read to cluster_io
                                 */
                                retval = cluster_io(vp, upl, upl_offset, upl_f_offset + upl_offset, io_size,
                                                    CL_ASYNC | CL_READ | CL_COMMIT | CL_AGE | bflag, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);

                                issued_io = 1;
                        }
                }
                if (issued_io == 0)
                        ubc_upl_abort(upl, 0);

                io_size = upl_size - start_offset;
                
                if (io_size > resid)
                        io_size = resid;
                f_offset += io_size;
                resid    -= io_size;
        }

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 60)) | DBG_FUNC_END,
                     (int)f_offset, resid, retval, 0, 0);

        return(retval);
}


int
cluster_push(vnode_t vp, int flags)
{
        return cluster_push_ext(vp, flags, NULL, NULL);
}


int
cluster_push_ext(vnode_t vp, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        return cluster_push_err(vp, flags, callback, callback_arg, NULL);
}

/* write errors via err, but return the number of clusters written */
int
cluster_push_err(vnode_t vp, int flags, int (*callback)(buf_t, void *), void *callback_arg, int *err)
{
        int     retval;
        int     my_sparse_wait = 0;
        struct  cl_writebehind *wbp;

        if (err)
                *err = 0;

        if ( !UBCINFOEXISTS(vp)) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 53)) | DBG_FUNC_NONE, kdebug_vnode(vp), flags, 0, -1, 0);
                return (0);
        }
        /* return if deferred write is set */
        if (((unsigned int)vfs_flags(vp->v_mount) & MNT_DEFWRITE) && (flags & IO_DEFWRITE)) {
                return (0);
        }
        if ((wbp = cluster_get_wbp(vp, CLW_RETURNLOCKED)) == NULL) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 53)) | DBG_FUNC_NONE, kdebug_vnode(vp), flags, 0, -2, 0);
                return (0);
        }
        if (!ISSET(flags, IO_SYNC) && wbp->cl_number == 0 && wbp->cl_scmap == NULL) {
                lck_mtx_unlock(&wbp->cl_lockw);

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 53)) | DBG_FUNC_NONE, kdebug_vnode(vp), flags, 0, -3, 0);
                return(0);
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 53)) | DBG_FUNC_START,
                     wbp->cl_scmap, wbp->cl_number, flags, 0, 0);

        /*
         * if we have an fsync in progress, we don't want to allow any additional
         * sync/fsync/close(s) to occur until it finishes.
         * note that its possible for writes to continue to occur to this file
         * while we're waiting and also once the fsync starts to clean if we're
         * in the sparse map case
         */
        while (wbp->cl_sparse_wait) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 97)) | DBG_FUNC_START, kdebug_vnode(vp), 0, 0, 0, 0);

                msleep((caddr_t)&wbp->cl_sparse_wait, &wbp->cl_lockw, PRIBIO + 1, "cluster_push_ext", NULL);

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 97)) | DBG_FUNC_END, kdebug_vnode(vp), 0, 0, 0, 0);
        }
        if (flags & IO_SYNC) {
                my_sparse_wait = 1;
                wbp->cl_sparse_wait = 1;

                /*
                 * this is an fsync (or equivalent)... we must wait for any existing async
                 * cleaning operations to complete before we evaulate the current state
                 * and finish cleaning... this insures that all writes issued before this
                 * fsync actually get cleaned to the disk before this fsync returns
                 */
                while (wbp->cl_sparse_pushes) {
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 98)) | DBG_FUNC_START, kdebug_vnode(vp), 0, 0, 0, 0);

                        msleep((caddr_t)&wbp->cl_sparse_pushes, &wbp->cl_lockw, PRIBIO + 1, "cluster_push_ext", NULL);

                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 98)) | DBG_FUNC_END, kdebug_vnode(vp), 0, 0, 0, 0);
                }
        }
        if (wbp->cl_scmap) {
                void    *scmap;

                if (wbp->cl_sparse_pushes < SPARSE_PUSH_LIMIT) {

                        scmap = wbp->cl_scmap;
                        wbp->cl_scmap = NULL;

                        wbp->cl_sparse_pushes++;

                        lck_mtx_unlock(&wbp->cl_lockw);

                        retval = sparse_cluster_push(&scmap, vp, ubc_getsize(vp), PUSH_ALL, flags, callback, callback_arg);

                        lck_mtx_lock(&wbp->cl_lockw);

                        wbp->cl_sparse_pushes--;
                        
                        if (wbp->cl_sparse_wait && wbp->cl_sparse_pushes == 0)
                                wakeup((caddr_t)&wbp->cl_sparse_pushes);
                } else {
                        retval = sparse_cluster_push(&(wbp->cl_scmap), vp, ubc_getsize(vp), PUSH_ALL, flags, callback, callback_arg);
                }
                if (err)
                        *err = retval;
                retval = 1;
        } else {
                retval = cluster_try_push(wbp, vp, ubc_getsize(vp), PUSH_ALL, flags, callback, callback_arg, err);
        }
        lck_mtx_unlock(&wbp->cl_lockw);

        if (flags & IO_SYNC)
                (void)vnode_waitforwrites(vp, 0, 0, 0, "cluster_push");

        if (my_sparse_wait) {
                /*
                 * I'm the owner of the serialization token
                 * clear it and wakeup anyone that is waiting
                 * for me to finish
                 */
                lck_mtx_lock(&wbp->cl_lockw);

                wbp->cl_sparse_wait = 0;
                wakeup((caddr_t)&wbp->cl_sparse_wait);

                lck_mtx_unlock(&wbp->cl_lockw);
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 53)) | DBG_FUNC_END,
                     wbp->cl_scmap, wbp->cl_number, retval, 0, 0);

        return (retval);
}


__private_extern__ void
cluster_release(struct ubc_info *ubc)
{
        struct cl_writebehind *wbp;
        struct cl_readahead   *rap;

        if ((wbp = ubc->cl_wbehind)) {

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 81)) | DBG_FUNC_START, ubc, wbp->cl_scmap, 0, 0, 0);

                if (wbp->cl_scmap)
                        vfs_drt_control(&(wbp->cl_scmap), 0);
        } else {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 81)) | DBG_FUNC_START, ubc, 0, 0, 0, 0);
        }

        rap = ubc->cl_rahead;

        if (wbp != NULL) {
                lck_mtx_destroy(&wbp->cl_lockw, cl_mtx_grp);
                FREE_ZONE((void *)wbp, sizeof *wbp, M_CLWRBEHIND);
        }
        if ((rap = ubc->cl_rahead)) {
                lck_mtx_destroy(&rap->cl_lockr, cl_mtx_grp);
                FREE_ZONE((void *)rap, sizeof *rap, M_CLRDAHEAD);
        }
        ubc->cl_rahead  = NULL;
        ubc->cl_wbehind = NULL;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 81)) | DBG_FUNC_END, ubc, rap, wbp, 0, 0);
}


static int
cluster_try_push(struct cl_writebehind *wbp, vnode_t vp, off_t EOF, int push_flag, int io_flags, int (*callback)(buf_t, void *), void *callback_arg, int *err)
{
        int cl_index;
        int cl_index1;
        int min_index;
        int cl_len;
        int cl_pushed = 0;
        struct cl_wextent l_clusters[MAX_CLUSTERS];
        u_int  max_cluster_pgcount;
        int error = 0;

        max_cluster_pgcount = MAX_CLUSTER_SIZE(vp) / PAGE_SIZE;
        /*
         * the write behind context exists and has
         * already been locked...
         */
        if (wbp->cl_number == 0)
                /*
                 * no clusters to push
                 * return number of empty slots
                 */
                return (MAX_CLUSTERS);
         
