xnu-6153.11.26.tar.gz

[apple/xnu.git] / osfmk / vm / vm_swapfile_pager.c

/*
 * Copyright (c) 2019 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@
 */

#include <mach/kern_return.h>
#include <mach/memory_object_control.h>
#include <mach/upl.h>

#include <kern/ipc_kobject.h>
#include <kern/kalloc.h>
#include <kern/queue.h>

#include <vm/memory_object.h>
#include <vm/vm_kern.h>
#include <vm/vm_map.h>
#include <vm/vm_pageout.h>
#include <vm/vm_protos.h>


/*
 * APPLE SWAPFILE MEMORY PAGER
 *
 * This external memory manager (EMM) handles mappings of the swap files.
 * Swap files are not regular files and are used solely to store contents of
 * anonymous memory mappings while not resident in memory.
 * There's no valid reason to map a swap file.  This just puts extra burden
 * on the system, is potentially a security issue and is not reliable since
 * the contents can change at any time with pageout operations.
 * Here are some of the issues with mapping a swap file.
 * * PERFORMANCE:
 *   Each page in the swap file belong to an anonymous memory object. Mapping
 *   the swap file makes those pages also accessible via a vnode memory
 *   object and each page can now be resident twice.
 * * SECURITY:
 *   Mapping a swap file allows access to other processes' memory.  Swap files
 *   are only accessible by the "root" super-user, who can already access any
 *   process's memory, so this is not a real issue but if permissions on the
 *   swap file got changed, it could become one.
 *   Swap files are not "zero-filled" on creation, so until their contents are
 *   overwritten with pageout operations, they still contain whatever was on
 *   the disk blocks they were allocated.  The "super-user" could see the
 *   contents of free blocks anyway, so this is not a new security issue but
 *   it may be perceive as one.
 *
 * We can't legitimately prevent a user process with appropriate privileges
 * from mapping a swap file, but we can prevent it from accessing its actual
 * contents.
 * This pager mostly handles page-in request (from memory_object_data_request())
 * for swap file mappings and just returns bogus data.
 * Pageouts are not handled, so mmap() has to make sure it does not allow
 * writable (i.e. MAP_SHARED and PROT_WRITE) mappings of swap files.
 */

/* forward declarations */
void swapfile_pager_reference(memory_object_t mem_obj);
void swapfile_pager_deallocate(memory_object_t mem_obj);
kern_return_t swapfile_pager_init(memory_object_t mem_obj,
    memory_object_control_t control,
    memory_object_cluster_size_t pg_size);
kern_return_t swapfile_pager_terminate(memory_object_t mem_obj);
kern_return_t swapfile_pager_data_request(memory_object_t mem_obj,
    memory_object_offset_t offset,
    memory_object_cluster_size_t length,
    vm_prot_t protection_required,
    memory_object_fault_info_t fault_info);
kern_return_t swapfile_pager_data_return(memory_object_t mem_obj,
    memory_object_offset_t offset,
    memory_object_cluster_size_t      data_cnt,
    memory_object_offset_t *resid_offset,
    int *io_error,
    boolean_t dirty,
    boolean_t kernel_copy,
    int upl_flags);
kern_return_t swapfile_pager_data_initialize(memory_object_t mem_obj,
    memory_object_offset_t offset,
    memory_object_cluster_size_t data_cnt);
kern_return_t swapfile_pager_data_unlock(memory_object_t mem_obj,
    memory_object_offset_t offset,
    memory_object_size_t size,
    vm_prot_t desired_access);
kern_return_t swapfile_pager_synchronize(memory_object_t mem_obj,
    memory_object_offset_t offset,
    memory_object_size_t length,
    vm_sync_t sync_flags);
kern_return_t swapfile_pager_map(memory_object_t mem_obj,
    vm_prot_t prot);
kern_return_t swapfile_pager_last_unmap(memory_object_t mem_obj);

