Add UPDATING entries and bump version.

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

/*-
 * SPDX-License-Identifier: (BSD-4-Clause AND MIT-CMU)
 *
 * 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/sdt.h>
#include <sys/signalvar.h>
#include <sys/smp.h>
#include <sys/time.h>
#include <sys/vnode.h>
#include <sys/vmmeter.h>
#include <sys/rwlock.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/vm_phys.h>
#include <vm/vm_pagequeue.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 void vm_pageout_init(void);
static int vm_pageout_clean(vm_page_t m, int *numpagedout);
static int vm_pageout_cluster(vm_page_t m);
static void vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
    int starting_page_shortage);

SYSINIT(pagedaemon_init, SI_SUB_KTHREAD_PAGE, SI_ORDER_FIRST, vm_pageout_init,
    NULL);

struct proc *pageproc;

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

SDT_PROVIDER_DEFINE(vm);
SDT_PROBE_DEFINE(vm, , , vm__lowmem_scan);

/* Pagedaemon activity rates, in subdivisions of one second. */
#define VM_LAUNDER_RATE         10
#define VM_INACT_SCAN_RATE      10

static int vm_pageout_oom_seq = 12;

static int vm_pageout_update_period;
static int disable_swap_pageouts;
static int lowmem_period = 10;
static int swapdev_enabled;

static int vm_panic_on_oom = 0;

SYSCTL_INT(_vm, OID_AUTO, panic_on_oom,
        CTLFLAG_RWTUN, &vm_panic_on_oom, 0,
        "panic on out of memory instead of killing the largest process");

SYSCTL_INT(_vm, OID_AUTO, pageout_update_period,
        CTLFLAG_RWTUN, &vm_pageout_update_period, 0,
        "Maximum active LRU update period");
  
SYSCTL_INT(_vm, OID_AUTO, lowmem_period, CTLFLAG_RWTUN, &lowmem_period, 0,
        "Low memory callback period");

SYSCTL_INT(_vm, OID_AUTO, disable_swapspace_pageouts,
        CTLFLAG_RWTUN, &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");

SYSCTL_INT(_vm, OID_AUTO, pageout_oom_seq,
        CTLFLAG_RWTUN, &vm_pageout_oom_seq, 0,
        "back-to-back calls to oom detector to start OOM");

static int act_scan_laundry_weight = 3;
SYSCTL_INT(_vm, OID_AUTO, act_scan_laundry_weight, CTLFLAG_RWTUN,
    &act_scan_laundry_weight, 0,
    "weight given to clean vs. dirty pages in active queue scans");

static u_int vm_background_launder_rate = 4096;
SYSCTL_UINT(_vm, OID_AUTO, background_launder_rate, CTLFLAG_RWTUN,
    &vm_background_launder_rate, 0,
    "background laundering rate, in kilobytes per second");

static u_int vm_background_launder_max = 20 * 1024;
SYSCTL_UINT(_vm, OID_AUTO, background_launder_max, CTLFLAG_RWTUN,
    &vm_background_launder_max, 0, "background laundering cap, in kilobytes");

int vm_pageout_page_count = 32;

u_int vm_page_max_user_wired;
SYSCTL_UINT(_vm, OID_AUTO, max_wired, CTLFLAG_RW,
    &vm_page_max_user_wired, 0,
    "system-wide limit to user-wired page count");

static u_int isqrt(u_int num);
static int vm_pageout_launder(struct vm_domain *vmd, int launder,
    bool in_shortfall);
static void vm_pageout_laundry_worker(void *arg);

struct scan_state {
        struct vm_batchqueue bq;
        struct vm_pagequeue *pq;
        vm_page_t       marker;
        int             maxscan;
        int             scanned;
};

static void
vm_pageout_init_scan(struct scan_state *ss, struct vm_pagequeue *pq,
    vm_page_t marker, vm_page_t after, int maxscan)
{

        vm_pagequeue_assert_locked(pq);
        KASSERT((marker->aflags & PGA_ENQUEUED) == 0,
            ("marker %p already enqueued", marker));

        if (after == NULL)
                TAILQ_INSERT_HEAD(&pq->pq_pl, marker, plinks.q);
        else
                TAILQ_INSERT_AFTER(&pq->pq_pl, after, marker, plinks.q);
        vm_page_aflag_set(marker, PGA_ENQUEUED);

        vm_batchqueue_init(&ss->bq);
        ss->pq = pq;
        ss->marker = marker;
        ss->maxscan = maxscan;
        ss->scanned = 0;
        vm_pagequeue_unlock(pq);
}

static void
vm_pageout_end_scan(struct scan_state *ss)
{
        struct vm_pagequeue *pq;

        pq = ss->pq;
        vm_pagequeue_assert_locked(pq);
        KASSERT((ss->marker->aflags & PGA_ENQUEUED) != 0,
            ("marker %p not enqueued", ss->marker));

        TAILQ_REMOVE(&pq->pq_pl, ss->marker, plinks.q);
        vm_page_aflag_clear(ss->marker, PGA_ENQUEUED);
        pq->pq_pdpages += ss->scanned;
}

/*
 * Add a small number of queued pages to a batch queue for later processing
 * without the corresponding queue lock held.  The caller must have enqueued a
 * marker page at the desired start point for the scan.  Pages will be
 * physically dequeued if the caller so requests.  Otherwise, the returned
 * batch may contain marker pages, and it is up to the caller to handle them.
 *
 * When processing the batch queue, vm_page_queue() must be used to
 * determine whether the page has been logically dequeued by another thread.
 * Once this check is performed, the page lock guarantees that the page will
 * not be disassociated from the queue.
 */
static __always_inline void
vm_pageout_collect_batch(struct scan_state *ss, const bool dequeue)
{
        struct vm_pagequeue *pq;
        vm_page_t m, marker, n;

        marker = ss->marker;
        pq = ss->pq;

        KASSERT((marker->aflags & PGA_ENQUEUED) != 0,
            ("marker %p not enqueued", ss->marker));

        vm_pagequeue_lock(pq);
        for (m = TAILQ_NEXT(marker, plinks.q); m != NULL &&
            ss->scanned < ss->maxscan && ss->bq.bq_cnt < VM_BATCHQUEUE_SIZE;
            m = n, ss->scanned++) {
                n = TAILQ_NEXT(m, plinks.q);
                if ((m->flags & PG_MARKER) == 0) {
                        KASSERT((m->aflags & PGA_ENQUEUED) != 0,
                            ("page %p not enqueued", m));
                        KASSERT((m->flags & PG_FICTITIOUS) == 0,
                            ("Fictitious page %p cannot be in page queue", m));
                        KASSERT((m->oflags & VPO_UNMANAGED) == 0,
                            ("Unmanaged page %p cannot be in page queue", m));
                } else if (dequeue)
                        continue;

                (void)vm_batchqueue_insert(&ss->bq, m);
                if (dequeue) {
                        TAILQ_REMOVE(&pq->pq_pl, m, plinks.q);
                        vm_page_aflag_clear(m, PGA_ENQUEUED);
                }
        }
        TAILQ_REMOVE(&pq->pq_pl, marker, plinks.q);
        if (__predict_true(m != NULL))
                TAILQ_INSERT_BEFORE(m, marker, plinks.q);
        else
                TAILQ_INSERT_TAIL(&pq->pq_pl, marker, plinks.q);
        if (dequeue)
                vm_pagequeue_cnt_add(pq, -ss->bq.bq_cnt);
        vm_pagequeue_unlock(pq);
}

/* Return the next page to be scanned, or NULL if the scan is complete. */
static __always_inline vm_page_t
vm_pageout_next(struct scan_state *ss, const bool dequeue)
{

        if (ss->bq.bq_cnt == 0)
                vm_pageout_collect_batch(ss, dequeue);
        return (vm_batchqueue_pop(&ss->bq));
}