        /*
         * make a local 'sorted' copy of the clusters
         * and clear wbp->cl_number so that new clusters can
         * be developed
         */
        for (cl_index = 0; cl_index < wbp->cl_number; cl_index++) {
                for (min_index = -1, cl_index1 = 0; cl_index1 < wbp->cl_number; cl_index1++) {
                        if (wbp->cl_clusters[cl_index1].b_addr == wbp->cl_clusters[cl_index1].e_addr)
                                continue;
                        if (min_index == -1)
                                min_index = cl_index1;
                        else if (wbp->cl_clusters[cl_index1].b_addr < wbp->cl_clusters[min_index].b_addr)
                                min_index = cl_index1;
                }
                if (min_index == -1)
                        break;

                l_clusters[cl_index].b_addr = wbp->cl_clusters[min_index].b_addr;
                l_clusters[cl_index].e_addr = wbp->cl_clusters[min_index].e_addr;
                l_clusters[cl_index].io_flags = wbp->cl_clusters[min_index].io_flags;

                wbp->cl_clusters[min_index].b_addr = wbp->cl_clusters[min_index].e_addr;
        }
        wbp->cl_number = 0;

        cl_len = cl_index;

        /* skip switching to the sparse cluster mechanism if on diskimage */
        if ( ((push_flag & PUSH_DELAY) && cl_len == MAX_CLUSTERS ) &&
            !(vp->v_mount->mnt_kern_flag & MNTK_VIRTUALDEV) ) {
                int   i;
                
                /*
                 * determine if we appear to be writing the file sequentially
                 * if not, by returning without having pushed any clusters
                 * we will cause this vnode to be pushed into the sparse cluster mechanism
                 * used for managing more random I/O patterns
                 *
                 * we know that we've got all clusters currently in use and the next write doesn't fit into one of them...
                 * that's why we're in try_push with PUSH_DELAY...
                 *
                 * check to make sure that all the clusters except the last one are 'full'... and that each cluster
                 * is adjacent to the next (i.e. we're looking for sequential writes) they were sorted above
                 * so we can just make a simple pass through, up to, but not including the last one...
                 * note that e_addr is not inclusive, so it will be equal to the b_addr of the next cluster if they
                 * are sequential
                 * 
                 * we let the last one be partial as long as it was adjacent to the previous one...
                 * we need to do this to deal with multi-threaded servers that might write an I/O or 2 out
                 * of order... if this occurs at the tail of the last cluster, we don't want to fall into the sparse cluster world...
                 */
                for (i = 0; i < MAX_CLUSTERS - 1; i++) {
                        if ((l_clusters[i].e_addr - l_clusters[i].b_addr) != max_cluster_pgcount)
                                goto dont_try;
                        if (l_clusters[i].e_addr != l_clusters[i+1].b_addr)
                                goto dont_try;
                }
        }
        for (cl_index = 0; cl_index < cl_len; cl_index++) {
                int     flags;
                struct  cl_extent cl;
                int retval;

                flags = io_flags & (IO_PASSIVE|IO_CLOSE);

                /*
                 * try to push each cluster in turn...
                 */
                if (l_clusters[cl_index].io_flags & CLW_IONOCACHE)
                        flags |= IO_NOCACHE;

                if (l_clusters[cl_index].io_flags & CLW_IOPASSIVE)
                        flags |= IO_PASSIVE;

                if (push_flag & PUSH_SYNC)
                        flags |= IO_SYNC;

                cl.b_addr = l_clusters[cl_index].b_addr;
                cl.e_addr = l_clusters[cl_index].e_addr;

                retval = cluster_push_now(vp, &cl, EOF, flags, callback, callback_arg);

                if (error == 0 && retval)
                        error = retval;

                l_clusters[cl_index].b_addr = 0;
                l_clusters[cl_index].e_addr = 0;

                cl_pushed++;

                if ( !(push_flag & PUSH_ALL) )
                        break;
        }
        if (err)
                *err = error;

dont_try:
        if (cl_len > cl_pushed) {
               /*
                * we didn't push all of the clusters, so
                * lets try to merge them back in to the vnode
                */
                if ((MAX_CLUSTERS - wbp->cl_number) < (cl_len - cl_pushed)) {
                        /*
                         * we picked up some new clusters while we were trying to
                         * push the old ones... this can happen because I've dropped
                         * the vnode lock... the sum of the
                         * leftovers plus the new cluster count exceeds our ability
                         * to represent them, so switch to the sparse cluster mechanism
                         *
                         * collect the active public clusters...
                         */
                        sparse_cluster_switch(wbp, vp, EOF, callback, callback_arg);

                        for (cl_index = 0, cl_index1 = 0; cl_index < cl_len; cl_index++) {
                                if (l_clusters[cl_index].b_addr == l_clusters[cl_index].e_addr)
                                        continue;
                                wbp->cl_clusters[cl_index1].b_addr = l_clusters[cl_index].b_addr;
                                wbp->cl_clusters[cl_index1].e_addr = l_clusters[cl_index].e_addr;
                                wbp->cl_clusters[cl_index1].io_flags = l_clusters[cl_index].io_flags;

                                cl_index1++;
                        }
                        /*
                         * update the cluster count
                         */
                        wbp->cl_number = cl_index1;

                        /*
                         * and collect the original clusters that were moved into the 
                         * local storage for sorting purposes
                         */
                        sparse_cluster_switch(wbp, vp, EOF, callback, callback_arg);

                } else {
                        /*
                         * we've got room to merge the leftovers back in
                         * just append them starting at the next 'hole'
                         * represented by wbp->cl_number
                         */
                        for (cl_index = 0, cl_index1 = wbp->cl_number; cl_index < cl_len; cl_index++) {
                                if (l_clusters[cl_index].b_addr == l_clusters[cl_index].e_addr)
                                        continue;

                                wbp->cl_clusters[cl_index1].b_addr = l_clusters[cl_index].b_addr;
                                wbp->cl_clusters[cl_index1].e_addr = l_clusters[cl_index].e_addr;
                                wbp->cl_clusters[cl_index1].io_flags = l_clusters[cl_index].io_flags;

                                cl_index1++;
                        }
                        /*
                         * update the cluster count
                         */
                        wbp->cl_number = cl_index1;
                }
        }
        return (MAX_CLUSTERS - wbp->cl_number);
}



static int
cluster_push_now(vnode_t vp, struct cl_extent *cl, off_t EOF, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        upl_page_info_t *pl;
        upl_t            upl;
        vm_offset_t      upl_offset;
        int              upl_size;
        off_t            upl_f_offset;
        int              pages_in_upl;
        int              start_pg;
        int              last_pg;
        int              io_size;
        int              io_flags;
        int              upl_flags;
        int              bflag;
        int              size;
        int              error = 0;
        int              retval;
        kern_return_t    kret;

        if (flags & IO_PASSIVE)
                bflag = CL_PASSIVE;
        else
                bflag = 0;

        if (flags & IO_SKIP_ENCRYPTION)
                bflag |= CL_ENCRYPTED;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 51)) | DBG_FUNC_START,
                     (int)cl->b_addr, (int)cl->e_addr, (int)EOF, flags, 0);

        if ((pages_in_upl = (int)(cl->e_addr - cl->b_addr)) == 0) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 51)) | DBG_FUNC_END, 1, 0, 0, 0, 0);

                return (0);
        }
        upl_size = pages_in_upl * PAGE_SIZE;
        upl_f_offset = (off_t)(cl->b_addr * PAGE_SIZE_64);

        if (upl_f_offset + upl_size >= EOF) {

                if (upl_f_offset >= EOF) {
                        /*
                         * must have truncated the file and missed 
                         * clearing a dangling cluster (i.e. it's completely
                         * beyond the new EOF
                         */
                        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 51)) | DBG_FUNC_END, 1, 1, 0, 0, 0);

