FreeBSD/Linux Kernel Cross Reference
sys/uvm/uvm_pdaemon.c

     /*      $NetBSD: uvm_pdaemon.c,v 1.93.4.2 2009/02/02 19:24:04 snj Exp $ */
     
     /*
      * Copyright (c) 1997 Charles D. Cranor and Washington University.
      * Copyright (c) 1991, 1993, The Regents of the University of California.
      *
      * 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 Charles D. Cranor,
     *      Washington University, 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.
     *
     *      @(#)vm_pageout.c        8.5 (Berkeley) 2/14/94
     * from: Id: uvm_pdaemon.c,v 1.1.2.32 1998/02/06 05:26:30 chs Exp
     *
     *
     * Copyright (c) 1987, 1990 Carnegie-Mellon University.
     * All rights reserved.
     *
     * 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.
     */
    
    /*
     * uvm_pdaemon.c: the page daemon
     */
    
    #include <sys/cdefs.h>
    __KERNEL_RCSID(0, "$NetBSD: uvm_pdaemon.c,v 1.93.4.2 2009/02/02 19:24:04 snj Exp $");
    
    #include "opt_uvmhist.h"
    #include "opt_readahead.h"
    
    #include <sys/param.h>
    #include <sys/proc.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/pool.h>
    #include <sys/buf.h>
    #include <sys/atomic.h>
    
    #include <uvm/uvm.h>
    #include <uvm/uvm_pdpolicy.h>
    
    /*
     * UVMPD_NUMDIRTYREACTS is how many dirty pages the pagedaemon will reactivate
     * in a pass thru the inactive list when swap is full.  the value should be
     * "small"... if it's too large we'll cycle the active pages thru the inactive
     * queue too quickly to for them to be referenced and avoid being freed.
     */
    
    #define UVMPD_NUMDIRTYREACTS    16
    
    #define UVMPD_NUMTRYLOCKOWNER   16
   
   /*
    * local prototypes
    */
   
   static void     uvmpd_scan(void);
   static void     uvmpd_scan_queue(void);
   static void     uvmpd_tune(void);
   
   unsigned int uvm_pagedaemon_waiters;
   
   /*
    * XXX hack to avoid hangs when large processes fork.
    */
   u_int uvm_extrapages;
   
   /*
    * uvm_wait: wait (sleep) for the page daemon to free some pages
    *
    * => should be called with all locks released
    * => should _not_ be called by the page daemon (to avoid deadlock)
    */
   
   void
   uvm_wait(const char *wmsg)
   {
           int timo = 0;
   
           mutex_spin_enter(&uvm_fpageqlock);
   
           /*
            * check for page daemon going to sleep (waiting for itself)
            */
   
           if (curlwp == uvm.pagedaemon_lwp && uvmexp.paging == 0) {
                   /*
                    * now we have a problem: the pagedaemon wants to go to
                    * sleep until it frees more memory.   but how can it
                    * free more memory if it is asleep?  that is a deadlock.
                    * we have two options:
                    *  [1] panic now
                    *  [2] put a timeout on the sleep, thus causing the
                    *      pagedaemon to only pause (rather than sleep forever)
                    *
                    * note that option [2] will only help us if we get lucky
                    * and some other process on the system breaks the deadlock
                    * by exiting or freeing memory (thus allowing the pagedaemon
                    * to continue).  for now we panic if DEBUG is defined,
                    * otherwise we hope for the best with option [2] (better
                    * yet, this should never happen in the first place!).
                    */
   
                   printf("pagedaemon: deadlock detected!\n");
                   timo = hz >> 3;         /* set timeout */
   #if defined(DEBUG)
                   /* DEBUG: panic so we can debug it */
                   panic("pagedaemon deadlock");
   #endif
           }
   
           uvm_pagedaemon_waiters++;
           wakeup(&uvm.pagedaemon);                /* wake the daemon! */
           UVM_UNLOCK_AND_WAIT(&uvmexp.free, &uvm_fpageqlock, false, wmsg, timo);
   }
   
