path: root/vm/vm_pageout.c

/*
 * Mach Operating System
 * Copyright (c) 1991,1990,1989,1988,1987 Carnegie Mellon University.
 * Copyright (c) 1993,1994 The University of Utah and
 * the Computer Systems Laboratory (CSL).
 * 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, THE UNIVERSITY OF UTAH AND CSL ALLOW FREE USE OF
 * THIS SOFTWARE IN ITS "AS IS" CONDITION, AND DISCLAIM 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 Mellon
 * the rights to redistribute these changes.
 */
/*
 *	File:	vm/vm_pageout.c
 *	Author:	Avadis Tevanian, Jr., Michael Wayne Young
 *	Date:	1985
 *
 *	The proverbial page-out daemon.
 */

#include <mach/mach_types.h>
#include <mach/memory_object.h>
#include <vm/memory_object_default.user.h>
#include <vm/memory_object_user.user.h>
#include <mach/vm_param.h>
#include <mach/vm_statistics.h>
#include <kern/counters.h>
#include <kern/debug.h>  /* for panic() */
#include <kern/thread.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_pageout.h>
#include <machine/vm_tuning.h>



#ifndef	VM_PAGEOUT_BURST_MAX
#define	VM_PAGEOUT_BURST_MAX	10		/* number of pages */
#endif	/* VM_PAGEOUT_BURST_MAX */

#ifndef	VM_PAGEOUT_BURST_MIN
#define	VM_PAGEOUT_BURST_MIN	5		/* number of pages */
#endif	/* VM_PAGEOUT_BURST_MIN */

#ifndef	VM_PAGEOUT_BURST_WAIT
#define	VM_PAGEOUT_BURST_WAIT	10		/* milliseconds per page */
#endif	/* VM_PAGEOUT_BURST_WAIT */

#ifndef	VM_PAGEOUT_EMPTY_WAIT
#define VM_PAGEOUT_EMPTY_WAIT	75		/* milliseconds */
#endif	/* VM_PAGEOUT_EMPTY_WAIT */

#ifndef	VM_PAGEOUT_PAUSE_MAX
#define	VM_PAGEOUT_PAUSE_MAX	10		/* number of pauses */
#endif	/* VM_PAGEOUT_PAUSE_MAX */

/*
 *	To obtain a reasonable LRU approximation, the inactive queue
 *	needs to be large enough to give pages on it a chance to be
 *	referenced a second time.  This macro defines the fraction
 *	of active+inactive pages that should be inactive.
 *	The pageout daemon uses it to update vm_page_inactive_target.
 *
 *	If vm_page_free_count falls below vm_page_free_target and
 *	vm_page_inactive_count is below vm_page_inactive_target,
 *	then the pageout daemon starts running.
 */

#ifndef	VM_PAGE_INACTIVE_TARGET
#define	VM_PAGE_INACTIVE_TARGET(avail)	((avail) * 2 / 3)
#endif	/* VM_PAGE_INACTIVE_TARGET */

/*
 *	Once the pageout daemon starts running, it keeps going
 *	until vm_page_free_count meets or exceeds vm_page_free_target.
 */

#ifndef	VM_PAGE_FREE_TARGET
#define	VM_PAGE_FREE_TARGET(free)	(15 + (free) / 80)
#endif	/* VM_PAGE_FREE_TARGET */

/*
 *	The pageout daemon always starts running once vm_page_free_count
 *	falls below vm_page_free_min.
 */

#ifndef	VM_PAGE_FREE_MIN
#define	VM_PAGE_FREE_MIN(free)	(10 + (free) / 100)
#endif	/* VM_PAGE_FREE_MIN */

/*      When vm_page_external_count exceeds vm_page_external_limit, 
 *	allocations of externally paged pages stops.
 */

#ifndef VM_PAGE_EXTERNAL_LIMIT
#define VM_PAGE_EXTERNAL_LIMIT(free)		((free) / 2)
#endif  /* VM_PAGE_EXTERNAL_LIMIT */

