NetBSD/sys/uvm/uvm_pdaemon.c

/*	$NetBSD: uvm_pdaemon.c,v 1.38 2001/09/26 07:08:41 chs 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.
 */

#include "opt_uvmhist.h"

/*
 * uvm_pdaemon.c: the page daemon
 */

#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/vnode.h>

#include <uvm/uvm.h>

/*
 * UVMPD_NUMDIRTYREACTS is how many dirty pages the pagedeamon 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


/*
 * local prototypes
 */

void		uvmpd_scan __P((void));
boolean_t	uvmpd_scan_inactive __P((struct pglist *));
void		uvmpd_tune __P((void));

/*
 * 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(wmsg)
	const char *wmsg;
{
	int timo = 0;
	int s = splbio();

	/*
	 * check for page daemon going to sleep (waiting for itself)
	 */

	if (curproc == uvm.pagedaemon_proc && 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
	}

	simple_lock(&uvm.pagedaemon_lock);
	wakeup(&uvm.pagedaemon);		/* wake the daemon! */
	UVM_UNLOCK_AND_WAIT(&uvmexp.free, &uvm.pagedaemon_lock, FALSE, wmsg,
	    timo);

	splx(s);
}


/*
 * uvmpd_tune: tune paging parameters
 *
 * => called when ever memory is added (or removed?) to the system
 * => caller must call with page queues locked
 */

void
uvmpd_tune(void)
{
	UVMHIST_FUNC("uvmpd_tune"); UVMHIST_CALLED(pdhist);

	uvmexp.freemin = uvmexp.npages / 20;

	/* between 16k and 256k */
	/* XXX:  what are these values good for? */
	uvmexp.freemin = MAX(uvmexp.freemin, (16*1024) >> PAGE_SHIFT);
	uvmexp.freemin = MIN(uvmexp.freemin, (256*1024) >> PAGE_SHIFT);

	/* Make sure there's always a user page free. */
	if (uvmexp.freemin < uvmexp.reserve_kernel + 1)
		uvmexp.freemin = uvmexp.reserve_kernel + 1;

	uvmexp.freetarg = (uvmexp.freemin * 4) / 3;
	if (uvmexp.freetarg <= uvmexp.freemin)
		uvmexp.freetarg = uvmexp.freemin + 1;

	/* uvmexp.inactarg: computed in main daemon loop */

	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 npages = 0;
	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_proc = curproc;
	uvm_lock_pageq();
	npages = uvmexp.npages;
	uvmpd_tune();
	uvm_unlock_pageq();

	/*
	 * main loop
	 */

	for (;;) {
		simple_lock(&uvm.pagedaemon_lock);

		UVMHIST_LOG(pdhist,"  <<SLEEPING>>",0,0,0,0);
		UVM_UNLOCK_AND_WAIT(&uvm.pagedaemon,
		    &uvm.pagedaemon_lock, FALSE, "pgdaemon", 0);
		uvmexp.pdwoke++;
		UVMHIST_LOG(pdhist,"  <<WOKE UP>>",0,0,0,0);

		/*
		 * now lock page queues and recompute inactive count
		 */

		uvm_lock_pageq();
		if (npages != uvmexp.npages) {	/* check for new pages? */
			npages = uvmexp.npages;
			uvmpd_tune();
		}

		uvmexp.inactarg = (uvmexp.active + uvmexp.inactive) / 3;
		if (uvmexp.inactarg <= uvmexp.freetarg) {
			uvmexp.inactarg = uvmexp.freetarg + 1;
		}

		UVMHIST_LOG(pdhist,"  free/ftarg=%d/%d, inact/itarg=%d/%d",
		    uvmexp.free, uvmexp.freetarg, uvmexp.inactive,
		    uvmexp.inactarg);

		/*
		 * scan if needed
		 */

		if (uvmexp.free + uvmexp.paging < uvmexp.freetarg ||
		    uvmexp.inactive < uvmexp.inactarg) {
			uvmpd_scan();
		}

		/*
		 * 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);
		}

		/*
		 * scan done.  unlock page queues (the only lock we are holding)
		 */

		uvm_unlock_pageq();

		/*
		 * drain pool resources now that we're not holding any locks
		 */

		pool_drain(0);
	}
	/*NOTREACHED*/
}


/*
 * uvm_aiodone_daemon:  main loop for the aiodone daemon.
 */

void
uvm_aiodone_daemon(void *arg)
{
	int s, free;
	struct buf *bp, *nbp;
	UVMHIST_FUNC("uvm_aiodoned"); UVMHIST_CALLED(pdhist);

	for (;;) {

		/*
		 * carefully attempt to go to sleep (without losing "wakeups"!).
		 * we need splbio because we want to make sure the aio_done list
		 * is totally empty before we go to sleep.
		 */

		s = splbio();
		simple_lock(&uvm.aiodoned_lock);
		if (TAILQ_FIRST(&uvm.aio_done) == NULL) {
			UVMHIST_LOG(pdhist,"  <<SLEEPING>>",0,0,0,0);
			UVM_UNLOCK_AND_WAIT(&uvm.aiodoned,
			    &uvm.aiodoned_lock, FALSE, "aiodoned", 0);
			UVMHIST_LOG(pdhist,"  <<WOKE UP>>",0,0,0,0);