   /*
    * uvm_kick_pdaemon: perform checks to determine if we need to
    * give the pagedaemon a nudge, and do so if necessary.
    *
    * => called with uvm_fpageqlock held.
    */
   
   void
   uvm_kick_pdaemon(void)
   {
   
           KASSERT(mutex_owned(&uvm_fpageqlock));
   
           if (uvmexp.free + uvmexp.paging < uvmexp.freemin ||
               (uvmexp.free + uvmexp.paging < uvmexp.freetarg &&
                uvmpdpol_needsscan_p())) {
                   wakeup(&uvm.pagedaemon);
           }
   }
   
   /*
    * uvmpd_tune: tune paging parameters
    *
    * => called when ever memory is added (or removed?) to the system
    * => caller must call with page queues locked
    */
   
   static void
   uvmpd_tune(void)
   {
           int val;
   
           UVMHIST_FUNC("uvmpd_tune"); UVMHIST_CALLED(pdhist);
   
           /*
            * try to keep 0.5% of available RAM free, but limit to between
            * 128k and 1024k per-CPU.  XXX: what are these values good for?
            */
           val = uvmexp.npages / 200;
           val = MAX(val, (128*1024) >> PAGE_SHIFT);
           val = MIN(val, (1024*1024) >> PAGE_SHIFT);
           val *= ncpu;
   
           /* Make sure there's always a user page free. */
           if (val < uvmexp.reserve_kernel + 1)
                   val = uvmexp.reserve_kernel + 1;
           uvmexp.freemin = val;
   
           /* Calculate free target. */
           val = (uvmexp.freemin * 4) / 3;
           if (val <= uvmexp.freemin)
                   val = uvmexp.freemin + 1;
           uvmexp.freetarg = val + atomic_swap_uint(&uvm_extrapages, 0);
   
           uvmexp.wiredmax = uvmexp.npages / 3;
           UVMHIST_LOG(pdhist, "<- done, freemin=%d, freetarg=%d, wiredmax=%d",
                 uvmexp.freemin, uvmexp.freetarg, uvmexp.wiredmax, 0);
   }
   
   /*
    * uvm_pageout: the main loop for the pagedaemon
    */
   
   void
   uvm_pageout(void *arg)
   {
           int bufcnt, npages = 0;
           int extrapages = 0;
           struct pool *pp;
           uint64_t where;
           UVMHIST_FUNC("uvm_pageout"); UVMHIST_CALLED(pdhist);
   
           UVMHIST_LOG(pdhist,"<starting uvm pagedaemon>", 0, 0, 0, 0);
   
           /*
            * ensure correct priority and set paging parameters...
            */
   
           uvm.pagedaemon_lwp = curlwp;
           mutex_enter(&uvm_pageqlock);
           npages = uvmexp.npages;
           uvmpd_tune();
           mutex_exit(&uvm_pageqlock);
   
           /*
            * main loop
            */
   
           for (;;) {
                   bool needsscan, needsfree;
   
                   mutex_spin_enter(&uvm_fpageqlock);
                   if (uvm_pagedaemon_waiters == 0 || uvmexp.paging > 0) {
                           UVMHIST_LOG(pdhist,"  <<SLEEPING>>",0,0,0,0);
                           UVM_UNLOCK_AND_WAIT(&uvm.pagedaemon,
                               &uvm_fpageqlock, false, "pgdaemon", 0);
                           uvmexp.pdwoke++;
                           UVMHIST_LOG(pdhist,"  <<WOKE UP>>",0,0,0,0);
                   } else {
                           mutex_spin_exit(&uvm_fpageqlock);
                   }
   
                   /*
                    * now lock page queues and recompute inactive count
                    */
   
                   mutex_enter(&uvm_pageqlock);
                   if (npages != uvmexp.npages || extrapages != uvm_extrapages) {
                           npages = uvmexp.npages;
                           extrapages = uvm_extrapages;
                           mutex_spin_enter(&uvm_fpageqlock);
                           uvmpd_tune();
                           mutex_spin_exit(&uvm_fpageqlock);
                   }
   
                   uvmpdpol_tune();
   
                   /*
                    * Estimate a hint.  Note that bufmem are returned to
                    * system only when entire pool page is empty.
                    */
                   mutex_spin_enter(&uvm_fpageqlock);
                   bufcnt = uvmexp.freetarg - uvmexp.free;
                   if (bufcnt < 0)
                           bufcnt = 0;
   
