FreeBSD/Linux Kernel Cross Reference
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/blockcount.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 the given number of out-of-memory errors 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");
   
   static int pageout_cpus_per_thread = 16;
   SYSCTL_INT(_vm, OID_AUTO, pageout_cpus_per_thread, CTLFLAG_RDTUN,
       &pageout_cpus_per_thread, 0,
       "Number of CPUs per pagedaemon worker thread");
     
   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_long vm_page_max_user_wired;
   SYSCTL_ULONG(_vm, OID_AUTO, max_user_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->a.flags & 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->a.flags & 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_pageout_defer() must be used to
    * determine whether the page has been logically dequeued since the batch was
    * collected.
    */
   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->a.flags & 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->a.flags & 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));
   }
   
   /*
    * Determine whether processing of a page should be deferred and ensure that any
    * outstanding queue operations are processed.
    */
   static __always_inline bool
   vm_pageout_defer(vm_page_t m, const uint8_t queue, const bool enqueued)
   {
           vm_page_astate_t as;
   
           as = vm_page_astate_load(m);
           if (__predict_false(as.queue != queue ||
               ((as.flags & PGA_ENQUEUED) != 0) != enqueued))
                   return (true);
           if ((as.flags & PGA_QUEUE_OP_MASK) != 0) {
                   vm_page_pqbatch_submit(m, queue);
                   return (true);
           }
           return (false);
   }
   
   /*
    * 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;
   
           object = m->object;
           VM_OBJECT_ASSERT_WLOCKED(object);
           pindex = m->pindex;
   
           vm_page_assert_xbusied(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_tryxbusy(p) == 0) {
                           ib = 0;
                           break;
                   }
                   if (vm_page_wired(p)) {
                           ib = 0;
                           vm_page_xunbusy(p);
                           break;
                   }
                   vm_page_test_dirty(p);
                   if (p->dirty == 0) {
                           ib = 0;
                           vm_page_xunbusy(p);
                           break;
                   }
                   if (!vm_page_in_laundry(p) || !vm_page_try_remove_write(p)) {
                           vm_page_xunbusy(p);
                           ib = 0;
                           break;
                   }
                   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_tryxbusy(p) == 0)
                           break;
                   if (vm_page_wired(p)) {
                           vm_page_xunbusy(p);
                           break;
                   }
                   vm_page_test_dirty(p);
                   if (p->dirty == 0) {
                           vm_page_xunbusy(p);
                           break;
                   }
                   if (!vm_page_in_laundry(p) || !vm_page_try_remove_write(p)) {
                           vm_page_xunbusy(p);
                           break;
                   }
                   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 shared 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(vm_page_all_valid(mc[i]),
                       ("vm_pageout_flush: partially invalid page %p index %d/%d",
                           mc[i], i, count));
                   KASSERT((mc[i]->a.flags & PGA_WRITEABLE) == 0,
                       ("vm_pageout_flush: writeable page %p", mc[i]));
                   vm_page_busy_downgrade(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:
                           /*
                            * The page may have moved since laundering started, in
                            * which case it should be left alone.
                            */
                           if (vm_page_in_laundry(mt))
                                   vm_page_deactivate_noreuse(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);
                           if (vm_page_in_laundry(mt))
                                   vm_page_deactivate_noreuse(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.)
                            */
                           if (object->type == OBJT_SWAP &&
                               pageout_status[i] == VM_PAGER_FAIL) {
                                   vm_page_unswappable(mt);
                                   numpagedout++;
                           } else
                                   vm_page_activate(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;
   
           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_xunbusy(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)) {
                           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;
                   }
   
                   /*
                    * While the object was 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) {
                           error = ENXIO;
                           goto unlock_all;
                   }
   
                   /*
                    * The page may have been busied while the object lock was
                    * released.
                    */
                   if (vm_page_tryxbusy(m) == 0) {
                           error = EBUSY;
                           goto unlock_all;
                   }
           }
   
           /*
            * Remove all writeable mappings, failing if the page is wired.
            */
           if (!vm_page_try_remove_write(m)) {
                   vm_page_xunbusy(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:
           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;
           vm_object_t object;
           vm_page_t m, marker;
           vm_page_astate_t new, old;
           int act_delta, error, numpagedout, queue, refs, starting_target;
           int vnodes_skipped;
           bool pageout_ok;
   
           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;
   
