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[FreeBSD/FreeBSD.git] / sys / vm / vm_pageout.c

/*-
 * Copyright (c) 1991 Regents of the University of California.
 * All rights reserved.
 * Copyright (c) 1994 John S. Dyson
 * All rights reserved.
 * Copyright (c) 1994 David Greenman
 * All rights reserved.
 * Copyright (c) 2005 Yahoo! Technologies Norway AS
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * The Mach Operating System project at Carnegie-Mellon University.
 *
 * 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.
 *
 *      from: @(#)vm_pageout.c  7.4 (Berkeley) 5/7/91
 *
 *
 * Copyright (c) 1987, 1990 Carnegie-Mellon University.
 * All rights reserved.
 *
 * Authors: Avadis Tevanian, Jr., Michael Wayne Young
 *
 * Permission to use, copy, modify and distribute this software and
 * its documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 *
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 *
 * Carnegie Mellon requests users of this software to return to
 *
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 *
 * any improvements or extensions that they make and grant Carnegie the
 * rights to redistribute these changes.
 */

/*
 *      The proverbial page-out daemon.
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include "opt_vm.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/eventhandler.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/kthread.h>
#include <sys/ktr.h>
#include <sys/mount.h>
#include <sys/resourcevar.h>
#include <sys/sched.h>
#include <sys/signalvar.h>
#include <sys/vnode.h>
#include <sys/vmmeter.h>
#include <sys/sx.h>
#include <sys/sysctl.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_map.h>
#include <vm/vm_pageout.h>
#include <vm/vm_pager.h>
#include <vm/swap_pager.h>
#include <vm/vm_extern.h>
#include <vm/uma.h>

#include <machine/mutex.h>

/*
 * System initialization
 */

/* the kernel process "vm_pageout"*/
static void vm_pageout(void);
static int vm_pageout_clean(vm_page_t);
static void vm_pageout_scan(int pass);

struct proc *pageproc;

static struct kproc_desc page_kp = {
        "pagedaemon",
        vm_pageout,
        &pageproc
};
SYSINIT(pagedaemon, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, kproc_start,
    &page_kp);

#if !defined(NO_SWAPPING)
/* the kernel process "vm_daemon"*/
static void vm_daemon(void);
static struct   proc *vmproc;

static struct kproc_desc vm_kp = {
        "vmdaemon",
        vm_daemon,
        &vmproc
};
SYSINIT(vmdaemon, SI_SUB_KTHREAD_VM, SI_ORDER_FIRST, kproc_start, &vm_kp);
#endif


int vm_pages_needed;            /* Event on which pageout daemon sleeps */
int vm_pageout_deficit;         /* Estimated number of pages deficit */
int vm_pageout_pages_needed;    /* flag saying that the pageout daemon needs pages */

#if !defined(NO_SWAPPING)
static int vm_pageout_req_swapout;      /* XXX */
static int vm_daemon_needed;
static struct mtx vm_daemon_mtx;
/* Allow for use by vm_pageout before vm_daemon is initialized. */
MTX_SYSINIT(vm_daemon, &vm_daemon_mtx, "vm daemon", MTX_DEF);
#endif
static int vm_max_launder = 32;
static int vm_pageout_stats_max=0, vm_pageout_stats_interval = 0;
static int vm_pageout_full_stats_interval = 0;
static int vm_pageout_algorithm=0;
static int defer_swap_pageouts=0;
static int disable_swap_pageouts=0;

#if defined(NO_SWAPPING)
static int vm_swap_enabled=0;
static int vm_swap_idle_enabled=0;
#else
static int vm_swap_enabled=1;
static int vm_swap_idle_enabled=0;
#endif

SYSCTL_INT(_vm, VM_PAGEOUT_ALGORITHM, pageout_algorithm,
        CTLFLAG_RW, &vm_pageout_algorithm, 0, "LRU page mgmt");

SYSCTL_INT(_vm, OID_AUTO, max_launder,
        CTLFLAG_RW, &vm_max_launder, 0, "Limit dirty flushes in pageout");

SYSCTL_INT(_vm, OID_AUTO, pageout_stats_max,
        CTLFLAG_RW, &vm_pageout_stats_max, 0, "Max pageout stats scan length");

SYSCTL_INT(_vm, OID_AUTO, pageout_full_stats_interval,
        CTLFLAG_RW, &vm_pageout_full_stats_interval, 0, "Interval for full stats scan");

SYSCTL_INT(_vm, OID_AUTO, pageout_stats_interval,
        CTLFLAG_RW, &vm_pageout_stats_interval, 0, "Interval for partial stats scan");

#if defined(NO_SWAPPING)
SYSCTL_INT(_vm, VM_SWAPPING_ENABLED, swap_enabled,
        CTLFLAG_RD, &vm_swap_enabled, 0, "");
SYSCTL_INT(_vm, OID_AUTO, swap_idle_enabled,
        CTLFLAG_RD, &vm_swap_idle_enabled, 0, "");
#else
SYSCTL_INT(_vm, VM_SWAPPING_ENABLED, swap_enabled,
        CTLFLAG_RW, &vm_swap_enabled, 0, "Enable entire process swapout");
SYSCTL_INT(_vm, OID_AUTO, swap_idle_enabled,
        CTLFLAG_RW, &vm_swap_idle_enabled, 0, "Allow swapout on idle criteria");
#endif

SYSCTL_INT(_vm, OID_AUTO, defer_swapspace_pageouts,
        CTLFLAG_RW, &defer_swap_pageouts, 0, "Give preference to dirty pages in mem");

SYSCTL_INT(_vm, OID_AUTO, disable_swapspace_pageouts,
        CTLFLAG_RW, &disable_swap_pageouts, 0, "Disallow swapout of dirty pages");

static int pageout_lock_miss;
SYSCTL_INT(_vm, OID_AUTO, pageout_lock_miss,
        CTLFLAG_RD, &pageout_lock_miss, 0, "vget() lock misses during pageout");

#define VM_PAGEOUT_PAGE_COUNT 16
int vm_pageout_page_count = VM_PAGEOUT_PAGE_COUNT;

int vm_page_max_wired;          /* XXX max # of wired pages system-wide */
SYSCTL_INT(_vm, OID_AUTO, max_wired,
        CTLFLAG_RW, &vm_page_max_wired, 0, "System-wide limit to wired page count");

#if !defined(NO_SWAPPING)
static void vm_pageout_map_deactivate_pages(vm_map_t, long);
static void vm_pageout_object_deactivate_pages(pmap_t, vm_object_t, long);
static void vm_req_vmdaemon(int req);
#endif
static void vm_pageout_page_stats(void);