                   UVMHIST_LOG(pdhist,"  free/ftarg=%d/%d",
                       uvmexp.free, uvmexp.freetarg, 0,0);
   
                   needsfree = uvmexp.free + uvmexp.paging < uvmexp.freetarg;
                   needsscan = needsfree || uvmpdpol_needsscan_p();
   
                   /*
                    * scan if needed
                    */
                   if (needsscan) {
                           mutex_spin_exit(&uvm_fpageqlock);
                           uvmpd_scan();
                           mutex_spin_enter(&uvm_fpageqlock);
                   }
   
                   /*
                    * if there's any free memory to be had,
                    * wake up any waiters.
                    */
                   if (uvmexp.free > uvmexp.reserve_kernel ||
                       uvmexp.paging == 0) {
                           wakeup(&uvmexp.free);
                           uvm_pagedaemon_waiters = 0;
                   }
                   mutex_spin_exit(&uvm_fpageqlock);
   
                   /*
                    * scan done.  unlock page queues (the only lock we are holding)
                    */
                   mutex_exit(&uvm_pageqlock);
   
                   /*
                    * if we don't need free memory, we're done.
                    */
   
                   if (!needsfree) 
                           continue;
   
                   /*
                    * start draining pool resources now that we're not
                    * holding any locks.
                    */
                   pool_drain_start(&pp, &where);
   
                   /*
                    * kill unused metadata buffers.
                    */
                   mutex_enter(&bufcache_lock);
                   buf_drain(bufcnt << PAGE_SHIFT);
                   mutex_exit(&bufcache_lock);
   
                   /*
                    * complete draining the pools.
                    */
                   pool_drain_end(pp, where);
           }
           /*NOTREACHED*/
   }
   
   
   /*
    * uvm_aiodone_worker: a workqueue callback for the aiodone daemon.
    */
   
   void
   uvm_aiodone_worker(struct work *wk, void *dummy)
   {
           struct buf *bp = (void *)wk;
   
           KASSERT(&bp->b_work == wk);
   
           /*
            * process an i/o that's done.
            */
   
           (*bp->b_iodone)(bp);
   }
   
   void
   uvm_pageout_start(int npages)
   {
   
           mutex_spin_enter(&uvm_fpageqlock);
           uvmexp.paging += npages;
           mutex_spin_exit(&uvm_fpageqlock);
   }
   
   void
   uvm_pageout_done(int npages)
   {
   
           mutex_spin_enter(&uvm_fpageqlock);
           KASSERT(uvmexp.paging >= npages);
           uvmexp.paging -= npages;
   
           /*
            * wake up either of pagedaemon or LWPs waiting for it.
            */
   
           if (uvmexp.free <= uvmexp.reserve_kernel) {
                   wakeup(&uvm.pagedaemon);
           } else {
                   wakeup(&uvmexp.free);
                   uvm_pagedaemon_waiters = 0;
           }
           mutex_spin_exit(&uvm_fpageqlock);
   }
   
   /*
    * uvmpd_trylockowner: trylock the page's owner.
    *
    * => called with pageq locked.
    * => resolve orphaned O->A loaned page.
    * => return the locked mutex on success.  otherwise, return NULL.
    */
   
   kmutex_t *
   uvmpd_trylockowner(struct vm_page *pg)
   {
           struct uvm_object *uobj = pg->uobject;
           kmutex_t *slock;
   
           KASSERT(mutex_owned(&uvm_pageqlock));
   
           if (uobj != NULL) {
                   slock = &uobj->vmobjlock;
           } else {
                   struct vm_anon *anon = pg->uanon;
   
                   KASSERT(anon != NULL);
                   slock = &anon->an_lock;
           }
   
           if (!mutex_tryenter(slock)) {
                   return NULL;
           }
   
           if (uobj == NULL) {
   
                   /*
                    * set PQ_ANON if it isn't set already.
                    */
   
                   if ((pg->pqflags & PQ_ANON) == 0) {
                           KASSERT(pg->loan_count > 0);
                           pg->loan_count--;
                           pg->pqflags |= PQ_ANON;
                           /* anon now owns it */
                   }
           }
   
           return slock;
   }
   
   #if defined(VMSWAP)
   struct swapcluster {
           int swc_slot;
           int swc_nallocated;
           int swc_nused;
           struct vm_page *swc_pages[howmany(MAXPHYS, MIN_PAGE_SIZE)];
   };
   