/*
 * Scan for pages at adjacent offsets within the given page's object that are
 * eligible for laundering, form a cluster of these pages and the given page,
 * and launder that cluster.
 */
static int
vm_pageout_cluster(vm_page_t m)
{
        vm_object_t object;
        vm_page_t mc[2 * vm_pageout_page_count], p, pb, ps;
        vm_pindex_t pindex;
        int ib, is, page_base, pageout_count;

        vm_page_assert_locked(m);
        object = m->object;
        VM_OBJECT_ASSERT_WLOCKED(object);
        pindex = m->pindex;

        vm_page_assert_unbusied(m);
        KASSERT(!vm_page_held(m), ("page %p is held", m));

        pmap_remove_write(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;

        /*
         * We can cluster only if the page is not clean, busy, or held, and
         * the page is in the laundry queue.
         *
         * During heavy mmap/modification loads the pageout
         * daemon can really fragment the underlying file
         * due to flushing pages out of order and not trying to
         * align the clusters (which leaves sporadic 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 != 0 && pageout_count < vm_pageout_page_count) {
                if (ib > pindex) {
                        ib = 0;
                        break;
                }
                if ((p = vm_page_prev(pb)) == NULL || vm_page_busied(p)) {
                        ib = 0;
                        break;
                }
                vm_page_test_dirty(p);
                if (p->dirty == 0) {
                        ib = 0;
                        break;
                }
                vm_page_lock(p);
                if (vm_page_held(p) || !vm_page_in_laundry(p)) {
                        vm_page_unlock(p);
                        ib = 0;
                        break;
                }
                pmap_remove_write(p);
                vm_page_unlock(p);
                mc[--page_base] = pb = p;
                ++pageout_count;
                ++ib;

                /*
                 * We are at an alignment boundary.  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) {
                if ((p = vm_page_next(ps)) == NULL || vm_page_busied(p))
                        break;
                vm_page_test_dirty(p);
                if (p->dirty == 0)
                        break;
                vm_page_lock(p);
                if (vm_page_held(p) || !vm_page_in_laundry(p)) {
                        vm_page_unlock(p);
                        break;
                }
                pmap_remove_write(p);
                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 an alignment boundary.  This catches
         * boundary conditions.
         */
        if (ib != 0 && pageout_count < vm_pageout_page_count)
                goto more;

        return (vm_pageout_flush(&mc[page_base], pageout_count,
            VM_PAGER_PUT_NOREUSE, 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_ASSERT_WLOCKED(object);

        /*
         * Initiate I/O.  Mark the pages busy and verify that they're valid
         * and 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));
                KASSERT((mc[i]->aflags & PGA_WRITEABLE) == 0,
                    ("vm_pageout_flush: writeable page %p", mc[i]));
                vm_page_sbusy(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:
                        vm_page_lock(mt);
                        if (vm_page_in_laundry(mt))
                                vm_page_deactivate_noreuse(mt);
                        vm_page_unlock(mt);
                        /* FALLTHROUGH */
                case VM_PAGER_PEND:
                        numpagedout++;
                        break;
                case VM_PAGER_BAD:
                        /*
                         * The page is outside the object's range.  We pretend
                         * that the page out worked and clean the page, so the
                         * changes will be lost if the page is reclaimed by
                         * the page daemon.
                         */
                        vm_page_undirty(mt);
                        vm_page_lock(mt);
                        if (vm_page_in_laundry(mt))
                                vm_page_deactivate_noreuse(mt);
                        vm_page_unlock(mt);
                        break;
                case VM_PAGER_ERROR:
                case VM_PAGER_FAIL:
                        /*
                         * If the page couldn't be paged out to swap because the
                         * pager wasn't able to find space, place the page in
                         * the PQ_UNSWAPPABLE holding queue.  This is an
                         * optimization that prevents the page daemon from
                         * wasting CPU cycles on pages that cannot be reclaimed
                         * becase no swap device is configured.
                         *
                         * Otherwise, reactivate the page so that it doesn't
                         * clog the laundry and inactive queues.  (We will try
                         * paging it out again later.)
                         */
                        vm_page_lock(mt);
                        if (object->type == OBJT_SWAP &&
                            pageout_status[i] == VM_PAGER_FAIL) {
                                vm_page_unswappable(mt);
                                numpagedout++;
                        } else
                                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_sunbusy(mt);
                }
        }
        if (prunlen != NULL)
                *prunlen = runlen;
        return (numpagedout);
}

static void
vm_pageout_swapon(void *arg __unused, struct swdevt *sp __unused)
{

        atomic_store_rel_int(&swapdev_enabled, 1);
}

static void
vm_pageout_swapoff(void *arg __unused, struct swdevt *sp __unused)
{

        if (swap_pager_nswapdev() == 1)
                atomic_store_rel_int(&swapdev_enabled, 0);
}

/*
 * Attempt to acquire all of the necessary locks to launder a page and
 * then call through the clustering layer to PUTPAGES.  Wait a short
 * time for a vnode lock.
 *
 * Requires the page and object lock on entry, releases both before return.
 * Returns 0 on success and an errno otherwise.
 */
static int
vm_pageout_clean(vm_page_t m, int *numpagedout)
{
        struct vnode *vp;
        struct mount *mp;
        vm_object_t object;
        vm_pindex_t pindex;
        int error, lockmode;

        vm_page_assert_locked(m);
        object = m->object;
        VM_OBJECT_ASSERT_WLOCKED(object);
        error = 0;
        vp = NULL;
        mp = NULL;

        /*
         * 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.
         *
         * 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;
                        error = EDEADLK;
                        goto unlock_all;
                }
                KASSERT(mp != NULL,
                    ("vp %p with NULL v_mount", vp));
                vm_object_reference_locked(object);
                pindex = m->pindex;
                VM_OBJECT_WUNLOCK(object);
                lockmode = MNT_SHARED_WRITES(vp->v_mount) ?
                    LK_SHARED : LK_EXCLUSIVE;
                if (vget(vp, lockmode | LK_TIMELOCK, curthread)) {
                        vp = NULL;
                        error = EDEADLK;
                        goto unlock_mp;
                }
                VM_OBJECT_WLOCK(object);

                /*
                 * Ensure that the object and vnode were not disassociated
                 * while locks were dropped.
                 */
                if (vp->v_object != object) {
                        error = ENOENT;
                        goto unlock_all;
                }
                vm_page_lock(m);

                /*
                 * While the object and page were unlocked, the page
                 * may have been:
                 * (1) moved to a different queue,
                 * (2) reallocated to a different object,
                 * (3) reallocated to a different offset, or
                 * (4) cleaned.
                 */
                if (!vm_page_in_laundry(m) || m->object != object ||
                    m->pindex != pindex || m->dirty == 0) {
                        vm_page_unlock(m);
                        error = ENXIO;
                        goto unlock_all;
                }

                /*
                 * The page may have been busied or referenced while the object
                 * and page locks were released.
                 */
                if (vm_page_busied(m) || vm_page_held(m)) {
                        vm_page_unlock(m);
                        error = EBUSY;
                        goto unlock_all;
                }
        }

        /*
         * 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. 
         */
        if ((*numpagedout = vm_pageout_cluster(m)) == 0)
                error = EIO;

unlock_all:
        VM_OBJECT_WUNLOCK(object);

unlock_mp:
        vm_page_lock_assert(m, MA_NOTOWNED);
        if (mp != NULL) {
                if (vp != NULL)
                        vput(vp);
                vm_object_deallocate(object);
                vn_finished_write(mp);
        }

        return (error);
}

/*
 * Attempt to launder the specified number of pages.
 *
 * Returns the number of pages successfully laundered.
 */
static int
vm_pageout_launder(struct vm_domain *vmd, int launder, bool in_shortfall)
{
        struct scan_state ss;
        struct vm_pagequeue *pq;
        struct mtx *mtx;
        vm_object_t object;
        vm_page_t m, marker;
        int act_delta, error, numpagedout, queue, starting_target;
        int vnodes_skipped;
        bool pageout_ok;

        mtx = NULL;
        object = NULL;
        starting_target = launder;
        vnodes_skipped = 0;