                   /*
                    * Don't touch a page that was removed from the queue after the
                    * page queue lock was released.  Otherwise, ensure that any
                    * pending queue operations, such as dequeues for wired pages,
                    * are handled.
                    */
                   if (vm_pageout_defer(m, queue, true))
                           continue;
   
                   /*
                    * Lock the page's object.
                    */
                   if (object == NULL || object != m->object) {
                           if (object != NULL)
                                   VM_OBJECT_WUNLOCK(object);
                           object = atomic_load_ptr(&m->object);
                           if (__predict_false(object == NULL))
                                   /* The page is being freed by another thread. */
                                   continue;
   
                           /* Depends on type-stability. */
                           VM_OBJECT_WLOCK(object);
                           if (__predict_false(m->object != object)) {
                                   VM_OBJECT_WUNLOCK(object);
                                   object = NULL;
                                   continue;
                           }
                   }
   
                   if (vm_page_tryxbusy(m) == 0)
                           continue;
   
                   /*
                    * Check for wirings now that we hold the object lock and have
                    * exclusively busied the page.  If the page is mapped, it may
                    * still be wired by pmap lookups.  The call to
                    * vm_page_try_remove_all() below atomically checks for such
                    * wirings and removes mappings.  If the page is unmapped, the
                    * wire count is guaranteed not to increase after this check.
                    */
                   if (__predict_false(vm_page_wired(m)))
                           goto skip_page;
   
                   /*
                    * Invalid pages can be easily freed.  They cannot be
                    * mapped; vm_page_free() asserts this.
                    */
                   if (vm_page_none_valid(m))
                           goto free_page;
   
                   refs = object->ref_count != 0 ? pmap_ts_referenced(m) : 0;
   
                   for (old = vm_page_astate_load(m);;) {
                           /*
                            * Check to see if the page has been removed from the
                            * queue since the first such check.  Leave it alone if
                            * so, discarding any references collected by
                            * pmap_ts_referenced().
                            */
                           if (__predict_false(_vm_page_queue(old) == PQ_NONE))
                                   goto skip_page;
   
                           new = old;
                           act_delta = refs;
                           if ((old.flags & PGA_REFERENCED) != 0) {
                                   new.flags &= ~PGA_REFERENCED;
                                   act_delta++;
                           }
                           if (act_delta == 0) {
                                   ;
                           } else if (object->ref_count != 0) {
                                   /*
                                    * 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 laundry queue.
                                    */
                                   new.act_count += ACT_ADVANCE +
                                       act_delta;
                                   if (new.act_count > ACT_MAX)
                                           new.act_count = ACT_MAX;
   
                                   new.flags &= ~PGA_QUEUE_OP_MASK;
                                   new.flags |= PGA_REQUEUE;
                                   new.queue = PQ_ACTIVE;
                                   if (!vm_page_pqstate_commit(m, &old, new))
                                           continue;
   
                                   /*
                                    * 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--;
                                   VM_CNT_INC(v_reactivated);
                                   goto skip_page;
                           } else if ((object->flags & OBJ_DEAD) == 0) {
                                   new.flags |= PGA_REQUEUE;
                                   if (!vm_page_pqstate_commit(m, &old, new))
                                           continue;
                                   goto skip_page;
                           }
                           break;
                   }
   
                   /*
                    * 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 && !vm_page_try_remove_all(m))
                                   goto skip_page;
                   }
   
                   /*
                    * 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:
                           /*
                            * Now we are guaranteed that no other threads are
                            * manipulating the page, check for a last-second
                            * reference.
                            */
                           if (vm_pageout_defer(m, queue, true))
                                   goto skip_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_launder(m);
                                   goto skip_page;
                           }
   
                           /*
                            * 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++;
                           }
                           object = NULL;
                   } else {
   skip_page:
                           vm_page_xunbusy(m);
                   }
           }
           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 = num != 0 ? (1u << ((fls(num) - 1) & ~1)) : 0;
           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;
          vm_object_t object;
          vm_page_t m, marker;
          struct vm_pagequeue *pq;
          vm_page_astate_t old, new;
          long min_scan;
          int act_delta, max_scan, ps_delta, refs, scan_tick;
          uint8_t nqueue;
  
          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;
  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;
  
                  /*
                   * Don't touch a page that was removed from the queue after the
                   * page queue lock was released.  Otherwise, ensure that any
                   * pending queue operations, such as dequeues for wired pages,
                   * are handled.
                   */
                  if (vm_pageout_defer(m, PQ_ACTIVE, true))
                          continue;
  
