xref: /src/sys/uvm/uvm_pdaemon.c

/*	$NetBSD: uvm_pdaemon.c,v 1.5 1998/02/07 17:00:44 mrg Exp $	*/

/*
 * XXXCDC: "ROUGH DRAFT" QUALITY UVM PRE-RELEASE FILE!
 *         >>>USE AT YOUR OWN RISK, WORK IS NOT FINISHED<<<
 */
/*
 * 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 (at) 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/param.h>
#include <sys/proc.h>
#include <sys/systm.h>
#include <sys/kernel.h>

#include <vm/vm.h>
#include <vm/vm_page.h>
#include <vm/vm_kern.h>

#include <uvm/uvm.h>

UVMHIST_DECL(pdhist);

/*
 * local prototypes
 */

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

char *wmsg;

{
  int timo = 0;
  int s = splbio();

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

  if (curproc == uvm.pagedaemon_proc) {
    /*
     * 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)
    panic("pagedaemon deadlock");	/* DEBUG: panic so we can debug it */
#endif
  }

  simple_lock(&uvm.pagedaemon_lock);
  thread_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
 */

static void uvmpd_tune()

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

  uvmexp.freemin = uvmexp.npages / 20;
  uvmexp.freemin = max(uvmexp.freemin, (16*1024)/PAGE_SIZE);  /* at least 16K */
  uvmexp.freemin = min(uvmexp.freemin, (256*1024)/PAGE_SIZE); /* at most 256K */

  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()

{
  int npages = 0;
  int s;
  struct uvm_aiodesc *aio, *nextaio;
  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;
  (void) spl0();
  uvm_lock_pageq();
  npages = uvmexp.npages;
  uvmpd_tune();
  uvm_unlock_pageq();

  /*
   * main loop
   */
  while (TRUE) {

    /*
     * 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.pagedaemon_lock);

    /*
     * if we've got done aio's, then bypass the sleep
     */

    if (uvm.aio_done.tqh_first == NULL) {
      UVMHIST_LOG(maphist,"  <<SLEEPING>>",0,0,0,0);
      UVM_UNLOCK_AND_WAIT(&uvm.pagedaemon, &uvm.pagedaemon_lock, FALSE,
			  "daemon_slp", 0);
      uvmexp.pdwoke++;
      UVMHIST_LOG(pdhist,"  <<WOKE UP>>",0,0,0,0);

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

    /*
     * check for done aio structures
     */

    aio = uvm.aio_done.tqh_first;		/* save current list (if any)*/
    if (aio) {
      TAILQ_INIT(&uvm.aio_done);		/* zero global list */
    }

    simple_unlock(&uvm.pagedaemon_lock);	/* unlock */
    splx(s);					/* drop splbio */

    /*
     * first clear out any pending aios (to free space in case we
     * want to pageout more stuff).
     */

    for (/*null*/; aio != NULL ; aio = nextaio) {

      uvmexp.paging -= aio->npages;
      nextaio = aio->aioq.tqe_next;
      aio->aiodone(aio);

    }

    /*
     * 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
     * [XXX: note we are reading uvm.free without locking]
     */
    if (uvmexp.free < uvmexp.freetarg || uvmexp.inactive < uvmexp.inactarg)
      uvmpd_scan();

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

    /*
     * done!    restart loop.
     */
    thread_wakeup(&uvmexp.free);
  }
  /*NOTREACHED*/
}

/*
 * uvmpd_scan_inactive: the first loop of uvmpd_scan broken out into
 * 	its own function for ease of reading.
 *
 * => 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
 */

static boolean_t uvmpd_scan_inactive(pglst)

struct pglist *pglst;

{
  boolean_t retval = FALSE;	/* assume we haven't hit target */
  int s, free, result;
  struct vm_page *p, *nextpg;
  struct uvm_object *uobj;
  struct vm_page *pps[MAXBSIZE/PAGE_SIZE], **ppsp;
  int npages;
  struct vm_page *swpps[MAXBSIZE/PAGE_SIZE]; 		/* XXX: see below */
  int swnpages, swcpages;				/* XXX: see below */
  int swslot, oldslot;
  struct vm_anon *anon;
  boolean_t swap_backed;
  vm_offset_t start;
  UVMHIST_FUNC("uvmpd_scan_inactive"); UVMHIST_CALLED(pdhist);

  /*
   * note: we currently keep swap-backed pages on a seperate inactive
   * list from object-backed pages.   however, merging the two lists
   * back together again hasn't been ruled out.   thus, we keep our
   * swap cluster in "swpps" rather than in pps (allows us to mix clustering
   * types in the event of a mixed inactive queue).
   */