                        return(0);
                }
                size = EOF - upl_f_offset;

                upl_size = (size + (PAGE_SIZE - 1)) & ~PAGE_MASK;
                pages_in_upl = upl_size / PAGE_SIZE;
        } else
                size = upl_size;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 41)) | DBG_FUNC_START, upl_size, size, 0, 0, 0);

        /*
         * by asking for UPL_COPYOUT_FROM and UPL_RET_ONLY_DIRTY, we get the following desirable behavior
         * 
         * - only pages that are currently dirty are returned... these are the ones we need to clean
         * - the hardware dirty bit is cleared when the page is gathered into the UPL... the software dirty bit is set
         * - if we have to abort the I/O for some reason, the software dirty bit is left set since we didn't clean the page
         * - when we commit the page, the software dirty bit is cleared... the hardware dirty bit is untouched so that if 
         *   someone dirties this page while the I/O is in progress, we don't lose track of the new state
         *
         * when the I/O completes, we no longer ask for an explicit clear of the DIRTY state (either soft or hard)
         */

        if ((vp->v_flag & VNOCACHE_DATA) || (flags & IO_NOCACHE))
                upl_flags = UPL_COPYOUT_FROM | UPL_RET_ONLY_DIRTY | UPL_SET_LITE | UPL_WILL_BE_DUMPED;
        else
                upl_flags = UPL_COPYOUT_FROM | UPL_RET_ONLY_DIRTY | UPL_SET_LITE;

        kret = ubc_create_upl_kernel(vp,
                                upl_f_offset,
                                upl_size,
                                &upl,
                                &pl,
                                upl_flags,
                                VM_KERN_MEMORY_FILE);
        if (kret != KERN_SUCCESS)
                panic("cluster_push: failed to get pagelist");

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 41)) | DBG_FUNC_END, upl, upl_f_offset, 0, 0, 0);

        /*
         * since we only asked for the dirty pages back
         * it's possible that we may only get a few or even none, so...
         * before we start marching forward, we must make sure we know
         * where the last present page is in the UPL, otherwise we could
         * end up working with a freed upl due to the FREE_ON_EMPTY semantics
         * employed by commit_range and abort_range.
         */
        for (last_pg = pages_in_upl - 1; last_pg >= 0; last_pg--) {
                if (upl_page_present(pl, last_pg))
                        break;
        }
        pages_in_upl = last_pg + 1;

        if (pages_in_upl == 0) {
                ubc_upl_abort(upl, 0);

                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 51)) | DBG_FUNC_END, 1, 2, 0, 0, 0);
                return(0);
        }         

        for (last_pg = 0; last_pg < pages_in_upl; ) {
                /*
                 * find the next dirty page in the UPL
                 * this will become the first page in the 
                 * next I/O to generate
                 */
                for (start_pg = last_pg; start_pg < pages_in_upl; start_pg++) {
                        if (upl_dirty_page(pl, start_pg))
                                break;
                        if (upl_page_present(pl, start_pg))
                                /*
                                 * RET_ONLY_DIRTY will return non-dirty 'precious' pages
                                 * just release these unchanged since we're not going
                                 * to steal them or change their state
                                 */
                                ubc_upl_abort_range(upl, start_pg * PAGE_SIZE, PAGE_SIZE, UPL_ABORT_FREE_ON_EMPTY);
                }
                if (start_pg >= pages_in_upl)
                        /*
                         * done... no more dirty pages to push
                         */
                        break;
                if (start_pg > last_pg)
                        /*
                         * skipped over some non-dirty pages
                         */
                        size -= ((start_pg - last_pg) * PAGE_SIZE);

                /*
                 * find a range of dirty pages to write
                 */
                for (last_pg = start_pg; last_pg < pages_in_upl; last_pg++) {
                        if (!upl_dirty_page(pl, last_pg))
                                break;
                }
                upl_offset = start_pg * PAGE_SIZE;

                io_size = min(size, (last_pg - start_pg) * PAGE_SIZE);

                io_flags = CL_THROTTLE | CL_COMMIT | CL_AGE | bflag;

                if ( !(flags & IO_SYNC))
                        io_flags |= CL_ASYNC;

                if (flags & IO_CLOSE)
                        io_flags |= CL_CLOSE;

                if (flags & IO_NOCACHE)
                        io_flags |= CL_NOCACHE;

                retval = cluster_io(vp, upl, upl_offset, upl_f_offset + upl_offset, io_size,
                                    io_flags, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);

                if (error == 0 && retval)
                        error = retval;

                size -= io_size;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 51)) | DBG_FUNC_END, 1, 3, 0, 0, 0);

        return(error);
}


/*
 * sparse_cluster_switch is called with the write behind lock held
 */
static void
sparse_cluster_switch(struct cl_writebehind *wbp, vnode_t vp, off_t EOF, int (*callback)(buf_t, void *), void *callback_arg)
{
        int     cl_index;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 78)) | DBG_FUNC_START, kdebug_vnode(vp), wbp->cl_scmap, 0, 0, 0);

        for (cl_index = 0; cl_index < wbp->cl_number; cl_index++) {
                int       flags;
                struct cl_extent cl;

                for (cl.b_addr = wbp->cl_clusters[cl_index].b_addr; cl.b_addr < wbp->cl_clusters[cl_index].e_addr; cl.b_addr++) {

                        if (ubc_page_op(vp, (off_t)(cl.b_addr * PAGE_SIZE_64), 0, NULL, &flags) == KERN_SUCCESS) {
                                if (flags & UPL_POP_DIRTY) {
                                        cl.e_addr = cl.b_addr + 1;

                                        sparse_cluster_add(&(wbp->cl_scmap), vp, &cl, EOF, callback, callback_arg);
                                }
                        }
                }
        }
        wbp->cl_number = 0;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 78)) | DBG_FUNC_END, kdebug_vnode(vp), wbp->cl_scmap, 0, 0, 0);
}


/*
 * sparse_cluster_push must be called with the write-behind lock held if the scmap is
 * still associated with the write-behind context... however, if the scmap has been disassociated
 * from the write-behind context (the cluster_push case), the wb lock is not held
 */
static int
sparse_cluster_push(void **scmap, vnode_t vp, off_t EOF, int push_flag, int io_flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        struct cl_extent cl;
        off_t           offset;
        u_int           length;
        int error = 0;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 79)) | DBG_FUNC_START, kdebug_vnode(vp), (*scmap), 0, push_flag, 0);

        if (push_flag & PUSH_ALL)
                vfs_drt_control(scmap, 1);

        for (;;) {
                int retval;
                if (vfs_drt_get_cluster(scmap, &offset, &length) != KERN_SUCCESS)
                        break;

                cl.b_addr = (daddr64_t)(offset / PAGE_SIZE_64);
                cl.e_addr = (daddr64_t)((offset + length) / PAGE_SIZE_64);

                retval = cluster_push_now(vp, &cl, EOF, io_flags & (IO_PASSIVE|IO_CLOSE), callback, callback_arg);
                if (error == 0 && retval)
                        error = retval;

                if ( !(push_flag & PUSH_ALL) )
                        break;
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 79)) | DBG_FUNC_END, kdebug_vnode(vp), (*scmap), 0, 0, 0);