        /*
         * Scan the laundry queues for pages eligible to be laundered.  We stop
         * once the target number of dirty pages have been laundered, or once
         * we've reached the end of the queue.  A single iteration of this loop
         * may cause more than one page to be laundered because of clustering.
         *
         * As an optimization, we avoid laundering from PQ_UNSWAPPABLE when no
         * swap devices are configured.
         */
        if (atomic_load_acq_int(&swapdev_enabled))
                queue = PQ_UNSWAPPABLE;
        else
                queue = PQ_LAUNDRY;

scan:
        marker = &vmd->vmd_markers[queue];
        pq = &vmd->vmd_pagequeues[queue];
        vm_pagequeue_lock(pq);
        vm_pageout_init_scan(&ss, pq, marker, NULL, pq->pq_cnt);
        while (launder > 0 && (m = vm_pageout_next(&ss, false)) != NULL) {
                if (__predict_false((m->flags & PG_MARKER) != 0))
                        continue;

                vm_page_change_lock(m, &mtx);

recheck:
                /*
                 * The page may have been disassociated from the queue
                 * while locks were dropped.
                 */
                if (vm_page_queue(m) != queue)
                        continue;

                /*
                 * A requeue was requested, so this page gets a second
                 * chance.
                 */
                if ((m->aflags & PGA_REQUEUE) != 0) {
                        vm_page_requeue(m);
                        continue;
                }

                /*
                 * Held pages are essentially stuck in the queue.
                 *
                 * Wired pages may not be freed.  Complete their removal
                 * from the queue now to avoid needless revisits during
                 * future scans.
                 */
                if (m->hold_count != 0)
                        continue;
                if (vm_page_wired(m)) {
                        vm_page_dequeue_deferred(m);
                        continue;
                }

                if (object != m->object) {
                        if (object != NULL)
                                VM_OBJECT_WUNLOCK(object);
                        object = m->object;
                        if (!VM_OBJECT_TRYWLOCK(object)) {
                                mtx_unlock(mtx);
                                /* Depends on type-stability. */
                                VM_OBJECT_WLOCK(object);
                                mtx_lock(mtx);
                                goto recheck;
                        }
                }

                if (vm_page_busied(m))
                        continue;

                /*
                 * Invalid pages can be easily freed.  They cannot be
                 * mapped; vm_page_free() asserts this.
                 */
                if (m->valid == 0)
                        goto free_page;

                /*
                 * If the page has been referenced and the object is not dead,
                 * reactivate or requeue the page depending on whether the
                 * object is mapped.
                 *
                 * Test PGA_REFERENCED after calling pmap_ts_referenced() so
                 * that a reference from a concurrently destroyed mapping is
                 * observed here and now.
                 */
                if (object->ref_count != 0)
                        act_delta = pmap_ts_referenced(m);
                else {
                        KASSERT(!pmap_page_is_mapped(m),
                            ("page %p is mapped", m));
                        act_delta = 0;
                }
                if ((m->aflags & PGA_REFERENCED) != 0) {
                        vm_page_aflag_clear(m, PGA_REFERENCED);
                        act_delta++;
                }
                if (act_delta != 0) {
                        if (object->ref_count != 0) {
                                VM_CNT_INC(v_reactivated);
                                vm_page_activate(m);

                                /*
                                 * Increase the activation count if the page
                                 * was referenced while in the laundry queue.
                                 * This makes it less likely that the page will
                                 * be returned prematurely to the inactive
                                 * queue.
                                 */
                                m->act_count += act_delta + ACT_ADVANCE;

                                /*
                                 * If this was a background laundering, count
                                 * activated pages towards our target.  The
                                 * purpose of background laundering is to ensure
                                 * that pages are eventually cycled through the
                                 * laundry queue, and an activation is a valid
                                 * way out.
                                 */
                                if (!in_shortfall)
                                        launder--;
                                continue;
                        } else if ((object->flags & OBJ_DEAD) == 0) {
                                vm_page_requeue(m);
                                continue;
                        }
                }

                /*
                 * If the page appears to be clean at the machine-independent
                 * layer, then remove all of its mappings from the pmap in
                 * anticipation of freeing it.  If, however, any of the page's
                 * mappings allow write access, then the page may still be
                 * modified until the last of those mappings are removed.
                 */
                if (object->ref_count != 0) {
                        vm_page_test_dirty(m);
                        if (m->dirty == 0)
                                pmap_remove_all(m);
                }

                /*
                 * Clean pages are freed, and dirty pages are paged out unless
                 * they belong to a dead object.  Requeueing dirty pages from
                 * dead objects is pointless, as they are being paged out and
                 * freed by the thread that destroyed the object.
                 */
                if (m->dirty == 0) {
free_page:
                        vm_page_free(m);
                        VM_CNT_INC(v_dfree);
                } else if ((object->flags & OBJ_DEAD) == 0) {
                        if (object->type != OBJT_SWAP &&
                            object->type != OBJT_DEFAULT)
                                pageout_ok = true;
                        else if (disable_swap_pageouts)
                                pageout_ok = false;
                        else
                                pageout_ok = true;
                        if (!pageout_ok) {
                                vm_page_requeue(m);
                                continue;
                        }

                        /*
                         * Form a cluster with adjacent, dirty pages from the
                         * same object, and page out that entire cluster.
                         *
                         * The adjacent, dirty pages must also be in the
                         * laundry.  However, their mappings are not checked
                         * for new references.  Consequently, a recently
                         * referenced page may be paged out.  However, that
                         * page will not be prematurely reclaimed.  After page
                         * out, the page will be placed in the inactive queue,
                         * where any new references will be detected and the
                         * page reactivated.
                         */
                        error = vm_pageout_clean(m, &numpagedout);
                        if (error == 0) {
                                launder -= numpagedout;
                                ss.scanned += numpagedout;
                        } else if (error == EDEADLK) {
                                pageout_lock_miss++;
                                vnodes_skipped++;
                        }
                        mtx = NULL;
                        object = NULL;
                }
        }
        if (mtx != NULL) {
                mtx_unlock(mtx);
                mtx = NULL;
        }
        if (object != NULL) {
                VM_OBJECT_WUNLOCK(object);
                object = NULL;
        }
        vm_pagequeue_lock(pq);
        vm_pageout_end_scan(&ss);
        vm_pagequeue_unlock(pq);

        if (launder > 0 && queue == PQ_UNSWAPPABLE) {
                queue = PQ_LAUNDRY;
                goto scan;
        }

        /*
         * Wakeup the sync daemon if we skipped a vnode in a writeable object
         * and we didn't launder enough pages.
         */
        if (vnodes_skipped > 0 && launder > 0)
                (void)speedup_syncer();

        return (starting_target - launder);
}

/*
 * Compute the integer square root.
 */
static u_int
isqrt(u_int num)
{
        u_int bit, root, tmp;

        bit = 1u << ((NBBY * sizeof(u_int)) - 2);
        while (bit > num)
                bit >>= 2;
        root = 0;
        while (bit != 0) {
                tmp = root + bit;
                root >>= 1;
                if (num >= tmp) {
                        num -= tmp;
                        root += bit;
                }
                bit >>= 2;
        }
        return (root);
}

/*
 * Perform the work of the laundry thread: periodically wake up and determine
 * whether any pages need to be laundered.  If so, determine the number of pages
 * that need to be laundered, and launder them.
 */
static void
vm_pageout_laundry_worker(void *arg)
{
        struct vm_domain *vmd;
        struct vm_pagequeue *pq;
        uint64_t nclean, ndirty, nfreed;
        int domain, last_target, launder, shortfall, shortfall_cycle, target;
        bool in_shortfall;

        domain = (uintptr_t)arg;
        vmd = VM_DOMAIN(domain);
        pq = &vmd->vmd_pagequeues[PQ_LAUNDRY];
        KASSERT(vmd->vmd_segs != 0, ("domain without segments"));

        shortfall = 0;
        in_shortfall = false;
        shortfall_cycle = 0;
        last_target = target = 0;
        nfreed = 0;