/*
 * Vector of VM operations for this EMM.
 * These routines are invoked by VM via the memory_object_*() interfaces.
 */
const struct memory_object_pager_ops swapfile_pager_ops = {
        .memory_object_reference = swapfile_pager_reference,
        .memory_object_deallocate = swapfile_pager_deallocate,
        .memory_object_init = swapfile_pager_init,
        .memory_object_terminate = swapfile_pager_terminate,
        .memory_object_data_request = swapfile_pager_data_request,
        .memory_object_data_return = swapfile_pager_data_return,
        .memory_object_data_initialize = swapfile_pager_data_initialize,
        .memory_object_data_unlock = swapfile_pager_data_unlock,
        .memory_object_synchronize = swapfile_pager_synchronize,
        .memory_object_map = swapfile_pager_map,
        .memory_object_last_unmap = swapfile_pager_last_unmap,
        .memory_object_data_reclaim = NULL,
        .memory_object_pager_name = "swapfile pager"
};

/*
 * The "swapfile_pager" describes a memory object backed by
 * the "swapfile" EMM.
 */
typedef struct swapfile_pager {
        /* mandatory generic header */
        struct memory_object swp_pgr_hdr;

        /* pager-specific data */
        queue_chain_t           pager_queue;    /* next & prev pagers */
        unsigned int            ref_count;      /* reference count */
        boolean_t               is_ready;       /* is this pager ready ? */
        boolean_t               is_mapped;      /* is this pager mapped ? */
        struct vnode            *swapfile_vnode;/* the swapfile's vnode */
} *swapfile_pager_t;
#define SWAPFILE_PAGER_NULL     ((swapfile_pager_t) NULL)

/*
 * List of memory objects managed by this EMM.
 * The list is protected by the "swapfile_pager_lock" lock.
 */
int swapfile_pager_count = 0;           /* number of pagers */
queue_head_t swapfile_pager_queue;
decl_lck_mtx_data(, swapfile_pager_lock);

/*
 * Statistics & counters.
 */
int swapfile_pager_count_max = 0;


lck_grp_t               swapfile_pager_lck_grp;
lck_grp_attr_t          swapfile_pager_lck_grp_attr;
lck_attr_t              swapfile_pager_lck_attr;


/* internal prototypes */
swapfile_pager_t swapfile_pager_create(struct vnode *vp);
swapfile_pager_t swapfile_pager_lookup(memory_object_t mem_obj);
void swapfile_pager_dequeue(swapfile_pager_t pager);
void swapfile_pager_deallocate_internal(swapfile_pager_t pager,
    boolean_t locked);
void swapfile_pager_terminate_internal(swapfile_pager_t pager);


#if DEBUG
int swapfile_pagerdebug = 0;
#define PAGER_ALL               0xffffffff
#define PAGER_INIT              0x00000001
#define PAGER_PAGEIN            0x00000002

#define PAGER_DEBUG(LEVEL, A)                                           \
        MACRO_BEGIN                                                     \
        if ((swapfile_pagerdebug & LEVEL)==LEVEL) {             \
                printf A;                                               \
        }                                                               \
        MACRO_END
#else
#define PAGER_DEBUG(LEVEL, A)
#endif


void
swapfile_pager_bootstrap(void)
{
        lck_grp_attr_setdefault(&swapfile_pager_lck_grp_attr);
        lck_grp_init(&swapfile_pager_lck_grp, "swapfile pager", &swapfile_pager_lck_grp_attr);
        lck_attr_setdefault(&swapfile_pager_lck_attr);
        lck_mtx_init(&swapfile_pager_lock, &swapfile_pager_lck_grp, &swapfile_pager_lck_attr);
        queue_init(&swapfile_pager_queue);
}

/*
 * swapfile_pager_init()
 *
 * Initialize the memory object and makes it ready to be used and mapped.
 */
kern_return_t
swapfile_pager_init(
        memory_object_t         mem_obj,
        memory_object_control_t control,
#if !DEBUG
        __unused
#endif
        memory_object_cluster_size_t pg_size)
{
        swapfile_pager_t        pager;
        kern_return_t           kr;
        memory_object_attr_info_data_t  attributes;