			/* relock aiodoned_lock, still at splbio */
			simple_lock(&uvm.aiodoned_lock);
		}

		/*
		 * check for done aio structures
		 */

		bp = TAILQ_FIRST(&uvm.aio_done);
		if (bp) {
			TAILQ_INIT(&uvm.aio_done);
		}

		simple_unlock(&uvm.aiodoned_lock);
		splx(s);

		/*
		 * process each i/o that's done.
		 */

		free = uvmexp.free;
		while (bp != NULL) {
			nbp = TAILQ_NEXT(bp, b_freelist);
			(*bp->b_iodone)(bp);
			bp = nbp;
		}
		if (free <= uvmexp.reserve_kernel) {
			s = uvm_lock_fpageq();
			wakeup(&uvm.pagedaemon);
			uvm_unlock_fpageq(s);
		} else {
			simple_lock(&uvm.pagedaemon_lock);
			wakeup(&uvmexp.free);
			simple_unlock(&uvm.pagedaemon_lock);
		}
	}
}

/*
 * uvmpd_scan_inactive: scan an inactive 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
 * => we return TRUE if we are exiting because we met our target
 */

boolean_t
uvmpd_scan_inactive(pglst)
	struct pglist *pglst;
{
	boolean_t retval = FALSE;	/* assume we haven't hit target */
	int error;
	struct vm_page *p, *nextpg;
	struct uvm_object *uobj;
	struct vm_anon *anon;
	struct vm_page *swpps[MAXBSIZE >> PAGE_SHIFT];
	struct simplelock *slock;
	int swnpages, swcpages;
	int swslot;
	int dirtyreacts, t, result;
	UVMHIST_FUNC("uvmpd_scan_inactive"); 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.
	 */

	swslot = 0;
	swnpages = swcpages = 0;
	dirtyreacts = 0;
	for (p = TAILQ_FIRST(pglst); p != NULL || swslot != 0; p = nextpg) {
		uobj = NULL;
		anon = NULL;
		if (p) {

			/*
			 * see if we've met the free target.
			 */

			if (uvmexp.free + uvmexp.paging >=
			    uvmexp.freetarg << 2 ||
			    dirtyreacts == UVMPD_NUMDIRTYREACTS) {
				UVMHIST_LOG(pdhist,"  met free target: "
					    "exit loop", 0, 0, 0, 0);
				retval = TRUE;

				if (swslot == 0) {
					/* exit now if no swap-i/o pending */
					break;
				}

				/* set p to null to signal final swap i/o */
				p = NULL;
				nextpg = NULL;
			}
		}
		if (p) {	/* if (we have a new page to consider) */

			/*
			 * we are below target and have a new page to consider.
			 */

			uvmexp.pdscans++;
			nextpg = TAILQ_NEXT(p, pageq);

			/*
			 * move referenced pages back to active queue and
			 * skip to next page.
			 */

			if (pmap_clear_reference(p)) {
				uvm_pageactivate(p);
				uvmexp.pdreact++;
				continue;
			}
			anon = p->uanon;
			uobj = p->uobject;

			/*
			 * enforce the minimum thresholds on different
			 * types of memory usage.  if reusing the current
			 * page would reduce that type of usage below its
			 * minimum, reactivate the page instead and move
			 * on to the next page.
			 */

			t = uvmexp.active + uvmexp.inactive + uvmexp.free;
			if (anon &&
			    uvmexp.anonpages <= (t * uvmexp.anonmin) >> 8) {
				uvm_pageactivate(p);
				uvmexp.pdreanon++;
				continue;
			}
			if (uobj && UVM_OBJ_IS_VTEXT(uobj) &&
			    uvmexp.vtextpages <= (t * uvmexp.vtextmin) >> 8) {
				uvm_pageactivate(p);
				uvmexp.pdrevtext++;
				continue;
			}
			if (uobj && UVM_OBJ_IS_VNODE(uobj) &&
			    !UVM_OBJ_IS_VTEXT(uobj) &&
			    uvmexp.vnodepages <= (t * uvmexp.vnodemin) >> 8) {
				uvm_pageactivate(p);
				uvmexp.pdrevnode++;
				continue;
			}

