Eliminate the conditional for releasing the page queues lock in

[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/racct.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>

/*
 * 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, "Enable entire process swapout");
SYSCTL_INT(_vm, OID_AUTO, swap_idle_enabled,
        CTLFLAG_RD, &vm_swap_idle_enabled, 0, "Allow swapout on idle criteria");
#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");

static boolean_t vm_pageout_fallback_object_lock(vm_page_t, vm_page_t *);
static boolean_t vm_pageout_launder(int, int, vm_paddr_t, vm_paddr_t);
#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 boolean_t vm_pageout_page_lock(vm_page_t, vm_page_t *);
static void vm_pageout_page_stats(void);

/*
 * Initialize a dummy page for marking the caller's place in the specified
 * paging queue.  In principle, this function only needs to set the flag
 * PG_MARKER.  Nonetheless, it sets the flag VPO_BUSY and initializes the hold
 * count to one as safety precautions.
 */ 
static void
vm_pageout_init_marker(vm_page_t marker, u_short queue)
{

        bzero(marker, sizeof(*marker));
        marker->flags = PG_MARKER;
        marker->oflags = VPO_BUSY;
        marker->queue = queue;
        marker->hold_count = 1;
}

/*
 * 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.
 */
static 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;

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

/*
 * Lock the page while holding the page queue lock.  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.  The page is locked on return. The page queue lock might
 * be dropped and reacquired.
 *
 * This function depends on normal struct vm_page being type stable.
 */
static boolean_t
vm_pageout_page_lock(vm_page_t m, vm_page_t *next)
{
        struct vm_page marker;
        boolean_t unchanged;
        u_short queue;

        vm_page_lock_assert(m, MA_NOTOWNED);
        mtx_assert(&vm_page_queue_mtx, MA_OWNED);

        if (vm_page_trylock(m))
                return (TRUE);

        queue = m->queue;
        vm_pageout_init_marker(&marker, queue);

        TAILQ_INSERT_AFTER(&vm_page_queues[queue].pl, m, &marker, pageq);
        vm_page_unlock_queues();
        vm_page_lock(m);
        vm_page_lock_queues();

        /* Page queue might have changed. */
        *next = TAILQ_NEXT(&marker, pageq);
        unchanged = (m->queue == queue && &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(vm_page_t m)
{
        vm_object_t object;
        vm_page_t mc[2*vm_pageout_page_count], pb, ps;
        int pageout_count;
        int ib, is, page_base;
        vm_pindex_t pindex = m->pindex;

        vm_page_lock_assert(m, MA_OWNED);
        object = m->object;
        VM_OBJECT_LOCK_ASSERT(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.
         */
        KASSERT(m->busy == 0 && (m->oflags & VPO_BUSY) == 0,
            ("vm_pageout_clean: page %p is busy", m));
        KASSERT(m->hold_count == 0, ("vm_pageout_clean: page %p is held", m));
        vm_page_unlock(m);

        mc[vm_pageout_page_count] = pb = ps = 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.
         */
more:
        while (ib && pageout_count < vm_pageout_page_count) {
                vm_page_t p;

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

                if ((p = vm_page_prev(pb)) == NULL ||
                    (p->oflags & VPO_BUSY) != 0 || p->busy != 0) {
                        ib = 0;
                        break;
                }
                vm_page_lock(p);
                vm_page_test_dirty(p);
                if (p->dirty == 0 ||
                    p->queue != PQ_INACTIVE ||
                    p->hold_count != 0) {       /* may be undergoing I/O */
                        vm_page_unlock(p);
                        ib = 0;
                        break;
                }
                vm_page_unlock(p);
                mc[--page_base] = pb = 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_next(ps)) == NULL ||
                    (p->oflags & VPO_BUSY) != 0 || p->busy != 0)
                        break;
                vm_page_lock(p);
                vm_page_test_dirty(p);
                if (p->dirty == 0 ||
                    p->queue != PQ_INACTIVE ||
                    p->hold_count != 0) {       /* may be undergoing I/O */
                        vm_page_unlock(p);
                        break;
                }
                vm_page_unlock(p);
                mc[page_base + pageout_count] = ps = 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, 0, NULL,
            NULL));
}