        return error;
}


/*
 * sparse_cluster_add is called with the write behind lock held
 */
static void
sparse_cluster_add(void **scmap, vnode_t vp, struct cl_extent *cl, off_t EOF, int (*callback)(buf_t, void *), void *callback_arg)
{
        u_int   new_dirty;
        u_int   length;
        off_t   offset;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 80)) | DBG_FUNC_START, (*scmap), 0, cl->b_addr, (int)cl->e_addr, 0);

        offset = (off_t)(cl->b_addr * PAGE_SIZE_64);
        length = ((u_int)(cl->e_addr - cl->b_addr)) * PAGE_SIZE;

        while (vfs_drt_mark_pages(scmap, offset, length, &new_dirty) != KERN_SUCCESS) {
                /*
                 * no room left in the map
                 * only a partial update was done
                 * push out some pages and try again
                 */
                sparse_cluster_push(scmap, vp, EOF, 0, 0, callback, callback_arg);

                offset += (new_dirty * PAGE_SIZE_64);
                length -= (new_dirty * PAGE_SIZE);
        }
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 80)) | DBG_FUNC_END, kdebug_vnode(vp), (*scmap), 0, 0, 0);
}


static int
cluster_align_phys_io(vnode_t vp, struct uio *uio, addr64_t usr_paddr, u_int32_t xsize, int flags, int (*callback)(buf_t, void *), void *callback_arg)
{
        upl_page_info_t  *pl;
        upl_t            upl;
        addr64_t         ubc_paddr;
        kern_return_t    kret;
        int              error = 0;
        int              did_read = 0;
        int              abort_flags;
        int              upl_flags;
        int              bflag;

        if (flags & IO_PASSIVE)
                bflag = CL_PASSIVE;
        else
                bflag = 0;

        if (flags & IO_NOCACHE)
                bflag |= CL_NOCACHE;

        upl_flags = UPL_SET_LITE;

        if ( !(flags & CL_READ) ) {
                /*
                 * "write" operation:  let the UPL subsystem know
                 * that we intend to modify the buffer cache pages
                 * we're gathering.
                 */
                upl_flags |= UPL_WILL_MODIFY;
        } else {
                /*
                 * indicate that there is no need to pull the
                 * mapping for this page... we're only going
                 * to read from it, not modify it.
                 */
                upl_flags |= UPL_FILE_IO;
        }
        kret = ubc_create_upl_kernel(vp,
                              uio->uio_offset & ~PAGE_MASK_64,
                              PAGE_SIZE,
                              &upl,
                              &pl,
                              upl_flags,
                              VM_KERN_MEMORY_FILE);

        if (kret != KERN_SUCCESS)
                return(EINVAL);

        if (!upl_valid_page(pl, 0)) {
                /*
                 * issue a synchronous read to cluster_io
                 */
                error = cluster_io(vp, upl, 0, uio->uio_offset & ~PAGE_MASK_64, PAGE_SIZE,
                                   CL_READ | bflag, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);
                if (error) {
                          ubc_upl_abort_range(upl, 0, PAGE_SIZE, UPL_ABORT_DUMP_PAGES | UPL_ABORT_FREE_ON_EMPTY);

                          return(error);
                }
                did_read = 1;
        }
        ubc_paddr = ((addr64_t)upl_phys_page(pl, 0) << PAGE_SHIFT) + (addr64_t)(uio->uio_offset & PAGE_MASK_64);

/*
 *      NOTE:  There is no prototype for the following in BSD. It, and the definitions
 *      of the defines for cppvPsrc, cppvPsnk, cppvFsnk, and cppvFsrc will be found in
 *      osfmk/ppc/mappings.h.  They are not included here because there appears to be no
 *      way to do so without exporting them to kexts as well.
 */
        if (flags & CL_READ)
//              copypv(ubc_paddr, usr_paddr, xsize, cppvPsrc | cppvPsnk | cppvFsnk);    /* Copy physical to physical and flush the destination */
                copypv(ubc_paddr, usr_paddr, xsize,        2 |        1 |        4);    /* Copy physical to physical and flush the destination */
        else
//              copypv(usr_paddr, ubc_paddr, xsize, cppvPsrc | cppvPsnk | cppvFsrc);    /* Copy physical to physical and flush the source */
                copypv(usr_paddr, ubc_paddr, xsize,        2 |        1 |        8);    /* Copy physical to physical and flush the source */
        
        if ( !(flags & CL_READ) || (upl_valid_page(pl, 0) && upl_dirty_page(pl, 0))) {
                /*
                 * issue a synchronous write to cluster_io
                 */
                error = cluster_io(vp, upl, 0, uio->uio_offset & ~PAGE_MASK_64, PAGE_SIZE,
                                   bflag, (buf_t)NULL, (struct clios *)NULL, callback, callback_arg);
        }
        if (error == 0) 
                uio_update(uio, (user_size_t)xsize);

        if (did_read)
                abort_flags = UPL_ABORT_FREE_ON_EMPTY;
        else
                abort_flags = UPL_ABORT_FREE_ON_EMPTY | UPL_ABORT_DUMP_PAGES;

        ubc_upl_abort_range(upl, 0, PAGE_SIZE, abort_flags);
        
        return (error);
}

int
cluster_copy_upl_data(struct uio *uio, upl_t upl, int upl_offset, int *io_resid)
{
        int       pg_offset;
        int       pg_index;
        int       csize;
        int       segflg;
        int       retval = 0;
        int       xsize;
        upl_page_info_t *pl;
        int       dirty_count;

        xsize = *io_resid;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 34)) | DBG_FUNC_START,
                     (int)uio->uio_offset, upl_offset, xsize, 0, 0);

        segflg = uio->uio_segflg;

        switch(segflg) {

          case UIO_USERSPACE32:
          case UIO_USERISPACE32:
                uio->uio_segflg = UIO_PHYS_USERSPACE32;
                break;

          case UIO_USERSPACE:
          case UIO_USERISPACE:
                uio->uio_segflg = UIO_PHYS_USERSPACE;
                break;

          case UIO_USERSPACE64:
          case UIO_USERISPACE64:
                uio->uio_segflg = UIO_PHYS_USERSPACE64;
                break;

          case UIO_SYSSPACE:
                uio->uio_segflg = UIO_PHYS_SYSSPACE;
                break;

        }
        pl = ubc_upl_pageinfo(upl);

        pg_index  = upl_offset / PAGE_SIZE;
        pg_offset = upl_offset & PAGE_MASK;
        csize     = min(PAGE_SIZE - pg_offset, xsize);

        dirty_count = 0;
        while (xsize && retval == 0) {
                addr64_t  paddr;

                paddr = ((addr64_t)upl_phys_page(pl, pg_index) << PAGE_SHIFT) + pg_offset;
                if ((uio->uio_rw == UIO_WRITE) && (upl_dirty_page(pl, pg_index) == FALSE)) 
                        dirty_count++;

                retval = uiomove64(paddr, csize, uio);

                pg_index += 1;
                pg_offset = 0;
                xsize    -= csize;
                csize     = min(PAGE_SIZE, xsize);
        }
        *io_resid = xsize;

        uio->uio_segflg = segflg;

        task_update_logical_writes(current_task(), (dirty_count * PAGE_SIZE), TASK_WRITE_DEFERRED, upl_lookup_vnode(upl));
        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 34)) | DBG_FUNC_END,
                     (int)uio->uio_offset, xsize, retval, segflg, 0);
        
        return (retval);
}


int
cluster_copy_ubc_data(vnode_t vp, struct uio *uio, int *io_resid, int mark_dirty)
{