/*	Attempt to keep the number of externally paged pages less
 *	than vm_pages_external_target.
 */
#ifndef VM_PAGE_EXTERNAL_TARGET
#define VM_PAGE_EXTERNAL_TARGET(free)		((free) / 4)
#endif  /* VM_PAGE_EXTERNAL_TARGET */

/*
 *	When vm_page_free_count falls below vm_page_free_reserved,
 *	only vm-privileged threads can allocate pages.  vm-privilege
 *	allows the pageout daemon and default pager (and any other
 *	associated threads needed for default pageout) to continue
 *	operation by dipping into the reserved pool of pages.  */

#ifndef	VM_PAGE_FREE_RESERVED
#define	VM_PAGE_FREE_RESERVED			50
#endif	/* VM_PAGE_FREE_RESERVED */

/*
 *	When vm_page_free_count falls below vm_pageout_reserved_internal,
 *	the pageout daemon no longer trusts external pagers to clean pages.
 *	External pagers are probably all wedged waiting for a free page.
 *	It forcibly double-pages dirty pages belonging to external objects,
 *	getting the pages to the default pager to clean.
 */

#ifndef	VM_PAGEOUT_RESERVED_INTERNAL
#define	VM_PAGEOUT_RESERVED_INTERNAL(reserve)	((reserve) - 25)
#endif	/* VM_PAGEOUT_RESERVED_INTERNAL */

/*
 *	When vm_page_free_count falls below vm_pageout_reserved_really,
 *	the pageout daemon stops work entirely to let the default pager
 *	catch up (assuming the default pager has pages to clean).
 *	Beyond this point, it is too dangerous to consume memory
 *	even for memory_object_data_write messages to the default pager.
 */

#ifndef	VM_PAGEOUT_RESERVED_REALLY
#define	VM_PAGEOUT_RESERVED_REALLY(reserve)	((reserve) - 40)
#endif	/* VM_PAGEOUT_RESERVED_REALLY */

extern void vm_pageout_continue();
extern void vm_pageout_scan_continue();

unsigned int vm_pageout_reserved_internal = 0;
unsigned int vm_pageout_reserved_really = 0;

unsigned int vm_page_external_target = 0;

unsigned int vm_pageout_burst_max = 0;
unsigned int vm_pageout_burst_min = 0;
unsigned int vm_pageout_burst_wait = 0;		/* milliseconds per page */
unsigned int vm_pageout_empty_wait = 0;		/* milliseconds */
unsigned int vm_pageout_pause_count = 0;
unsigned int vm_pageout_pause_max = 0;

/*
 *	These variables record the pageout daemon's actions:
 *	how many pages it looks at and what happens to those pages.
 *	No locking needed because only one thread modifies the variables.
 */

unsigned int vm_pageout_active = 0;		/* debugging */
unsigned int vm_pageout_inactive = 0;		/* debugging */
unsigned int vm_pageout_inactive_nolock = 0;	/* debugging */
unsigned int vm_pageout_inactive_busy = 0;	/* debugging */
unsigned int vm_pageout_inactive_absent = 0;	/* debugging */
unsigned int vm_pageout_inactive_used = 0;	/* debugging */
unsigned int vm_pageout_inactive_clean = 0;	/* debugging */
unsigned int vm_pageout_inactive_dirty = 0;	/* debugging */
unsigned int vm_pageout_inactive_double = 0;	/* debugging */
unsigned int vm_pageout_inactive_cleaned_external = 0;

