xnu-2050.9.2.tar.gz

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

/*
 * Copyright (c) 2007 Apple Inc. All rights reserved.
 *
 * @APPLE_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. 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_LICENSE_HEADER_END@
 */

#include <mach/mach_types.h>
#include <vm/vm_page.h>
#include <vm/vm_kern.h>                         /* kmem_alloc */
#include <vm/vm_purgeable_internal.h>
#include <sys/kdebug.h>
#include <kern/sched_prim.h>

struct token {
        token_cnt_t     count;
        token_idx_t     next;
};

struct token    *tokens;
token_idx_t     token_q_max_cnt = 0;
vm_size_t       token_q_cur_size = 0;

token_idx_t     token_free_idx = 0;             /* head of free queue */
token_idx_t     token_init_idx = 1;             /* token 0 is reserved!! */
int32_t         token_new_pagecount = 0;        /* count of pages that will
                                                 * be added onto token queue */

int             available_for_purge = 0;        /* increase when ripe token
                                                 * added, decrease when ripe
                                                 * token removed.
                                                 * protected by page_queue_lock 
                                                 */

static int token_q_allocating = 0;              /* flag for singlethreading 
                                                 * allocator */

struct purgeable_q purgeable_queues[PURGEABLE_Q_TYPE_MAX];

decl_lck_mtx_data(,vm_purgeable_queue_lock)

#define TOKEN_ADD               0x40    /* 0x100 */
#define TOKEN_DELETE            0x41    /* 0x104 */
#define TOKEN_RIPEN             0x42    /* 0x108 */
#define OBJECT_ADD              0x48    /* 0x120 */
#define OBJECT_REMOVE           0x49    /* 0x124 */
#define OBJECT_PURGE            0x4a    /* 0x128 */
#define OBJECT_PURGE_ALL        0x4b    /* 0x12c */

static token_idx_t vm_purgeable_token_remove_first(purgeable_q_t queue);

#if MACH_ASSERT
static void
vm_purgeable_token_check_queue(purgeable_q_t queue)
{
        int             token_cnt = 0, page_cnt = 0;
        token_idx_t     token = queue->token_q_head;
        token_idx_t     unripe = 0;
        int             our_inactive_count;

        while (token) {
                if (tokens[token].count != 0) {
                        assert(queue->token_q_unripe);
                        if (unripe == 0) {
                                assert(token == queue->token_q_unripe);
                                unripe = token;
                        }
                        page_cnt += tokens[token].count;
                }
                if (tokens[token].next == 0)
                        assert(queue->token_q_tail == token);

                token_cnt++;
                token = tokens[token].next;
        }

        if (unripe)
                assert(queue->token_q_unripe == unripe);
        assert(token_cnt == queue->debug_count_tokens);
        
        /* obsolete queue doesn't maintain token counts */
        if(queue->type != PURGEABLE_Q_TYPE_OBSOLETE)
        {
                our_inactive_count = page_cnt + queue->new_pages + token_new_pagecount;
                assert(our_inactive_count >= 0);
                assert((uint32_t) our_inactive_count == vm_page_inactive_count - vm_page_cleaned_count);
        }
}
#endif

/*
 * Add a token. Allocate token queue memory if necessary.
 * Call with page queue locked.
 */
kern_return_t
vm_purgeable_token_add(purgeable_q_t queue)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        
        /* new token */
        token_idx_t     token;
        enum purgeable_q_type i;

find_available_token:

        if (token_free_idx) {                           /* unused tokens available */
                token = token_free_idx;
                token_free_idx = tokens[token_free_idx].next;
        } else if (token_init_idx < token_q_max_cnt) {  /* lazy token array init */
                token = token_init_idx;
                token_init_idx++;
        } else {                                        /* allocate more memory */
                /* Wait if another thread is inside the memory alloc section */
                while(token_q_allocating) {
                        wait_result_t res = lck_mtx_sleep(&vm_page_queue_lock,
                                                          LCK_SLEEP_DEFAULT,
                                                          (event_t)&token_q_allocating,
                                                          THREAD_UNINT);
                        if(res != THREAD_AWAKENED) return KERN_ABORTED;
                };
                