/*
 * 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.
 *
 *      Returned runlen is the count of pages between mreq and first
 *      page after mreq with status VM_PAGER_AGAIN.
 *      *eio is set to TRUE if pager returned VM_PAGER_ERROR or VM_PAGER_FAIL
 *      for any page in runlen set.
 */
int
vm_pageout_flush(vm_page_t *mc, int count, int flags, int mreq, int *prunlen,
    boolean_t *eio)
{
        vm_object_t object = mc[0]->object;
        int pageout_status[count];
        int numpagedout = 0;
        int i, runlen;

        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_object_pip_add(object, count);

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

        runlen = count - mreq;
        if (eio != NULL)
                *eio = FALSE;
        for (i = 0; i < count; i++) {
                vm_page_t mt = mc[i];

                KASSERT(pageout_status[i] == VM_PAGER_PEND ||
                    !pmap_page_is_write_mapped(mt),
                    ("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.
                         */
                        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_lock(mt);
                        vm_page_activate(mt);
                        vm_page_unlock(mt);
                        if (eio != NULL && i >= mreq && i - mreq < runlen)
                                *eio = TRUE;
                        break;
                case VM_PAGER_AGAIN:
                        if (i >= mreq && i - mreq < runlen)
                                runlen = i - mreq;
                        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_lock(mt);
                                vm_page_try_to_cache(mt);
                                vm_page_unlock(mt);
                        }
                }
        }
        if (prunlen != NULL)
                *prunlen = runlen;
        return (numpagedout);
}

static boolean_t
vm_pageout_launder(int queue, int tries, vm_paddr_t low, vm_paddr_t high)
{
        struct mount *mp;
        struct vnode *vp;
        vm_object_t object;
        vm_paddr_t pa;
        vm_page_t m, m_tmp, next;
        int vfslocked;

        vm_page_lock_queues();
        TAILQ_FOREACH_SAFE(m, &vm_page_queues[queue].pl, pageq, next) {
                KASSERT(m->queue == queue,
                    ("vm_pageout_launder: page %p's queue is not %d", m,
                    queue));
                if ((m->flags & PG_MARKER) != 0)
                        continue;
                pa = VM_PAGE_TO_PHYS(m);
                if (pa < low || pa + PAGE_SIZE > high)
                        continue;
                if (!vm_pageout_page_lock(m, &next) || m->hold_count != 0) {
                        vm_page_unlock(m);
                        continue;
                }
                object = m->object;
                if ((!VM_OBJECT_TRYLOCK(object) &&
                    (!vm_pageout_fallback_object_lock(m, &next) ||
                    m->hold_count != 0)) || (m->oflags & VPO_BUSY) != 0 ||
                    m->busy != 0) {
                        vm_page_unlock(m);
                        VM_OBJECT_UNLOCK(object);
                        continue;
                }
                vm_page_test_dirty(m);
                if (m->dirty == 0)
                        pmap_remove_all(m);
                if (m->dirty != 0) {
                        vm_page_unlock(m);
                        if (tries == 0 || (object->flags & OBJ_DEAD) != 0) {
                                VM_OBJECT_UNLOCK(object);
                                continue;
                        }
                        if (object->type == OBJT_VNODE) {
                                vm_page_unlock_queues();
                                vp = object->handle;
                                vm_object_reference_locked(object);
                                VM_OBJECT_UNLOCK(object);
                                (void)vn_start_write(vp, &mp, V_WAIT);
                                vfslocked = VFS_LOCK_GIANT(vp->v_mount);
                                vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
                                VM_OBJECT_LOCK(object);
                                vm_object_page_clean(object, 0, 0, OBJPC_SYNC);
                                VM_OBJECT_UNLOCK(object);
                                VOP_UNLOCK(vp, 0);
                                VFS_UNLOCK_GIANT(vfslocked);
                                vm_object_deallocate(object);
                                vn_finished_write(mp);
                                return (TRUE);
                        } else if (object->type == OBJT_SWAP ||
                            object->type == OBJT_DEFAULT) {
                                vm_page_unlock_queues();
                                m_tmp = m;
                                vm_pageout_flush(&m_tmp, 1, VM_PAGER_PUT_SYNC,
                                    0, NULL, NULL);
                                VM_OBJECT_UNLOCK(object);
                                return (TRUE);
                        }
                } else {
                        vm_page_cache(m);
                        vm_page_unlock(m);
                }
                VM_OBJECT_UNLOCK(object);
        }
        vm_page_unlock_queues();
        return (FALSE);
}