/*
 * vm_pageout_fallback_object_lock:
 * 
 * Lock vm object currently associated with `m'. VM_OBJECT_TRYLOCK is
 * known to have failed and page queue must be either PQ_ACTIVE or
 * PQ_INACTIVE.  To avoid lock order violation, unlock the page queues
 * while locking the vm object.  Use marker page to detect page queue
 * changes and maintain notion of next page on page queue.  Return
 * TRUE if no changes were detected, FALSE otherwise.  vm object is
 * locked on return.
 * 
 * This function depends on both the lock portion of struct vm_object
 * and normal struct vm_page being type stable.
 */
boolean_t
vm_pageout_fallback_object_lock(vm_page_t m, vm_page_t *next)
{
        struct vm_page marker;
        boolean_t unchanged;
        u_short queue;
        vm_object_t object;

        /*
         * Initialize our marker
         */
        bzero(&marker, sizeof(marker));
        marker.flags = PG_FICTITIOUS | PG_MARKER;
        marker.oflags = VPO_BUSY;
        marker.queue = m->queue;
        marker.wire_count = 1;

        queue = m->queue;
        object = m->object;
        
        TAILQ_INSERT_AFTER(&vm_page_queues[queue].pl,
                           m, &marker, pageq);
        vm_page_unlock_queues();
        VM_OBJECT_LOCK(object);
        vm_page_lock_queues();

        /* Page queue might have changed. */
        *next = TAILQ_NEXT(&marker, pageq);
        unchanged = (m->queue == queue &&
                     m->object == object &&
                     &marker == TAILQ_NEXT(m, pageq));
        TAILQ_REMOVE(&vm_page_queues[queue].pl,
                     &marker, pageq);
        return (unchanged);
}

/*
 * vm_pageout_clean:
 *
 * Clean the page and remove it from the laundry.
 * 
 * We set the busy bit to cause potential page faults on this page to
 * block.  Note the careful timing, however, the busy bit isn't set till
 * late and we cannot do anything that will mess with the page.
 */
static int
vm_pageout_clean(m)
        vm_page_t m;
{
        vm_object_t object;
        vm_page_t mc[2*vm_pageout_page_count];
        int pageout_count;
        int ib, is, page_base;
        vm_pindex_t pindex = m->pindex;

        mtx_assert(&vm_page_queue_mtx, MA_OWNED);
        VM_OBJECT_LOCK_ASSERT(m->object, MA_OWNED);

        /*
         * It doesn't cost us anything to pageout OBJT_DEFAULT or OBJT_SWAP
         * with the new swapper, but we could have serious problems paging
         * out other object types if there is insufficient memory.  
         *
         * Unfortunately, checking free memory here is far too late, so the
         * check has been moved up a procedural level.
         */

        /*
         * Can't clean the page if it's busy or held.
         */
        if ((m->hold_count != 0) ||
            ((m->busy != 0) || (m->oflags & VPO_BUSY))) {
                return 0;
        }

        mc[vm_pageout_page_count] = m;
        pageout_count = 1;
        page_base = vm_pageout_page_count;
        ib = 1;
        is = 1;

        /*
         * Scan object for clusterable pages.
         *
         * We can cluster ONLY if: ->> the page is NOT
         * clean, wired, busy, held, or mapped into a
         * buffer, and one of the following:
         * 1) The page is inactive, or a seldom used
         *    active page.
         * -or-
         * 2) we force the issue.
         *
         * During heavy mmap/modification loads the pageout
         * daemon can really fragment the underlying file
         * due to flushing pages out of order and not trying
         * align the clusters (which leave sporatic out-of-order
         * holes).  To solve this problem we do the reverse scan
         * first and attempt to align our cluster, then do a 
         * forward scan if room remains.
         */
        object = m->object;
more:
        while (ib && pageout_count < vm_pageout_page_count) {
                vm_page_t p;

                if (ib > pindex) {
                        ib = 0;
                        break;
                }

                if ((p = vm_page_lookup(object, pindex - ib)) == NULL) {
                        ib = 0;
                        break;
                }
                if ((p->oflags & VPO_BUSY) || p->busy) {
                        ib = 0;
                        break;
                }
                vm_page_test_dirty(p);
                if ((p->dirty & p->valid) == 0 ||
                    p->queue != PQ_INACTIVE ||
                    p->wire_count != 0 ||       /* may be held by buf cache */
                    p->hold_count != 0) {       /* may be undergoing I/O */
                        ib = 0;
                        break;
                }
                mc[--page_base] = p;
                ++pageout_count;
                ++ib;
                /*
                 * alignment boundry, stop here and switch directions.  Do
                 * not clear ib.
                 */
                if ((pindex - (ib - 1)) % vm_pageout_page_count == 0)
                        break;
        }

        while (pageout_count < vm_pageout_page_count && 
            pindex + is < object->size) {
                vm_page_t p;

                if ((p = vm_page_lookup(object, pindex + is)) == NULL)
                        break;
                if ((p->oflags & VPO_BUSY) || p->busy) {
                        break;
                }
                vm_page_test_dirty(p);
                if ((p->dirty & p->valid) == 0 ||
                    p->queue != PQ_INACTIVE ||
                    p->wire_count != 0 ||       /* may be held by buf cache */
                    p->hold_count != 0) {       /* may be undergoing I/O */
                        break;
                }
                mc[page_base + pageout_count] = p;
                ++pageout_count;
                ++is;
        }

        /*
         * If we exhausted our forward scan, continue with the reverse scan
         * when possible, even past a page boundry.  This catches boundry
         * conditions.
         */
        if (ib && pageout_count < vm_pageout_page_count)
                goto more;

        /*
         * we allow reads during pageouts...
         */
        return (vm_pageout_flush(&mc[page_base], pageout_count, 0));
}