        PAGER_DEBUG(PAGER_ALL,
            ("swapfile_pager_init: %p, %p, %x\n",
            mem_obj, control, pg_size));

        if (control == MEMORY_OBJECT_CONTROL_NULL) {
                return KERN_INVALID_ARGUMENT;
        }

        pager = swapfile_pager_lookup(mem_obj);

        memory_object_control_reference(control);

        pager->swp_pgr_hdr.mo_control = control;

        attributes.copy_strategy = MEMORY_OBJECT_COPY_DELAY;
        attributes.cluster_size = (1 << (PAGE_SHIFT));
        attributes.may_cache_object = FALSE;
        attributes.temporary = TRUE;

        kr = memory_object_change_attributes(
                control,
                MEMORY_OBJECT_ATTRIBUTE_INFO,
                (memory_object_info_t) &attributes,
                MEMORY_OBJECT_ATTR_INFO_COUNT);
        if (kr != KERN_SUCCESS) {
                panic("swapfile_pager_init: "
                    "memory_object_change_attributes() failed");
        }

        return KERN_SUCCESS;
}

/*
 * swapfile_data_return()
 *
 * Handles page-out requests from VM.  This should never happen since
 * the pages provided by this EMM are not supposed to be dirty or dirtied
 * and VM should simply discard the contents and reclaim the pages if it
 * needs to.
 */
kern_return_t
swapfile_pager_data_return(
        __unused memory_object_t        mem_obj,
        __unused memory_object_offset_t offset,
        __unused memory_object_cluster_size_t           data_cnt,
        __unused memory_object_offset_t *resid_offset,
        __unused int                    *io_error,
        __unused boolean_t              dirty,
        __unused boolean_t              kernel_copy,
        __unused int                    upl_flags)
{
        panic("swapfile_pager_data_return: should never get called");
        return KERN_FAILURE;
}

kern_return_t
swapfile_pager_data_initialize(
        __unused memory_object_t        mem_obj,
        __unused memory_object_offset_t offset,
        __unused memory_object_cluster_size_t           data_cnt)
{
        panic("swapfile_pager_data_initialize: should never get called");
        return KERN_FAILURE;
}

kern_return_t
swapfile_pager_data_unlock(
        __unused memory_object_t        mem_obj,
        __unused memory_object_offset_t offset,
        __unused memory_object_size_t           size,
        __unused vm_prot_t              desired_access)
{
        return KERN_FAILURE;
}

/*
 * swapfile_pager_data_request()
 *
 * Handles page-in requests from VM.
 */
kern_return_t
swapfile_pager_data_request(
        memory_object_t         mem_obj,
        memory_object_offset_t  offset,
        memory_object_cluster_size_t            length,
#if !DEBUG
        __unused
#endif
        vm_prot_t               protection_required,
        __unused memory_object_fault_info_t mo_fault_info)
{
        swapfile_pager_t        pager;
        memory_object_control_t mo_control;
        upl_t                   upl;
        int                     upl_flags;
        upl_size_t              upl_size;
        upl_page_info_t         *upl_pl = NULL;
        unsigned int            pl_count;
        vm_object_t             dst_object;
        kern_return_t           kr, retval;
        vm_map_offset_t         kernel_mapping;
        vm_offset_t             dst_vaddr;
        char                    *dst_ptr;
        vm_offset_t             cur_offset;
        vm_map_entry_t          map_entry;

        PAGER_DEBUG(PAGER_ALL, ("swapfile_pager_data_request: %p, %llx, %x, %x\n", mem_obj, offset, length, protection_required));

        kernel_mapping = 0;
        upl = NULL;
        upl_pl = NULL;

        pager = swapfile_pager_lookup(mem_obj);
        assert(pager->is_ready);
        assert(pager->ref_count > 1); /* pager is alive and mapped */

        PAGER_DEBUG(PAGER_PAGEIN, ("swapfile_pager_data_request: %p, %llx, %x, %x, pager %p\n", mem_obj, offset, length, protection_required, pager));