                  /*
                   * A page's object pointer may be set to NULL before
                   * the object lock is acquired.
                   */
                  object = atomic_load_ptr(&m->object);
                  if (__predict_false(object == NULL))
                          /*
                           * The page has been removed from its object.
                           */
                          continue;
  
                  /* Deferred free of swap space. */
                  if ((m->a.flags & PGA_SWAP_FREE) != 0 &&
                      VM_OBJECT_TRYWLOCK(object)) {
                          if (m->object == object)
                                  vm_pager_page_unswapped(m);
                          VM_OBJECT_WUNLOCK(object);
                  }
  
                  /*
                   * 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.
                   */
                  refs = object->ref_count != 0 ? pmap_ts_referenced(m) : 0;
  
                  old = vm_page_astate_load(m);
                  do {
                          /*
                           * Check to see if the page has been removed from the
                           * queue since the first such check.  Leave it alone if
                           * so, discarding any references collected by
                           * pmap_ts_referenced().
                           */
                          if (__predict_false(_vm_page_queue(old) == PQ_NONE)) {
                                  ps_delta = 0;
                                  break;
                          }
  
                          /*
                           * Advance or decay the act_count based on recent usage.
                           */
                          new = old;
                          act_delta = refs;
                          if ((old.flags & PGA_REFERENCED) != 0) {
                                  new.flags &= ~PGA_REFERENCED;
                                  act_delta++;
                          }
                          if (act_delta != 0) {
                                  new.act_count += ACT_ADVANCE + act_delta;
                                  if (new.act_count > ACT_MAX)
                                          new.act_count = ACT_MAX;
                          } else {
                                  new.act_count -= min(new.act_count,
                                      ACT_DECLINE);
                          }
  
                          if (new.act_count > 0) {
                                  /*
                                   * Adjust the activation count and keep the page
                                   * in the active queue.  The count might be left
                                   * unchanged if it is saturated.  The page may
                                   * have been moved to a different queue since we
                                   * started the scan, in which case we move it
                                   * back.
                                   */
                                  ps_delta = 0;
                                  if (old.queue != PQ_ACTIVE) {
                                          new.flags &= ~PGA_QUEUE_OP_MASK;
                                          new.flags |= PGA_REQUEUE;
                                          new.queue = PQ_ACTIVE;
                                  }
                          } else {
                                  /*
                                   * When not short for inactive pages, let dirty
                                   * pages go through the inactive queue before
                                   * moving to the laundry queue.  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.
                                   *
                                   * 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 (page_shortage <= 0) {
                                          nqueue = PQ_INACTIVE;
                                          ps_delta = 0;
                                  } else if (m->dirty == 0) {
                                          nqueue = PQ_INACTIVE;
                                          ps_delta = act_scan_laundry_weight;
                                  } else {
                                          nqueue = PQ_LAUNDRY;
                                          ps_delta = 1;
                                  }
  
                                  new.flags &= ~PGA_QUEUE_OP_MASK;
                                  new.flags |= PGA_REQUEUE;
                                  new.queue = nqueue;
                          }
                  } while (!vm_page_pqstate_commit(m, &old, new));
  
                  page_shortage -= ps_delta;
          }
          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 vm_pagequeue *pq, vm_page_t marker,
      vm_page_t m)
  {
          vm_page_astate_t as;
  
          vm_pagequeue_assert_locked(pq);
  
          as = vm_page_astate_load(m);
          if (as.queue != PQ_INACTIVE || (as.flags & PGA_ENQUEUED) != 0)
                  return (0);
          vm_page_aflag_set(m, PGA_ENQUEUED);
          TAILQ_INSERT_BEFORE(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;
          vm_page_t marker;
          int delta;
  
          delta = 0;
          marker = ss->marker;
          pq = ss->pq;
  
          if (m != NULL) {
                  if (vm_batchqueue_insert(bq, m))
                          return;
                  vm_pagequeue_lock(pq);
                  delta += vm_pageout_reinsert_inactive_page(pq, marker, m);
          } else
                  vm_pagequeue_lock(pq);
          while ((m = vm_batchqueue_pop(bq)) != NULL)
                  delta += vm_pageout_reinsert_inactive_page(pq, marker, m);
          vm_pagequeue_cnt_add(pq, delta);
          vm_pagequeue_unlock(pq);
          vm_batchqueue_init(bq);
  }
  
  static void
  vm_pageout_scan_inactive(struct vm_domain *vmd, int page_shortage)
  {
          struct timeval start, end;
          struct scan_state ss;
          struct vm_batchqueue rq;
          struct vm_page marker_page;
          vm_page_t m, marker;
          struct vm_pagequeue *pq;
          vm_object_t object;
          vm_page_astate_t old, new;
          int act_delta, addl_page_shortage, starting_page_shortage, refs;
  
          object = NULL;
          vm_batchqueue_init(&rq);
          getmicrouptime(&start);
  