/*
 * vm_pageout_flush() - launder the given pages
 *
 *      The given pages are laundered.  Note that we setup for the start of
 *      I/O ( i.e. busy the page ), mark it read-only, and bump the object
 *      reference count all in here rather then in the parent.  If we want
 *      the parent to do more sophisticated things we may have to change
 *      the ordering.
 */
int
vm_pageout_flush(vm_page_t *mc, int count, int flags)
{
        vm_object_t object = mc[0]->object;
        int pageout_status[count];
        int numpagedout = 0;
        int i;

        mtx_assert(&vm_page_queue_mtx, MA_OWNED);
        VM_OBJECT_LOCK_ASSERT(object, MA_OWNED);
        /*
         * Initiate I/O.  Bump the vm_page_t->busy counter and
         * mark the pages read-only.
         *
         * We do not have to fixup the clean/dirty bits here... we can
         * allow the pager to do it after the I/O completes.
         *
         * NOTE! mc[i]->dirty may be partial or fragmented due to an
         * edge case with file fragments.
         */
        for (i = 0; i < count; i++) {
                KASSERT(mc[i]->valid == VM_PAGE_BITS_ALL,
                    ("vm_pageout_flush: partially invalid page %p index %d/%d",
                        mc[i], i, count));
                vm_page_io_start(mc[i]);
                pmap_remove_write(mc[i]);
        }
        vm_page_unlock_queues();
        vm_object_pip_add(object, count);

        vm_pager_put_pages(object, mc, count, flags, pageout_status);

        vm_page_lock_queues();
        for (i = 0; i < count; i++) {
                vm_page_t mt = mc[i];

                KASSERT(pageout_status[i] == VM_PAGER_PEND ||
                    (mt->flags & PG_WRITEABLE) == 0,
                    ("vm_pageout_flush: page %p is not write protected", mt));
                switch (pageout_status[i]) {
                case VM_PAGER_OK:
                case VM_PAGER_PEND:
                        numpagedout++;
                        break;
                case VM_PAGER_BAD:
                        /*
                         * Page outside of range of object. Right now we
                         * essentially lose the changes by pretending it
                         * worked.
                         */
                        pmap_clear_modify(mt);
                        vm_page_undirty(mt);
                        break;
                case VM_PAGER_ERROR:
                case VM_PAGER_FAIL:
                        /*
                         * If page couldn't be paged out, then reactivate the
                         * page so it doesn't clog the inactive list.  (We
                         * will try paging out it again later).
                         */
                        vm_page_activate(mt);
                        break;
                case VM_PAGER_AGAIN:
                        break;
                }

                /*
                 * If the operation is still going, leave the page busy to
                 * block all other accesses. Also, leave the paging in
                 * progress indicator set so that we don't attempt an object
                 * collapse.
                 */
                if (pageout_status[i] != VM_PAGER_PEND) {
                        vm_object_pip_wakeup(object);
                        vm_page_io_finish(mt);
                        if (vm_page_count_severe())
                                vm_page_try_to_cache(mt);
                }
        }
        return numpagedout;
}

#if !defined(NO_SWAPPING)
/*
 *      vm_pageout_object_deactivate_pages
 *
 *      deactivate enough pages to satisfy the inactive target
 *      requirements or if vm_page_proc_limit is set, then
 *      deactivate all of the pages in the object and its
 *      backing_objects.
 *
 *      The object and map must be locked.
 */
static void
vm_pageout_object_deactivate_pages(pmap, first_object, desired)
        pmap_t pmap;
        vm_object_t first_object;
        long desired;
{
        vm_object_t backing_object, object;
        vm_page_t p, next;
        int actcount, rcount, remove_mode;

        VM_OBJECT_LOCK_ASSERT(first_object, MA_OWNED);
        if (first_object->type == OBJT_DEVICE || first_object->type == OBJT_PHYS)
                return;
        for (object = first_object;; object = backing_object) {
                if (pmap_resident_count(pmap) <= desired)
                        goto unlock_return;
                if (object->paging_in_progress)
                        goto unlock_return;

                remove_mode = 0;
                if (object->shadow_count > 1)
                        remove_mode = 1;
                /*
                 * scan the objects entire memory queue
                 */
                rcount = object->resident_page_count;
                p = TAILQ_FIRST(&object->memq);
                vm_page_lock_queues();
                while (p && (rcount-- > 0)) {
                        if (pmap_resident_count(pmap) <= desired) {
                                vm_page_unlock_queues();
                                goto unlock_return;
                        }
                        next = TAILQ_NEXT(p, listq);
                        cnt.v_pdpages++;
                        if (p->wire_count != 0 ||
                            p->hold_count != 0 ||
                            p->busy != 0 ||
                            (p->oflags & VPO_BUSY) ||
                            (p->flags & PG_UNMANAGED) ||
                            !pmap_page_exists_quick(pmap, p)) {
                                p = next;
                                continue;
                        }
                        actcount = pmap_ts_referenced(p);
                        if (actcount) {
                                vm_page_flag_set(p, PG_REFERENCED);
                        } else if (p->flags & PG_REFERENCED) {
                                actcount = 1;
                        }
                        if ((p->queue != PQ_ACTIVE) &&
                                (p->flags & PG_REFERENCED)) {
                                vm_page_activate(p);
                                p->act_count += actcount;
                                vm_page_flag_clear(p, PG_REFERENCED);
                        } else if (p->queue == PQ_ACTIVE) {
                                if ((p->flags & PG_REFERENCED) == 0) {
                                        p->act_count -= min(p->act_count, ACT_DECLINE);
                                        if (!remove_mode && (vm_pageout_algorithm || (p->act_count == 0))) {
                                                pmap_remove_all(p);
                                                vm_page_deactivate(p);
                                        } else {
                                                vm_page_requeue(p);
                                        }
                                } else {
                                        vm_page_activate(p);
                                        vm_page_flag_clear(p, PG_REFERENCED);
                                        if (p->act_count < (ACT_MAX - ACT_ADVANCE))
                                                p->act_count += ACT_ADVANCE;
                                        vm_page_requeue(p);
                                }
                        } else if (p->queue == PQ_INACTIVE) {
                                pmap_remove_all(p);
                        }
                        p = next;
                }
                vm_page_unlock_queues();
                if ((backing_object = object->backing_object) == NULL)
                        goto unlock_return;
                VM_OBJECT_LOCK(backing_object);
                if (object != first_object)
                        VM_OBJECT_UNLOCK(object);
        }
unlock_return:
        if (object != first_object)
                VM_OBJECT_UNLOCK(object);
}

/*
 * deactivate some number of pages in a map, try to do it fairly, but
 * that is really hard to do.
 */
static void
vm_pageout_map_deactivate_pages(map, desired)
        vm_map_t map;
        long desired;
{
        vm_map_entry_t tmpe;
        vm_object_t obj, bigobj;
        int nothingwired;

        if (!vm_map_trylock(map))
                return;

        bigobj = NULL;
        nothingwired = TRUE;