/*
 * Increase the number of cached pages.  The specified value, "tries",
 * determines which categories of pages are cached:
 *
 *  0: All clean, inactive pages within the specified physical address range
 *     are cached.  Will not sleep.
 *  1: The vm_lowmem handlers are called.  All inactive pages within
 *     the specified physical address range are cached.  May sleep.
 *  2: The vm_lowmem handlers are called.  All inactive and active pages
 *     within the specified physical address range are cached.  May sleep.
 */
void
vm_pageout_grow_cache(int tries, vm_paddr_t low, vm_paddr_t high)
{
        int actl, actmax, inactl, inactmax;

        if (tries > 0) {
                /*
                 * Decrease registered cache sizes.  The vm_lowmem handlers
                 * may acquire locks and/or sleep, so they can only be invoked
                 * when "tries" is greater than zero.
                 */
                EVENTHANDLER_INVOKE(vm_lowmem, 0);

                /*
                 * We do this explicitly after the caches have been drained
                 * above.
                 */
                uma_reclaim();
        }
        inactl = 0;
        inactmax = cnt.v_inactive_count;
        actl = 0;
        actmax = tries < 2 ? 0 : cnt.v_active_count;
again:
        if (inactl < inactmax && vm_pageout_launder(PQ_INACTIVE, tries, low,
            high)) {
                inactl++;
                goto again;
        }
        if (actl < actmax && vm_pageout_launder(PQ_ACTIVE, tries, low, high)) {
                actl++;
                goto again;
        }
}

#if !defined(NO_SWAPPING)
/*
 *      vm_pageout_object_deactivate_pages
 *
 *      Deactivate enough pages to satisfy the inactive target
 *      requirements.
 *
 *      The object and map must be locked.
 */
static void
vm_pageout_object_deactivate_pages(pmap_t pmap, vm_object_t first_object,
    long desired)
{
        vm_object_t backing_object, object;
        vm_page_t p;
        int actcount, remove_mode;

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

                remove_mode = 0;
                if (object->shadow_count > 1)
                        remove_mode = 1;
                /*
                 * Scan the object's entire memory queue.
                 */
                TAILQ_FOREACH(p, &object->memq, listq) {
                        if (pmap_resident_count(pmap) <= desired)
                                goto unlock_return;
                        if ((p->oflags & VPO_BUSY) != 0 || p->busy != 0)
                                continue;
                        PCPU_INC(cnt.v_pdpages);
                        vm_page_lock(p);
                        if (p->wire_count != 0 || p->hold_count != 0 ||
                            !pmap_page_exists_quick(pmap, p)) {
                                vm_page_unlock(p);
                                continue;
                        }
                        actcount = pmap_ts_referenced(p);
                        if ((p->aflags & PGA_REFERENCED) != 0) {
                                if (actcount == 0)
                                        actcount = 1;
                                vm_page_aflag_clear(p, PGA_REFERENCED);
                        }
                        if (p->queue != PQ_ACTIVE && actcount != 0) {
                                vm_page_activate(p);
                                p->act_count += actcount;
                        } else if (p->queue == PQ_ACTIVE) {
                                if (actcount == 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_lock_queues();
                                                vm_page_requeue(p);
                                                vm_page_unlock_queues();
                                        }
                                } else {
                                        vm_page_activate(p);
                                        if (p->act_count < ACT_MAX -
                                            ACT_ADVANCE)
                                                p->act_count += ACT_ADVANCE;
                                        vm_page_lock_queues();
                                        vm_page_requeue(p);
                                        vm_page_unlock_queues();
                                }
                        } else if (p->queue == PQ_INACTIVE)
                                pmap_remove_all(p);
                        vm_page_unlock(p);
                }
                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;
        vm_object_t object;
        int actcount;
        int vnodes_skipped = 0;
        int maxlaunder;
        boolean_t queues_locked;

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

        /*
         * The addl_page_shortage is the number of temporarily
         * stuck pages in the inactive queue.  In other words, the
         * number of pages from cnt.v_inactive_count that should be
         * discounted in setting the target for the active queue scan.
         */
        addl_page_shortage = 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;

        vm_pageout_init_marker(&marker, PQ_INACTIVE);