          /*
           * 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;
  
          /*
           * 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->a.act_count is not used to make
           * decisions for the inactive queue, only for the active queue.)
           */
          starting_page_shortage = page_shortage;
          marker = &marker_page;
          vm_page_init_marker(marker, PQ_INACTIVE, 0);
          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));
  
                  /*
                   * Don't touch a page that was removed from the queue after the
                   * page queue lock was released.  Otherwise, ensure that any
                   * pending queue operations, such as dequeues for wired pages,
                   * are handled.
                   */
                  if (vm_pageout_defer(m, PQ_INACTIVE, false))
                          continue;
  
                  /*
                   * Lock the page's object.
                   */
                  if (object == NULL || object != m->object) {
                          if (object != NULL)
                                  VM_OBJECT_WUNLOCK(object);
                          object = atomic_load_ptr(&m->object);
                          if (__predict_false(object == NULL))
                                  /* The page is being freed by another thread. */
                                  continue;
  
                          /* Depends on type-stability. */
                          VM_OBJECT_WLOCK(object);
                          if (__predict_false(m->object != object)) {
                                  VM_OBJECT_WUNLOCK(object);
                                  object = NULL;
                                  goto reinsert;
                          }
                  }
  
                  if (vm_page_tryxbusy(m) == 0) {
                          /*
                           * 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;
                  }
  
                  /* Deferred free of swap space. */
                  if ((m->a.flags & PGA_SWAP_FREE) != 0)
                          vm_pager_page_unswapped(m);
  
                  /*
                   * Check for wirings now that we hold the object lock and have
                   * exclusively busied the page.  If the page is mapped, it may
                   * still be wired by pmap lookups.  The call to
                   * vm_page_try_remove_all() below atomically checks for such
                   * wirings and removes mappings.  If the page is unmapped, the
                   * wire count is guaranteed not to increase after this check.
                   */
                  if (__predict_false(vm_page_wired(m)))
                          goto skip_page;
  
                  /*
                   * Invalid pages can be easily freed. They cannot be
                   * mapped, vm_page_free() asserts this.
                   */
                  if (vm_page_none_valid(m))
                          goto free_page;
  
                  refs = object->ref_count != 0 ? pmap_ts_referenced(m) : 0;
  
                  for (old = vm_page_astate_load(m);;) {
                          /*
                           * Check to see if the page has been removed from the
                           * queue since the first such check.  Leave it alone if
                           * so, discarding any references collected by
                           * pmap_ts_referenced().
                           */
                          if (__predict_false(_vm_page_queue(old) == PQ_NONE))
                                  goto skip_page;
  
                          new = old;
                          act_delta = refs;
                          if ((old.flags & PGA_REFERENCED) != 0) {
                                  new.flags &= ~PGA_REFERENCED;
                                  act_delta++;
                          }
                          if (act_delta == 0) {
                                  ;
                          } else if (object->ref_count != 0) {
                                  /*
                                   * 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.
                                   */
                                  new.act_count += ACT_ADVANCE +
                                      act_delta;
                                  if (new.act_count > ACT_MAX)
                                          new.act_count = ACT_MAX;
  
                                  new.flags &= ~PGA_QUEUE_OP_MASK;
                                  new.flags |= PGA_REQUEUE;
                                  new.queue = PQ_ACTIVE;
                                  if (!vm_page_pqstate_commit(m, &old, new))
                                          continue;
  
                                  VM_CNT_INC(v_reactivated);
                                  goto skip_page;
                          } else if ((object->flags & OBJ_DEAD) == 0) {
                                  new.queue = PQ_INACTIVE;
                                  new.flags |= PGA_REQUEUE;
                                  if (!vm_page_pqstate_commit(m, &old, new))
                                          continue;
                                  goto skip_page;
                          }
                          break;
                  }
  
                  /*
                   * 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 && !vm_page_try_remove_all(m))
                                  goto skip_page;
                  }
  