        /*
         * first, search out the biggest object, and try to free pages from
         * that.
         */
        tmpe = map->header.next;
        while (tmpe != &map->header) {
                if ((tmpe->eflags & MAP_ENTRY_IS_SUB_MAP) == 0) {
                        obj = tmpe->object.vm_object;
                        if (obj != NULL && VM_OBJECT_TRYLOCK(obj)) {
                                if (obj->shadow_count <= 1 &&
                                    (bigobj == NULL ||
                                     bigobj->resident_page_count < obj->resident_page_count)) {
                                        if (bigobj != NULL)
                                                VM_OBJECT_UNLOCK(bigobj);
                                        bigobj = obj;
                                } else
                                        VM_OBJECT_UNLOCK(obj);
                        }
                }
                if (tmpe->wired_count > 0)
                        nothingwired = FALSE;
                tmpe = tmpe->next;
        }

        if (bigobj != NULL) {
                vm_pageout_object_deactivate_pages(map->pmap, bigobj, desired);
                VM_OBJECT_UNLOCK(bigobj);
        }
        /*
         * Next, hunt around for other pages to deactivate.  We actually
         * do this search sort of wrong -- .text first is not the best idea.
         */
        tmpe = map->header.next;
        while (tmpe != &map->header) {
                if (pmap_resident_count(vm_map_pmap(map)) <= desired)
                        break;
                if ((tmpe->eflags & MAP_ENTRY_IS_SUB_MAP) == 0) {
                        obj = tmpe->object.vm_object;
                        if (obj != NULL) {
                                VM_OBJECT_LOCK(obj);
                                vm_pageout_object_deactivate_pages(map->pmap, obj, desired);
                                VM_OBJECT_UNLOCK(obj);
                        }
                }
                tmpe = tmpe->next;
        }

        /*
         * Remove all mappings if a process is swapped out, this will free page
         * table pages.
         */
        if (desired == 0 && nothingwired) {
                pmap_remove(vm_map_pmap(map), vm_map_min(map),
                    vm_map_max(map));
        }
        vm_map_unlock(map);
}
#endif          /* !defined(NO_SWAPPING) */

/*
 *      vm_pageout_scan does the dirty work for the pageout daemon.
 */
static void
vm_pageout_scan(int pass)
{
        vm_page_t m, next;
        struct vm_page marker;
        int page_shortage, maxscan, pcount;
        int addl_page_shortage, addl_page_shortage_init;
        struct proc *p, *bigproc;
        struct thread *td;
        vm_offset_t size, bigsize;
        vm_object_t object;
        int actcount;
        int vnodes_skipped = 0;
        int maxlaunder;

        /*
         * Decrease registered cache sizes.
         */
        EVENTHANDLER_INVOKE(vm_lowmem, 0);
        /*
         * We do this explicitly after the caches have been drained above.
         */
        uma_reclaim();

        addl_page_shortage_init = atomic_readandclear_int(&vm_pageout_deficit);

        /*
         * Calculate the number of pages we want to either free or move
         * to the cache.
         */
        page_shortage = vm_paging_target() + addl_page_shortage_init;

        /*
         * Initialize our marker
         */
        bzero(&marker, sizeof(marker));
        marker.flags = PG_FICTITIOUS | PG_MARKER;
        marker.oflags = VPO_BUSY;
        marker.queue = PQ_INACTIVE;
        marker.wire_count = 1;

        /*
         * Start scanning the inactive queue for pages we can move to the
         * cache or free.  The scan will stop when the target is reached or
         * we have scanned the entire inactive queue.  Note that m->act_count
         * is not used to form decisions for the inactive queue, only for the
         * active queue.
         *
         * maxlaunder limits the number of dirty pages we flush per scan.
         * For most systems a smaller value (16 or 32) is more robust under
         * extreme memory and disk pressure because any unnecessary writes
         * to disk can result in extreme performance degredation.  However,
         * systems with excessive dirty pages (especially when MAP_NOSYNC is
         * used) will die horribly with limited laundering.  If the pageout
         * daemon cannot clean enough pages in the first pass, we let it go
         * all out in succeeding passes.
         */
        if ((maxlaunder = vm_max_launder) <= 1)
                maxlaunder = 1;
        if (pass)
                maxlaunder = 10000;
        vm_page_lock_queues();
rescan0:
        addl_page_shortage = addl_page_shortage_init;
        maxscan = cnt.v_inactive_count;

        for (m = TAILQ_FIRST(&vm_page_queues[PQ_INACTIVE].pl);
             m != NULL && maxscan-- > 0 && page_shortage > 0;
             m = next) {

                cnt.v_pdpages++;

                if (VM_PAGE_GETQUEUE(m) != PQ_INACTIVE) {
                        goto rescan0;
                }

                next = TAILQ_NEXT(m, pageq);
                object = m->object;

                /*
                 * skip marker pages
                 */
                if (m->flags & PG_MARKER)
                        continue;

                /*
                 * A held page may be undergoing I/O, so skip it.
                 */
                if (m->hold_count) {
                        vm_page_requeue(m);
                        addl_page_shortage++;
                        continue;
                }
                /*
                 * Don't mess with busy pages, keep in the front of the
                 * queue, most likely are being paged out.
                 */
                if (!VM_OBJECT_TRYLOCK(object) &&
                    (!vm_pageout_fallback_object_lock(m, &next) ||
                     m->hold_count != 0)) {
                        VM_OBJECT_UNLOCK(object);
                        addl_page_shortage++;
                        continue;
                }
                if (m->busy || (m->oflags & VPO_BUSY)) {
                        VM_OBJECT_UNLOCK(object);
                        addl_page_shortage++;
                        continue;
                }

                /*
                 * If the object is not being used, we ignore previous 
                 * references.
                 */
                if (object->ref_count == 0) {
                        vm_page_flag_clear(m, PG_REFERENCED);
                        pmap_clear_reference(m);

                /*
                 * Otherwise, if the page has been referenced while in the 
                 * inactive queue, we bump the "activation count" upwards, 
                 * making it less likely that the page will be added back to 
                 * the inactive queue prematurely again.  Here we check the 
                 * page tables (or emulated bits, if any), given the upper 
                 * level VM system not knowing anything about existing 
                 * references.
                 */
                } else if (((m->flags & PG_REFERENCED) == 0) &&
                        (actcount = pmap_ts_referenced(m))) {
                        vm_page_activate(m);
                        VM_OBJECT_UNLOCK(object);
                        m->act_count += (actcount + ACT_ADVANCE);
                        continue;
                }