/*
 *	Routine:	vm_pageout_setup
 *	Purpose:
 *		Set up a page for pageout.
 *
 *		Move or copy the page to a new object, as part
 *		of which it will be sent to its memory manager
 *		in a memory_object_data_write or memory_object_initialize
 *		message.
 *
 *		The "paging_offset" argument specifies the offset
 *		of the page within its external memory object.
 *
 *		The "new_object" and "new_offset" arguments
 *		indicate where the page should be moved.
 *
 *		The "flush" argument specifies whether the page
 *		should be flushed from its object.  If not, a
 *		copy of the page is moved to the new object.
 *
 *	In/Out conditions:
 *		The page in question must not be on any pageout queues,
 *		and must be busy.  The object to which it belongs
 *		must be unlocked, and the caller must hold a paging
 *		reference to it.  The new_object must not be locked.
 *
 *		If the page is flushed from its original object,
 *		this routine returns a pointer to a place-holder page,
 *		inserted at the same offset, to block out-of-order
 *		requests for the page.  The place-holder page must
 *		be freed after the data_write or initialize message
 *		has been sent.  If the page is copied,
 *		the holding page is VM_PAGE_NULL.
 *
 *		The original page is put on a paging queue and marked
 *		not busy on exit.
 */
vm_page_t
vm_pageout_setup(m, paging_offset, new_object, new_offset, flush)
	register vm_page_t	m;
	vm_offset_t		paging_offset;
	register vm_object_t	new_object;
	vm_offset_t		new_offset;
	boolean_t		flush;
{
	register vm_object_t	old_object = m->object;
	register vm_page_t	holding_page = 0; /*'=0'to quiet gcc warnings*/
	register vm_page_t	new_m;

	assert(m->busy && !m->absent && !m->fictitious);

	/*
	 *	If we are not flushing the page, allocate a
	 *	page in the object.  If we cannot get the
	 *	page, flush instead.
	 */
	if (!flush) {
		vm_object_lock(new_object);
		new_m = vm_page_alloc(new_object, new_offset);
		if (new_m == VM_PAGE_NULL)
			flush = TRUE;
		vm_object_unlock(new_object);
	}

	if (flush) {
		/*
		 *	Create a place-holder page where the old one was,
		 *	to prevent anyone from attempting to page in this
		 *	page while we`re unlocked.
		 */
		while ((holding_page = vm_page_grab_fictitious())
							== VM_PAGE_NULL)
			vm_page_more_fictitious();

		vm_object_lock(old_object);
		vm_page_lock_queues();
		vm_page_remove(m);
		vm_page_unlock_queues();
		PAGE_WAKEUP_DONE(m);

		vm_page_lock_queues();
		vm_page_insert(holding_page, old_object, m->offset);
		vm_page_unlock_queues();

		/*
		 *	Record that this page has been written out
		 */
#if	MACH_PAGEMAP
		vm_external_state_set(old_object->existence_info,
					paging_offset,
					VM_EXTERNAL_STATE_EXISTS);
#endif	/* MACH_PAGEMAP */

		vm_object_unlock(old_object);

		vm_object_lock(new_object);

		/*
		 *	Move this page into the new object
		 */

		vm_page_lock_queues();
		vm_page_insert(m, new_object, new_offset);
		vm_page_unlock_queues();

		m->dirty = TRUE;
		m->precious = FALSE;
		m->page_lock = VM_PROT_NONE;
		m->unlock_request = VM_PROT_NONE;
	}
	else {
		/*
		 *	Copy the data into the new page,
		 *	and mark the new page as clean.
		 */
		vm_page_copy(m, new_m);

		vm_object_lock(old_object);
		m->dirty = FALSE;
		pmap_clear_modify(m->phys_addr);

		/*
		 *	Deactivate old page.
		 */
		vm_page_lock_queues();
		vm_page_deactivate(m);
		vm_page_unlock_queues();

		PAGE_WAKEUP_DONE(m);

		/*
		 *	Record that this page has been written out
		 */

#if	MACH_PAGEMAP
		vm_external_state_set(old_object->existence_info,
					paging_offset,
					VM_EXTERNAL_STATE_EXISTS);
#endif	/* MACH_PAGEMAP */

		vm_object_unlock(old_object);

		vm_object_lock(new_object);

		/*
		 *	Use the new page below.
		 */
		m = new_m;
		m->dirty = TRUE;
		assert(!m->precious);
		PAGE_WAKEUP_DONE(m);
	}