        /*
         * 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();
        queues_locked = TRUE;
        maxscan = cnt.v_inactive_count;

        for (m = TAILQ_FIRST(&vm_page_queues[PQ_INACTIVE].pl);
             m != NULL && maxscan-- > 0 && page_shortage > 0;
             m = next) {
                KASSERT(queues_locked, ("unlocked queues"));
                mtx_assert(&vm_page_queue_mtx, MA_OWNED);
                KASSERT(m->queue == PQ_INACTIVE, ("Inactive queue %p", m));

                cnt.v_pdpages++;
                next = TAILQ_NEXT(m, pageq);

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

                KASSERT((m->flags & PG_FICTITIOUS) == 0,
                    ("Fictitious page %p cannot be in inactive queue", m));
                KASSERT((m->oflags & VPO_UNMANAGED) == 0,
                    ("Unmanaged page %p cannot be in inactive queue", m));

                /*
                 * The page or object lock acquisitions fail if the
                 * page was removed from the queue or moved to a
                 * different position within the queue.  In either
                 * case, addl_page_shortage should not be incremented.
                 */
                if (!vm_pageout_page_lock(m, &next)) {
                        vm_page_unlock(m);
                        continue;
                }
                object = m->object;
                if (!VM_OBJECT_TRYLOCK(object) &&
                    !vm_pageout_fallback_object_lock(m, &next)) {
                        vm_page_unlock(m);
                        VM_OBJECT_UNLOCK(object);
                        continue;
                }

                /*
                 * Don't mess with busy pages, keep them at at the
                 * front of the queue, most likely they are being
                 * paged out.  Increment addl_page_shortage for busy
                 * pages, because they may leave the inactive queue
                 * shortly after page scan is finished.
                 */
                if (m->busy != 0 || (m->oflags & VPO_BUSY) != 0) {
                        vm_page_unlock(m);
                        VM_OBJECT_UNLOCK(object);
                        addl_page_shortage++;
                        continue;
                }

                /*
                 * We unlock vm_page_queue_mtx, invalidating the
                 * 'next' pointer.  Use our marker to remember our
                 * place.
                 */
                TAILQ_INSERT_AFTER(&vm_page_queues[PQ_INACTIVE].pl,
                    m, &marker, pageq);
                vm_page_unlock_queues();
                queues_locked = FALSE;

                /*
                 * If the object is not being used, we ignore previous 
                 * references.
                 */
                if (object->ref_count == 0) {
                        vm_page_aflag_clear(m, PGA_REFERENCED);
                        KASSERT(!pmap_page_is_mapped(m),
                            ("vm_pageout_scan: page %p is mapped", 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->aflags & PGA_REFERENCED) == 0 &&
                    (actcount = pmap_ts_referenced(m)) != 0) {
                        vm_page_activate(m);
                        vm_page_unlock(m);
                        m->act_count += actcount + ACT_ADVANCE;
                        VM_OBJECT_UNLOCK(object);
                        goto relock_queues;
                }

                /*
                 * 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->aflags & PGA_REFERENCED) != 0) {
                        vm_page_aflag_clear(m, PGA_REFERENCED);
                        actcount = pmap_ts_referenced(m);
                        vm_page_activate(m);
                        vm_page_unlock(m);
                        m->act_count += actcount + ACT_ADVANCE + 1;
                        VM_OBJECT_UNLOCK(object);
                        goto relock_queues;
                }

                if (m->hold_count != 0) {
                        vm_page_unlock(m);
                        VM_OBJECT_UNLOCK(object);

                        /*
                         * Held pages are essentially stuck in the
                         * queue.  So, they ought to be discounted
                         * from cnt.v_inactive_count.  See the
                         * calculation of the page_shortage for the
                         * loop over the active queue below.
                         */
                        addl_page_shortage++;
                        goto relock_queues;
                }

                /*
                 * If the upper level VM system does not believe that the page
                 * is fully dirty, but it is mapped for write access, then we
                 * consult the pmap to see if the page's dirty status should
                 * be updated.
                 */
                if (m->dirty != VM_PAGE_BITS_ALL &&
                    pmap_page_is_write_mapped(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 (pmap_is_modified(m))
                                vm_page_dirty(m);
                        else if (m->dirty == 0)
                                pmap_remove_all(m);
                }

                if (m->valid == 0) {
                        /*
                         * Invalid pages can be easily freed
                         */
                        vm_page_free(m);
                        PCPU_INC(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.
                         */
                        m->flags |= PG_WINATCFLS;
                        vm_page_lock_queues();
                        queues_locked = TRUE;
                        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_page_lock_queues();
                                vm_page_unlock(m);
                                VM_OBJECT_UNLOCK(object);
                                queues_locked = TRUE;
                                vm_page_requeue(m);
                                goto relock_queues;
                        }