   static void
   swapcluster_init(struct swapcluster *swc)
   {
   
           swc->swc_slot = 0;
           swc->swc_nused = 0;
   }
   
   static int
   swapcluster_allocslots(struct swapcluster *swc)
   {
           int slot;
           int npages;
   
           if (swc->swc_slot != 0) {
                   return 0;
           }
   
           /* Even with strange MAXPHYS, the shift
              implicitly rounds down to a page. */
           npages = MAXPHYS >> PAGE_SHIFT;
           slot = uvm_swap_alloc(&npages, true);
           if (slot == 0) {
                   return ENOMEM;
           }
           swc->swc_slot = slot;
           swc->swc_nallocated = npages;
           swc->swc_nused = 0;
   
           return 0;
   }
   
   static int
   swapcluster_add(struct swapcluster *swc, struct vm_page *pg)
   {
           int slot;
           struct uvm_object *uobj;
   
           KASSERT(swc->swc_slot != 0);
           KASSERT(swc->swc_nused < swc->swc_nallocated);
           KASSERT((pg->pqflags & PQ_SWAPBACKED) != 0);
   
           slot = swc->swc_slot + swc->swc_nused;
           uobj = pg->uobject;
           if (uobj == NULL) {
                   KASSERT(mutex_owned(&pg->uanon->an_lock));
                   pg->uanon->an_swslot = slot;
           } else {
                   int result;
   
                   KASSERT(mutex_owned(&uobj->vmobjlock));
                   result = uao_set_swslot(uobj, pg->offset >> PAGE_SHIFT, slot);
                   if (result == -1) {
                           return ENOMEM;
                   }
           }
           swc->swc_pages[swc->swc_nused] = pg;
           swc->swc_nused++;
   
           return 0;
   }
   
   static void
   swapcluster_flush(struct swapcluster *swc, bool now)
   {
           int slot;
           int nused;
           int nallocated;
           int error;
   
           if (swc->swc_slot == 0) {
                   return;
           }
           KASSERT(swc->swc_nused <= swc->swc_nallocated);
   
           slot = swc->swc_slot;
           nused = swc->swc_nused;
           nallocated = swc->swc_nallocated;
   
           /*
            * if this is the final pageout we could have a few
            * unused swap blocks.  if so, free them now.
            */
   
           if (nused < nallocated) {
                   if (!now) {
                           return;
                   }
                   uvm_swap_free(slot + nused, nallocated - nused);
           }
   
           /*
            * now start the pageout.
            */
   
           if (nused > 0) {
                   uvmexp.pdpageouts++;
                   uvm_pageout_start(nused);
                   error = uvm_swap_put(slot, swc->swc_pages, nused, 0);
                   KASSERT(error == 0 || error == ENOMEM);
           }
   
           /*
            * zero swslot to indicate that we are
            * no longer building a swap-backed cluster.
            */
   
           swc->swc_slot = 0;
           swc->swc_nused = 0;
   }
   
   static int
   swapcluster_nused(struct swapcluster *swc)
   {
   
           return swc->swc_nused;
   }
   
   /*
    * uvmpd_dropswap: free any swap allocated to this page.
    *
    * => called with owner locked.
    * => return true if a page had an associated slot.
    */
   
   static bool
   uvmpd_dropswap(struct vm_page *pg)
   {
           bool result = false;
           struct vm_anon *anon = pg->uanon;
   
           if ((pg->pqflags & PQ_ANON) && anon->an_swslot) {
                   uvm_swap_free(anon->an_swslot, 1);
                   anon->an_swslot = 0;
                   pg->flags &= ~PG_CLEAN;
                   result = true;
           } else if (pg->pqflags & PQ_AOBJ) {
                   int slot = uao_set_swslot(pg->uobject,
                       pg->offset >> PAGE_SHIFT, 0);
                   if (slot) {
                           uvm_swap_free(slot, 1);
                           pg->flags &= ~PG_CLEAN;
                           result = true;
                   }
           }
   
           return result;
   }
   
   /*
    * uvmpd_trydropswap: try to free any swap allocated to this page.
    *
    * => return true if a slot is successfully freed.
    */
   
   bool
   uvmpd_trydropswap(struct vm_page *pg)
   {
           kmutex_t *slock;
           bool result;
   
           if ((pg->flags & PG_BUSY) != 0) {
                   return false;
           }
   
           /*
            * lock the page's owner.
            */
   
           slock = uvmpd_trylockowner(pg);
           if (slock == NULL) {
                   return false;
           }
   