  /*
   * 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;
  free = 0;

  for (p = pglst->tqh_first ; p != NULL || swslot != 0 ; p = nextpg) {

    /*
     * note that p can be NULL iff we have traversed the whole
     * list and need to do one final swap-backed clustered pageout.
     */
    if (p) {
      /*
       * update our copy of "free" and see if we've met our target
       */
      s = splimp();
      uvm_lock_fpageq();
      free = uvmexp.free;
      uvm_unlock_fpageq();
      splx(s);

      if (free >= uvmexp.freetarg) {
	UVMHIST_LOG(pdhist,"  met free target: exit loop", 0, 0, 0, 0);
	retval = TRUE;		/* hit the target! */
	if (swslot == 0)
	  break;	/* exit now if no swap-i/o pending */
	p = NULL;	/* set p to null to signal final swap i/o */
      }
    }

    uobj = NULL;	/* be safe and shut gcc up */
    anon = NULL;	/* be safe and shut gcc up */

    if (p) {	/* if (we have a new page to consider) */
      /*
       * we are below target and have a new page to consider.
       */
      uvmexp.pdscans++;
      nextpg = p->pageq.tqe_next;

      /*
       * move referenced pages back to active queue and skip to next page
       * (unlikely to happen since inactive pages shouldn't have any
       *  valid mappings and we cleared reference before deactivating).
       */
      if (pmap_is_referenced(PMAP_PGARG(p))) {
	uvm_pageactivate(p);
	uvmexp.pdreact++;
	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 get deadlocked.
       *
       * the only time we exepct 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) || p->uobject == NULL) {

	anon = p->uanon;

#ifdef DIAGNOSTIC
        /* to be on inactive q, page must be part of _something_ */
        if (anon == NULL)
          panic("pagedaemon: page with no anon or object detected - loop 1");
#endif

	if (!simple_lock_try(&anon->an_lock))
	  continue;		/* lock failed, skip this page */

        /* if the page is ownerless, claim it in the name of "anon"! */
        if ((p->pqflags & PQ_ANON) == 0) {
#ifdef DIAGNOSTIC
          if (p->loan_count < 1)
            panic("pagedaemon: non-loaned ownerless page detected - loop 1");
#endif
          p->loan_count--;
          p->pqflags |= PQ_ANON;      /* anon now owns it */
        }

	if (p->flags & PG_BUSY) {
	  simple_unlock(&anon->an_lock);
	  uvmexp.pdbusy++;
	  continue;		/* someone else owns page, skip it */
	}

	uvmexp.pdanscan++;

      } else {

	uobj = p->uobject;

	if (!simple_lock_try(&uobj->vmobjlock))
	  continue;		/* lock failed, skip this page */

	if (p->flags & PG_BUSY) {
	  simple_unlock(&uobj->vmobjlock);
	  uvmexp.pdbusy++;
	  continue;		/* someone else owns page, skip it */
	}

	uvmexp.pdobscan++;

      }

      /*
       * we now have the object and the page queues locked.  the page is
       * not busy.   if the page is clean we can free it now and continue.
       */

      if (p->flags & PG_CLEAN) {
	/* zap all mappings with pmap_page_protect... */
	pmap_page_protect(PMAP_PGARG(p), VM_PROT_NONE);
	uvm_pagefree(p);
	uvmexp.pdfreed++;

	if (anon) {
#ifdef DIAGNOSTIC
	  /*
	   * an anonymous page can only be clean if it has valid
	   * backing store.
	   */
	  if (anon->an_swslot == 0)
	    panic("pagedaemon: clean anon page without backing store?");
#endif
	  anon->u.an_page = NULL;		/* remove from object */
	  simple_unlock(&anon->an_lock);
	} else {
	  /* pagefree has already removed the page from the object */
	  simple_unlock(&uobj->vmobjlock);
	}
	continue;
      }

      /*
       * this page is dirty, skip it if we'll have met
       * our free target when all the current pageouts complete.
       */
      if (free + uvmexp.paging > uvmexp.freetarg)
      {
	if (anon) {
	  simple_unlock(&anon->an_lock);
	} else {
	  simple_unlock(&uobj->vmobjlock);
	}
	continue;
      }