                /* Check whether memory is still maxed out */
                if(token_init_idx < token_q_max_cnt)
                        goto find_available_token;
                
                /* Still no memory. Allocate some. */
                token_q_allocating = 1;
                
                /* Drop page queue lock so we can allocate */
                vm_page_unlock_queues();
                
                struct token *new_loc;
                vm_size_t alloc_size = token_q_cur_size + PAGE_SIZE;
                kern_return_t result;
                
                if (alloc_size / sizeof (struct token) > TOKEN_COUNT_MAX) {
                        result = KERN_RESOURCE_SHORTAGE;
                } else {
                        if (token_q_cur_size) {
                                result = kmem_realloc(kernel_map,
                                                      (vm_offset_t) tokens,
                                                      token_q_cur_size,
                                                      (vm_offset_t *) &new_loc,
                                                      alloc_size);
                        } else {
                                result = kmem_alloc(kernel_map,
                                                    (vm_offset_t *) &new_loc,
                                                    alloc_size);
                        }
                }
                
                vm_page_lock_queues();
                
                if (result) {
                        /* Unblock waiting threads */
                        token_q_allocating = 0;
                        thread_wakeup((event_t)&token_q_allocating);
                        return result;
                }
                
                /* If we get here, we allocated new memory. Update pointers and
                 * dealloc old range */
                struct token *old_tokens=tokens;
                tokens=new_loc;
                vm_size_t old_token_q_cur_size=token_q_cur_size;
                token_q_cur_size=alloc_size;
                token_q_max_cnt = (token_idx_t) (token_q_cur_size /
                                                 sizeof(struct token));
                assert (token_init_idx < token_q_max_cnt);      /* We must have a free token now */
                
                if (old_token_q_cur_size) {     /* clean up old mapping */
                        vm_page_unlock_queues();
                        /* kmem_realloc leaves the old region mapped. Get rid of it. */
                        kmem_free(kernel_map, (vm_offset_t)old_tokens, old_token_q_cur_size);
                        vm_page_lock_queues();
                }
                
                /* Unblock waiting threads */
                token_q_allocating = 0;
                thread_wakeup((event_t)&token_q_allocating);
                
                goto find_available_token;
        }
        
        assert (token);
        
        /*
         * the new pagecount we got need to be applied to all queues except
         * obsolete
         */
        for (i = PURGEABLE_Q_TYPE_FIFO; i < PURGEABLE_Q_TYPE_MAX; i++) {
                int64_t pages = purgeable_queues[i].new_pages += token_new_pagecount;
                assert(pages >= 0);
                assert(pages <= TOKEN_COUNT_MAX);
                purgeable_queues[i].new_pages = (int32_t) pages;
                assert(purgeable_queues[i].new_pages == pages);
        }
        token_new_pagecount = 0;

        /* set token counter value */
        if (queue->type != PURGEABLE_Q_TYPE_OBSOLETE)
                tokens[token].count = queue->new_pages;
        else
                tokens[token].count = 0;        /* all obsolete items are
                                                 * ripe immediately */
        queue->new_pages = 0;

        /* put token on token counter list */
        tokens[token].next = 0;
        if (queue->token_q_tail == 0) {
                assert(queue->token_q_head == 0 && queue->token_q_unripe == 0);
                queue->token_q_head = token;
        } else {
                tokens[queue->token_q_tail].next = token;
        }
        if (queue->token_q_unripe == 0) {       /* only ripe tokens (token
                                                 * count == 0) in queue */
                if (tokens[token].count > 0)
                        queue->token_q_unripe = token;  /* first unripe token */
                else
                        available_for_purge++;  /* added a ripe token?
                                                 * increase available count */
        }
        queue->token_q_tail = token;