        return (cluster_copy_ubc_data_internal(vp, uio, io_resid, mark_dirty, 1));
}


static int
cluster_copy_ubc_data_internal(vnode_t vp, struct uio *uio, int *io_resid, int mark_dirty, int take_reference)
{
        int       segflg;
        int       io_size;
        int       xsize;
        int       start_offset;
        int       retval = 0;
        memory_object_control_t  control;

        io_size = *io_resid;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 34)) | DBG_FUNC_START,
                     (int)uio->uio_offset, io_size, mark_dirty, take_reference, 0);

        control = ubc_getobject(vp, UBC_FLAGS_NONE);

        if (control == MEMORY_OBJECT_CONTROL_NULL) {
                KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 34)) | DBG_FUNC_END,
                             (int)uio->uio_offset, io_size, retval, 3, 0);

                return(0);
        }
        segflg = uio->uio_segflg;

        switch(segflg) {

          case UIO_USERSPACE32:
          case UIO_USERISPACE32:
                uio->uio_segflg = UIO_PHYS_USERSPACE32;
                break;

          case UIO_USERSPACE64:
          case UIO_USERISPACE64:
                uio->uio_segflg = UIO_PHYS_USERSPACE64;
                break;

          case UIO_USERSPACE:
          case UIO_USERISPACE:
                uio->uio_segflg = UIO_PHYS_USERSPACE;
                break;

          case UIO_SYSSPACE:
                uio->uio_segflg = UIO_PHYS_SYSSPACE;
                break;
        }

        if ( (io_size = *io_resid) ) {
                start_offset = (int)(uio->uio_offset & PAGE_MASK_64);
                xsize = uio_resid(uio);

                retval = memory_object_control_uiomove(control, uio->uio_offset - start_offset, uio,
                                                       start_offset, io_size, mark_dirty, take_reference);
                xsize -= uio_resid(uio);
                io_size -= xsize;
        }
        uio->uio_segflg = segflg;
        *io_resid       = io_size;

        KERNEL_DEBUG((FSDBG_CODE(DBG_FSRW, 34)) | DBG_FUNC_END,
                     (int)uio->uio_offset, io_size, retval, 0x80000000 | segflg, 0);

        return(retval);
}


int
is_file_clean(vnode_t vp, off_t filesize)
{
        off_t f_offset;
        int   flags;
        int   total_dirty = 0;

        for (f_offset = 0; f_offset < filesize; f_offset += PAGE_SIZE_64) {
                if (ubc_page_op(vp, f_offset, 0, NULL, &flags) == KERN_SUCCESS) {
                        if (flags & UPL_POP_DIRTY) {
                                total_dirty++;
                        }
                }
        }
        if (total_dirty)
                return(EINVAL);

        return (0);
}



/*
 * Dirty region tracking/clustering mechanism.
 *
 * This code (vfs_drt_*) provides a mechanism for tracking and clustering
 * dirty regions within a larger space (file).  It is primarily intended to
 * support clustering in large files with many dirty areas.
 *
 * The implementation assumes that the dirty regions are pages.
 *
 * To represent dirty pages within the file, we store bit vectors in a
 * variable-size circular hash.
 */

/*
 * Bitvector size.  This determines the number of pages we group in a
 * single hashtable entry.  Each hashtable entry is aligned to this
 * size within the file.
 */
#define DRT_BITVECTOR_PAGES             ((1024 * 1024) / PAGE_SIZE)

/*
 * File offset handling.
 *
  * DRT_ADDRESS_MASK is dependent on DRT_BITVECTOR_PAGES;
 * the correct formula is  (~((DRT_BITVECTOR_PAGES * PAGE_SIZE) - 1))
 */
#define DRT_ADDRESS_MASK                (~((DRT_BITVECTOR_PAGES * PAGE_SIZE) - 1))
#define DRT_ALIGN_ADDRESS(addr)         ((addr) & DRT_ADDRESS_MASK)

/*
 * Hashtable address field handling.
 *
 * The low-order bits of the hashtable address are used to conserve
 * space.
 *
 * DRT_HASH_COUNT_MASK must be large enough to store the range
 * 0-DRT_BITVECTOR_PAGES inclusive, as well as have one value
 * to indicate that the bucket is actually unoccupied.
 */
#define DRT_HASH_GET_ADDRESS(scm, i)    ((scm)->scm_hashtable[(i)].dhe_control & DRT_ADDRESS_MASK)
#define DRT_HASH_SET_ADDRESS(scm, i, a)                                                                 \
        do {                                                                                            \
                (scm)->scm_hashtable[(i)].dhe_control =                                                 \
                    ((scm)->scm_hashtable[(i)].dhe_control & ~DRT_ADDRESS_MASK) | DRT_ALIGN_ADDRESS(a); \
        } while (0)
#define DRT_HASH_COUNT_MASK             0x1ff
#define DRT_HASH_GET_COUNT(scm, i)      ((scm)->scm_hashtable[(i)].dhe_control & DRT_HASH_COUNT_MASK)
#define DRT_HASH_SET_COUNT(scm, i, c)                                                                                   \
        do {                                                                                                            \
                (scm)->scm_hashtable[(i)].dhe_control =                                                                 \
                    ((scm)->scm_hashtable[(i)].dhe_control & ~DRT_HASH_COUNT_MASK) | ((c) & DRT_HASH_COUNT_MASK);       \
        } while (0)
#define DRT_HASH_CLEAR(scm, i)                                                                                          \
        do {                                                                                                            \
                (scm)->scm_hashtable[(i)].dhe_control = 0;                                                              \
        } while (0)
#define DRT_HASH_VACATE(scm, i)         DRT_HASH_SET_COUNT((scm), (i), DRT_HASH_COUNT_MASK)
#define DRT_HASH_VACANT(scm, i)         (DRT_HASH_GET_COUNT((scm), (i)) == DRT_HASH_COUNT_MASK)
#define DRT_HASH_COPY(oscm, oi, scm, i)                                                                 \
        do {                                                                                            \
                (scm)->scm_hashtable[(i)].dhe_control = (oscm)->scm_hashtable[(oi)].dhe_control;        \
                DRT_BITVECTOR_COPY(oscm, oi, scm, i);                                                   \
        } while(0);


/*
 * Hash table moduli.
 *
 * Since the hashtable entry's size is dependent on the size of
 * the bitvector, and since the hashtable size is constrained to
 * both being prime and fitting within the desired allocation
 * size, these values need to be manually determined.
 *
 * For DRT_BITVECTOR_SIZE = 256, the entry size is 40 bytes.
 *
 * The small hashtable allocation is 1024 bytes, so the modulus is 23.
 * The large hashtable allocation is 16384 bytes, so the modulus is 401.
 */
#define DRT_HASH_SMALL_MODULUS  23
#define DRT_HASH_LARGE_MODULUS  401

/*
 * Physical memory required before the large hash modulus is permitted.
 *
 * On small memory systems, the large hash modulus can lead to phsyical
 * memory starvation, so we avoid using it there.
 */
#define DRT_HASH_LARGE_MEMORY_REQUIRED  (1024LL * 1024LL * 1024LL)      /* 1GiB */

#define DRT_SMALL_ALLOCATION    1024    /* 104 bytes spare */
#define DRT_LARGE_ALLOCATION    16384   /* 344 bytes spare */

/* *** nothing below here has secret dependencies on DRT_BITVECTOR_PAGES *** */

/*
 * Hashtable bitvector handling.
 *
 * Bitvector fields are 32 bits long.
 */

#define DRT_HASH_SET_BIT(scm, i, bit)                           \
        (scm)->scm_hashtable[(i)].dhe_bitvector[(bit) / 32] |= (1 << ((bit) % 32))