           /*
            * skip this page if it's busy.
            */
   
           if ((pg->flags & PG_BUSY) != 0) {
                   mutex_exit(slock);
                   return false;
           }
   
           result = uvmpd_dropswap(pg);
   
           mutex_exit(slock);
   
           return result;
   }
   
   #endif /* defined(VMSWAP) */
   
   /*
    * uvmpd_scan_queue: scan an replace candidate list for pages
    * to clean or free.
    *
    * => called with page queues locked
    * => we work on meeting our free target by converting inactive pages
    *    into free pages.
    * => we handle the building of swap-backed clusters
    */
   
   static void
   uvmpd_scan_queue(void)
   {
           struct vm_page *p;
           struct uvm_object *uobj;
           struct vm_anon *anon;
   #if defined(VMSWAP)
           struct swapcluster swc;
   #endif /* defined(VMSWAP) */
           int dirtyreacts;
           int lockownerfail;
           kmutex_t *slock;
           UVMHIST_FUNC("uvmpd_scan_queue"); UVMHIST_CALLED(pdhist);
   
           /*
            * swslot is non-zero if we are building a swap cluster.  we want
            * to stay in the loop while we have a page to scan or we have
            * a swap-cluster to build.
            */
   
   #if defined(VMSWAP)
           swapcluster_init(&swc);
   #endif /* defined(VMSWAP) */
   
           dirtyreacts = 0;
           lockownerfail = 0;
           uvmpdpol_scaninit();
   
           while (/* CONSTCOND */ 1) {
   
                   /*
                    * see if we've met the free target.
                    */
   
                   if (uvmexp.free + uvmexp.paging
   #if defined(VMSWAP)
                       + swapcluster_nused(&swc)
   #endif /* defined(VMSWAP) */
                       >= uvmexp.freetarg << 2 ||
                       dirtyreacts == UVMPD_NUMDIRTYREACTS) {
                           UVMHIST_LOG(pdhist,"  met free target: "
                                       "exit loop", 0, 0, 0, 0);
                           break;
                   }
   
                   p = uvmpdpol_selectvictim();
                   if (p == NULL) {
                           break;
                   }
                   KASSERT(uvmpdpol_pageisqueued_p(p));
                   KASSERT(p->wire_count == 0);
   
                   /*
                    * we are below target and have a new page to consider.
                    */
   
                   anon = p->uanon;
                   uobj = p->uobject;
   
                   /*
                    * first we attempt to lock the object that this page
                    * belongs to.  if our attempt fails we skip on to
                    * the next page (no harm done).  it is important to
                    * "try" locking the object as we are locking in the
                    * wrong order (pageq -> object) and we don't want to
                    * deadlock.
                    *
                    * the only time we expect to see an ownerless page
                    * (i.e. a page with no uobject and !PQ_ANON) is if an
                    * anon has loaned a page from a uvm_object and the
                    * uvm_object has dropped the ownership.  in that
                    * case, the anon can "take over" the loaned page
                    * and make it its own.
                    */
   
                   slock = uvmpd_trylockowner(p);
                   if (slock == NULL) {
                           /*
                            * yield cpu to make a chance for an LWP holding
                            * the lock run.  otherwise we can busy-loop too long
                            * if the page queue is filled with a lot of pages
                            * from few objects.
                            */
                           lockownerfail++;
                           if (lockownerfail > UVMPD_NUMTRYLOCKOWNER) {
                                   mutex_exit(&uvm_pageqlock);
                                   /* XXX Better than yielding but inadequate. */
                                   kpause("livelock", false, 1, NULL);
                                   mutex_enter(&uvm_pageqlock);
                                   lockownerfail = 0;
                           }
                           continue;
                   }
                   if (p->flags & PG_BUSY) {
                           mutex_exit(slock);
                           uvmexp.pdbusy++;
                           continue;
                   }
   