        /*
         * Gather in a UPL all the VM pages requested by VM.
         */
        mo_control = pager->swp_pgr_hdr.mo_control;

        upl_size = length;
        upl_flags =
            UPL_RET_ONLY_ABSENT |
            UPL_SET_LITE |
            UPL_NO_SYNC |
            UPL_CLEAN_IN_PLACE |        /* triggers UPL_CLEAR_DIRTY */
            UPL_SET_INTERNAL;
        pl_count = 0;
        kr = memory_object_upl_request(mo_control,
            offset, upl_size,
            &upl, NULL, NULL, upl_flags, VM_KERN_MEMORY_OSFMK);
        if (kr != KERN_SUCCESS) {
                retval = kr;
                goto done;
        }
        dst_object = mo_control->moc_object;
        assert(dst_object != VM_OBJECT_NULL);


        /*
         * Reserve a virtual page in the kernel address space to map each
         * destination physical page when it's its turn to be processed.
         */
        vm_object_reference(kernel_object);     /* ref. for mapping */
        kr = vm_map_find_space(kernel_map,
            &kernel_mapping,
            PAGE_SIZE_64,
            0,
            0,
            VM_MAP_KERNEL_FLAGS_NONE,
            VM_KERN_MEMORY_NONE,
            &map_entry);
        if (kr != KERN_SUCCESS) {
                vm_object_deallocate(kernel_object);
                retval = kr;
                goto done;
        }
        VME_OBJECT_SET(map_entry, kernel_object);
        VME_OFFSET_SET(map_entry, kernel_mapping - VM_MIN_KERNEL_ADDRESS);
        vm_map_unlock(kernel_map);
        dst_vaddr = CAST_DOWN(vm_offset_t, kernel_mapping);
        dst_ptr = (char *) dst_vaddr;

        /*
         * Fill in the contents of the pages requested by VM.
         */
        upl_pl = UPL_GET_INTERNAL_PAGE_LIST(upl);
        pl_count = length / PAGE_SIZE;
        for (cur_offset = 0; cur_offset < length; cur_offset += PAGE_SIZE) {
                ppnum_t dst_pnum;

                if (!upl_page_present(upl_pl, (int)(cur_offset / PAGE_SIZE))) {
                        /* this page is not in the UPL: skip it */
                        continue;
                }

                /*
                 * Establish an explicit pmap mapping of the destination
                 * physical page.
                 * We can't do a regular VM mapping because the VM page
                 * is "busy".
                 */
                dst_pnum = (ppnum_t)
                    upl_phys_page(upl_pl, (int)(cur_offset / PAGE_SIZE));
                assert(dst_pnum != 0);
                retval = pmap_enter(kernel_pmap,
                    kernel_mapping,
                    dst_pnum,
                    VM_PROT_READ | VM_PROT_WRITE,
                    VM_PROT_NONE,
                    0,
                    TRUE);

                assert(retval == KERN_SUCCESS);

                if (retval != KERN_SUCCESS) {
                        goto done;
                }

                memset(dst_ptr, '\0', PAGE_SIZE);
                /* add an end-of-line to keep line counters happy */
                dst_ptr[PAGE_SIZE - 1] = '\n';

                /*
                 * Remove the pmap mapping of the destination page
                 * in the kernel.
                 */
                pmap_remove(kernel_pmap,
                    (addr64_t) kernel_mapping,
                    (addr64_t) (kernel_mapping + PAGE_SIZE_64));
        }

        retval = KERN_SUCCESS;
done:
        if (upl != NULL) {
                /* clean up the UPL */

                /*
                 * The pages are currently dirty because we've just been
                 * writing on them, but as far as we're concerned, they're
                 * clean since they contain their "original" contents as
                 * provided by us, the pager.
                 * Tell the UPL to mark them "clean".
                 */
                upl_clear_dirty(upl, TRUE);

                /* abort or commit the UPL */
                if (retval != KERN_SUCCESS) {
                        upl_abort(upl, 0);
                } else {
                        boolean_t empty;
                        upl_commit_range(upl, 0, upl->size,
                            UPL_COMMIT_CS_VALIDATED,
                            upl_pl, pl_count, &empty);
                }