#define DRT_HASH_CLEAR_BIT(scm, i, bit)                         \
        (scm)->scm_hashtable[(i)].dhe_bitvector[(bit) / 32] &= ~(1 << ((bit) % 32))
    
#define DRT_HASH_TEST_BIT(scm, i, bit)                          \
        ((scm)->scm_hashtable[(i)].dhe_bitvector[(bit) / 32] & (1 << ((bit) % 32)))
    
#define DRT_BITVECTOR_CLEAR(scm, i)                             \
        bzero(&(scm)->scm_hashtable[(i)].dhe_bitvector[0], (DRT_BITVECTOR_PAGES / 32) * sizeof(u_int32_t))

#define DRT_BITVECTOR_COPY(oscm, oi, scm, i)                    \
        bcopy(&(oscm)->scm_hashtable[(oi)].dhe_bitvector[0],    \
            &(scm)->scm_hashtable[(i)].dhe_bitvector[0],        \
            (DRT_BITVECTOR_PAGES / 32) * sizeof(u_int32_t))


 
/*
 * Hashtable entry.
 */
struct vfs_drt_hashentry {
        u_int64_t       dhe_control;
/*
* dhe_bitvector was declared as dhe_bitvector[DRT_BITVECTOR_PAGES / 32];
* DRT_BITVECTOR_PAGES is defined as ((1024 * 1024) / PAGE_SIZE)
* Since PAGE_SIZE is only known at boot time, 
*       -define MAX_DRT_BITVECTOR_PAGES for smallest supported page size (4k) 
*       -declare dhe_bitvector array for largest possible length
*/
#define MAX_DRT_BITVECTOR_PAGES (1024 * 1024)/( 4 * 1024)
        u_int32_t       dhe_bitvector[MAX_DRT_BITVECTOR_PAGES/32];
};

/*
 * Dirty Region Tracking structure.
 *
 * The hashtable is allocated entirely inside the DRT structure.
 *
 * The hash is a simple circular prime modulus arrangement, the structure
 * is resized from small to large if it overflows.
 */

struct vfs_drt_clustermap {
        u_int32_t               scm_magic;      /* sanity/detection */
#define DRT_SCM_MAGIC           0x12020003
        u_int32_t               scm_modulus;    /* current ring size */
        u_int32_t               scm_buckets;    /* number of occupied buckets */
        u_int32_t               scm_lastclean;  /* last entry we cleaned */
        u_int32_t               scm_iskips;     /* number of slot skips */

        struct vfs_drt_hashentry scm_hashtable[0];
};


#define DRT_HASH(scm, addr)             ((addr) % (scm)->scm_modulus)
#define DRT_HASH_NEXT(scm, addr)        (((addr) + 1) % (scm)->scm_modulus)

/*
 * Debugging codes and arguments.
 */
#define DRT_DEBUG_EMPTYFREE     (FSDBG_CODE(DBG_FSRW, 82)) /* nil */
#define DRT_DEBUG_RETCLUSTER    (FSDBG_CODE(DBG_FSRW, 83)) /* offset, length */
#define DRT_DEBUG_ALLOC         (FSDBG_CODE(DBG_FSRW, 84)) /* copycount */
#define DRT_DEBUG_INSERT        (FSDBG_CODE(DBG_FSRW, 85)) /* offset, iskip */
#define DRT_DEBUG_MARK          (FSDBG_CODE(DBG_FSRW, 86)) /* offset, length,
                                                            * dirty */
                                                           /* 0, setcount */
                                                           /* 1 (clean, no map) */
                                                           /* 2 (map alloc fail) */
                                                           /* 3, resid (partial) */
#define DRT_DEBUG_6             (FSDBG_CODE(DBG_FSRW, 87))
#define DRT_DEBUG_SCMDATA       (FSDBG_CODE(DBG_FSRW, 88)) /* modulus, buckets,
                                                            * lastclean, iskips */


static kern_return_t    vfs_drt_alloc_map(struct vfs_drt_clustermap **cmapp);
static kern_return_t    vfs_drt_free_map(struct vfs_drt_clustermap *cmap);
static kern_return_t    vfs_drt_search_index(struct vfs_drt_clustermap *cmap,
        u_int64_t offset, int *indexp);
static kern_return_t    vfs_drt_get_index(struct vfs_drt_clustermap **cmapp,
        u_int64_t offset,
        int *indexp,
        int recursed);
static kern_return_t    vfs_drt_do_mark_pages(
        void            **cmapp,
        u_int64_t       offset,
        u_int           length,
        u_int           *setcountp,
        int             dirty);
static void             vfs_drt_trace(
        struct vfs_drt_clustermap *cmap,
        int code,
        int arg1,
        int arg2,
        int arg3,
        int arg4);


/*
 * Allocate and initialise a sparse cluster map.
 *
 * Will allocate a new map, resize or compact an existing map.
 *
 * XXX we should probably have at least one intermediate map size,
 * as the 1:16 ratio seems a bit drastic.
 */
static kern_return_t
vfs_drt_alloc_map(struct vfs_drt_clustermap **cmapp)
{
        struct vfs_drt_clustermap *cmap, *ocmap;
        kern_return_t   kret;
        u_int64_t       offset;
        u_int32_t       i;
        int             nsize, active_buckets, index, copycount;

        ocmap = NULL;
        if (cmapp != NULL)
                ocmap = *cmapp;
        
        /*
         * Decide on the size of the new map.
         */
        if (ocmap == NULL) {
                nsize = DRT_HASH_SMALL_MODULUS;
        } else {
                /* count the number of active buckets in the old map */
                active_buckets = 0;
                for (i = 0; i < ocmap->scm_modulus; i++) {
                        if (!DRT_HASH_VACANT(ocmap, i) &&
                            (DRT_HASH_GET_COUNT(ocmap, i) != 0))
                                active_buckets++;
                }
                /*
                 * If we're currently using the small allocation, check to
                 * see whether we should grow to the large one.
                 */
                if (ocmap->scm_modulus == DRT_HASH_SMALL_MODULUS) {
                        /* 
                         * If the ring is nearly full and we are allowed to
                         * use the large modulus, upgrade.
                         */
                        if ((active_buckets > (DRT_HASH_SMALL_MODULUS - 5)) &&
                            (max_mem >= DRT_HASH_LARGE_MEMORY_REQUIRED)) {
                                nsize = DRT_HASH_LARGE_MODULUS;
                        } else {
                                nsize = DRT_HASH_SMALL_MODULUS;
                        }
                } else {
                        /* already using the large modulus */
                        nsize = DRT_HASH_LARGE_MODULUS;
                        /*
                         * If the ring is completely full, there's
                         * nothing useful for us to do.  Behave as
                         * though we had compacted into the new
                         * array and return.
                         */
                        if (active_buckets >= DRT_HASH_LARGE_MODULUS)
                                return(KERN_SUCCESS);
                }
        }

        /*
         * Allocate and initialise the new map.
         */

        kret = kmem_alloc(kernel_map, (vm_offset_t *)&cmap,
            (nsize == DRT_HASH_SMALL_MODULUS) ? DRT_SMALL_ALLOCATION : DRT_LARGE_ALLOCATION, VM_KERN_MEMORY_FILE);
        if (kret != KERN_SUCCESS)
                return(kret);
        cmap->scm_magic = DRT_SCM_MAGIC;
        cmap->scm_modulus = nsize;
        cmap->scm_buckets = 0;
        cmap->scm_lastclean = 0;
        cmap->scm_iskips = 0;
        for (i = 0; i < cmap->scm_modulus; i++) {
                DRT_HASH_CLEAR(cmap, i);
                DRT_HASH_VACATE(cmap, i);
                DRT_BITVECTOR_CLEAR(cmap, i);
        }