	/*
	 *	Make the old page eligible for replacement again; if a
	 *	user-supplied memory manager fails to release the page,
	 *	it will be paged out again to the default memory manager.
	 *
	 *	Note that pages written to the default memory manager
	 *	must be wired down -- in return, it guarantees to free
	 *	this page, rather than reusing it.
	 */

	vm_page_lock_queues();
	vm_stat.pageouts++;
	if (m->laundry) {
		/*
		 *	vm_pageout_scan is telling us to put this page
		 *	at the front of the inactive queue, so it will
		 *	be immediately paged out to the default pager.
		 */

		assert(!old_object->internal);
		m->laundry = FALSE;

		queue_enter_first(&vm_page_queue_inactive, m,
				  vm_page_t, pageq);
		m->inactive = TRUE;
		vm_page_inactive_count++;
	} else if (old_object->internal) {
		m->laundry = TRUE;
		vm_page_laundry_count++;

		vm_page_wire(m);
	} else
		vm_page_activate(m);
	vm_page_unlock_queues();

	/*
	 *	Since IPC operations may block, we drop locks now.
	 *	[The placeholder page is busy, and we still have
	 *	paging_in_progress incremented.]
	 */

	vm_object_unlock(new_object);

	/*
	 *	Return the placeholder page to simplify cleanup.
	 */
	return (flush ? holding_page : VM_PAGE_NULL);
}

/*
 *	Routine:	vm_pageout_page
 *	Purpose:
 *		Causes the specified page to be written back to
 *		the appropriate memory object.
 *
 *		The "initial" argument specifies whether this
 *		data is an initialization only, and should use
 *		memory_object_data_initialize instead of
 *		memory_object_data_write.
 *
 *		The "flush" argument specifies whether the page
 *		should be flushed from the object.  If not, a
 *		copy of the data is sent to the memory object.
 *
 *	In/out conditions:
 *		The page in question must not be on any pageout queues.
 *		The object to which it belongs must be locked.
 *	Implementation:
 *		Move this page to a completely new object, if flushing;
 *		copy to a new page in a new object, if not.
 */
void
vm_pageout_page(m, initial, flush)
	register vm_page_t	m;
	boolean_t		initial;
	boolean_t		flush;
{
	vm_map_copy_t		copy;
	register vm_object_t	old_object;
	register vm_object_t	new_object;
	register vm_page_t	holding_page;
	vm_offset_t		paging_offset;
	kern_return_t		rc;
	boolean_t		precious_clean;

	assert(m->busy);

	/*
	 *	Cleaning but not flushing a clean precious page is a
	 *	no-op.  Remember whether page is clean and precious now
	 *	because vm_pageout_setup will mark it dirty and not precious.
	 *
	 * XXX Check if precious_clean && !flush can really happen.
	 */
	precious_clean = (!m->dirty) && m->precious;
	if (precious_clean && !flush) {
		PAGE_WAKEUP_DONE(m);
		return;
	}

	/*
	 *	Verify that we really want to clean this page.
	 */
	if (m->absent || m->error || (!m->dirty && !m->precious)) {
		VM_PAGE_FREE(m);
		return;
	}

	/*
	 *	Create a paging reference to let us play with the object.
	 */
	old_object = m->object;
	paging_offset = m->offset + old_object->paging_offset;
	vm_object_paging_begin(old_object);
	vm_object_unlock(old_object);

	/*
	 *	Allocate a new object into which we can put the page.
	 */
	new_object = vm_object_allocate(PAGE_SIZE);