#if MACH_ASSERT
        queue->debug_count_tokens++;
        /* Check both queues, since we modified the new_pages count on each */
        vm_purgeable_token_check_queue(&purgeable_queues[PURGEABLE_Q_TYPE_FIFO]);
        vm_purgeable_token_check_queue(&purgeable_queues[PURGEABLE_Q_TYPE_LIFO]);

        KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, TOKEN_ADD)),
                              queue->type,
                              tokens[token].count,      /* num pages on token
                                                         * (last token) */
                              queue->debug_count_tokens,
                              0,
                              0);
#endif

        return KERN_SUCCESS;
}

/*
 * Remove first token from queue and return its index. Add its count to the
 * count of the next token.
 * Call with page queue locked. 
 */
static token_idx_t 
vm_purgeable_token_remove_first(purgeable_q_t queue)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        
        token_idx_t     token;
        token = queue->token_q_head;

        assert(token);

        if (token) {
                assert(queue->token_q_tail);
                if (queue->token_q_head == queue->token_q_unripe) {
                        /* no ripe tokens... must move unripe pointer */
                        queue->token_q_unripe = tokens[token].next;
                } else {
                        /* we're removing a ripe token. decrease count */
                        available_for_purge--;
                        assert(available_for_purge >= 0);
                }

                if (queue->token_q_tail == queue->token_q_head)
                        assert(tokens[token].next == 0);

                queue->token_q_head = tokens[token].next;
                if (queue->token_q_head) {
                        tokens[queue->token_q_head].count += tokens[token].count;
                } else {
                        /* currently no other tokens in the queue */
                        /*
                         * the page count must be added to the next newly
                         * created token
                         */
                        queue->new_pages += tokens[token].count;
                        /* if head is zero, tail is too */
                        queue->token_q_tail = 0;
                }

#if MACH_ASSERT
                queue->debug_count_tokens--;
                vm_purgeable_token_check_queue(queue);

                KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, TOKEN_DELETE)),
                                      queue->type,
                                      tokens[queue->token_q_head].count,        /* num pages on new
                                                                                 * first token */
                                      token_new_pagecount,      /* num pages waiting for
                                                                 * next token */
                                      available_for_purge,
                                      0);
#endif
        }
        return token;
}

static token_idx_t 
vm_purgeable_token_remove_last(purgeable_q_t queue)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        
        token_idx_t     token;
        token = queue->token_q_tail;

        assert(token);

        if (token) {
                assert(queue->token_q_head);

                if (queue->token_q_tail == queue->token_q_head)
                        assert(tokens[token].next == 0);

                if (queue->token_q_unripe == 0) {
                        /* we're removing a ripe token. decrease count */
                        available_for_purge--;
                        assert(available_for_purge >= 0);
                } else if (queue->token_q_unripe == token) {
                        /* we're removing the only unripe token */
                        queue->token_q_unripe = 0;
                }
                        
                if (token == queue->token_q_head) {
                        /* token is the last one in the queue */
                        queue->token_q_head = 0;
                        queue->token_q_tail = 0;
                } else {
                        token_idx_t new_tail;

                        for (new_tail = queue->token_q_head;
                             tokens[new_tail].next != token && new_tail != 0;
                             new_tail = tokens[new_tail].next) {
                        }
                        assert(tokens[new_tail].next == token);
                        queue->token_q_tail = new_tail;
                        tokens[new_tail].next = 0;
                }

                queue->new_pages += tokens[token].count;

#if MACH_ASSERT
                queue->debug_count_tokens--;
                vm_purgeable_token_check_queue(queue);

                KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, TOKEN_DELETE)),
                                      queue->type,
                                      tokens[queue->token_q_head].count,        /* num pages on new
                                                                                 * first token */
                                      token_new_pagecount,      /* num pages waiting for
                                                                 * next token */
                                      available_for_purge,
                                      0);
#endif
        }
        return token;
}