        /*
         * Calls to these handlers are serialized by the swap syscall lock.
         */
        (void)EVENTHANDLER_REGISTER(swapon, vm_pageout_swapon, vmd,
            EVENTHANDLER_PRI_ANY);
        (void)EVENTHANDLER_REGISTER(swapoff, vm_pageout_swapoff, vmd,
            EVENTHANDLER_PRI_ANY);

        /*
         * The pageout laundry worker is never done, so loop forever.
         */
        for (;;) {
                KASSERT(target >= 0, ("negative target %d", target));
                KASSERT(shortfall_cycle >= 0,
                    ("negative cycle %d", shortfall_cycle));
                launder = 0;

                /*
                 * First determine whether we need to launder pages to meet a
                 * shortage of free pages.
                 */
                if (shortfall > 0) {
                        in_shortfall = true;
                        shortfall_cycle = VM_LAUNDER_RATE / VM_INACT_SCAN_RATE;
                        target = shortfall;
                } else if (!in_shortfall)
                        goto trybackground;
                else if (shortfall_cycle == 0 || vm_laundry_target(vmd) <= 0) {
                        /*
                         * We recently entered shortfall and began laundering
                         * pages.  If we have completed that laundering run
                         * (and we are no longer in shortfall) or we have met
                         * our laundry target through other activity, then we
                         * can stop laundering pages.
                         */
                        in_shortfall = false;
                        target = 0;
                        goto trybackground;
                }
                launder = target / shortfall_cycle--;
                goto dolaundry;

                /*
                 * There's no immediate need to launder any pages; see if we
                 * meet the conditions to perform background laundering:
                 *
                 * 1. The ratio of dirty to clean inactive pages exceeds the
                 *    background laundering threshold, or
                 * 2. we haven't yet reached the target of the current
                 *    background laundering run.
                 *
                 * The background laundering threshold is not a constant.
                 * Instead, it is a slowly growing function of the number of
                 * clean pages freed by the page daemon since the last
                 * background laundering.  Thus, as the ratio of dirty to
                 * clean inactive pages grows, the amount of memory pressure
                 * required to trigger laundering decreases.  We ensure
                 * that the threshold is non-zero after an inactive queue
                 * scan, even if that scan failed to free a single clean page.
                 */
trybackground:
                nclean = vmd->vmd_free_count +
                    vmd->vmd_pagequeues[PQ_INACTIVE].pq_cnt;
                ndirty = vmd->vmd_pagequeues[PQ_LAUNDRY].pq_cnt;
                if (target == 0 && ndirty * isqrt(howmany(nfreed + 1,
                    vmd->vmd_free_target - vmd->vmd_free_min)) >= nclean) {
                        target = vmd->vmd_background_launder_target;
                }

                /*
                 * We have a non-zero background laundering target.  If we've
                 * laundered up to our maximum without observing a page daemon
                 * request, just stop.  This is a safety belt that ensures we
                 * don't launder an excessive amount if memory pressure is low
                 * and the ratio of dirty to clean pages is large.  Otherwise,
                 * proceed at the background laundering rate.
                 */
                if (target > 0) {
                        if (nfreed > 0) {
                                nfreed = 0;
                                last_target = target;
                        } else if (last_target - target >=
                            vm_background_launder_max * PAGE_SIZE / 1024) {
                                target = 0;
                        }
                        launder = vm_background_launder_rate * PAGE_SIZE / 1024;
                        launder /= VM_LAUNDER_RATE;
                        if (launder > target)
                                launder = target;
                }

dolaundry:
                if (launder > 0) {
                        /*
                         * Because of I/O clustering, the number of laundered
                         * pages could exceed "target" by the maximum size of
                         * a cluster minus one. 
                         */
                        target -= min(vm_pageout_launder(vmd, launder,
                            in_shortfall), target);
                        pause("laundp", hz / VM_LAUNDER_RATE);
                }

                /*
                 * If we're not currently laundering pages and the page daemon
                 * hasn't posted a new request, sleep until the page daemon
                 * kicks us.
                 */
                vm_pagequeue_lock(pq);
                if (target == 0 && vmd->vmd_laundry_request == VM_LAUNDRY_IDLE)
                        (void)mtx_sleep(&vmd->vmd_laundry_request,
                            vm_pagequeue_lockptr(pq), PVM, "launds", 0);

                /*
                 * If the pagedaemon has indicated that it's in shortfall, start
                 * a shortfall laundering unless we're already in the middle of
                 * one.  This may preempt a background laundering.
                 */
                if (vmd->vmd_laundry_request == VM_LAUNDRY_SHORTFALL &&
                    (!in_shortfall || shortfall_cycle == 0)) {
                        shortfall = vm_laundry_target(vmd) +
                            vmd->vmd_pageout_deficit;
                        target = 0;
                } else
                        shortfall = 0;

                if (target == 0)
                        vmd->vmd_laundry_request = VM_LAUNDRY_IDLE;
                nfreed += vmd->vmd_clean_pages_freed;
                vmd->vmd_clean_pages_freed = 0;
                vm_pagequeue_unlock(pq);
        }
}

/*
 * Compute the number of pages we want to try to move from the
 * active queue to either the inactive or laundry queue.
 *
 * When scanning active pages during a shortage, we make clean pages
 * count more heavily towards the page shortage than dirty pages.
 * This is because dirty pages must be laundered before they can be
 * reused and thus have less utility when attempting to quickly
 * alleviate a free page shortage.  However, this weighting also
 * causes the scan to deactivate dirty pages more aggressively,
 * improving the effectiveness of clustering.
 */
static int
vm_pageout_active_target(struct vm_domain *vmd)
{
        int shortage;

        shortage = vmd->vmd_inactive_target + vm_paging_target(vmd) -
            (vmd->vmd_pagequeues[PQ_INACTIVE].pq_cnt +
            vmd->vmd_pagequeues[PQ_LAUNDRY].pq_cnt / act_scan_laundry_weight);
        shortage *= act_scan_laundry_weight;
        return (shortage);
}

/*
 * Scan the active queue.  If there is no shortage of inactive pages, scan a
 * small portion of the queue in order to maintain quasi-LRU.
 */
static void
vm_pageout_scan_active(struct vm_domain *vmd, int page_shortage)
{
        struct scan_state ss;
        struct mtx *mtx;
        vm_page_t m, marker;
        struct vm_pagequeue *pq;
        long min_scan;
        int act_delta, max_scan, scan_tick;

        marker = &vmd->vmd_markers[PQ_ACTIVE];
        pq = &vmd->vmd_pagequeues[PQ_ACTIVE];
        vm_pagequeue_lock(pq);

        /*
         * If we're just idle polling attempt to visit every
         * active page within 'update_period' seconds.
         */
        scan_tick = ticks;
        if (vm_pageout_update_period != 0) {
                min_scan = pq->pq_cnt;
                min_scan *= scan_tick - vmd->vmd_last_active_scan;
                min_scan /= hz * vm_pageout_update_period;
        } else
                min_scan = 0;
        if (min_scan > 0 || (page_shortage > 0 && pq->pq_cnt > 0))
                vmd->vmd_last_active_scan = scan_tick;