                /* and deallocate the UPL */
                upl_deallocate(upl);
                upl = NULL;
        }
        if (kernel_mapping != 0) {
                /* clean up the mapping of the source and destination pages */
                kr = vm_map_remove(kernel_map,
                    kernel_mapping,
                    kernel_mapping + PAGE_SIZE_64,
                    VM_MAP_REMOVE_NO_FLAGS);
                assert(kr == KERN_SUCCESS);
                kernel_mapping = 0;
                dst_vaddr = 0;
        }

        return retval;
}

/*
 * swapfile_pager_reference()
 *
 * Get a reference on this memory object.
 * For external usage only.  Assumes that the initial reference count is not 0,
 * i.e one should not "revive" a dead pager this way.
 */
void
swapfile_pager_reference(
        memory_object_t         mem_obj)
{
        swapfile_pager_t        pager;

        pager = swapfile_pager_lookup(mem_obj);

        lck_mtx_lock(&swapfile_pager_lock);
        assert(pager->ref_count > 0);
        pager->ref_count++;
        lck_mtx_unlock(&swapfile_pager_lock);
}


/*
 * swapfile_pager_dequeue:
 *
 * Removes a pager from the list of pagers.
 *
 * The caller must hold "swapfile_pager_lock".
 */
void
swapfile_pager_dequeue(
        swapfile_pager_t pager)
{
        assert(!pager->is_mapped);

        queue_remove(&swapfile_pager_queue,
            pager,
            swapfile_pager_t,
            pager_queue);
        pager->pager_queue.next = NULL;
        pager->pager_queue.prev = NULL;

        swapfile_pager_count--;
}

/*
 * swapfile_pager_terminate_internal:
 *
 * Trigger the asynchronous termination of the memory object associated
 * with this pager.
 * When the memory object is terminated, there will be one more call
 * to memory_object_deallocate() (i.e. swapfile_pager_deallocate())
 * to finish the clean up.
 *
 * "swapfile_pager_lock" should not be held by the caller.
 * We don't need the lock because the pager has already been removed from
 * the pagers' list and is now ours exclusively.
 */
void
swapfile_pager_terminate_internal(
        swapfile_pager_t pager)
{
        assert(pager->is_ready);
        assert(!pager->is_mapped);

        if (pager->swapfile_vnode != NULL) {
                pager->swapfile_vnode = NULL;
        }

        /* trigger the destruction of the memory object */
        memory_object_destroy(pager->swp_pgr_hdr.mo_control, 0);
}

/*
 * swapfile_pager_deallocate_internal()
 *
 * Release a reference on this pager and free it when the last
 * reference goes away.
 * Can be called with swapfile_pager_lock held or not but always returns
 * with it unlocked.
 */
void
swapfile_pager_deallocate_internal(
        swapfile_pager_t        pager,
        boolean_t               locked)
{
        if (!locked) {
                lck_mtx_lock(&swapfile_pager_lock);
        }

        /* drop a reference on this pager */
        pager->ref_count--;

        if (pager->ref_count == 1) {
                /*
                 * Only the "named" reference is left, which means that
                 * no one is really holding on to this pager anymore.
                 * Terminate it.
                 */
                swapfile_pager_dequeue(pager);
                /* the pager is all ours: no need for the lock now */
                lck_mtx_unlock(&swapfile_pager_lock);
                swapfile_pager_terminate_internal(pager);
        } else if (pager->ref_count == 0) {
                /*
                 * Dropped the existence reference;  the memory object has
                 * been terminated.  Do some final cleanup and release the
                 * pager structure.
                 */
                lck_mtx_unlock(&swapfile_pager_lock);
                if (pager->swp_pgr_hdr.mo_control != MEMORY_OBJECT_CONTROL_NULL) {
                        memory_object_control_deallocate(pager->swp_pgr_hdr.mo_control);
                        pager->swp_pgr_hdr.mo_control = MEMORY_OBJECT_CONTROL_NULL;
                }
                kfree(pager, sizeof(*pager));
                pager = SWAPFILE_PAGER_NULL;
        } else {
                /* there are still plenty of references:  keep going... */
                lck_mtx_unlock(&swapfile_pager_lock);
        }

        /* caution: lock is not held on return... */
}

/*
 * swapfile_pager_deallocate()
 *
 * Release a reference on this pager and free it when the last
 * reference goes away.
 */
void
swapfile_pager_deallocate(
        memory_object_t         mem_obj)
{
        swapfile_pager_t        pager;