      /*
       * the page we are looking at is dirty.   we must clean it before
       * it can be freed.  to do this we first mark the page busy so that
       * no one else will touch the page.   we write protect all the mappings
       * of the page so that no one touches it while it is in I/O.
       */

      swap_backed = ((p->pqflags & PQ_SWAPBACKED) != 0);
      uvmexp.pdpageouts++;
      p->flags |= PG_BUSY;		/* now we own it */
      UVM_PAGE_OWN(p, "scan_inactive");
      pmap_page_protect(PMAP_PGARG(p), VM_PROT_READ);

      /*
       * for swap-backed pages we need to (re)allocate swap space.
       */
      if (swap_backed) {

	/*
	 * free old swap slot (if any)
	 */
	if (anon) {
	  if (anon->an_swslot) {
	    uvm_swap_free(anon->an_swslot, 1);
	    anon->an_swslot = 0;
	  }
	} else {
	  oldslot = uao_set_swslot(uobj, p->offset/PAGE_SIZE, 0); /* remove */
	  if (oldslot)
	    uvm_swap_free(oldslot, 1); /* free */
	}

	/*
	 * start new cluster (if necessary)
	 */
	if (swslot == 0) {
	  swnpages = MAXBSIZE/PAGE_SIZE;	/* want this much */
	  swslot = uvm_swap_alloc(&swnpages, TRUE);

	  if (swslot == 0) {
	    /* no swap?  give up! */
	    p->flags &= ~PG_BUSY;
            UVM_PAGE_OWN(p, NULL);
	    if (anon)
	      simple_unlock(&anon->an_lock);
	    else
	      simple_unlock(&uobj->vmobjlock);
	    continue;
	  }
	  swcpages = 0;	/* cluster is empty */
	}

	/*
	 * add block to cluster
	 */
	swpps[swcpages] = p;
	uvmexp.pageouts++;
	if (anon)
	  anon->an_swslot = swslot + swcpages;
	else
	  uao_set_swslot(uobj, p->offset/PAGE_SIZE, swslot + swcpages);
	swcpages++;

	/* done (swap-backed) */
      }

      /* end: if (p) [end of "if we have new page to consider"] */
    } else {

      swap_backed = TRUE; /* if p == NULL we must be doing a last swap i/o */

    }

    /*
     * now consider doing the pageout.
     *
     * for swap-backed pages, we do the pageout if we have either
     * filled the cluster (in which case (swnpages == swcpages) or
     * run out of pages (p == NULL).
     *
     * for object pages, we always do the pageout.
     */
    if (swap_backed) {

      if (p) {	/* if we just added a page to cluster */
	if (anon)
	  simple_unlock(&anon->an_lock);
	else
	  simple_unlock(&uobj->vmobjlock);
	if (swcpages < swnpages)	/* cluster not full yet? */
	  continue;
      }

      /* starting I/O now... set up for it */
      npages = swcpages;
      ppsp = swpps;
      start = (vm_offset_t) swslot;	/* for swap-backed pages only */

      /* if this is final pageout we could have a few extra swap blocks */
      if (swcpages < swnpages) {
	uvm_swap_free(swslot + swcpages, (swnpages - swcpages));
      }

    } else {

      /* normal object pageout */
      ppsp = pps;
      npages = sizeof(pps) / sizeof(struct vm_page *);
      start = 0;	/* not looked at because PGO_ALLPAGES is set */

    }

    /*
     * now do the pageout.
     *
     * for swap_backed pages we have already built the cluster.
     * for !swap_backed pages, uvm_pager_put will call the object's
     * "make put cluster" function to build a cluster on our behalf.
     *
     * we pass the PGO_PDFREECLUST flag to uvm_pager_put to instruct
     * it to free the cluster pages for us on a successful I/O (it always
     * does this for un-successful I/O requests).  this allows us to
     * do clustered pageout without having to deal with cluster pages
     * at this level.
     *
     * note locking semantics of uvm_pager_put with PGO_PDFREECLUST:
     *  IN:  locked: uobj (if !swap_backed), page queues
     * OUT:  locked: uobj (if !swap_backed && result != VM_PAGER_PEND)
     *      !locked: page queues, uobj (if swap_backed || VM_PAGER_PEND)
     *
     * [the bit about VM_PAGER_PEND saves us one lock-unlock pair]
     */

    /* locked: uobj (if !swap_backed), page queues */
    result = uvm_pager_put((swap_backed) ? NULL : uobj, p, &ppsp, &npages,
			   PGO_ALLPAGES|PGO_PDFREECLUST, start, 0);
    /* locked: uobj (if !swap_backed && result != PEND) */
    /* unlocked: page queues, object (if swap_backed || result == PEND) */