        /*
         * Scan the active queue for pages that can be deactivated.  Update
         * the per-page activity counter and use it to identify deactivation
         * candidates.  Held pages may be deactivated.
         *
         * To avoid requeuing each page that remains in the active queue, we
         * implement the CLOCK algorithm.  To keep the implementation of the
         * enqueue operation consistent for all page queues, we use two hands,
         * represented by marker pages. Scans begin at the first hand, which
         * precedes the second hand in the queue.  When the two hands meet,
         * they are moved back to the head and tail of the queue, respectively,
         * and scanning resumes.
         */
        max_scan = page_shortage > 0 ? pq->pq_cnt : min_scan;
        mtx = NULL;
act_scan:
        vm_pageout_init_scan(&ss, pq, marker, &vmd->vmd_clock[0], max_scan);
        while ((m = vm_pageout_next(&ss, false)) != NULL) {
                if (__predict_false(m == &vmd->vmd_clock[1])) {
                        vm_pagequeue_lock(pq);
                        TAILQ_REMOVE(&pq->pq_pl, &vmd->vmd_clock[0], plinks.q);
                        TAILQ_REMOVE(&pq->pq_pl, &vmd->vmd_clock[1], plinks.q);
                        TAILQ_INSERT_HEAD(&pq->pq_pl, &vmd->vmd_clock[0],
                            plinks.q);
                        TAILQ_INSERT_TAIL(&pq->pq_pl, &vmd->vmd_clock[1],
                            plinks.q);
                        max_scan -= ss.scanned;
                        vm_pageout_end_scan(&ss);
                        goto act_scan;
                }
                if (__predict_false((m->flags & PG_MARKER) != 0))
                        continue;

                vm_page_change_lock(m, &mtx);

                /*
                 * The page may have been disassociated from the queue
                 * while locks were dropped.
                 */
                if (vm_page_queue(m) != PQ_ACTIVE)
                        continue;

                /*
                 * Wired pages are dequeued lazily.
                 */
                if (vm_page_wired(m)) {
                        vm_page_dequeue_deferred(m);
                        continue;
                }

                /*
                 * Check to see "how much" the page has been used.
                 *
                 * Test PGA_REFERENCED after calling pmap_ts_referenced() so
                 * that a reference from a concurrently destroyed mapping is
                 * observed here and now.
                 *
                 * Perform an unsynchronized object ref count check.  While
                 * the page lock ensures that the page is not reallocated to
                 * another object, in particular, one with unmanaged mappings
                 * that cannot support pmap_ts_referenced(), two races are,
                 * nonetheless, possible:
                 * 1) The count was transitioning to zero, but we saw a non-
                 *    zero value.  pmap_ts_referenced() will return zero
                 *    because the page is not mapped.
                 * 2) The count was transitioning to one, but we saw zero.
                 *    This race delays the detection of a new reference.  At
                 *    worst, we will deactivate and reactivate the page.
                 */
                if (m->object->ref_count != 0)
                        act_delta = pmap_ts_referenced(m);
                else
                        act_delta = 0;
                if ((m->aflags & PGA_REFERENCED) != 0) {
                        vm_page_aflag_clear(m, PGA_REFERENCED);
                        act_delta++;
                }

                /*
                 * Advance or decay the act_count based on recent usage.
                 */
                if (act_delta != 0) {
                        m->act_count += ACT_ADVANCE + act_delta;
                        if (m->act_count > ACT_MAX)
                                m->act_count = ACT_MAX;
                } else
                        m->act_count -= min(m->act_count, ACT_DECLINE);

                if (m->act_count == 0) {
                        /*
                         * When not short for inactive pages, let dirty pages go
                         * through the inactive queue before moving to the
                         * laundry queues.  This gives them some extra time to
                         * be reactivated, potentially avoiding an expensive
                         * pageout.  However, during a page shortage, the
                         * inactive queue is necessarily small, and so dirty
                         * pages would only spend a trivial amount of time in
                         * the inactive queue.  Therefore, we might as well
                         * place them directly in the laundry queue to reduce
                         * queuing overhead.
                         */
                        if (page_shortage <= 0)
                                vm_page_deactivate(m);
                        else {
                                /*
                                 * Calling vm_page_test_dirty() here would
                                 * require acquisition of the object's write
                                 * lock.  However, during a page shortage,
                                 * directing dirty pages into the laundry
                                 * queue is only an optimization and not a
                                 * requirement.  Therefore, we simply rely on
                                 * the opportunistic updates to the page's
                                 * dirty field by the pmap.
                                 */
                                if (m->dirty == 0) {
                                        vm_page_deactivate(m);
                                        page_shortage -=
                                            act_scan_laundry_weight;
                                } else {
                                        vm_page_launder(m);
                                        page_shortage--;
                                }
                        }
                }
        }
        if (mtx != NULL) {
                mtx_unlock(mtx);
                mtx = NULL;
        }
        vm_pagequeue_lock(pq);
        TAILQ_REMOVE(&pq->pq_pl, &vmd->vmd_clock[0], plinks.q);
        TAILQ_INSERT_AFTER(&pq->pq_pl, marker, &vmd->vmd_clock[0], plinks.q);
        vm_pageout_end_scan(&ss);
        vm_pagequeue_unlock(pq);
}

static int
vm_pageout_reinsert_inactive_page(struct scan_state *ss, vm_page_t m)
{
        struct vm_domain *vmd;

        if (m->queue != PQ_INACTIVE || (m->aflags & PGA_ENQUEUED) != 0)
                return (0);
        vm_page_aflag_set(m, PGA_ENQUEUED);
        if ((m->aflags & PGA_REQUEUE_HEAD) != 0) {
                vmd = vm_pagequeue_domain(m);
                TAILQ_INSERT_BEFORE(&vmd->vmd_inacthead, m, plinks.q);
                vm_page_aflag_clear(m, PGA_REQUEUE | PGA_REQUEUE_HEAD);
        } else if ((m->aflags & PGA_REQUEUE) != 0) {
                TAILQ_INSERT_TAIL(&ss->pq->pq_pl, m, plinks.q);
                vm_page_aflag_clear(m, PGA_REQUEUE | PGA_REQUEUE_HEAD);
        } else
                TAILQ_INSERT_BEFORE(ss->marker, m, plinks.q);
        return (1);
}

/*
 * Re-add stuck pages to the inactive queue.  We will examine them again
 * during the next scan.  If the queue state of a page has changed since
 * it was physically removed from the page queue in
 * vm_pageout_collect_batch(), don't do anything with that page.
 */
static void
vm_pageout_reinsert_inactive(struct scan_state *ss, struct vm_batchqueue *bq,
    vm_page_t m)
{
        struct vm_pagequeue *pq;
        int delta;

        delta = 0;
        pq = ss->pq;

        if (m != NULL) {
                if (vm_batchqueue_insert(bq, m))
                        return;
                vm_pagequeue_lock(pq);
                delta += vm_pageout_reinsert_inactive_page(ss, m);
        } else
                vm_pagequeue_lock(pq);
        while ((m = vm_batchqueue_pop(bq)) != NULL)
                delta += vm_pageout_reinsert_inactive_page(ss, m);
        vm_pagequeue_cnt_add(pq, delta);
        vm_pagequeue_unlock(pq);
        vm_batchqueue_init(bq);
}

/*
 * Attempt to reclaim the requested number of pages from the inactive queue.
 * Returns true if the shortage was addressed.
 */
static int
vm_pageout_scan_inactive(struct vm_domain *vmd, int shortage,
    int *addl_shortage)
{
        struct scan_state ss;
        struct vm_batchqueue rq;
        struct mtx *mtx;
        vm_page_t m, marker;
        struct vm_pagequeue *pq;
        vm_object_t object;
        int act_delta, addl_page_shortage, deficit, page_shortage;
        int starting_page_shortage;

        /*
         * The addl_page_shortage is an estimate of the number of temporarily
         * stuck pages in the inactive queue.  In other words, the
         * number of pages from the inactive count that should be
         * discounted in setting the target for the active queue scan.
         */
        addl_page_shortage = 0;

        /*
         * vmd_pageout_deficit counts the number of pages requested in
         * allocations that failed because of a free page shortage.  We assume
         * that the allocations will be reattempted and thus include the deficit
         * in our scan target.
         */
        deficit = atomic_readandclear_int(&vmd->vmd_pageout_deficit);
        starting_page_shortage = page_shortage = shortage + deficit;

        mtx = NULL;
        object = NULL;
        vm_batchqueue_init(&rq);