        PAGER_DEBUG(PAGER_ALL, ("swapfile_pager_deallocate: %p\n", mem_obj));
        pager = swapfile_pager_lookup(mem_obj);
        swapfile_pager_deallocate_internal(pager, FALSE);
}

/*
 *
 */
kern_return_t
swapfile_pager_terminate(
#if !DEBUG
        __unused
#endif
        memory_object_t mem_obj)
{
        PAGER_DEBUG(PAGER_ALL, ("swapfile_pager_terminate: %p\n", mem_obj));

        return KERN_SUCCESS;
}

/*
 *
 */
kern_return_t
swapfile_pager_synchronize(
        __unused memory_object_t        mem_obbj,
        __unused memory_object_offset_t offset,
        __unused memory_object_size_t   length,
        __unused vm_sync_t              sync_flags)
{
        panic("swapfile_pager_synchronize: memory_object_synchronize no longer supported\n");
        return KERN_FAILURE;
}

/*
 * swapfile_pager_map()
 *
 * This allows VM to let us, the EMM, know that this memory object
 * is currently mapped one or more times.  This is called by VM each time
 * the memory object gets mapped and we take one extra reference on the
 * memory object to account for all its mappings.
 */
kern_return_t
swapfile_pager_map(
        memory_object_t         mem_obj,
        __unused vm_prot_t      prot)
{
        swapfile_pager_t        pager;

        PAGER_DEBUG(PAGER_ALL, ("swapfile_pager_map: %p\n", mem_obj));

        pager = swapfile_pager_lookup(mem_obj);

        lck_mtx_lock(&swapfile_pager_lock);
        assert(pager->is_ready);
        assert(pager->ref_count > 0); /* pager is alive */
        if (pager->is_mapped == FALSE) {
                /*
                 * First mapping of this pager:  take an extra reference
                 * that will remain until all the mappings of this pager
                 * are removed.
                 */
                pager->is_mapped = TRUE;
                pager->ref_count++;
        }
        lck_mtx_unlock(&swapfile_pager_lock);

        return KERN_SUCCESS;
}

/*
 * swapfile_pager_last_unmap()
 *
 * This is called by VM when this memory object is no longer mapped anywhere.
 */
kern_return_t
swapfile_pager_last_unmap(
        memory_object_t         mem_obj)
{
        swapfile_pager_t        pager;

        PAGER_DEBUG(PAGER_ALL,
            ("swapfile_pager_last_unmap: %p\n", mem_obj));

        pager = swapfile_pager_lookup(mem_obj);

        lck_mtx_lock(&swapfile_pager_lock);
        if (pager->is_mapped) {
                /*
                 * All the mappings are gone, so let go of the one extra
                 * reference that represents all the mappings of this pager.
                 */
                pager->is_mapped = FALSE;
                swapfile_pager_deallocate_internal(pager, TRUE);
                /* caution: deallocate_internal() released the lock ! */
        } else {
                lck_mtx_unlock(&swapfile_pager_lock);
        }

        return KERN_SUCCESS;
}


/*
 *
 */
swapfile_pager_t
swapfile_pager_lookup(
        memory_object_t  mem_obj)
{
        swapfile_pager_t        pager;

        assert(mem_obj->mo_pager_ops == &swapfile_pager_ops);
        __IGNORE_WCASTALIGN(pager = (swapfile_pager_t) mem_obj);
        assert(pager->ref_count > 0);
        return pager;
}

swapfile_pager_t
swapfile_pager_create(
        struct vnode            *vp)
{
        swapfile_pager_t        pager, pager2;
        memory_object_control_t control;
        kern_return_t           kr;

        pager = (swapfile_pager_t) kalloc(sizeof(*pager));
        if (pager == SWAPFILE_PAGER_NULL) {
                return SWAPFILE_PAGER_NULL;
        }