    /*
     * if we did i/o to swap, zero swslot to indicate that we are
     * no longer building a swap-backed cluster.
     */

    if (swap_backed)
      swslot = 0;		/* done with this cluster */

    /*
     * first, we check for VM_PAGER_PEND which means that the async I/O
     * is in progress and the async I/O done routine will clean up
     * after us.   in this case we move on to the next page.
     *
     * there is a very remote chance that the pending async i/o can
     * finish _before_ we get here.   if that happens, our page "p"
     * may no longer be on the inactive queue.   so we verify this
     * when determining the next page (starting over at the head if
     * we've lost our inactive page).
     */

    if (result == VM_PAGER_PEND) {
      uvmexp.paging += npages;
      uvm_lock_pageq();				/* relock page queues */
      uvmexp.pdpending++;
      if (p) {
	if (p->pqflags & PQ_INACTIVE)
	  nextpg = p->pageq.tqe_next;		/* reload! */
	else
	  nextpg = pglst->tqh_first;		/* reload! */
      } else {
	nextpg = NULL;				/* done list */
      }
      continue;
    }

    /*
     * clean up "p" if we have one
     */

    if (p) {
      /*
       * the I/O request to "p" is done and uvm_pager_put has freed
       * any cluster pages it may have allocated during I/O.  all
       * that is left for us to do is clean up page "p" (which is
       * still PG_BUSY).
       *
       * our result could be one of the following:
       *   VM_PAGER_OK: successful pageout
       *
       *   VM_PAGER_AGAIN: tmp resource shortage, we skip to next page
       *   VM_PAGER_{FAIL,ERROR,BAD}: an error.   we "reactivate"
       *		page to get it out of the way (it will eventually
       *		drift back into the inactive queue for a retry).
       *   VM_PAGER_UNLOCK: should never see this as it is only
       *		valid for "get" operations
       */

      /* relock p's object: page queues not lock yet, so no need for "try" */
      if (swap_backed) {	/* !swap_backed case: already locked... */
	if (anon)
	  simple_lock(&anon->an_lock);
	else
	  simple_lock(&uobj->vmobjlock);
      }

#ifdef DIAGNOSTIC
      if (result == VM_PAGER_UNLOCK)
	panic("pagedaemon: pageout returned invalid 'unlock' code");
#endif

      /* handle PG_WANTED now */
      if (p->flags & PG_WANTED)
	thread_wakeup(p);			/* still holding object lock */
      p->flags &= ~(PG_BUSY|PG_WANTED);
      UVM_PAGE_OWN(p, NULL);

      /* released during I/O? */
      if (p->flags & PG_RELEASED) {
	if (anon) {
	  anon->u.an_page = NULL;	/* remove page so we can get nextpg */
	  simple_unlock(&anon->an_lock);/* XXX needed? */
	  uvm_anfree(anon);		/* kills anon */
	  pmap_page_protect(PMAP_PGARG(p), VM_PROT_NONE);
	  anon = NULL;
	  uvm_lock_pageq();
	  nextpg = p->pageq.tqe_next;
	  uvm_pagefree(p);			/* free released page */

	} else {

#ifdef DIAGNOSTIC
	  if (uobj->pgops->pgo_releasepg == NULL)
	    panic("pagedaemon: no pgo_releasepg function");
#endif

	  /*
	   * pgo_releasepg nukes the page and gets "nextpg" for us.
	   * it returns with the page queues locked (when given nextpg ptr).
	   */
	  if (!uobj->pgops->pgo_releasepg(p, &nextpg))
	    uobj = NULL;			/* uobj died after release */

	  /*
	   * lock page queues here so that they're always locked
	   * at the end of the loop.
	   */
	  uvm_lock_pageq();
	}

      } else {	/* page was not released during I/O */

	uvm_lock_pageq();
	nextpg = p->pageq.tqe_next;

	if (result != VM_PAGER_OK) {

	  /* pageout was a failure... */
	  if (result != VM_PAGER_AGAIN)
	    uvm_pageactivate(p);
	  pmap_clear_reference(PMAP_PGARG(p));
	  /* XXXCDC: if (swap_backed) FREE p's swap block? */

	} else {

	  /* pageout was a success... */
	  pmap_clear_reference(PMAP_PGARG(p));
	  pmap_clear_modify(PMAP_PGARG(p));
	  p->flags |= PG_CLEAN;
	  /* XXX: could free page here, but old pagedaemon does not */