        /*
         * Start scanning the inactive queue for pages that we can free.  The
         * scan will stop when we reach the target or we have scanned the
         * entire queue.  (Note that m->act_count is not used to make
         * decisions for the inactive queue, only for the active queue.)
         */
        marker = &vmd->vmd_markers[PQ_INACTIVE];
        pq = &vmd->vmd_pagequeues[PQ_INACTIVE];
        vm_pagequeue_lock(pq);
        vm_pageout_init_scan(&ss, pq, marker, NULL, pq->pq_cnt);
        while (page_shortage > 0 && (m = vm_pageout_next(&ss, true)) != NULL) {
                KASSERT((m->flags & PG_MARKER) == 0,
                    ("marker page %p was dequeued", m));

                vm_page_change_lock(m, &mtx);

recheck:
                /*
                 * The page may have been disassociated from the queue
                 * while locks were dropped.
                 */
                if (vm_page_queue(m) != PQ_INACTIVE) {
                        addl_page_shortage++;
                        continue;
                }

                /*
                 * The page was re-enqueued after the page queue lock was
                 * dropped, or a requeue was requested.  This page gets a second
                 * chance.
                 */
                if ((m->aflags & (PGA_ENQUEUED | PGA_REQUEUE |
                    PGA_REQUEUE_HEAD)) != 0)
                        goto reinsert;

                /*
                 * Held pages are essentially stuck in the queue.  So,
                 * they ought to be discounted from the inactive count.
                 * See the description of addl_page_shortage above.
                 *
                 * Wired pages may not be freed.  Complete their removal
                 * from the queue now to avoid needless revisits during
                 * future scans.
                 */
                if (m->hold_count != 0) {
                        addl_page_shortage++;
                        goto reinsert;
                }
                if (vm_page_wired(m)) {
                        vm_page_dequeue_deferred(m);
                        continue;
                }

                if (object != m->object) {
                        if (object != NULL)
                                VM_OBJECT_WUNLOCK(object);
                        object = m->object;
                        if (!VM_OBJECT_TRYWLOCK(object)) {
                                mtx_unlock(mtx);
                                /* Depends on type-stability. */
                                VM_OBJECT_WLOCK(object);
                                mtx_lock(mtx);
                                goto recheck;
                        }
                }

                if (vm_page_busied(m)) {
                        /*
                         * Don't mess with busy pages.  Leave them at
                         * the front of the queue.  Most likely, they
                         * are being paged out and will leave the
                         * queue shortly after the scan finishes.  So,
                         * they ought to be discounted from the
                         * inactive count.
                         */
                        addl_page_shortage++;
                        goto reinsert;
                }

                /*
                 * Invalid pages can be easily freed. They cannot be
                 * mapped, vm_page_free() asserts this.
                 */
                if (m->valid == 0)
                        goto free_page;

                /*
                 * If the page has been referenced and the object is not dead,
                 * reactivate or requeue the page depending on whether the
                 * object is mapped.
                 *
                 * Test PGA_REFERENCED after calling pmap_ts_referenced() so
                 * that a reference from a concurrently destroyed mapping is
                 * observed here and now.
                 */
                if (object->ref_count != 0)
                        act_delta = pmap_ts_referenced(m);
                else {
                        KASSERT(!pmap_page_is_mapped(m),
                            ("page %p is mapped", m));
                        act_delta = 0;
                }
                if ((m->aflags & PGA_REFERENCED) != 0) {
                        vm_page_aflag_clear(m, PGA_REFERENCED);
                        act_delta++;
                }
                if (act_delta != 0) {
                        if (object->ref_count != 0) {
                                VM_CNT_INC(v_reactivated);
                                vm_page_activate(m);

                                /*
                                 * Increase the activation count if the page
                                 * was referenced while in the inactive queue.
                                 * This makes it less likely that the page will
                                 * be returned prematurely to the inactive
                                 * queue.
                                 */
                                m->act_count += act_delta + ACT_ADVANCE;
                                continue;
                        } else if ((object->flags & OBJ_DEAD) == 0) {
                                vm_page_aflag_set(m, PGA_REQUEUE);
                                goto reinsert;
                        }
                }

                /*
                 * If the page appears to be clean at the machine-independent
                 * layer, then remove all of its mappings from the pmap in
                 * anticipation of freeing it.  If, however, any of the page's
                 * mappings allow write access, then the page may still be
                 * modified until the last of those mappings are removed.
                 */
                if (object->ref_count != 0) {
                        vm_page_test_dirty(m);
                        if (m->dirty == 0)
                                pmap_remove_all(m);
                }

                /*
                 * Clean pages can be freed, but dirty pages must be sent back
                 * to the laundry, unless they belong to a dead object.
                 * Requeueing dirty pages from dead objects is pointless, as
                 * they are being paged out and freed by the thread that
                 * destroyed the object.
                 */
                if (m->dirty == 0) {
free_page:
                        /*
                         * Because we dequeued the page and have already
                         * checked for concurrent dequeue and enqueue
                         * requests, we can safely disassociate the page
                         * from the inactive queue.
                         */
                        KASSERT((m->aflags & PGA_QUEUE_STATE_MASK) == 0,
                            ("page %p has queue state", m));
                        m->queue = PQ_NONE;
                        vm_page_free(m);
                        page_shortage--;
                } else if ((object->flags & OBJ_DEAD) == 0)
                        vm_page_launder(m);
                continue;
reinsert:
                vm_pageout_reinsert_inactive(&ss, &rq, m);
        }
        if (mtx != NULL)
                mtx_unlock(mtx);
        if (object != NULL)
                VM_OBJECT_WUNLOCK(object);
        vm_pageout_reinsert_inactive(&ss, &rq, NULL);
        vm_pageout_reinsert_inactive(&ss, &ss.bq, NULL);
        vm_pagequeue_lock(pq);
        vm_pageout_end_scan(&ss);
        vm_pagequeue_unlock(pq);

        VM_CNT_ADD(v_dfree, starting_page_shortage - page_shortage);

        /*
         * Wake up the laundry thread so that it can perform any needed
         * laundering.  If we didn't meet our target, we're in shortfall and
         * need to launder more aggressively.  If PQ_LAUNDRY is empty and no
         * swap devices are configured, the laundry thread has no work to do, so
         * don't bother waking it up.
         *
         * The laundry thread uses the number of inactive queue scans elapsed
         * since the last laundering to determine whether to launder again, so
         * keep count.
         */
        if (starting_page_shortage > 0) {
                pq = &vmd->vmd_pagequeues[PQ_LAUNDRY];
                vm_pagequeue_lock(pq);
                if (vmd->vmd_laundry_request == VM_LAUNDRY_IDLE &&
                    (pq->pq_cnt > 0 || atomic_load_acq_int(&swapdev_enabled))) {
                        if (page_shortage > 0) {
                                vmd->vmd_laundry_request = VM_LAUNDRY_SHORTFALL;
                                VM_CNT_INC(v_pdshortfalls);
                        } else if (vmd->vmd_laundry_request !=
                            VM_LAUNDRY_SHORTFALL)
                                vmd->vmd_laundry_request =
                                    VM_LAUNDRY_BACKGROUND;
                        wakeup(&vmd->vmd_laundry_request);
                }
                vmd->vmd_clean_pages_freed +=
                    starting_page_shortage - page_shortage;
                vm_pagequeue_unlock(pq);
        }

        /*
         * Wakeup the swapout daemon if we didn't free the targeted number of
         * pages.
         */
        if (page_shortage > 0)
                vm_swapout_run();

        /*
         * If the inactive queue scan fails repeatedly to meet its
         * target, kill the largest process.
         */
        vm_pageout_mightbe_oom(vmd, page_shortage, starting_page_shortage);

        /*
         * Reclaim pages by swapping out idle processes, if configured to do so.
         */
        vm_swapout_run_idle();

        /*
         * See the description of addl_page_shortage above.
         */
        *addl_shortage = addl_page_shortage + deficit;

        return (page_shortage <= 0);
}

static int vm_pageout_oom_vote;