	/*
	 *	Move the page into the new object.
	 */
	holding_page = vm_pageout_setup(m,
				paging_offset,
				new_object,
				0,		/* new offset */
				flush);		/* flush */

	rc = vm_map_copyin_object(new_object, 0, PAGE_SIZE, &copy);
	assert(rc == KERN_SUCCESS);

	if (initial || old_object->use_old_pageout) {
		rc = (*(initial ? memory_object_data_initialize
			     : memory_object_data_write))
			(old_object->pager,
			 old_object->pager_request,
			 paging_offset, (pointer_t) copy, PAGE_SIZE);
	}
	else {
		rc = memory_object_data_return(
			 old_object->pager,
			 old_object->pager_request,
			 paging_offset, (pointer_t) copy, PAGE_SIZE,
			 !precious_clean, !flush);
	}

	if (rc != KERN_SUCCESS)
		vm_map_copy_discard(copy);

	/*
	 *	Clean up.
	 */
	vm_object_lock(old_object);
	if (holding_page != VM_PAGE_NULL)
	    VM_PAGE_FREE(holding_page);
	vm_object_paging_end(old_object);
}

/*
 *	vm_pageout_scan does the dirty work for the pageout daemon.
 *	It returns with vm_page_queue_free_lock held and
 *	vm_page_free_wanted == 0.
 */

void vm_pageout_scan()
{
	unsigned int burst_count;
	unsigned int want_pages;

	/*
	 *	We want to gradually dribble pages from the active queue
	 *	to the inactive queue.  If we let the inactive queue get
	 *	very small, and then suddenly dump many pages into it,
	 *	those pages won't get a sufficient chance to be referenced
	 *	before we start taking them from the inactive queue.
	 *
	 *	We must limit the rate at which we send pages to the pagers.
	 *	data_write messages consume memory, for message buffers and
	 *	for map-copy objects.  If we get too far ahead of the pagers,
	 *	we can potentially run out of memory.
	 *
	 *	We can use the laundry count to limit directly the number
	 *	of pages outstanding to the default pager.  A similar
	 *	strategy for external pagers doesn't work, because
	 *	external pagers don't have to deallocate the pages sent them,
	 *	and because we might have to send pages to external pagers
	 *	even if they aren't processing writes.  So we also
	 *	use a burst count to limit writes to external pagers.
	 *
	 *	When memory is very tight, we can't rely on external pagers to
	 *	clean pages.  They probably aren't running, because they
	 *	aren't vm-privileged.  If we kept sending dirty pages to them,
	 *	we could exhaust the free list.  However, we can't just ignore
	 *	pages belonging to external objects, because there might be no
	 *	pages belonging to internal objects.  Hence, we get the page
	 *	into an internal object and then immediately double-page it,
	 *	sending it to the default pager.
	 *
	 *	consider_zone_gc should be last, because the other operations
	 *	might return memory to zones.  When we pause we use
	 *	vm_pageout_scan_continue as our continuation, so we will
	 *	reenter vm_pageout_scan periodically and attempt to reclaim
	 *	internal memory even if we never reach vm_page_free_target.
	 */

    Restart:
	stack_collect();
	net_kmsg_collect();
	consider_task_collect();
	consider_thread_collect();
	consider_zone_gc();

	for (burst_count = 0;;) {
		register vm_page_t m;
		register vm_object_t object;
		unsigned int free_count;

		/*
		 *	Recalculate vm_page_inactivate_target.
		 */

		vm_page_lock_queues();
		vm_page_inactive_target =
			VM_PAGE_INACTIVE_TARGET(vm_page_active_count +
						vm_page_inactive_count);

		/*
		 *	Move pages from active to inactive.
		 */

		while ((vm_page_inactive_count < vm_page_inactive_target) &&
		       !queue_empty(&vm_page_queue_active)) {
			register vm_object_t obj;

			vm_pageout_active++;
			m = (vm_page_t) queue_first(&vm_page_queue_active);
			assert(m->active && !m->inactive);

			obj = m->object;
			if (!vm_object_lock_try(obj)) {
				/*
				 *	Move page to end and continue.
				 */

				queue_remove(&vm_page_queue_active, m,
					     vm_page_t, pageq);
				queue_enter(&vm_page_queue_active, m,
					    vm_page_t, pageq);
				vm_page_unlock_queues();
				vm_page_lock_queues();
				continue;
			}