                /*
                 * If the upper level VM system knows about any page 
                 * references, we activate the page.  We also set the 
                 * "activation count" higher than normal so that we will less 
                 * likely place pages back onto the inactive queue again.
                 */
                if ((m->flags & PG_REFERENCED) != 0) {
                        vm_page_flag_clear(m, PG_REFERENCED);
                        actcount = pmap_ts_referenced(m);
                        vm_page_activate(m);
                        VM_OBJECT_UNLOCK(object);
                        m->act_count += (actcount + ACT_ADVANCE + 1);
                        continue;
                }

                /*
                 * If the upper level VM system doesn't know anything about 
                 * the page being dirty, we have to check for it again.  As 
                 * far as the VM code knows, any partially dirty pages are 
                 * fully dirty.
                 */
                if (m->dirty == 0 && !pmap_is_modified(m)) {
                        /*
                         * Avoid a race condition: Unless write access is
                         * removed from the page, another processor could
                         * modify it before all access is removed by the call
                         * to vm_page_cache() below.  If vm_page_cache() finds
                         * that the page has been modified when it removes all
                         * access, it panics because it cannot cache dirty
                         * pages.  In principle, we could eliminate just write
                         * access here rather than all access.  In the expected
                         * case, when there are no last instant modifications
                         * to the page, removing all access will be cheaper
                         * overall.
                         */
                        if ((m->flags & PG_WRITEABLE) != 0)
                                pmap_remove_all(m);
                } else {
                        vm_page_dirty(m);
                }

                if (m->valid == 0) {
                        /*
                         * Invalid pages can be easily freed
                         */
                        vm_page_free(m);
                        cnt.v_dfree++;
                        --page_shortage;
                } else if (m->dirty == 0) {
                        /*
                         * Clean pages can be placed onto the cache queue.
                         * This effectively frees them.
                         */
                        vm_page_cache(m);
                        --page_shortage;
                } else if ((m->flags & PG_WINATCFLS) == 0 && pass == 0) {
                        /*
                         * Dirty pages need to be paged out, but flushing
                         * a page is extremely expensive verses freeing
                         * a clean page.  Rather then artificially limiting
                         * the number of pages we can flush, we instead give
                         * dirty pages extra priority on the inactive queue
                         * by forcing them to be cycled through the queue
                         * twice before being flushed, after which the
                         * (now clean) page will cycle through once more
                         * before being freed.  This significantly extends
                         * the thrash point for a heavily loaded machine.
                         */
                        vm_page_flag_set(m, PG_WINATCFLS);
                        vm_page_requeue(m);
                } else if (maxlaunder > 0) {
                        /*
                         * We always want to try to flush some dirty pages if
                         * we encounter them, to keep the system stable.
                         * Normally this number is small, but under extreme
                         * pressure where there are insufficient clean pages
                         * on the inactive queue, we may have to go all out.
                         */
                        int swap_pageouts_ok, vfslocked = 0;
                        struct vnode *vp = NULL;
                        struct mount *mp = NULL;

                        if ((object->type != OBJT_SWAP) && (object->type != OBJT_DEFAULT)) {
                                swap_pageouts_ok = 1;
                        } else {
                                swap_pageouts_ok = !(defer_swap_pageouts || disable_swap_pageouts);
                                swap_pageouts_ok |= (!disable_swap_pageouts && defer_swap_pageouts &&
                                vm_page_count_min());
                                                                                
                        }

                        /*
                         * We don't bother paging objects that are "dead".  
                         * Those objects are in a "rundown" state.
                         */
                        if (!swap_pageouts_ok || (object->flags & OBJ_DEAD)) {
                                VM_OBJECT_UNLOCK(object);
                                vm_page_requeue(m);
                                continue;
                        }

                        /*
                         * Following operations may unlock
                         * vm_page_queue_mtx, invalidating the 'next'
                         * pointer.  To prevent an inordinate number
                         * of restarts we use our marker to remember
                         * our place.
                         *
                         */
                        TAILQ_INSERT_AFTER(&vm_page_queues[PQ_INACTIVE].pl,
                                           m, &marker, pageq);
                        /*
                         * The object is already known NOT to be dead.   It
                         * is possible for the vget() to block the whole
                         * pageout daemon, but the new low-memory handling
                         * code should prevent it.
                         *
                         * The previous code skipped locked vnodes and, worse,
                         * reordered pages in the queue.  This results in
                         * completely non-deterministic operation and, on a
                         * busy system, can lead to extremely non-optimal
                         * pageouts.  For example, it can cause clean pages
                         * to be freed and dirty pages to be moved to the end
                         * of the queue.  Since dirty pages are also moved to
                         * the end of the queue once-cleaned, this gives
                         * way too large a weighting to defering the freeing
                         * of dirty pages.
                         *
                         * We can't wait forever for the vnode lock, we might
                         * deadlock due to a vn_read() getting stuck in
                         * vm_wait while holding this vnode.  We skip the 
                         * vnode if we can't get it in a reasonable amount
                         * of time.
                         */
                        if (object->type == OBJT_VNODE) {
                                vp = object->handle;
                                if (vp->v_type == VREG &&
                                    vn_start_write(vp, &mp, V_NOWAIT) != 0) {
                                        KASSERT(mp == NULL,
                                            ("vm_pageout_scan: mp != NULL"));
                                        ++pageout_lock_miss;
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        goto unlock_and_continue;
                                }
                                vm_page_unlock_queues();
                                vm_object_reference_locked(object);
                                VM_OBJECT_UNLOCK(object);
                                vfslocked = VFS_LOCK_GIANT(vp->v_mount);
                                if (vget(vp, LK_EXCLUSIVE | LK_TIMELOCK,
                                    curthread)) {
                                        VM_OBJECT_LOCK(object);
                                        vm_page_lock_queues();
                                        ++pageout_lock_miss;
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        vp = NULL;
                                        goto unlock_and_continue;
                                }
                                VM_OBJECT_LOCK(object);
                                vm_page_lock_queues();
                                /*
                                 * The page might have been moved to another
                                 * queue during potential blocking in vget()
                                 * above.  The page might have been freed and
                                 * reused for another vnode.
                                 */
                                if (VM_PAGE_GETQUEUE(m) != PQ_INACTIVE ||
                                    m->object != object ||
                                    TAILQ_NEXT(m, pageq) != &marker) {
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        goto unlock_and_continue;
                                }
        
                                /*
                                 * The page may have been busied during the
                                 * blocking in vget().  We don't move the
                                 * page back onto the end of the queue so that
                                 * statistics are more correct if we don't.
                                 */
                                if (m->busy || (m->oflags & VPO_BUSY)) {
                                        goto unlock_and_continue;
                                }