/*
 * The pagedaemon threads randlomly select one to perform the
 * OOM.  Trying to kill processes before all pagedaemons
 * failed to reach free target is premature.
 */
static void
vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
    int starting_page_shortage)
{
        int old_vote;

        if (starting_page_shortage <= 0 || starting_page_shortage !=
            page_shortage)
                vmd->vmd_oom_seq = 0;
        else
                vmd->vmd_oom_seq++;
        if (vmd->vmd_oom_seq < vm_pageout_oom_seq) {
                if (vmd->vmd_oom) {
                        vmd->vmd_oom = FALSE;
                        atomic_subtract_int(&vm_pageout_oom_vote, 1);
                }
                return;
        }

        /*
         * Do not follow the call sequence until OOM condition is
         * cleared.
         */
        vmd->vmd_oom_seq = 0;

        if (vmd->vmd_oom)
                return;

        vmd->vmd_oom = TRUE;
        old_vote = atomic_fetchadd_int(&vm_pageout_oom_vote, 1);
        if (old_vote != vm_ndomains - 1)
                return;

        /*
         * The current pagedaemon thread is the last in the quorum to
         * start OOM.  Initiate the selection and signaling of the
         * victim.
         */
        vm_pageout_oom(VM_OOM_MEM);

        /*
         * After one round of OOM terror, recall our vote.  On the
         * next pass, current pagedaemon would vote again if the low
         * memory condition is still there, due to vmd_oom being
         * false.
         */
        vmd->vmd_oom = FALSE;
        atomic_subtract_int(&vm_pageout_oom_vote, 1);
}

/*
 * The OOM killer is the page daemon's action of last resort when
 * memory allocation requests have been stalled for a prolonged period
 * of time because it cannot reclaim memory.  This function computes
 * the approximate number of physical pages that could be reclaimed if
 * the specified address space is destroyed.
 *
 * Private, anonymous memory owned by the address space is the
 * principal resource that we expect to recover after an OOM kill.
 * Since the physical pages mapped by the address space's COW entries
 * are typically shared pages, they are unlikely to be released and so
 * they are not counted.
 *
 * To get to the point where the page daemon runs the OOM killer, its
 * efforts to write-back vnode-backed pages may have stalled.  This
 * could be caused by a memory allocation deadlock in the write path
 * that might be resolved by an OOM kill.  Therefore, physical pages
 * belonging to vnode-backed objects are counted, because they might
 * be freed without being written out first if the address space holds
 * the last reference to an unlinked vnode.
 *
 * Similarly, physical pages belonging to OBJT_PHYS objects are
 * counted because the address space might hold the last reference to
 * the object.
 */
static long
vm_pageout_oom_pagecount(struct vmspace *vmspace)
{
        vm_map_t map;
        vm_map_entry_t entry;
        vm_object_t obj;
        long res;

        map = &vmspace->vm_map;
        KASSERT(!map->system_map, ("system map"));
        sx_assert(&map->lock, SA_LOCKED);
        res = 0;
        for (entry = map->header.next; entry != &map->header;
            entry = entry->next) {
                if ((entry->eflags & MAP_ENTRY_IS_SUB_MAP) != 0)
                        continue;
                obj = entry->object.vm_object;
                if (obj == NULL)
                        continue;
                if ((entry->eflags & MAP_ENTRY_NEEDS_COPY) != 0 &&
                    obj->ref_count != 1)
                        continue;
                switch (obj->type) {
                case OBJT_DEFAULT:
                case OBJT_SWAP:
                case OBJT_PHYS:
                case OBJT_VNODE:
                        res += obj->resident_page_count;
                        break;
                }
        }
        return (res);
}

static int vm_oom_ratelim_last;
static int vm_oom_pf_secs = 10;
SYSCTL_INT(_vm, OID_AUTO, oom_pf_secs, CTLFLAG_RWTUN, &vm_oom_pf_secs, 0,
    "");
static struct mtx vm_oom_ratelim_mtx;

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

        /*
         * For OOM requests originating from vm_fault(), there is a high
         * chance that a single large process faults simultaneously in
         * several threads.  Also, on an active system running many
         * processes of middle-size, like buildworld, all of them
         * could fault almost simultaneously as well.
         *
         * To avoid killing too many processes, rate-limit OOMs
         * initiated by vm_fault() time-outs on the waits for free
         * pages.
         */
        mtx_lock(&vm_oom_ratelim_mtx);
        now = ticks;
        if (shortage == VM_OOM_MEM_PF &&
            (u_int)(now - vm_oom_ratelim_last) < hz * vm_oom_pf_secs) {
                mtx_unlock(&vm_oom_ratelim_mtx);
                return;
        }
        vm_oom_ratelim_last = now;
        mtx_unlock(&vm_oom_ratelim_mtx);

        /*
         * 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 its 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) {
                PROC_LOCK(p);

                /*
                 * 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_WEXIT)) != 0 ||
                    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 = false;
                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) &&
                            !TD_IS_SWAPPED(td)) {
                                thread_unlock(td);
                                breakout = true;
                                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;
                }
                _PHOLD_LITE(p);
                PROC_UNLOCK(p);
                sx_sunlock(&allproc_lock);
                if (!vm_map_trylock_read(&vm->vm_map)) {
                        vmspace_free(vm);
                        sx_slock(&allproc_lock);
                        PRELE(p);
                        continue;
                }
                size = vmspace_swap_count(vm);
                if (shortage == VM_OOM_MEM || shortage == VM_OOM_MEM_PF)
                        size += vm_pageout_oom_pagecount(vm);
                vm_map_unlock_read(&vm->vm_map);
                vmspace_free(vm);
                sx_slock(&allproc_lock);

                /*
                 * If this process is bigger than the biggest one,
                 * remember it.
                 */
                if (size > bigsize) {
                        if (bigproc != NULL)
                                PRELE(bigproc);
                        bigproc = p;
                        bigsize = size;
                } else {
                        PRELE(p);
                }
        }
        sx_sunlock(&allproc_lock);
        if (bigproc != NULL) {
                if (vm_panic_on_oom != 0)
                        panic("out of swap space");
                PROC_LOCK(bigproc);
                killproc(bigproc, "out of swap space");
                sched_nice(bigproc, PRIO_MIN);
                _PRELE(bigproc);
                PROC_UNLOCK(bigproc);
        }
}

static bool
vm_pageout_lowmem(void)
{
        static int lowmem_ticks = 0;
        int last;

        last = atomic_load_int(&lowmem_ticks);
        while ((u_int)(ticks - last) / hz >= lowmem_period) {
                if (atomic_fcmpset_int(&lowmem_ticks, &last, ticks) == 0)
                        continue;

                /*
                 * Decrease registered cache sizes.
                 */
                SDT_PROBE0(vm, , , vm__lowmem_scan);
                EVENTHANDLER_INVOKE(vm_lowmem, VM_LOW_PAGES);

                /*
                 * We do this explicitly after the caches have been
                 * drained above.
                 */
                uma_reclaim();
                return (true);
        }
        return (false);
}

static void
vm_pageout_worker(void *arg)
{
        struct vm_domain *vmd;
        u_int ofree;
        int addl_shortage, domain, shortage;
        bool target_met;

        domain = (uintptr_t)arg;
        vmd = VM_DOMAIN(domain);
        shortage = 0;
        target_met = true;

        /*
         * XXXKIB It could be useful to bind pageout daemon threads to
         * the cores belonging to the domain, from which vm_page_array
         * is allocated.
         */

        KASSERT(vmd->vmd_segs != 0, ("domain without segments"));
        vmd->vmd_last_active_scan = ticks;

        /*
         * The pageout daemon worker is never done, so loop forever.
         */
        while (TRUE) {
                vm_domain_pageout_lock(vmd);

                /*
                 * We need to clear wanted before we check the limits.  This
                 * prevents races with wakers who will check wanted after they
                 * reach the limit.
                 */
                atomic_store_int(&vmd->vmd_pageout_wanted, 0);