                        /*
                         * 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) {
                                vm_page_unlock(m);
                                vp = object->handle;
                                if (vp->v_type == VREG &&
                                    vn_start_write(vp, &mp, V_NOWAIT) != 0) {
                                        mp = NULL;
                                        ++pageout_lock_miss;
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        goto unlock_and_continue;
                                }
                                KASSERT(mp != NULL,
                                    ("vp %p with NULL v_mount", vp));
                                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);
                                        ++pageout_lock_miss;
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        vp = NULL;
                                        goto unlock_and_continue;
                                }
                                VM_OBJECT_LOCK(object);
                                vm_page_lock(m);
                                vm_page_lock_queues();
                                queues_locked = TRUE;
                                /*
                                 * 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 (m->queue != PQ_INACTIVE ||
                                    m->object != object ||
                                    TAILQ_NEXT(m, pageq) != &marker) {
                                        vm_page_unlock(m);
                                        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)) {
                                        vm_page_unlock(m);
                                        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_unlock(m);
                                        vm_page_requeue(m);
                                        if (object->flags & OBJ_MIGHTBEDIRTY)
                                                vnodes_skipped++;
                                        goto unlock_and_continue;
                                }
                                vm_page_unlock_queues();
                                queues_locked = FALSE;
                        }

                        /*
                         * 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_page_lock_assert(m, MA_NOTOWNED);
                        VM_OBJECT_UNLOCK(object);
                        if (mp != NULL) {
                                if (queues_locked) {
                                        vm_page_unlock_queues();
                                        queues_locked = FALSE;
                                }
                                if (vp != NULL)
                                        vput(vp);
                                VFS_UNLOCK_GIANT(vfslocked);
                                vm_object_deallocate(object);
                                vn_finished_write(mp);
                        }
                        vm_page_lock_assert(m, MA_NOTOWNED);
                        goto relock_queues;
                }
                vm_page_unlock(m);
                VM_OBJECT_UNLOCK(object);
relock_queues:
                if (!queues_locked) {
                        vm_page_lock_queues();
                        queues_locked = TRUE;
                }
                next = TAILQ_NEXT(&marker, pageq);
                TAILQ_REMOVE(&vm_page_queues[PQ_INACTIVE].pl,
                    &marker, pageq);
        }

        /*
         * 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);
        mtx_assert(&vm_page_queue_mtx, MA_OWNED);

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

                KASSERT(m->queue == PQ_ACTIVE,
                    ("vm_pageout_scan: page %p isn't active", m));

                next = TAILQ_NEXT(m, pageq);
                if ((m->flags & PG_MARKER) != 0) {
                        m = next;
                        continue;
                }
                KASSERT((m->flags & PG_FICTITIOUS) == 0,
                    ("Fictitious page %p cannot be in active queue", m));
                KASSERT((m->oflags & VPO_UNMANAGED) == 0,
                    ("Unmanaged page %p cannot be in active queue", m));
                if (!vm_pageout_page_lock(m, &next)) {
                        vm_page_unlock(m);
                        m = next;
                        continue;
                }
                object = m->object;
                if (!VM_OBJECT_TRYLOCK(object) &&
                    !vm_pageout_fallback_object_lock(m, &next)) {
                        VM_OBJECT_UNLOCK(object);
                        vm_page_unlock(m);
                        m = next;
                        continue;
                }

                /*
                 * Don't deactivate pages that are busy.
                 */
                if ((m->busy != 0) ||
                    (m->oflags & VPO_BUSY) ||
                    (m->hold_count != 0)) {
                        vm_page_unlock(m);
                        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->aflags & PGA_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_aflag_clear(m, PGA_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) {
                                        KASSERT(!pmap_page_is_mapped(m),
                                    ("vm_pageout_scan: page %p is mapped", 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_page_unlock(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.
         */
        if (pass != 0 &&
            ((swap_pager_avail < 64 && vm_page_count_min()) ||
             (swap_pager_full && vm_paging_target() > 0)))
                vm_pageout_oom(VM_OOM_MEM);
}


void
vm_pageout_oom(int shortage)
{
        struct proc *p, *bigproc;
        vm_offset_t size, bigsize;
        struct thread *td;
        struct vmspace *vm;