			/*
			 *	If the page is busy, then we pull it
			 *	off the active queue and leave it alone.
			 */

			if (m->busy) {
				vm_object_unlock(obj);
				queue_remove(&vm_page_queue_active, m,
					     vm_page_t, pageq);
				m->active = FALSE;
				vm_page_active_count--;
				continue;
			}

			/*
			 *	Deactivate the page while holding the object
			 *	locked, so we know the page is still not busy.
			 *	This should prevent races between pmap_enter
			 *	and pmap_clear_reference.  The page might be
			 *	absent or fictitious, but vm_page_deactivate
			 *	can handle that.
			 */

			vm_page_deactivate(m);
			vm_object_unlock(obj);
		}

		/*
		 *	We are done if we have met our targets *and*
		 *	nobody is still waiting for a page.
		 */

		simple_lock(&vm_page_queue_free_lock);
		free_count = vm_page_free_count;
		if ((free_count >= vm_page_free_target) &&
		    (vm_page_external_count <= vm_page_external_target) &&
		    (vm_page_free_wanted == 0)) {
			vm_page_unlock_queues();
			break;
		}
		want_pages = ((free_count < vm_page_free_target) ||
			      vm_page_free_wanted);
		simple_unlock(&vm_page_queue_free_lock);

		/*
		 * Sometimes we have to pause:
		 *	1) No inactive pages - nothing to do.
		 *	2) Flow control - wait for pagers to catch up.
		 *	3) Extremely low memory - sending out dirty pages
		 *	consumes memory.  We don't take the risk of doing
		 *	this if the default pager already has work to do.
		 */
	pause:
		if (queue_empty(&vm_page_queue_inactive) ||
		    (burst_count >= vm_pageout_burst_max) ||
		    (vm_page_laundry_count >= vm_pageout_burst_max) ||
		    ((free_count < vm_pageout_reserved_really) &&
		     (vm_page_laundry_count > 0))) {
			unsigned int pages, msecs;

			/*
			 *	vm_pageout_burst_wait is msecs/page.
			 *	If there is nothing for us to do, we wait
			 *	at least vm_pageout_empty_wait msecs.
			 */

			if (vm_page_laundry_count > burst_count)
				pages = vm_page_laundry_count;
			else
				pages = burst_count;
			msecs = pages * vm_pageout_burst_wait;

			if (queue_empty(&vm_page_queue_inactive) &&
			    (msecs < vm_pageout_empty_wait))
				msecs = vm_pageout_empty_wait;
			vm_page_unlock_queues();

			thread_will_wait_with_timeout(current_thread(), msecs);
			counter(c_vm_pageout_scan_block++);
			thread_block(vm_pageout_scan_continue);
#ifndef CONTINUATIONS
			/*
			 *	Unfortunately, we don't have call_continuation
			 *	so we can't rely on tail-recursion.
			 */

			vm_pageout_scan_continue();
			goto Restart;
#else	/* CONTINUATIONS */
			call_continuation(vm_pageout_scan_continue);
			/*NOTREACHED*/
#endif	/* CONTINUATIONS */
		}

		vm_pageout_inactive++;

		/* Find a page we are interested in paging out.  If we
		   need pages, then we'll page anything out; otherwise
		   we only page out external pages. */
		m = (vm_page_t) queue_first (&vm_page_queue_inactive);
		while (1)
		  {
		    assert (!m->active && m->inactive);
		    if (want_pages || m->external)
		      break;
		    
		    m = (vm_page_t) queue_next (m);
		    if (!m)
		      goto pause;
		  }
		
		object = m->object;

		/*
		 *	Try to lock object; since we've got the
		 *	page queues lock, we can only try for this one.
		 */

		if (!vm_object_lock_try(object)) {
			/*
			 *	Move page to end and continue.
			 */

			queue_remove(&vm_page_queue_inactive, m,
				     vm_page_t, pageq);
			queue_enter(&vm_page_queue_inactive, m,
				    vm_page_t, pageq);
			vm_page_unlock_queues();
			vm_pageout_inactive_nolock++;
			continue;
		}