                /*
                 * Might the page daemon need to run again?
                 */
                if (vm_paging_needed(vmd, vmd->vmd_free_count)) {
                        /*
                         * Yes.  If the scan failed to produce enough free
                         * pages, sleep uninterruptibly for some time in the
                         * hope that the laundry thread will clean some pages.
                         */
                        vm_domain_pageout_unlock(vmd);
                        if (!target_met)
                                pause("pwait", hz / VM_INACT_SCAN_RATE);
                } else {
                        /*
                         * No, sleep until the next wakeup or until pages
                         * need to have their reference stats updated.
                         */
                        if (mtx_sleep(&vmd->vmd_pageout_wanted,
                            vm_domain_pageout_lockptr(vmd), PDROP | PVM,
                            "psleep", hz / VM_INACT_SCAN_RATE) == 0)
                                VM_CNT_INC(v_pdwakeups);
                }

                /* Prevent spurious wakeups by ensuring that wanted is set. */
                atomic_store_int(&vmd->vmd_pageout_wanted, 1);

                /*
                 * Use the controller to calculate how many pages to free in
                 * this interval, and scan the inactive queue.  If the lowmem
                 * handlers appear to have freed up some pages, subtract the
                 * difference from the inactive queue scan target.
                 */
                shortage = pidctrl_daemon(&vmd->vmd_pid, vmd->vmd_free_count);
                if (shortage > 0) {
                        ofree = vmd->vmd_free_count;
                        if (vm_pageout_lowmem() && vmd->vmd_free_count > ofree)
                                shortage -= min(vmd->vmd_free_count - ofree,
                                    (u_int)shortage);
                        target_met = vm_pageout_scan_inactive(vmd, shortage,
                            &addl_shortage);
                } else
                        addl_shortage = 0;

                /*
                 * Scan the active queue.  A positive value for shortage
                 * indicates that we must aggressively deactivate pages to avoid
                 * a shortfall.
                 */
                shortage = vm_pageout_active_target(vmd) + addl_shortage;
                vm_pageout_scan_active(vmd, shortage);
        }
}

/*
 *      vm_pageout_init initialises basic pageout daemon settings.
 */
static void
vm_pageout_init_domain(int domain)
{
        struct vm_domain *vmd;
        struct sysctl_oid *oid;

        vmd = VM_DOMAIN(domain);
        vmd->vmd_interrupt_free_min = 2;

        /*
         * v_free_reserved needs to include enough for the largest
         * swap pager structures plus enough for any pv_entry structs
         * when paging. 
         */
        if (vmd->vmd_page_count > 1024)
                vmd->vmd_free_min = 4 + (vmd->vmd_page_count - 1024) / 200;
        else
                vmd->vmd_free_min = 4;
        vmd->vmd_pageout_free_min = (2*MAXBSIZE)/PAGE_SIZE +
            vmd->vmd_interrupt_free_min;
        vmd->vmd_free_reserved = vm_pageout_page_count +
            vmd->vmd_pageout_free_min + (vmd->vmd_page_count / 768);
        vmd->vmd_free_severe = vmd->vmd_free_min / 2;
        vmd->vmd_free_target = 4 * vmd->vmd_free_min + vmd->vmd_free_reserved;
        vmd->vmd_free_min += vmd->vmd_free_reserved;
        vmd->vmd_free_severe += vmd->vmd_free_reserved;
        vmd->vmd_inactive_target = (3 * vmd->vmd_free_target) / 2;
        if (vmd->vmd_inactive_target > vmd->vmd_free_count / 3)
                vmd->vmd_inactive_target = vmd->vmd_free_count / 3;

        /*
         * Set the default wakeup threshold to be 10% below the paging
         * target.  This keeps the steady state out of shortfall.
         */
        vmd->vmd_pageout_wakeup_thresh = (vmd->vmd_free_target / 10) * 9;

        /*
         * Target amount of memory to move out of the laundry queue during a
         * background laundering.  This is proportional to the amount of system
         * memory.
         */
        vmd->vmd_background_launder_target = (vmd->vmd_free_target -
            vmd->vmd_free_min) / 10;

        /* Initialize the pageout daemon pid controller. */
        pidctrl_init(&vmd->vmd_pid, hz / VM_INACT_SCAN_RATE,
            vmd->vmd_free_target, PIDCTRL_BOUND,
            PIDCTRL_KPD, PIDCTRL_KID, PIDCTRL_KDD);
        oid = SYSCTL_ADD_NODE(NULL, SYSCTL_CHILDREN(vmd->vmd_oid), OID_AUTO,
            "pidctrl", CTLFLAG_RD, NULL, "");
        pidctrl_init_sysctl(&vmd->vmd_pid, SYSCTL_CHILDREN(oid));
}

static void
vm_pageout_init(void)
{
        u_long freecount;
        int i;

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

        freecount = 0;
        for (i = 0; i < vm_ndomains; i++) {
                struct vm_domain *vmd;

                vm_pageout_init_domain(i);
                vmd = VM_DOMAIN(i);
                vm_cnt.v_free_reserved += vmd->vmd_free_reserved;
                vm_cnt.v_free_target += vmd->vmd_free_target;
                vm_cnt.v_free_min += vmd->vmd_free_min;
                vm_cnt.v_inactive_target += vmd->vmd_inactive_target;
                vm_cnt.v_pageout_free_min += vmd->vmd_pageout_free_min;
                vm_cnt.v_interrupt_free_min += vmd->vmd_interrupt_free_min;
                vm_cnt.v_free_severe += vmd->vmd_free_severe;
                freecount += vmd->vmd_free_count;
        }

        /*
         * Set interval in seconds for active scan.  We want to visit each
         * page at least once every ten minutes.  This is to prevent worst
         * case paging behaviors with stale active LRU.
         */
        if (vm_pageout_update_period == 0)
                vm_pageout_update_period = 600;

        /*
         * Set the maximum number of user-wired virtual pages.  Historically the
         * main source of such pages was mlock(2) and mlockall(2).  Hypervisors
         * may also request user-wired memory.
         */
        if (vm_page_max_user_wired == 0)
                vm_page_max_user_wired = 4 * freecount / 5;
}

/*
 *     vm_pageout is the high level pageout daemon.
 */
static void
vm_pageout(void)
{
        struct proc *p;
        struct thread *td;
        int error, first, i;

        p = curproc;
        td = curthread;

        mtx_init(&vm_oom_ratelim_mtx, "vmoomr", NULL, MTX_DEF);
        swap_pager_swap_init();
        for (first = -1, i = 0; i < vm_ndomains; i++) {
                if (VM_DOMAIN_EMPTY(i)) {
                        if (bootverbose)
                                printf("domain %d empty; skipping pageout\n",
                                    i);
                        continue;
                }
                if (first == -1)
                        first = i;
                else {
                        error = kthread_add(vm_pageout_worker,
                            (void *)(uintptr_t)i, p, NULL, 0, 0, "dom%d", i);
                        if (error != 0)
                                panic("starting pageout for domain %d: %d\n",
                                    i, error);
                }
                error = kthread_add(vm_pageout_laundry_worker,
                    (void *)(uintptr_t)i, p, NULL, 0, 0, "laundry: dom%d", i);
                if (error != 0)
                        panic("starting laundry for domain %d: %d", i, error);
        }
        error = kthread_add(uma_reclaim_worker, NULL, p, NULL, 0, 0, "uma");
        if (error != 0)
                panic("starting uma_reclaim helper, error %d\n", error);

        snprintf(td->td_name, sizeof(td->td_name), "dom%d", first);
        vm_pageout_worker((void *)(uintptr_t)first);
}

/*
 * Perform an advisory wakeup of the page daemon.
 */
void
pagedaemon_wakeup(int domain)
{
        struct vm_domain *vmd;

        vmd = VM_DOMAIN(domain);
        vm_domain_pageout_assert_unlocked(vmd);
        if (curproc == pageproc)
                return;

        if (atomic_fetchadd_int(&vmd->vmd_pageout_wanted, 1) == 0) {
                vm_domain_pageout_lock(vmd);
                atomic_store_int(&vmd->vmd_pageout_wanted, 1);
                wakeup(&vmd->vmd_pageout_wanted);
                vm_domain_pageout_unlock(vmd);
        }
}