/* 
 * Delete first token from queue. Return token to token queue.
 * Call with page queue locked. 
 */
void
vm_purgeable_token_delete_first(purgeable_q_t queue)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        token_idx_t     token = vm_purgeable_token_remove_first(queue);

        if (token) {
                /* stick removed token on free queue */
                tokens[token].next = token_free_idx;
                token_free_idx = token;
        }
}

void
vm_purgeable_token_delete_last(purgeable_q_t queue)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        token_idx_t     token = vm_purgeable_token_remove_last(queue);

        if (token) {
                /* stick removed token on free queue */
                tokens[token].next = token_free_idx;
                token_free_idx = token;
        }
}


/* Call with page queue locked. */
void
vm_purgeable_q_advance_all()
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        
        /* check queue counters - if they get really large, scale them back.
         * They tend to get that large when there is no purgeable queue action */
        int i;
        if(token_new_pagecount > (TOKEN_NEW_PAGECOUNT_MAX >> 1))        /* a system idling years might get there */
        {
                for (i = PURGEABLE_Q_TYPE_FIFO; i < PURGEABLE_Q_TYPE_MAX; i++) {
                        int64_t pages = purgeable_queues[i].new_pages += token_new_pagecount;
                        assert(pages >= 0);
                        assert(pages <= TOKEN_COUNT_MAX);
                        purgeable_queues[i].new_pages = (int32_t) pages;
                        assert(purgeable_queues[i].new_pages == pages);
                }
                token_new_pagecount = 0;
        }
        
        /*
         * Decrement token counters. A token counter can be zero, this means the
         * object is ripe to be purged. It is not purged immediately, because that
         * could cause several objects to be purged even if purging one would satisfy
         * the memory needs. Instead, the pageout thread purges one after the other
         * by calling vm_purgeable_object_purge_one and then rechecking the memory
         * balance.
         *
         * No need to advance obsolete queue - all items are ripe there,
         * always
         */
        for (i = PURGEABLE_Q_TYPE_FIFO; i < PURGEABLE_Q_TYPE_MAX; i++) {
                purgeable_q_t queue = &purgeable_queues[i];
                uint32_t num_pages = 1;
                
                /* Iterate over tokens as long as there are unripe tokens. */
                while (queue->token_q_unripe) {
                        if (tokens[queue->token_q_unripe].count && num_pages)
                        {
                                tokens[queue->token_q_unripe].count -= 1;
                                num_pages -= 1;
                        }

                        if (tokens[queue->token_q_unripe].count == 0) {
                                queue->token_q_unripe = tokens[queue->token_q_unripe].next;
                                available_for_purge++;
                                KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, TOKEN_RIPEN)),
                                                      queue->type,
                                                      tokens[queue->token_q_head].count,        /* num pages on new
                                                                                         * first token */
                                                      0,
                                                      available_for_purge,
                                                      0);
                                continue;       /* One token ripened. Make sure to
                                                 * check the next. */
                        }
                        if (num_pages == 0)
                                break;  /* Current token not ripe and no more pages.
                                         * Work done. */
                }

                /*
                 * if there are no unripe tokens in the queue, decrement the
                 * new_pages counter instead new_pages can be negative, but must be
                 * canceled out by token_new_pagecount -- since inactive queue as a
                 * whole always contains a nonnegative number of pages
                 */
                if (!queue->token_q_unripe) {
                        queue->new_pages -= num_pages;
                        assert((int32_t) token_new_pagecount + queue->new_pages >= 0);
                }
#if MACH_ASSERT
                vm_purgeable_token_check_queue(queue);
#endif
        }
}

/*
 * grab any ripe object and purge it obsolete queue first. then, go through
 * each volatile group. Select a queue with a ripe token.
 * Start with first group (0)
 * 1. Look at queue. Is there an object?
 *   Yes - purge it. Remove token.
 *   No - check other queue. Is there an object?
 *     No - increment group, then go to (1)
 *     Yes - purge it. Remove token. If there is no ripe token, remove ripe
 *      token from other queue and migrate unripe token from this
 *      queue to other queue.
 * Call with page queue locked.
 */
static void
vm_purgeable_token_remove_ripe(purgeable_q_t queue)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        assert(queue->token_q_head && tokens[queue->token_q_head].count == 0);
        /* return token to free list. advance token list. */
        token_idx_t     new_head = tokens[queue->token_q_head].next;
        tokens[queue->token_q_head].next = token_free_idx;
        token_free_idx = queue->token_q_head;
        queue->token_q_head = new_head;
        if (new_head == 0)
                queue->token_q_tail = 0;