                   /* does the page belong to an object? */
                   if (uobj != NULL) {
                           uvmexp.pdobscan++;
                   } else {
   #if defined(VMSWAP)
                           KASSERT(anon != NULL);
                           uvmexp.pdanscan++;
   #else /* defined(VMSWAP) */
                           panic("%s: anon", __func__);
   #endif /* defined(VMSWAP) */
                   }
   
   
                   /*
                    * we now have the object and the page queues locked.
                    * if the page is not swap-backed, call the object's
                    * pager to flush and free the page.
                    */
   
   #if defined(READAHEAD_STATS)
                   if ((p->pqflags & PQ_READAHEAD) != 0) {
                           p->pqflags &= ~PQ_READAHEAD;
                           uvm_ra_miss.ev_count++;
                   }
   #endif /* defined(READAHEAD_STATS) */
   
                   if ((p->pqflags & PQ_SWAPBACKED) == 0) {
                           KASSERT(uobj != NULL);
                           mutex_exit(&uvm_pageqlock);
                           (void) (uobj->pgops->pgo_put)(uobj, p->offset,
                               p->offset + PAGE_SIZE, PGO_CLEANIT|PGO_FREE);
                           mutex_enter(&uvm_pageqlock);
                           continue;
                   }
   
                   /*
                    * the page is swap-backed.  remove all the permissions
                    * from the page so we can sync the modified info
                    * without any race conditions.  if the page is clean
                    * we can free it now and continue.
                    */
   
                   pmap_page_protect(p, VM_PROT_NONE);
                   if ((p->flags & PG_CLEAN) && pmap_clear_modify(p)) {
                           p->flags &= ~(PG_CLEAN);
                   }
                   if (p->flags & PG_CLEAN) {
                           int slot;
                           int pageidx;
   
                           pageidx = p->offset >> PAGE_SHIFT;
                           uvm_pagefree(p);
                           uvmexp.pdfreed++;
   
                           /*
                            * for anons, we need to remove the page
                            * from the anon ourselves.  for aobjs,
                            * pagefree did that for us.
                            */
   
                           if (anon) {
                                   KASSERT(anon->an_swslot != 0);
                                   anon->an_page = NULL;
                                   slot = anon->an_swslot;
                           } else {
                                   slot = uao_find_swslot(uobj, pageidx);
                           }
                           mutex_exit(slock);
   
                           if (slot > 0) {
                                   /* this page is now only in swap. */
                                   mutex_enter(&uvm_swap_data_lock);
                                   KASSERT(uvmexp.swpgonly < uvmexp.swpginuse);
                                   uvmexp.swpgonly++;
                                   mutex_exit(&uvm_swap_data_lock);
                           }
                           continue;
                   }
   
   #if defined(VMSWAP)
                   /*
                    * this page is dirty, skip it if we'll have met our
                    * free target when all the current pageouts complete.
                    */
   
                   if (uvmexp.free + uvmexp.paging > uvmexp.freetarg << 2) {
                           mutex_exit(slock);
                           continue;
                   }
   
                   /*
                    * free any swap space allocated to the page since
                    * we'll have to write it again with its new data.
                    */
   
                   uvmpd_dropswap(p);
   
                   /*
                    * start new swap pageout cluster (if necessary).
                    *
                    * if swap is full reactivate this page so that
                    * we eventually cycle all pages through the
                    * inactive queue.
                    */
   
                   if (swapcluster_allocslots(&swc)) {
                           dirtyreacts++;
                           uvm_pageactivate(p);
                           mutex_exit(slock);
                           continue;
                   }
   
                   /*
                    * at this point, we're definitely going reuse this
                    * page.  mark the page busy and delayed-free.
                    * we should remove the page from the page queues
                    * so we don't ever look at it again.
                    * adjust counters and such.
                    */
   
                   p->flags |= PG_BUSY;
                   UVM_PAGE_OWN(p, "scan_queue");
   
                   p->flags |= PG_PAGEOUT;
                   uvm_pagedequeue(p);
   
                   uvmexp.pgswapout++;
                   mutex_exit(&uvm_pageqlock);
   