                                /*
                                 * If the page has become held it might
                                 * be undergoing I/O, so skip it
                                 */
                                if (m->hold_count) {
                                        vm_page_requeue(m);
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        goto unlock_and_continue;
                                }
                        }

                        /*
                         * If a page is dirty, then it is either being washed
                         * (but not yet cleaned) or it is still in the
                         * laundry.  If it is still in the laundry, then we
                         * start the cleaning operation. 
                         *
                         * decrement page_shortage on success to account for
                         * the (future) cleaned page.  Otherwise we could wind
                         * up laundering or cleaning too many pages.
                         */
                        if (vm_pageout_clean(m) != 0) {
                                --page_shortage;
                                --maxlaunder;
                        }
unlock_and_continue:
                        VM_OBJECT_UNLOCK(object);
                        if (mp != NULL) {
                                vm_page_unlock_queues();
                                if (vp != NULL)
                                        vput(vp);
                                VFS_UNLOCK_GIANT(vfslocked);
                                vm_object_deallocate(object);
                                vn_finished_write(mp);
                                vm_page_lock_queues();
                        }
                        next = TAILQ_NEXT(&marker, pageq);
                        TAILQ_REMOVE(&vm_page_queues[PQ_INACTIVE].pl,
                                     &marker, pageq);
                        continue;
                }
                VM_OBJECT_UNLOCK(object);
        }

        /*
         * Compute the number of pages we want to try to move from the
         * active queue to the inactive queue.
         */
        page_shortage = vm_paging_target() +
                cnt.v_inactive_target - cnt.v_inactive_count;
        page_shortage += addl_page_shortage;

        /*
         * Scan the active queue for things we can deactivate. We nominally
         * track the per-page activity counter and use it to locate
         * deactivation candidates.
         */
        pcount = cnt.v_active_count;
        m = TAILQ_FIRST(&vm_page_queues[PQ_ACTIVE].pl);

        while ((m != NULL) && (pcount-- > 0) && (page_shortage > 0)) {

                KASSERT(VM_PAGE_INQUEUE2(m, PQ_ACTIVE),
                    ("vm_pageout_scan: page %p isn't active", m));

                next = TAILQ_NEXT(m, pageq);
                object = m->object;
                if ((m->flags & PG_MARKER) != 0) {
                        m = next;
                        continue;
                }
                if (!VM_OBJECT_TRYLOCK(object) &&
                    !vm_pageout_fallback_object_lock(m, &next)) {
                        VM_OBJECT_UNLOCK(object);
                        m = next;
                        continue;
                }

                /*
                 * Don't deactivate pages that are busy.
                 */
                if ((m->busy != 0) ||
                    (m->oflags & VPO_BUSY) ||
                    (m->hold_count != 0)) {
                        VM_OBJECT_UNLOCK(object);
                        vm_page_requeue(m);
                        m = next;
                        continue;
                }

                /*
                 * The count for pagedaemon pages is done after checking the
                 * page for eligibility...
                 */
                cnt.v_pdpages++;

                /*
                 * Check to see "how much" the page has been used.
                 */
                actcount = 0;
                if (object->ref_count != 0) {
                        if (m->flags & PG_REFERENCED) {
                                actcount += 1;
                        }
                        actcount += pmap_ts_referenced(m);
                        if (actcount) {
                                m->act_count += ACT_ADVANCE + actcount;
                                if (m->act_count > ACT_MAX)
                                        m->act_count = ACT_MAX;
                        }
                }

                /*
                 * Since we have "tested" this bit, we need to clear it now.
                 */
                vm_page_flag_clear(m, PG_REFERENCED);

                /*
                 * Only if an object is currently being used, do we use the
                 * page activation count stats.
                 */
                if (actcount && (object->ref_count != 0)) {
                        vm_page_requeue(m);
                } else {
                        m->act_count -= min(m->act_count, ACT_DECLINE);
                        if (vm_pageout_algorithm ||
                            object->ref_count == 0 ||
                            m->act_count == 0) {
                                page_shortage--;
                                if (object->ref_count == 0) {
                                        pmap_remove_all(m);
                                        if (m->dirty == 0)
                                                vm_page_cache(m);
                                        else
                                                vm_page_deactivate(m);
                                } else {
                                        vm_page_deactivate(m);
                                }
                        } else {
                                vm_page_requeue(m);
                        }
                }
                VM_OBJECT_UNLOCK(object);
                m = next;
        }
        vm_page_unlock_queues();
#if !defined(NO_SWAPPING)
        /*
         * Idle process swapout -- run once per second.
         */
        if (vm_swap_idle_enabled) {
                static long lsec;
                if (time_second != lsec) {
                        vm_req_vmdaemon(VM_SWAP_IDLE);
                        lsec = time_second;
                }
        }
#endif
                
        /*
         * If we didn't get enough free pages, and we have skipped a vnode
         * in a writeable object, wakeup the sync daemon.  And kick swapout
         * if we did not get enough free pages.
         */
        if (vm_paging_target() > 0) {
                if (vnodes_skipped && vm_page_count_min())
                        (void) speedup_syncer();
#if !defined(NO_SWAPPING)
                if (vm_swap_enabled && vm_page_count_target())
                        vm_req_vmdaemon(VM_SWAP_NORMAL);
#endif
        }

        /*
         * If we are critically low on one of RAM or swap and low on
         * the other, kill the largest process.  However, we avoid
         * doing this on the first pass in order to give ourselves a
         * chance to flush out dirty vnode-backed pages and to allow
         * active pages to be moved to the inactive queue and reclaimed.
         *
         * We keep the process bigproc locked once we find it to keep anyone
         * from messing with it; however, there is a possibility of
         * deadlock if process B is bigproc and one of it's child processes
         * attempts to propagate a signal to B while we are waiting for A's
         * lock while walking this list.  To avoid this, we don't block on
         * the process lock but just skip a process if it is already locked.
         */
        if (pass != 0 &&
            ((swap_pager_avail < 64 && vm_page_count_min()) ||
             (swap_pager_full && vm_paging_target() > 0))) {
                bigproc = NULL;
                bigsize = 0;
                sx_slock(&allproc_lock);
                FOREACH_PROC_IN_SYSTEM(p) {
                        int breakout;