#if MACH_ASSERT
        queue->debug_count_tokens--;
        vm_purgeable_token_check_queue(queue);
#endif

        available_for_purge--;
        assert(available_for_purge >= 0);
}

/*
 * Delete a ripe token from the given queue. If there are no ripe tokens on
 * that queue, delete a ripe token from queue2, and migrate an unripe token
 * from queue to queue2
 * Call with page queue locked.
 */
static void
vm_purgeable_token_choose_and_delete_ripe(purgeable_q_t queue, purgeable_q_t queue2)
{
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        assert(queue->token_q_head);

        if (tokens[queue->token_q_head].count == 0) {
                /* This queue has a ripe token. Remove. */
                vm_purgeable_token_remove_ripe(queue);
        } else {
                assert(queue2);
                /*
                 * queue2 must have a ripe token. Remove, and migrate one
                 * from queue to queue2.
                 */
                vm_purgeable_token_remove_ripe(queue2);
                /* migrate unripe token */
                token_idx_t     token;
                token_cnt_t     count;

                /* remove token from queue1 */
                assert(queue->token_q_unripe == queue->token_q_head);   /* queue1 had no unripe
                                                                         * tokens, remember? */
                token = vm_purgeable_token_remove_first(queue);
                assert(token);

                count = tokens[token].count;

                /* migrate to queue2 */
                /* go to migration target loc */
                token_idx_t    *token_in_queue2 = &queue2->token_q_head;
                while (*token_in_queue2 && count > tokens[*token_in_queue2].count) {
                        count -= tokens[*token_in_queue2].count;
                        token_in_queue2 = &tokens[*token_in_queue2].next;
                }

                if ((*token_in_queue2 == queue2->token_q_unripe) ||     /* becomes the first
                                                                         * unripe token */
                    (queue2->token_q_unripe == 0))
                        queue2->token_q_unripe = token; /* must update unripe
                                                         * pointer */

                /* insert token */
                tokens[token].count = count;
                tokens[token].next = *token_in_queue2;

                /*
                 * if inserting at end, reduce new_pages by that value if
                 * inserting before token, reduce counter of that token
                 */
                if (*token_in_queue2 == 0) {    /* insertion at end of queue2 */
                        queue2->token_q_tail = token;   /* must update tail
                                                         * pointer */
                        assert(queue2->new_pages >= (int32_t) count);
                        queue2->new_pages -= count;
                } else {
                        assert(tokens[*token_in_queue2].count >= count);
                        tokens[*token_in_queue2].count -= count;
                }
                *token_in_queue2 = token;

#if MACH_ASSERT
                queue2->debug_count_tokens++;
                vm_purgeable_token_check_queue(queue2);
#endif
        }
}

/* Find an object that can be locked. Returns locked object. */
/* Call with purgeable queue locked. */
static          vm_object_t
vm_purgeable_object_find_and_lock(purgeable_q_t queue, int group)
{
        lck_mtx_assert(&vm_purgeable_queue_lock, LCK_MTX_ASSERT_OWNED);
        /*
         * Usually we would pick the first element from a queue. However, we
         * might not be able to get a lock on it, in which case we try the
         * remaining elements in order.
         */

        vm_object_t     object;
        for (object = (vm_object_t) queue_first(&queue->objq[group]);
             !queue_end(&queue->objq[group], (queue_entry_t) object);
             object = (vm_object_t) queue_next(&object->objq)) {
                if (vm_object_lock_try(object)) {
                        /* Locked. Great. We'll take it. Remove and return. */
                        queue_remove(&queue->objq[group], object,
                                     vm_object_t, objq);
                        object->objq.next = 0;
                        object->objq.prev = 0;
#if MACH_ASSERT
                        queue->debug_count_objects--;
#endif
                        return object;
                }
        }