        /*
         * If there's an old map, re-hash entries from it into the new map.
         */
        copycount = 0;
        if (ocmap != NULL) {
                for (i = 0; i < ocmap->scm_modulus; i++) {
                        /* skip empty buckets */
                        if (DRT_HASH_VACANT(ocmap, i) ||
                            (DRT_HASH_GET_COUNT(ocmap, i) == 0))
                                continue;
                        /* get new index */
                        offset = DRT_HASH_GET_ADDRESS(ocmap, i);
                        kret = vfs_drt_get_index(&cmap, offset, &index, 1);
                        if (kret != KERN_SUCCESS) {
                                /* XXX need to bail out gracefully here */
                                panic("vfs_drt: new cluster map mysteriously too small");
                                index = 0;
                        }
                        /* copy */
                        DRT_HASH_COPY(ocmap, i, cmap, index);
                        copycount++;
                }
        }

        /* log what we've done */
        vfs_drt_trace(cmap, DRT_DEBUG_ALLOC, copycount, 0, 0, 0);
        
        /*
         * It's important to ensure that *cmapp always points to 
         * a valid map, so we must overwrite it before freeing
         * the old map.
         */
        *cmapp = cmap;
        if (ocmap != NULL) {
                /* emit stats into trace buffer */
                vfs_drt_trace(ocmap, DRT_DEBUG_SCMDATA,
                              ocmap->scm_modulus,
                              ocmap->scm_buckets,
                              ocmap->scm_lastclean,
                              ocmap->scm_iskips);

                vfs_drt_free_map(ocmap);
        }
        return(KERN_SUCCESS);
}


/*
 * Free a sparse cluster map.
 */
static kern_return_t
vfs_drt_free_map(struct vfs_drt_clustermap *cmap)
{
        kmem_free(kernel_map, (vm_offset_t)cmap, 
                  (cmap->scm_modulus == DRT_HASH_SMALL_MODULUS) ? DRT_SMALL_ALLOCATION : DRT_LARGE_ALLOCATION);
        return(KERN_SUCCESS);
}


/*
 * Find the hashtable slot currently occupied by an entry for the supplied offset.
 */
static kern_return_t
vfs_drt_search_index(struct vfs_drt_clustermap *cmap, u_int64_t offset, int *indexp)
{
        int             index;
        u_int32_t       i;

        offset = DRT_ALIGN_ADDRESS(offset);
        index = DRT_HASH(cmap, offset);

        /* traverse the hashtable */
        for (i = 0; i < cmap->scm_modulus; i++) {

                /*
                 * If the slot is vacant, we can stop.
                 */
                if (DRT_HASH_VACANT(cmap, index))
                        break;

                /*
                 * If the address matches our offset, we have success.
                 */
                if (DRT_HASH_GET_ADDRESS(cmap, index) == offset) {
                        *indexp = index;
                        return(KERN_SUCCESS);
                }

                /*
                 * Move to the next slot, try again.
                 */
                index = DRT_HASH_NEXT(cmap, index);
        }
        /*
         * It's not there.
         */
        return(KERN_FAILURE);
}

/*
 * Find the hashtable slot for the supplied offset.  If we haven't allocated
 * one yet, allocate one and populate the address field.  Note that it will
 * not have a nonzero page count and thus will still technically be free, so
 * in the case where we are called to clean pages, the slot will remain free.
 */
static kern_return_t
vfs_drt_get_index(struct vfs_drt_clustermap **cmapp, u_int64_t offset, int *indexp, int recursed)
{
        struct vfs_drt_clustermap *cmap;
        kern_return_t   kret;
        u_int32_t       index;
        u_int32_t       i;

        cmap = *cmapp;

        /* look for an existing entry */
        kret = vfs_drt_search_index(cmap, offset, indexp);
        if (kret == KERN_SUCCESS)
                return(kret);

        /* need to allocate an entry */
        offset = DRT_ALIGN_ADDRESS(offset);
        index = DRT_HASH(cmap, offset);

        /* scan from the index forwards looking for a vacant slot */
        for (i = 0; i < cmap->scm_modulus; i++) {
                /* slot vacant? */
                if (DRT_HASH_VACANT(cmap, index) || DRT_HASH_GET_COUNT(cmap,index) == 0) {
                        cmap->scm_buckets++;
                        if (index < cmap->scm_lastclean)
                                cmap->scm_lastclean = index;
                        DRT_HASH_SET_ADDRESS(cmap, index, offset);
                        DRT_HASH_SET_COUNT(cmap, index, 0);
                        DRT_BITVECTOR_CLEAR(cmap, index);
                        *indexp = index;
                        vfs_drt_trace(cmap, DRT_DEBUG_INSERT, (int)offset, i, 0, 0);
                        return(KERN_SUCCESS);
                }
                cmap->scm_iskips += i;
                index = DRT_HASH_NEXT(cmap, index);
        }

        /*
         * We haven't found a vacant slot, so the map is full.  If we're not
         * already recursed, try reallocating/compacting it.
         */
        if (recursed)
                return(KERN_FAILURE);
        kret = vfs_drt_alloc_map(cmapp);
        if (kret == KERN_SUCCESS) {
                /* now try to insert again */
                kret = vfs_drt_get_index(cmapp, offset, indexp, 1);
        }
        return(kret);
}

/*
 * Implementation of set dirty/clean.
 *
 * In the 'clean' case, not finding a map is OK.
 */
static kern_return_t
vfs_drt_do_mark_pages(
        void            **private,
        u_int64_t       offset,
        u_int           length,
        u_int           *setcountp,
        int             dirty)
{
        struct vfs_drt_clustermap *cmap, **cmapp;
        kern_return_t   kret;
        int             i, index, pgoff, pgcount, setcount, ecount;

        cmapp = (struct vfs_drt_clustermap **)private;
        cmap = *cmapp;

        vfs_drt_trace(cmap, DRT_DEBUG_MARK | DBG_FUNC_START, (int)offset, (int)length, dirty, 0);

        if (setcountp != NULL)
                *setcountp = 0;
        
        /* allocate a cluster map if we don't already have one */
        if (cmap == NULL) {
                /* no cluster map, nothing to clean */
                if (!dirty) {
                        vfs_drt_trace(cmap, DRT_DEBUG_MARK | DBG_FUNC_END, 1, 0, 0, 0);
                        return(KERN_SUCCESS);
                }
                kret = vfs_drt_alloc_map(cmapp);
                if (kret != KERN_SUCCESS) {
                        vfs_drt_trace(cmap, DRT_DEBUG_MARK | DBG_FUNC_END, 2, 0, 0, 0);
                        return(kret);
                }
        }
        setcount = 0;

        /*
         * Iterate over the length of the region.
         */
        while (length > 0) {
                /*
                 * Get the hashtable index for this offset.
                 *
                 * XXX this will add blank entries if we are clearing a range
                 * that hasn't been dirtied.
                 */
                kret = vfs_drt_get_index(cmapp, offset, &index, 0);
                cmap = *cmapp;  /* may have changed! */
                /* this may be a partial-success return */
                if (kret != KERN_SUCCESS) {
                        if (setcountp != NULL)
                                *setcountp = setcount;
                        vfs_drt_trace(cmap, DRT_DEBUG_MARK | DBG_FUNC_END, 3, (int)length, 0, 0);

                        return(kret);
                }

                /*
                 * Work out how many pages we're modifying in this
                 * hashtable entry.
                 */
                pgoff = (offset - DRT_ALIGN_ADDRESS(offset)) / PAGE_SIZE;
                pgcount = min((length / PAGE_SIZE), (DRT_BITVECTOR_PAGES - pgoff));