        /*
         * 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.
         */
        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, protected or killed process, skip it.
                 */
                if (p->p_state != PRS_NORMAL ||
                    (p->p_flag & (P_INEXEC | P_PROTECTED | P_SYSTEM)) ||
                    (p->p_pid == 1) || P_KILLED(p) ||
                    ((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) &&
                            !TD_IS_SUSPENDED(td)) {
                                thread_unlock(td);
                                breakout = 1;
                                break;
                        }
                        thread_unlock(td);
                }
                if (breakout) {
                        PROC_UNLOCK(p);
                        continue;
                }
                /*
                 * get the process size
                 */
                vm = vmspace_acquire_ref(p);
                if (vm == NULL) {
                        PROC_UNLOCK(p);
                        continue;
                }
                if (!vm_map_trylock_read(&vm->vm_map)) {
                        vmspace_free(vm);
                        PROC_UNLOCK(p);
                        continue;
                }
                size = vmspace_swap_count(vm);
                vm_map_unlock_read(&vm->vm_map);
                if (shortage == VM_OOM_MEM)
                        size += vmspace_resident_count(vm);
                vmspace_free(vm);
                /*
                 * 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;

        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;

        vm_page_lock_queues();
        pcount = cnt.v_active_count;
        fullintervalcount += vm_pageout_stats_interval;
        if (fullintervalcount < vm_pageout_full_stats_interval) {
                tpcount = (int64_t)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(m->queue == PQ_ACTIVE,
                    ("vm_pageout_page_stats: page %p isn't active", m));

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

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

                actcount = 0;
                if (m->aflags & PGA_REFERENCED) {
                        vm_page_aflag_clear(m, PGA_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_page_unlock(m);
                VM_OBJECT_UNLOCK(object);
                m = next;
        }
        vm_page_unlock_queues();
}

/*
 *      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_pageout_page_stats();
                                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;
        struct vmspace *vm;
        int breakout, swapout_flags, tryagain, attempts;
#ifdef RACCT
        uint64_t rsize, ravailable;
#endif

        while (TRUE) {
                mtx_lock(&vm_daemon_mtx);
#ifdef RACCT
                msleep(&vm_daemon_needed, &vm_daemon_mtx, PPAUSE, "psleep", hz);
#else
                msleep(&vm_daemon_needed, &vm_daemon_mtx, PPAUSE, "psleep", 0);
#endif
                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
                 */
                tryagain = 0;
                attempts = 0;
again:
                attempts++;
                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_state != PRS_NORMAL ||
                            p->p_flag & (P_INEXEC | 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) &&
                                    !TD_IS_SUSPENDED(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 */
                        vm = vmspace_acquire_ref(p);
                        PROC_UNLOCK(p);
                        if (vm == NULL)
                                continue;

                        size = vmspace_resident_count(vm);
                        if (limit >= 0 && size >= limit) {
                                vm_pageout_map_deactivate_pages(
                                    &vm->vm_map, limit);
                        }
#ifdef RACCT
                        rsize = IDX_TO_OFF(size);
                        PROC_LOCK(p);
                        racct_set(p, RACCT_RSS, rsize);
                        ravailable = racct_get_available(p, RACCT_RSS);
                        PROC_UNLOCK(p);
                        if (rsize > ravailable) {
                                /*
                                 * Don't be overly aggressive; this might be
                                 * an innocent process, and the limit could've
                                 * been exceeded by some memory hog.  Don't
                                 * try to deactivate more than 1/4th of process'
                                 * resident set size.
                                 */
                                if (attempts <= 8) {
                                        if (ravailable < rsize - (rsize / 4))
                                                ravailable = rsize - (rsize / 4);
                                }
                                vm_pageout_map_deactivate_pages(
                                    &vm->vm_map, OFF_TO_IDX(ravailable));
                                /* Update RSS usage after paging out. */
                                size = vmspace_resident_count(vm);
                                rsize = IDX_TO_OFF(size);
                                PROC_LOCK(p);
                                racct_set(p, RACCT_RSS, rsize);
                                PROC_UNLOCK(p);
                                if (rsize > ravailable)
                                        tryagain = 1;
                        }
#endif
                        vmspace_free(vm);
                }
                sx_sunlock(&allproc_lock);
                if (tryagain != 0 && attempts <= 10)
                        goto again;
        }
}
#endif                  /* !defined(NO_SWAPPING) */