        return 0;
}

/* Can be called without holding locks */
void
vm_purgeable_object_purge_all(void)
{
        enum purgeable_q_type i;
        int             group;
        vm_object_t     object;
        unsigned int    purged_count;
        uint32_t        collisions;

        purged_count = 0;
        collisions = 0;

restart:
        lck_mtx_lock(&vm_purgeable_queue_lock);
        /* Cycle through all queues */
        for (i = PURGEABLE_Q_TYPE_OBSOLETE; i < PURGEABLE_Q_TYPE_MAX; i++) {
                purgeable_q_t   queue;

                queue = &purgeable_queues[i];

                /*
                 * Look through all groups, starting from the lowest. If
                 * we find an object in that group, try to lock it (this can
                 * fail). If locking is successful, we can drop the queue
                 * lock, remove a token and then purge the object.
                 */
                for (group = 0; group < NUM_VOLATILE_GROUPS; group++) {
                        while (!queue_empty(&queue->objq[group])) {
                                object = vm_purgeable_object_find_and_lock(queue, group);
                                if (object == VM_OBJECT_NULL) {
                                        lck_mtx_unlock(&vm_purgeable_queue_lock);
                                        mutex_pause(collisions++);
                                        goto restart;
                                }

                                lck_mtx_unlock(&vm_purgeable_queue_lock);
                                
                                /* Lock the page queue here so we don't hold it
                                 * over the whole, legthy operation */
                                vm_page_lock_queues();
                                vm_purgeable_token_remove_first(queue);
                                vm_page_unlock_queues();
                                
                                assert(object->purgable == VM_PURGABLE_VOLATILE);
                                (void) vm_object_purge(object);
                                vm_object_unlock(object);
                                purged_count++;
                                goto restart;
                        }
                        assert(queue->debug_count_objects >= 0);
                }
        }
        KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, OBJECT_PURGE_ALL)),
                              purged_count, /* # of purged objects */
                              0,
                              available_for_purge,
                              0,
                              0);
        lck_mtx_unlock(&vm_purgeable_queue_lock);
        return;
}

boolean_t
vm_purgeable_object_purge_one(void)
{
        enum purgeable_q_type i;
        int             group;
        vm_object_t     object = 0;
        purgeable_q_t   queue, queue2;

        /* Need the page queue lock since we'll be changing the token queue. */
#if MACH_ASSERT
        lck_mtx_assert(&vm_page_queue_lock, LCK_MTX_ASSERT_OWNED);
#endif
        lck_mtx_lock(&vm_purgeable_queue_lock);
        
        /* Cycle through all queues */
        for (i = PURGEABLE_Q_TYPE_OBSOLETE; i < PURGEABLE_Q_TYPE_MAX; i++) {
                queue = &purgeable_queues[i];

                /*
                 * Are there any ripe tokens on this queue? If yes, we'll
                 * find an object to purge there
                 */
                if (!(queue->token_q_head && tokens[queue->token_q_head].count == 0))
                        continue;       /* no token? Look at next purgeable
                                         * queue */

                /*
                 * Now look through all groups, starting from the lowest. If
                 * we find an object in that group, try to lock it (this can
                 * fail). If locking is successful, we can drop the queue
                 * lock, remove a token and then purge the object.
                 */
                for (group = 0; group < NUM_VOLATILE_GROUPS; group++) {
                        if (!queue_empty(&queue->objq[group]) && 
                            (object = vm_purgeable_object_find_and_lock(queue, group))) {
                                lck_mtx_unlock(&vm_purgeable_queue_lock);
                                vm_purgeable_token_choose_and_delete_ripe(queue, 0);
                                goto purge_now;
                        }
                        if (i != PURGEABLE_Q_TYPE_OBSOLETE) { 
                                /* This is the token migration case, and it works between
                                 * FIFO and LIFO only */
                                queue2 = &purgeable_queues[i != PURGEABLE_Q_TYPE_FIFO ? 
                                                           PURGEABLE_Q_TYPE_FIFO : 
                                                           PURGEABLE_Q_TYPE_LIFO];