                /*
                 * Iterate over pages, dirty/clearing as we go.
                 */
                ecount = DRT_HASH_GET_COUNT(cmap, index);
                for (i = 0; i < pgcount; i++) {
                        if (dirty) {
                                if (!DRT_HASH_TEST_BIT(cmap, index, pgoff + i)) {
                                        DRT_HASH_SET_BIT(cmap, index, pgoff + i);
                                        ecount++;
                                        setcount++;
                                }
                        } else {
                                if (DRT_HASH_TEST_BIT(cmap, index, pgoff + i)) {
                                        DRT_HASH_CLEAR_BIT(cmap, index, pgoff + i);
                                        ecount--;
                                        setcount++;
                                }
                        }
                }
                DRT_HASH_SET_COUNT(cmap, index, ecount);

                offset += pgcount * PAGE_SIZE;
                length -= pgcount * PAGE_SIZE;
        }
        if (setcountp != NULL)
                *setcountp = setcount;

        vfs_drt_trace(cmap, DRT_DEBUG_MARK | DBG_FUNC_END, 0, setcount, 0, 0);

        return(KERN_SUCCESS);
}

/*
 * Mark a set of pages as dirty/clean.
 *
 * This is a public interface.
 *
 * cmapp
 *      Pointer to storage suitable for holding a pointer.  Note that
 *      this must either be NULL or a value set by this function.
 *
 * size
 *      Current file size in bytes.
 *
 * offset
 *      Offset of the first page to be marked as dirty, in bytes.  Must be
 *      page-aligned.
 *
 * length
 *      Length of dirty region, in bytes.  Must be a multiple of PAGE_SIZE.
 *
 * setcountp
 *      Number of pages newly marked dirty by this call (optional).
 *
 * Returns KERN_SUCCESS if all the pages were successfully marked.
 */
static kern_return_t
vfs_drt_mark_pages(void **cmapp, off_t offset, u_int length, u_int *setcountp)
{
        /* XXX size unused, drop from interface */
        return(vfs_drt_do_mark_pages(cmapp, offset, length, setcountp, 1));
}

#if 0
static kern_return_t
vfs_drt_unmark_pages(void **cmapp, off_t offset, u_int length)
{
        return(vfs_drt_do_mark_pages(cmapp, offset, length, NULL, 0));
}
#endif

/*
 * Get a cluster of dirty pages.
 *
 * This is a public interface.
 *
 * cmapp
 *      Pointer to storage managed by drt_mark_pages.  Note that this must
 *      be NULL or a value set by drt_mark_pages.
 *
 * offsetp
 *      Returns the byte offset into the file of the first page in the cluster.
 *
 * lengthp
 *      Returns the length in bytes of the cluster of dirty pages.
 *
 * Returns success if a cluster was found.  If KERN_FAILURE is returned, there
 * are no dirty pages meeting the minmum size criteria.  Private storage will
 * be released if there are no more dirty pages left in the map
 *
 */
static kern_return_t
vfs_drt_get_cluster(void **cmapp, off_t *offsetp, u_int *lengthp)
{
        struct vfs_drt_clustermap *cmap;
        u_int64_t       offset;
        u_int           length;
        u_int32_t       j;
        int             index, i, fs, ls;

        /* sanity */
        if ((cmapp == NULL) || (*cmapp == NULL))
                return(KERN_FAILURE);
        cmap = *cmapp;

        /* walk the hashtable */
        for (offset = 0, j = 0; j < cmap->scm_modulus; offset += (DRT_BITVECTOR_PAGES * PAGE_SIZE), j++) {
                index = DRT_HASH(cmap, offset);

                if (DRT_HASH_VACANT(cmap, index) || (DRT_HASH_GET_COUNT(cmap, index) == 0))
                        continue;

                /* scan the bitfield for a string of bits */
                fs = -1;

                for (i = 0; i < DRT_BITVECTOR_PAGES; i++) {
                        if (DRT_HASH_TEST_BIT(cmap, index, i)) {
                                fs = i;
                                break;
                        }
                }
                if (fs == -1) {
                        /*  didn't find any bits set */
                        panic("vfs_drt: entry summary count > 0 but no bits set in map");
                }
                for (ls = 0; i < DRT_BITVECTOR_PAGES; i++, ls++) {
                        if (!DRT_HASH_TEST_BIT(cmap, index, i))
                                break;
                }
                
                /* compute offset and length, mark pages clean */
                offset = DRT_HASH_GET_ADDRESS(cmap, index) + (PAGE_SIZE * fs);
                length = ls * PAGE_SIZE;
                vfs_drt_do_mark_pages(cmapp, offset, length, NULL, 0);
                cmap->scm_lastclean = index;

                /* return successful */
                *offsetp = (off_t)offset;
                *lengthp = length;

                vfs_drt_trace(cmap, DRT_DEBUG_RETCLUSTER, (int)offset, (int)length, 0, 0);
                return(KERN_SUCCESS);
        }
        /*
         * We didn't find anything... hashtable is empty
         * emit stats into trace buffer and
         * then free it
         */
        vfs_drt_trace(cmap, DRT_DEBUG_SCMDATA,
                      cmap->scm_modulus,
                      cmap->scm_buckets,
                      cmap->scm_lastclean,
                      cmap->scm_iskips);
        
        vfs_drt_free_map(cmap);
        *cmapp = NULL;

        return(KERN_FAILURE);
}


static kern_return_t
vfs_drt_control(void **cmapp, int op_type)
{
        struct vfs_drt_clustermap *cmap;

        /* sanity */
        if ((cmapp == NULL) || (*cmapp == NULL))
                return(KERN_FAILURE);
        cmap = *cmapp;

        switch (op_type) {
        case 0:
                /* emit stats into trace buffer */
                vfs_drt_trace(cmap, DRT_DEBUG_SCMDATA,
                              cmap->scm_modulus,
                              cmap->scm_buckets,
                              cmap->scm_lastclean,
                              cmap->scm_iskips);

                vfs_drt_free_map(cmap);
                *cmapp = NULL;
                break;

        case 1:
                cmap->scm_lastclean = 0;
                break;
        }
        return(KERN_SUCCESS);
}



/*
 * Emit a summary of the state of the clustermap into the trace buffer
 * along with some caller-provided data.
 */
#if KDEBUG
static void
vfs_drt_trace(__unused struct vfs_drt_clustermap *cmap, int code, int arg1, int arg2, int arg3, int arg4)
{
        KERNEL_DEBUG(code, arg1, arg2, arg3, arg4, 0);
}
#else
static void
vfs_drt_trace(__unused struct vfs_drt_clustermap *cmap, __unused int code, 
                          __unused int arg1, __unused int arg2, __unused int arg3, 
                          __unused int arg4)
{
}
#endif 

#if 0
/*
 * Perform basic sanity check on the hash entry summary count
 * vs. the actual bits set in the entry.
 */
static void
vfs_drt_sanity(struct vfs_drt_clustermap *cmap)
{
        int index, i;
        int bits_on;
        
        for (index = 0; index < cmap->scm_modulus; index++) {
                if (DRT_HASH_VACANT(cmap, index))
                        continue;

                for (bits_on = 0, i = 0; i < DRT_BITVECTOR_PAGES; i++) {
                        if (DRT_HASH_TEST_BIT(cmap, index, i))
                                bits_on++;
                }
                if (bits_on != DRT_HASH_GET_COUNT(cmap, index))
                        panic("bits_on = %d,  index = %d\n", bits_on, index);
        }               
}
#endif