			/*
			 * 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.
			 */

			/* is page part of an anon or ownerless ? */
			if ((p->pqflags & PQ_ANON) || uobj == NULL) {
				KASSERT(anon != NULL);
				slock = &anon->an_lock;
				if (!simple_lock_try(slock)) {
					/* lock failed, skip this page */
					continue;
				}

				/*
				 * if the page is ownerless, claim it in the
				 * name of "anon"!
				 */

				if ((p->pqflags & PQ_ANON) == 0) {
					KASSERT(p->loan_count > 0);
					p->loan_count--;
					p->pqflags |= PQ_ANON;
					/* anon now owns it */
				}
				if (p->flags & PG_BUSY) {
					simple_unlock(slock);
					uvmexp.pdbusy++;
					continue;
				}
				uvmexp.pdanscan++;
			} else {
				KASSERT(uobj != NULL);
				slock = &uobj->vmobjlock;
				if (!simple_lock_try(slock)) {
					continue;
				}
				if (p->flags & PG_BUSY) {
					simple_unlock(slock);
					uvmexp.pdbusy++;
					continue;
				}
				uvmexp.pdobscan++;
			}


			/*
			 * 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 ((p->pqflags & PQ_SWAPBACKED) == 0) {
				uvm_unlock_pageq();
				error = (uobj->pgops->pgo_put)(uobj, p->offset,
				    p->offset + PAGE_SIZE,
				    PGO_CLEANIT|PGO_FREE);
				uvm_lock_pageq();
				if (nextpg &&
				    (nextpg->flags & PQ_INACTIVE) == 0) {
					nextpg = TAILQ_FIRST(pglst);
				}
				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) {
				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->u.an_page = NULL;
				}
				simple_unlock(slock);
				continue;
			}

			/*
			 * 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) {
				simple_unlock(slock);
				continue;
			}

			/*
			 * free any swap space allocated to the page since
			 * we'll have to write it again with its new data.
			 */

			if ((p->pqflags & PQ_ANON) && anon->an_swslot) {
				uvm_swap_free(anon->an_swslot, 1);
				anon->an_swslot = 0;
			} else if (p->pqflags & PQ_AOBJ) {
				uao_dropswap(uobj, p->offset >> PAGE_SHIFT);
			}

			/*
			 * if all pages in swap are only in swap,
			 * the swap space is full and we can't page out
			 * any more swap-backed pages.  reactivate this page
			 * so that we eventually cycle all pages through
			 * the inactive queue.
			 */

			KASSERT(uvmexp.swpgonly <= uvmexp.swpages);
			if (uvmexp.swpgonly == uvmexp.swpages) {
				dirtyreacts++;
				uvm_pageactivate(p);
				simple_unlock(slock);
				continue;
			}

			/*
			 * start new swap pageout cluster (if necessary).
			 */

			if (swslot == 0) {
				swnpages = MAXBSIZE >> PAGE_SHIFT;
				swslot = uvm_swap_alloc(&swnpages, TRUE);
				if (swslot == 0) {
					simple_unlock(slock);
					continue;
				}
				swcpages = 0;
			}

			/*
			 * 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_inactive");

			p->flags |= PG_PAGEOUT;
			uvmexp.paging++;
			uvm_pagedequeue(p);

			uvmexp.pgswapout++;

			/*
			 * add the new page to the cluster.
			 */

			if (anon) {
				anon->an_swslot = swslot + swcpages;
				simple_unlock(slock);
			} else {
				result = uao_set_swslot(uobj,
				    p->offset >> PAGE_SHIFT, swslot + swcpages);
				if (result == -1) {
					p->flags &= ~(PG_BUSY|PG_PAGEOUT);
					UVM_PAGE_OWN(p, NULL);
					uvmexp.paging--;
					uvm_pageactivate(p);
					simple_unlock(slock);
					continue;
				}
				simple_unlock(slock);
			}
			swpps[swcpages] = p;
			swcpages++;

			/*
			 * if the cluster isn't full, look for more pages
			 * before starting the i/o.
			 */

			if (swcpages < swnpages) {
				continue;
			}
		}

		/*
		 * if this is the final pageout we could have a few
		 * unused swap blocks.  if so, free them now.
		 */

		if (swcpages < swnpages) {
			uvm_swap_free(swslot + swcpages, (swnpages - swcpages));
		}

		/*
		 * now start the pageout.
		 */

		uvm_unlock_pageq();
		uvmexp.pdpageouts++;
		error = uvm_swap_put(swslot, swpps, swcpages, 0);
		KASSERT(error == 0);
		uvm_lock_pageq();

		/*
		 * zero swslot to indicate that we are
		 * no longer building a swap-backed cluster.
		 */

		swslot = 0;