                                if (!queue_empty(&queue2->objq[group]) && 
                                    (object = vm_purgeable_object_find_and_lock(queue2, group))) {
                                        lck_mtx_unlock(&vm_purgeable_queue_lock);
                                        vm_purgeable_token_choose_and_delete_ripe(queue2, queue);
                                        goto purge_now;
                                }
                        }
                        assert(queue->debug_count_objects >= 0);
                }
        }
        /*
         * because we have to do a try_lock on the objects which could fail,
         * we could end up with no object to purge at this time, even though
         * we have objects in a purgeable state
         */
        lck_mtx_unlock(&vm_purgeable_queue_lock);
        return FALSE;

purge_now:

        assert(object);
        assert(object->purgable == VM_PURGABLE_VOLATILE);
        vm_page_unlock_queues();  /* Unlock for call to vm_object_purge() */
        (void) vm_object_purge(object);
        vm_object_unlock(object);
        vm_page_lock_queues();

        KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, OBJECT_PURGE)),
                              object,   /* purged object */
                              0,
                              available_for_purge,
                              0,
                              0);
        
        return TRUE;
}

/* Called with object lock held */
void
vm_purgeable_object_add(vm_object_t object, purgeable_q_t queue, int group)
{
        vm_object_lock_assert_exclusive(object);
        lck_mtx_lock(&vm_purgeable_queue_lock);

        if (queue->type == PURGEABLE_Q_TYPE_OBSOLETE)
                group = 0;
        if (queue->type != PURGEABLE_Q_TYPE_LIFO)       /* fifo and obsolete are
                                                         * fifo-queued */
                queue_enter(&queue->objq[group], object, vm_object_t, objq);    /* last to die */
        else
                queue_enter_first(&queue->objq[group], object, vm_object_t, objq);      /* first to die */

#if MACH_ASSERT
        queue->debug_count_objects++;
        KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, OBJECT_ADD)),
                              0,
                              tokens[queue->token_q_head].count,
                              queue->type,
                              group,
                              0);
#endif

        lck_mtx_unlock(&vm_purgeable_queue_lock);
}

/* Look for object. If found, remove from purgeable queue. */
/* Called with object lock held */
purgeable_q_t
vm_purgeable_object_remove(vm_object_t object)
{
        enum purgeable_q_type i;
        int             group;

        vm_object_lock_assert_exclusive(object);
        lck_mtx_lock(&vm_purgeable_queue_lock);
        
        for (i = PURGEABLE_Q_TYPE_OBSOLETE; i < PURGEABLE_Q_TYPE_MAX; i++) {
                purgeable_q_t   queue = &purgeable_queues[i];
                for (group = 0; group < NUM_VOLATILE_GROUPS; group++) {
                        vm_object_t     o;
                        for (o = (vm_object_t) queue_first(&queue->objq[group]);
                         !queue_end(&queue->objq[group], (queue_entry_t) o);
                             o = (vm_object_t) queue_next(&o->objq)) {
                                if (o == object) {
                                        queue_remove(&queue->objq[group], object,
                                                     vm_object_t, objq);
#if MACH_ASSERT
                                        queue->debug_count_objects--;
                                        KERNEL_DEBUG_CONSTANT((MACHDBG_CODE(DBG_MACH_VM, OBJECT_REMOVE)),
                                                              0,
                                          tokens[queue->token_q_head].count,
                                                              queue->type,
                                                              group,
                                                              0);
#endif
                                        lck_mtx_unlock(&vm_purgeable_queue_lock);
                                        object->objq.next = 0;
                                        object->objq.prev = 0;
                                        return &purgeable_queues[i];
                                }
                        }
                }
        }
        lck_mtx_unlock(&vm_purgeable_queue_lock);
        return 0;
}