                        if (PROC_TRYLOCK(p) == 0)
                                continue;
                        /*
                         * If this is a system or protected process, skip it.
                         */
                        if ((p->p_flag & P_SYSTEM) || (p->p_pid == 1) ||
                            (p->p_flag & P_PROTECTED) ||
                            ((p->p_pid < 48) && (swap_pager_avail != 0))) {
                                PROC_UNLOCK(p);
                                continue;
                        }
                        /*
                         * If the process is in a non-running type state,
                         * don't touch it.  Check all the threads individually.
                         */
                        breakout = 0;
                        FOREACH_THREAD_IN_PROC(p, td) {
                                thread_lock(td);
                                if (!TD_ON_RUNQ(td) &&
                                    !TD_IS_RUNNING(td) &&
                                    !TD_IS_SLEEPING(td)) {
                                        thread_unlock(td);
                                        breakout = 1;
                                        break;
                                }
                                thread_unlock(td);
                        }
                        if (breakout) {
                                PROC_UNLOCK(p);
                                continue;
                        }
                        /*
                         * get the process size
                         */
                        if (!vm_map_trylock_read(&p->p_vmspace->vm_map)) {
                                PROC_UNLOCK(p);
                                continue;
                        }
                        size = vmspace_swap_count(p->p_vmspace);
                        vm_map_unlock_read(&p->p_vmspace->vm_map);
                        size += vmspace_resident_count(p->p_vmspace);
                        /*
                         * if the this process is bigger than the biggest one
                         * remember it.
                         */
                        if (size > bigsize) {
                                if (bigproc != NULL)
                                        PROC_UNLOCK(bigproc);
                                bigproc = p;
                                bigsize = size;
                        } else
                                PROC_UNLOCK(p);
                }
                sx_sunlock(&allproc_lock);
                if (bigproc != NULL) {
                        killproc(bigproc, "out of swap space");
                        sched_nice(bigproc, PRIO_MIN);
                        PROC_UNLOCK(bigproc);
                        wakeup(&cnt.v_free_count);
                }
        }
}

/*
 * This routine tries to maintain the pseudo LRU active queue,
 * so that during long periods of time where there is no paging,
 * that some statistic accumulation still occurs.  This code
 * helps the situation where paging just starts to occur.
 */
static void
vm_pageout_page_stats()
{
        vm_object_t object;
        vm_page_t m,next;
        int pcount,tpcount;             /* Number of pages to check */
        static int fullintervalcount = 0;
        int page_shortage;

        mtx_assert(&vm_page_queue_mtx, MA_OWNED);
        page_shortage = 
            (cnt.v_inactive_target + cnt.v_cache_max + cnt.v_free_min) -
            (cnt.v_free_count + cnt.v_inactive_count + cnt.v_cache_count);

        if (page_shortage <= 0)
                return;

        pcount = cnt.v_active_count;
        fullintervalcount += vm_pageout_stats_interval;
        if (fullintervalcount < vm_pageout_full_stats_interval) {
                tpcount = (vm_pageout_stats_max * cnt.v_active_count) / cnt.v_page_count;
                if (pcount > tpcount)
                        pcount = tpcount;
        } else {
                fullintervalcount = 0;
        }

        m = TAILQ_FIRST(&vm_page_queues[PQ_ACTIVE].pl);
        while ((m != NULL) && (pcount-- > 0)) {
                int actcount;

                KASSERT(VM_PAGE_INQUEUE2(m, PQ_ACTIVE),
                    ("vm_pageout_page_stats: page %p isn't active", m));

                next = TAILQ_NEXT(m, pageq);
                object = m->object;

                if ((m->flags & PG_MARKER) != 0) {
                        m = next;
                        continue;
                }
                if (!VM_OBJECT_TRYLOCK(object) &&
                    !vm_pageout_fallback_object_lock(m, &next)) {
                        VM_OBJECT_UNLOCK(object);
                        m = next;
                        continue;
                }

                /*
                 * Don't deactivate pages that are busy.
                 */
                if ((m->busy != 0) ||
                    (m->oflags & VPO_BUSY) ||
                    (m->hold_count != 0)) {
                        VM_OBJECT_UNLOCK(object);
                        vm_page_requeue(m);
                        m = next;
                        continue;
                }

                actcount = 0;
                if (m->flags & PG_REFERENCED) {
                        vm_page_flag_clear(m, PG_REFERENCED);
                        actcount += 1;
                }

                actcount += pmap_ts_referenced(m);
                if (actcount) {
                        m->act_count += ACT_ADVANCE + actcount;
                        if (m->act_count > ACT_MAX)
                                m->act_count = ACT_MAX;
                        vm_page_requeue(m);
                } else {
                        if (m->act_count == 0) {
                                /*
                                 * We turn off page access, so that we have
                                 * more accurate RSS stats.  We don't do this
                                 * in the normal page deactivation when the
                                 * system is loaded VM wise, because the
                                 * cost of the large number of page protect
                                 * operations would be higher than the value
                                 * of doing the operation.
                                 */
                                pmap_remove_all(m);
                                vm_page_deactivate(m);
                        } else {
                                m->act_count -= min(m->act_count, ACT_DECLINE);
                                vm_page_requeue(m);
                        }
                }
                VM_OBJECT_UNLOCK(object);
                m = next;
        }
}

/*
 *      vm_pageout is the high level pageout daemon.
 */
static void
vm_pageout()
{
        int error, pass;

        /*
         * Initialize some paging parameters.
         */
        cnt.v_interrupt_free_min = 2;
        if (cnt.v_page_count < 2000)
                vm_pageout_page_count = 8;

        /*
         * v_free_reserved needs to include enough for the largest
         * swap pager structures plus enough for any pv_entry structs
         * when paging. 
         */
        if (cnt.v_page_count > 1024)
                cnt.v_free_min = 4 + (cnt.v_page_count - 1024) / 200;
        else
                cnt.v_free_min = 4;
        cnt.v_pageout_free_min = (2*MAXBSIZE)/PAGE_SIZE +
            cnt.v_interrupt_free_min;
        cnt.v_free_reserved = vm_pageout_page_count +
            cnt.v_pageout_free_min + (cnt.v_page_count / 768);
        cnt.v_free_severe = cnt.v_free_min / 2;
        cnt.v_free_min += cnt.v_free_reserved;
        cnt.v_free_severe += cnt.v_free_reserved;