                   /*
                    * add the new page to the cluster.
                    */
   
                   if (swapcluster_add(&swc, p)) {
                           p->flags &= ~(PG_BUSY|PG_PAGEOUT);
                           UVM_PAGE_OWN(p, NULL);
                           mutex_enter(&uvm_pageqlock);
                           dirtyreacts++;
                           uvm_pageactivate(p);
                           mutex_exit(slock);
                           continue;
                   }
                   mutex_exit(slock);
   
                   swapcluster_flush(&swc, false);
                   mutex_enter(&uvm_pageqlock);
   
                   /*
                    * the pageout is in progress.  bump counters and set up
                    * for the next loop.
                    */
   
                   uvmexp.pdpending++;
   
   #else /* defined(VMSWAP) */
                   uvm_pageactivate(p);
                   mutex_exit(slock);
   #endif /* defined(VMSWAP) */
           }
   
   #if defined(VMSWAP)
           mutex_exit(&uvm_pageqlock);
           swapcluster_flush(&swc, true);
           mutex_enter(&uvm_pageqlock);
   #endif /* defined(VMSWAP) */
   }
   
   /*
    * uvmpd_scan: scan the page queues and attempt to meet our targets.
    *
    * => called with pageq's locked
    */
   
   static void
   uvmpd_scan(void)
   {
           int swap_shortage, pages_freed;
           UVMHIST_FUNC("uvmpd_scan"); UVMHIST_CALLED(pdhist);
   
           uvmexp.pdrevs++;
   
           /*
            * work on meeting our targets.   first we work on our free target
            * by converting inactive pages into free pages.  then we work on
            * meeting our inactive target by converting active pages to
            * inactive ones.
            */
   
           UVMHIST_LOG(pdhist, "  starting 'free' loop",0,0,0,0);
   
           pages_freed = uvmexp.pdfreed;
           uvmpd_scan_queue();
           pages_freed = uvmexp.pdfreed - pages_freed;
   
           /*
            * detect if we're not going to be able to page anything out
            * until we free some swap resources from active pages.
            */
   
           swap_shortage = 0;
           if (uvmexp.free < uvmexp.freetarg &&
               uvmexp.swpginuse >= uvmexp.swpgavail &&
               !uvm_swapisfull() &&
               pages_freed == 0) {
                   swap_shortage = uvmexp.freetarg - uvmexp.free;
           }
   
           uvmpdpol_balancequeue(swap_shortage);
   
           /*
            * swap out some processes if we are still below the minimum
            * free target.  we need to unlock the page queues for this.
            */
   
           if (uvmexp.free < uvmexp.freemin && uvmexp.nswapdev != 0 &&
               uvm.swapout_enabled) {
                   uvmexp.pdswout++;
                   UVMHIST_LOG(pdhist,"  free %d < min %d: swapout",
                       uvmexp.free, uvmexp.freemin, 0, 0);
                   mutex_exit(&uvm_pageqlock);
                   uvm_swapout_threads();
                   mutex_enter(&uvm_pageqlock);
   
           }
   }
   
   /*
    * uvm_reclaimable: decide whether to wait for pagedaemon.
    *
    * => return true if it seems to be worth to do uvm_wait.
    *
    * XXX should be tunable.
    * XXX should consider pools, etc?
    */
   
   bool
   uvm_reclaimable(void)
   {
           int filepages;
           int active, inactive;
   
           /*
            * if swap is not full, no problem.
            */
   
           if (!uvm_swapisfull()) {
                  return true;
          }
  
          /*
           * file-backed pages can be reclaimed even when swap is full.
           * if we have more than 1/16 of pageable memory or 5MB, try to reclaim.
           *
           * XXX assume the worst case, ie. all wired pages are file-backed.
           *
           * XXX should consider about other reclaimable memory.
           * XXX ie. pools, traditional buffer cache.
           */
  
          filepages = uvmexp.filepages + uvmexp.execpages - uvmexp.wired;
          uvm_estimatepageable(&active, &inactive);
          if (filepages >= MIN((active + inactive) >> 4,
              5 * 1024 * 1024 >> PAGE_SHIFT)) {
                  return true;
          }
  
          /*
           * kill the process, fail allocation, etc..
           */
  
          return false;
  }
  
  void
  uvm_estimatepageable(int *active, int *inactive)
  {
  
          uvmpdpol_estimatepageable(active, inactive);
  }

Cache object: cf88822de864450905bb5db2db68aabe