		/*
		 * the pageout is in progress.  bump counters and set up
		 * for the next loop.
		 */

		uvmexp.pdpending++;
		if (nextpg && (nextpg->pqflags & PQ_INACTIVE) == 0) {
			nextpg = TAILQ_FIRST(pglst);
		}
	}
	return (error);
}

/*
 * uvmpd_scan: scan the page queues and attempt to meet our targets.
 *
 * => called with pageq's locked
 */

void
uvmpd_scan(void)
{
	int inactive_shortage, swap_shortage, pages_freed;
	struct vm_page *p, *nextpg;
	struct uvm_object *uobj;
	struct vm_anon *anon;
	boolean_t got_it;
	UVMHIST_FUNC("uvmpd_scan"); UVMHIST_CALLED(pdhist);

	uvmexp.pdrevs++;
	uobj = NULL;
	anon = NULL;

#ifndef __SWAP_BROKEN
	/*
	 * swap out some processes if we are below our free target.
	 * we need to unlock the page queues for this.
	 */
	if (uvmexp.free < uvmexp.freetarg) {
		uvmexp.pdswout++;
		UVMHIST_LOG(pdhist,"  free %d < target %d: swapout",
		    uvmexp.free, uvmexp.freetarg, 0, 0);
		uvm_unlock_pageq();
		uvm_swapout_threads();
		uvm_lock_pageq();

	}
#endif

	/*
	 * now we want to 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);

	/*
	 * alternate starting queue between swap and object based on the
	 * low bit of uvmexp.pdrevs (which we bump by one each call).
	 */

	got_it = FALSE;
	pages_freed = uvmexp.pdfreed;
	(void) uvmpd_scan_inactive(&uvm.page_inactive);
	pages_freed = uvmexp.pdfreed - pages_freed;

	/*
	 * we have done the scan to get free pages.   now we work on meeting
	 * our inactive target.
	 */

	inactive_shortage = uvmexp.inactarg - uvmexp.inactive;

	/*
	 * 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.swpages &&
	    uvmexp.swpgonly < uvmexp.swpages &&
	    pages_freed == 0) {
		swap_shortage = uvmexp.freetarg - uvmexp.free;
	}

	UVMHIST_LOG(pdhist, "  loop 2: inactive_shortage=%d swap_shortage=%d",
		    inactive_shortage, swap_shortage,0,0);
	for (p = TAILQ_FIRST(&uvm.page_active);
	     p != NULL && (inactive_shortage > 0 || swap_shortage > 0);
	     p = nextpg) {
		nextpg = TAILQ_NEXT(p, pageq);
		if (p->flags & PG_BUSY) {
			continue;
		}

		/*
		 * lock the page's owner.
		 */
		/* is page anon owned or ownerless? */
		if ((p->pqflags & PQ_ANON) || p->uobject == NULL) {
			anon = p->uanon;
			KASSERT(anon != NULL);
			if (!simple_lock_try(&anon->an_lock)) {
				continue;
			}

			/* take over the page? */
			if ((p->pqflags & PQ_ANON) == 0) {
				KASSERT(p->loan_count > 0);
				p->loan_count--;
				p->pqflags |= PQ_ANON;
			}
		} else {
			uobj = p->uobject;
			if (!simple_lock_try(&uobj->vmobjlock)) {
				continue;
			}
		}

		/*
		 * skip this page if it's busy.
		 */

		if ((p->flags & PG_BUSY) != 0) {
			if (p->pqflags & PQ_ANON)
				simple_unlock(&anon->an_lock);
			else
				simple_unlock(&uobj->vmobjlock);
			continue;
		}

		/*
		 * if there's a shortage of swap, free any swap allocated
		 * to this page so that other pages can be paged out.
		 */

		if (swap_shortage > 0) {
			if ((p->pqflags & PQ_ANON) && anon->an_swslot) {
				uvm_swap_free(anon->an_swslot, 1);
				anon->an_swslot = 0;
				p->flags &= ~PG_CLEAN;
				swap_shortage--;
			} else if (p->pqflags & PQ_AOBJ) {
				int slot = uao_set_swslot(uobj,
					p->offset >> PAGE_SHIFT, 0);
				if (slot) {
					uvm_swap_free(slot, 1);
					p->flags &= ~PG_CLEAN;
					swap_shortage--;
				}
			}
		}

		/*
		 * if there's a shortage of inactive pages, deactivate.
		 */

		if (inactive_shortage > 0) {
			/* no need to check wire_count as pg is "active" */
			uvm_pagedeactivate(p);
			uvmexp.pddeact++;
			inactive_shortage--;
		}

		/*
		 * we're done with this page.
		 */

		if (p->pqflags & PQ_ANON)
			simple_unlock(&anon->an_lock);
		else
			simple_unlock(&uobj->vmobjlock);
	}
}