        /*
         * The vm_map call takes both named entry ports and raw memory
         * objects in the same parameter.  We need to make sure that
         * vm_map does not see this object as a named entry port.  So,
         * we reserve the second word in the object for a fake ip_kotype
         * setting - that will tell vm_map to use it as a memory object.
         */
        pager->swp_pgr_hdr.mo_ikot = IKOT_MEMORY_OBJECT;
        pager->swp_pgr_hdr.mo_pager_ops = &swapfile_pager_ops;
        pager->swp_pgr_hdr.mo_control = MEMORY_OBJECT_CONTROL_NULL;

        pager->is_ready = FALSE;/* not ready until it has a "name" */
        pager->ref_count = 1;   /* setup reference */
        pager->is_mapped = FALSE;
        pager->swapfile_vnode = vp;

        lck_mtx_lock(&swapfile_pager_lock);
        /* see if anyone raced us to create a pager for the same object */
        queue_iterate(&swapfile_pager_queue,
            pager2,
            swapfile_pager_t,
            pager_queue) {
                if (pager2->swapfile_vnode == vp) {
                        break;
                }
        }
        if (!queue_end(&swapfile_pager_queue,
            (queue_entry_t) pager2)) {
                /* while we hold the lock, transfer our setup ref to winner */
                pager2->ref_count++;
                /* we lost the race, down with the loser... */
                lck_mtx_unlock(&swapfile_pager_lock);
                pager->swapfile_vnode = NULL;
                kfree(pager, sizeof(*pager));
                /* ... and go with the winner */
                pager = pager2;
                /* let the winner make sure the pager gets ready */
                return pager;
        }

        /* enter new pager at the head of our list of pagers */
        queue_enter_first(&swapfile_pager_queue,
            pager,
            swapfile_pager_t,
            pager_queue);
        swapfile_pager_count++;
        if (swapfile_pager_count > swapfile_pager_count_max) {
                swapfile_pager_count_max = swapfile_pager_count;
        }
        lck_mtx_unlock(&swapfile_pager_lock);

        kr = memory_object_create_named((memory_object_t) pager,
            0,
            &control);
        assert(kr == KERN_SUCCESS);

        memory_object_mark_trusted(control);

        lck_mtx_lock(&swapfile_pager_lock);
        /* the new pager is now ready to be used */
        pager->is_ready = TRUE;
        lck_mtx_unlock(&swapfile_pager_lock);

        /* wakeup anyone waiting for this pager to be ready */
        thread_wakeup(&pager->is_ready);

        return pager;
}

/*
 * swapfile_pager_setup()
 *
 * Provide the caller with a memory object backed by the provided
 * "backing_object" VM object.  If such a memory object already exists,
 * re-use it, otherwise create a new memory object.
 */
memory_object_t
swapfile_pager_setup(
        struct vnode *vp)
{
        swapfile_pager_t        pager;

        lck_mtx_lock(&swapfile_pager_lock);

        queue_iterate(&swapfile_pager_queue,
            pager,
            swapfile_pager_t,
            pager_queue) {
                if (pager->swapfile_vnode == vp) {
                        break;
                }
        }
        if (queue_end(&swapfile_pager_queue,
            (queue_entry_t) pager)) {
                /* no existing pager for this backing object */
                pager = SWAPFILE_PAGER_NULL;
        } else {
                /* make sure pager doesn't disappear */
                pager->ref_count++;
        }

        lck_mtx_unlock(&swapfile_pager_lock);

        if (pager == SWAPFILE_PAGER_NULL) {
                pager = swapfile_pager_create(vp);
                if (pager == SWAPFILE_PAGER_NULL) {
                        return MEMORY_OBJECT_NULL;
                }
        }

        lck_mtx_lock(&swapfile_pager_lock);
        while (!pager->is_ready) {
                lck_mtx_sleep(&swapfile_pager_lock,
                    LCK_SLEEP_DEFAULT,
                    &pager->is_ready,
                    THREAD_UNINT);
        }
        lck_mtx_unlock(&swapfile_pager_lock);

        return (memory_object_t) pager;
}

memory_object_control_t
swapfile_pager_control(
        memory_object_t mem_obj)
{
        swapfile_pager_t        pager;

        if (mem_obj == MEMORY_OBJECT_NULL ||
            mem_obj->mo_pager_ops != &swapfile_pager_ops) {
                return MEMORY_OBJECT_CONTROL_NULL;
        }
        pager = swapfile_pager_lookup(mem_obj);
        return pager->swp_pgr_hdr.mo_control;
}