                  /*
                   * 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:
                          /*
                           * Now we are guaranteed that no other threads are
                           * manipulating the page, check for a last-second
                           * reference that would save it from doom.
                           */
                          if (vm_pageout_defer(m, PQ_INACTIVE, false))
                                  goto skip_page;
  
                          /*
                           * Because we dequeued the page and have already checked
                           * for pending dequeue and enqueue requests, we can
                           * safely disassociate the page from the inactive queue
                           * without holding the queue lock.
                           */
                          m->a.queue = PQ_NONE;
                          vm_page_free(m);
                          page_shortage--;
                          continue;
                  }
                  if ((object->flags & OBJ_DEAD) == 0)
                          vm_page_launder(m);
  skip_page:
                  vm_page_xunbusy(m);
                  continue;
  reinsert:
                  vm_pageout_reinsert_inactive(&ss, &rq, m);
          }
          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);
  
          /*
           * Record the remaining shortage and the progress and rate it was made.
           */
          atomic_add_int(&vmd->vmd_addl_shortage, addl_page_shortage);
          getmicrouptime(&end);
          timevalsub(&end, &start);
          atomic_add_int(&vmd->vmd_inactive_us,
              end.tv_sec * 1000000 + end.tv_usec);
          atomic_add_int(&vmd->vmd_inactive_freed,
              starting_page_shortage - page_shortage);
  }
  
  /*
   * Dispatch a number of inactive threads according to load and collect the
   * results to present a coherent view of paging activity on this domain.
   */
  static int
  vm_pageout_inactive_dispatch(struct vm_domain *vmd, int shortage)
  {
          u_int freed, pps, slop, threads, us;
  
          vmd->vmd_inactive_shortage = shortage;
          slop = 0;
  
          /*
           * If we have more work than we can do in a quarter of our interval, we
           * fire off multiple threads to process it.
           */
          threads = vmd->vmd_inactive_threads;
          if (threads > 1 && vmd->vmd_inactive_pps != 0 &&
              shortage > vmd->vmd_inactive_pps / VM_INACT_SCAN_RATE / 4) {
                  vmd->vmd_inactive_shortage /= threads;
                  slop = shortage % threads;
                  vm_domain_pageout_lock(vmd);
                  blockcount_acquire(&vmd->vmd_inactive_starting, threads - 1);
                  blockcount_acquire(&vmd->vmd_inactive_running, threads - 1);
                  wakeup(&vmd->vmd_inactive_shortage);
                  vm_domain_pageout_unlock(vmd);
          }
  
          /* Run the local thread scan. */
          vm_pageout_scan_inactive(vmd, vmd->vmd_inactive_shortage + slop);
  
          /*
           * Block until helper threads report results and then accumulate
           * totals.
           */
          blockcount_wait(&vmd->vmd_inactive_running, NULL, "vmpoid", PVM);
          freed = atomic_readandclear_int(&vmd->vmd_inactive_freed);
          VM_CNT_ADD(v_dfree, freed);
  
          /*
           * Calculate the per-thread paging rate with an exponential decay of
           * prior results.  Careful to avoid integer rounding errors with large
           * us values.
           */
          us = max(atomic_readandclear_int(&vmd->vmd_inactive_us), 1);
          if (us > 1000000)
                  /* Keep rounding to tenths */
                  pps = (freed * 10) / ((us * 10) / 1000000);
          else
                  pps = (1000000 / us) * freed;
          vmd->vmd_inactive_pps = (vmd->vmd_inactive_pps / 2) + (pps / 2);
  
          return (shortage - freed);
  }
  
  /*
   * Attempt to reclaim the requested number of pages from the inactive queue.
   * Returns true if the shortage was addressed.
   */
  static int
  vm_pageout_inactive(struct vm_domain *vmd, int shortage, int *addl_shortage)
  {
          struct vm_pagequeue *pq;
          u_int addl_page_shortage, deficit, page_shortage;
          u_int starting_page_shortage;
  
          /*
           * 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 = shortage + deficit;
  
          /*
           * Run the inactive scan on as many threads as is necessary.
           */
          page_shortage = vm_pageout_inactive_dispatch(vmd, starting_page_shortage);
          addl_page_shortage = atomic_readandclear_int(&vmd->vmd_addl_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;
          VM_MAP_ENTRY_FOREACH(entry, map) {
                  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 && --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);
          }
  }
  
  /*
   * Signal a free page shortage to subsystems that have registered an event
   * handler.  Reclaim memory from UMA in the event of a severe shortage.
   * Return true if the free page count should be re-evaluated.
   */
  static bool
  vm_pageout_lowmem(void)
  {
          static int lowmem_ticks = 0;
          int last;
          bool ret;
  
          ret = false;
  