        /*
         * v_free_target and v_cache_min control pageout hysteresis.  Note
         * that these are more a measure of the VM cache queue hysteresis
         * then the VM free queue.  Specifically, v_free_target is the
         * high water mark (free+cache pages).
         *
         * v_free_reserved + v_cache_min (mostly means v_cache_min) is the
         * low water mark, while v_free_min is the stop.  v_cache_min must
         * be big enough to handle memory needs while the pageout daemon
         * is signalled and run to free more pages.
         */
        if (cnt.v_free_count > 6144)
                cnt.v_free_target = 4 * cnt.v_free_min + cnt.v_free_reserved;
        else
                cnt.v_free_target = 2 * cnt.v_free_min + cnt.v_free_reserved;

        if (cnt.v_free_count > 2048) {
                cnt.v_cache_min = cnt.v_free_target;
                cnt.v_cache_max = 2 * cnt.v_cache_min;
                cnt.v_inactive_target = (3 * cnt.v_free_target) / 2;
        } else {
                cnt.v_cache_min = 0;
                cnt.v_cache_max = 0;
                cnt.v_inactive_target = cnt.v_free_count / 4;
        }
        if (cnt.v_inactive_target > cnt.v_free_count / 3)
                cnt.v_inactive_target = cnt.v_free_count / 3;

        /* XXX does not really belong here */
        if (vm_page_max_wired == 0)
                vm_page_max_wired = cnt.v_free_count / 3;

        if (vm_pageout_stats_max == 0)
                vm_pageout_stats_max = cnt.v_free_target;

        /*
         * Set interval in seconds for stats scan.
         */
        if (vm_pageout_stats_interval == 0)
                vm_pageout_stats_interval = 5;
        if (vm_pageout_full_stats_interval == 0)
                vm_pageout_full_stats_interval = vm_pageout_stats_interval * 4;

        swap_pager_swap_init();
        pass = 0;
        /*
         * The pageout daemon is never done, so loop forever.
         */
        while (TRUE) {
                /*
                 * If we have enough free memory, wakeup waiters.  Do
                 * not clear vm_pages_needed until we reach our target,
                 * otherwise we may be woken up over and over again and
                 * waste a lot of cpu.
                 */
                mtx_lock(&vm_page_queue_free_mtx);
                if (vm_pages_needed && !vm_page_count_min()) {
                        if (!vm_paging_needed())
                                vm_pages_needed = 0;
                        wakeup(&cnt.v_free_count);
                }
                if (vm_pages_needed) {
                        /*
                         * Still not done, take a second pass without waiting
                         * (unlimited dirty cleaning), otherwise sleep a bit
                         * and try again.
                         */
                        ++pass;
                        if (pass > 1)
                                msleep(&vm_pages_needed,
                                    &vm_page_queue_free_mtx, PVM, "psleep",
                                    hz / 2);
                } else {
                        /*
                         * Good enough, sleep & handle stats.  Prime the pass
                         * for the next run.
                         */
                        if (pass > 1)
                                pass = 1;
                        else
                                pass = 0;
                        error = msleep(&vm_pages_needed,
                            &vm_page_queue_free_mtx, PVM, "psleep",
                            vm_pageout_stats_interval * hz);
                        if (error && !vm_pages_needed) {
                                mtx_unlock(&vm_page_queue_free_mtx);
                                pass = 0;
                                vm_page_lock_queues();
                                vm_pageout_page_stats();
                                vm_page_unlock_queues();
                                continue;
                        }
                }
                if (vm_pages_needed)
                        cnt.v_pdwakeups++;
                mtx_unlock(&vm_page_queue_free_mtx);
                vm_pageout_scan(pass);
        }
}

/*
 * Unless the free page queue lock is held by the caller, this function
 * should be regarded as advisory.  Specifically, the caller should
 * not msleep() on &cnt.v_free_count following this function unless
 * the free page queue lock is held until the msleep() is performed.
 */
void
pagedaemon_wakeup()
{

        if (!vm_pages_needed && curthread->td_proc != pageproc) {
                vm_pages_needed = 1;
                wakeup(&vm_pages_needed);
        }
}

#if !defined(NO_SWAPPING)
static void
vm_req_vmdaemon(int req)
{
        static int lastrun = 0;

        mtx_lock(&vm_daemon_mtx);
        vm_pageout_req_swapout |= req;
        if ((ticks > (lastrun + hz)) || (ticks < lastrun)) {
                wakeup(&vm_daemon_needed);
                lastrun = ticks;
        }
        mtx_unlock(&vm_daemon_mtx);
}

static void
vm_daemon()
{
        struct rlimit rsslim;
        struct proc *p;
        struct thread *td;
        int breakout, swapout_flags;

        while (TRUE) {
                mtx_lock(&vm_daemon_mtx);
                msleep(&vm_daemon_needed, &vm_daemon_mtx, PPAUSE, "psleep", 0);
                swapout_flags = vm_pageout_req_swapout;
                vm_pageout_req_swapout = 0;
                mtx_unlock(&vm_daemon_mtx);
                if (swapout_flags)
                        swapout_procs(swapout_flags);

                /*
                 * scan the processes for exceeding their rlimits or if
                 * process is swapped out -- deactivate pages
                 */
                sx_slock(&allproc_lock);
                FOREACH_PROC_IN_SYSTEM(p) {
                        vm_pindex_t limit, size;

                        /*
                         * if this is a system process or if we have already
                         * looked at this process, skip it.
                         */
                        PROC_LOCK(p);
                        if (p->p_flag & (P_SYSTEM | P_WEXIT)) {
                                PROC_UNLOCK(p);
                                continue;
                        }
                        /*
                         * if the process is in a non-running type state,
                         * don't touch it.
                         */
                        breakout = 0;
                        FOREACH_THREAD_IN_PROC(p, td) {
                                thread_lock(td);
                                if (!TD_ON_RUNQ(td) &&
                                    !TD_IS_RUNNING(td) &&
                                    !TD_IS_SLEEPING(td)) {
                                        thread_unlock(td);
                                        breakout = 1;
                                        break;
                                }
                                thread_unlock(td);
                        }
                        if (breakout) {
                                PROC_UNLOCK(p);
                                continue;
                        }
                        /*
                         * get a limit
                         */
                        lim_rlimit(p, RLIMIT_RSS, &rsslim);
                        limit = OFF_TO_IDX(
                            qmin(rsslim.rlim_cur, rsslim.rlim_max));

                        /*
                         * let processes that are swapped out really be
                         * swapped out set the limit to nothing (will force a
                         * swap-out.)
                         */
                        if ((p->p_flag & P_INMEM) == 0)
                                limit = 0;      /* XXX */
                        PROC_UNLOCK(p);

                        size = vmspace_resident_count(p->p_vmspace);
                        if (limit >= 0 && size >= limit) {
                                vm_pageout_map_deactivate_pages(
                                    &p->p_vmspace->vm_map, limit);
                        }
                }
                sx_sunlock(&allproc_lock);
        }
}
#endif                  /* !defined(NO_SWAPPING) */