	}
      }

      /*
       * drop object lock (if there is an object left).   do a safety
       * check of nextpg to make sure it is on the inactive queue
       * (it should be since PG_BUSY pages on the inactive queue can't
       * be re-queued [note: not true for active queue]).
       */

      if (anon)
	simple_unlock(&anon->an_lock);
      else if (uobj)
	simple_unlock(&uobj->vmobjlock);

    } /* if (p) */ else {

      /* if p is null in this loop, make sure it stays null in next loop */
      nextpg = NULL;

      /*
       * lock page queues here just so they're always locked
       * at the end of the loop.
       */
      uvm_lock_pageq();
    }

    if (nextpg && (nextpg->pqflags & PQ_INACTIVE) == 0) {
      printf("pagedaemon: invalid nextpg!   reverting to queue head\n");
      nextpg = pglst->tqh_first;	/* reload! */
    }

  }		/* end of "inactive" 'for' loop */
  return(retval);
}

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

void uvmpd_scan()

{
  int s, free, pages_freed, page_shortage;
  struct vm_page *p, *nextpg;
  struct uvm_object *uobj;
  boolean_t got_it;
  UVMHIST_FUNC("uvmpd_scan"); UVMHIST_CALLED(pdhist);

  uvmexp.pdrevs++;		/* counter */

#ifdef __GNUC__
  uobj = NULL;	/* XXX gcc */
#endif
  /*
   * get current "free" page count
   */
  s = splimp();
  uvm_lock_fpageq();
  free = uvmexp.free;
  uvm_unlock_fpageq();
  splx(s);

#ifndef __SWAP_BROKEN
  /*
   * swap out some processes if we are below our free target.
   * we need to unlock the page queues for this.
   */
  if (free < uvmexp.freetarg) {

    uvmexp.pdswout++;
    UVMHIST_LOG(pdhist,"  free %d < target %d: swapout", free, uvmexp.freetarg,
		0,0);
    uvm_unlock_pageq();
    uvm_swapout_threads();
    pmap_update();		/* update so we can scan inactive q */
    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);
  pages_freed = uvmexp.pdfreed;	/* so far... */

  /*
   * do loop #1!   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;
  if ((uvmexp.pdrevs & 1) != 0 && uvmexp.nswapdev != 0)
    got_it = uvmpd_scan_inactive(&uvm.page_inactive_swp);
  if (!got_it)
    got_it = uvmpd_scan_inactive(&uvm.page_inactive_obj);
  if (!got_it && (uvmexp.pdrevs & 1) == 0 && uvmexp.nswapdev != 0)
    (void) uvmpd_scan_inactive(&uvm.page_inactive_swp);

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

  page_shortage = uvmexp.inactarg - uvmexp.inactive;
  pages_freed = uvmexp.pdfreed - pages_freed; /* # pages freed in loop */
  if (page_shortage <= 0 && pages_freed == 0)
    page_shortage = 1;

  UVMHIST_LOG(pdhist, "  second loop: page_shortage=%d", page_shortage,0,0,0);
  for (p = uvm.page_active.tqh_first ;
       p != NULL && page_shortage > 0 ; p = nextpg) {

    nextpg = p->pageq.tqe_next;
    if (p->flags & PG_BUSY)
      continue;			/* quick check before trying to lock */

    /*
     * lock owner
     */
    /* is page anon owned or ownerless? */
    if ((p->pqflags & PQ_ANON) || p->uobject == NULL) {

#ifdef DIAGNOSTIC
      if (p->uanon == NULL)
        panic("pagedaemon: page with no anon or object detected - loop 2");
#endif

      if (!simple_lock_try(&p->uanon->an_lock))
        continue;

      /* take over the page? */
      if ((p->pqflags & PQ_ANON) == 0) {

#ifdef DIAGNOSTIC
        if (p->loan_count < 1)
          panic("pagedaemon: non-loaned ownerless page detected - loop 2");
#endif

        p->loan_count--;
        p->pqflags |= PQ_ANON;
      }

    } else {

      if (!simple_lock_try(&p->uobject->vmobjlock))
        continue;

    }

    if ((p->flags & PG_BUSY) == 0) {
      pmap_page_protect(PMAP_PGARG(p), VM_PROT_NONE);
      /* no need to check wire_count as pg is "active" */
      uvm_pagedeactivate(p);
      uvmexp.pddeact++;
      page_shortage--;
    }

    if (p->pqflags & PQ_ANON)
      simple_unlock(&p->uanon->an_lock);
    else
      simple_unlock(&p->uobject->vmobjlock);
  }

  /*
   * done scan
   */
}