          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(UMA_RECLAIM_TRIM);
                  ret = true;
                  break;
          }
  
          /*
           * Kick off an asynchronous reclaim of cached memory if one of the
           * page daemons is failing to keep up with demand.  Use the "severe"
           * threshold instead of "min" to ensure that we do not blow away the
           * caches if a subset of the NUMA domains are depleted by kernel memory
           * allocations; the domainset iterators automatically skip domains
           * below the "min" threshold on the first pass.
           *
           * UMA reclaim worker has its own rate-limiting mechanism, so don't
           * worry about kicking it too often.
           */
          if (vm_page_count_severe())
                  uma_reclaim_wakeup();
  
          return (ret);
  }
  
  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_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_helper runs additional pageout daemons in times of high paging
   * activity.
   */
  static void
  vm_pageout_helper(void *arg)
  {
          struct vm_domain *vmd;
          int domain;
  
          domain = (uintptr_t)arg;
          vmd = VM_DOMAIN(domain);
  
          vm_domain_pageout_lock(vmd);
          for (;;) {
                  msleep(&vmd->vmd_inactive_shortage,
                      vm_domain_pageout_lockptr(vmd), PVM, "psleep", 0);
                  blockcount_release(&vmd->vmd_inactive_starting, 1);
  
                  vm_domain_pageout_unlock(vmd);
                  vm_pageout_scan_inactive(vmd, vmd->vmd_inactive_shortage);
                  vm_domain_pageout_lock(vmd);
  
                  /*
                   * Release the running count while the pageout lock is held to
                   * prevent wakeup races.
                   */
                  blockcount_release(&vmd->vmd_inactive_running, 1);
          }
  }
  
  static int
  get_pageout_threads_per_domain(const struct vm_domain *vmd)
  {
          unsigned total_pageout_threads, eligible_cpus, domain_cpus;
  
          if (VM_DOMAIN_EMPTY(vmd->vmd_domain))
                  return (0);
  
          /*
           * Semi-arbitrarily constrain pagedaemon threads to less than half the
           * total number of CPUs in the system as an upper limit.
           */
          if (pageout_cpus_per_thread < 2)
                  pageout_cpus_per_thread = 2;
          else if (pageout_cpus_per_thread > mp_ncpus)
                  pageout_cpus_per_thread = mp_ncpus;
  
          total_pageout_threads = howmany(mp_ncpus, pageout_cpus_per_thread);
          domain_cpus = CPU_COUNT(&cpuset_domain[vmd->vmd_domain]);
  
          /* Pagedaemons are not run in empty domains. */
          eligible_cpus = mp_ncpus;
          for (unsigned i = 0; i < vm_ndomains; i++)
                  if (VM_DOMAIN_EMPTY(i))
                          eligible_cpus -= CPU_COUNT(&cpuset_domain[i]);
  
          /*
           * Assign a portion of the total pageout threads to this domain
           * corresponding to the fraction of pagedaemon-eligible CPUs in the
           * domain.  In asymmetric NUMA systems, domains with more CPUs may be
           * allocated more threads than domains with fewer CPUs.
           */
          return (howmany(total_pageout_threads * domain_cpus, eligible_cpus));
  }
  
  /*
   * Initialize basic pageout daemon settings.  See the comment above the
   * definition of vm_domain for some explanation of how these thresholds are
   * used.
   */
  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. 
           */
          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_min = vmd->vmd_page_count / 200;
          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 | CTLFLAG_MPSAFE, NULL, "");
          pidctrl_init_sysctl(&vmd->vmd_pid, SYSCTL_CHILDREN(oid));
  
          vmd->vmd_inactive_threads = get_pageout_threads_per_domain(vmd);
  }
  
  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, j, pageout_threads;
  
          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);
                  }
                  pageout_threads = VM_DOMAIN(i)->vmd_inactive_threads;
                  for (j = 0; j < pageout_threads - 1; j++) {
                          error = kthread_add(vm_pageout_helper,
                              (void *)(uintptr_t)i, p, NULL, 0, 0,
                              "dom%d helper%d", i, j);
                          if (error != 0)
                                  panic("starting pageout helper %d for domain "
                                      "%d: %d\n", j, 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);
          }
  }

Cache object: d347f9db3a1fd27bceb7c19c6e1c2bdc