		/*
		 *	Remove the page from the inactive list.
		 */

		queue_remove(&vm_page_queue_inactive, m, vm_page_t, pageq);
		vm_page_inactive_count--;
		m->inactive = FALSE;

		if (m->busy || !object->alive) {
			/*
			 *	Somebody is already playing with this page.
			 *	Leave it off the pageout queues.
			 */

			vm_page_unlock_queues();
			vm_object_unlock(object);
			vm_pageout_inactive_busy++;
			continue;
		}

		/*
		 *	If it's absent, we can reclaim the page.
		 */

		if (want_pages && m->absent) {
			vm_pageout_inactive_absent++;
		    reclaim_page:
			vm_page_free(m);
			vm_page_unlock_queues();
			vm_object_unlock(object);
			continue;
		}

		/*
		 *	If it's being used, reactivate.
		 *	(Fictitious pages are either busy or absent.)
		 */

		assert(!m->fictitious);
		if (m->reference || pmap_is_referenced(m->phys_addr)) {
			vm_object_unlock(object);
			vm_page_activate(m);
			vm_stat.reactivations++;
			vm_page_unlock_queues();
			vm_pageout_inactive_used++;
			continue;
		}

		/*
		 *	Eliminate all mappings.
		 */

		m->busy = TRUE;
		pmap_page_protect(m->phys_addr, VM_PROT_NONE);
		if (!m->dirty)
			m->dirty = pmap_is_modified(m->phys_addr);

		if (m->external) {
			/* Figure out if we still care about this
			page in the limit of externally managed pages.
			Clean pages don't actually cause system hosage,
			so it's ok to stop considering them as
			"consumers" of memory. */
			if (m->dirty && !m->extcounted) {
				m->extcounted = TRUE;
				vm_page_external_count++;
			} else if (!m->dirty && m->extcounted) {
				m->extcounted = FALSE;
				vm_page_external_count--;
			}
		}
		
		/* If we don't actually need more memory, and the page
		   is not dirty, put it on the tail of the inactive queue
		   and move on to the next page. */
		if (!want_pages && !m->dirty) {
			queue_remove (&vm_page_queue_inactive, m, 
				      vm_page_t, pageq);
			queue_enter (&vm_page_queue_inactive, m,
				     vm_page_t, pageq);
			vm_page_unlock_queues();
			vm_pageout_inactive_cleaned_external++;
			continue;
		}			

		/*
		 *	If it's clean and not precious, we can free the page.
		 */

		if (!m->dirty && !m->precious) {
			vm_pageout_inactive_clean++;
			goto reclaim_page;
		}

		/*
		 *	If we are very low on memory, then we can't
		 *	rely on an external pager to clean a dirty page,
		 *	because external pagers are not vm-privileged.
		 *
		 *	The laundry bit tells vm_pageout_setup to
		 *	put the page back at the front of the inactive
		 *	queue instead of activating the page.  Hence,
		 *	we will pick the page up again immediately and
		 *	resend it to the default pager.
		 */

		assert(!m->laundry);
		if ((free_count < vm_pageout_reserved_internal) &&
		    !object->internal) {
			m->laundry = TRUE;
			vm_pageout_inactive_double++;
		}
		vm_page_unlock_queues();

		/*
		 *	If there is no memory object for the page, create
		 *	one and hand it to the default pager.
		 *	[First try to collapse, so we don't create
		 *	one unnecessarily.]
		 */

		if (!object->pager_initialized)
			vm_object_collapse(object);
		if (!object->pager_initialized)
			vm_object_pager_create(object);
		if (!object->pager_initialized)
			panic("vm_pageout_scan");

		vm_pageout_inactive_dirty++;
		vm_pageout_page(m, FALSE, TRUE);	/* flush it */
		vm_object_unlock(object);
		burst_count++;
	}
}

void vm_pageout_scan_continue()
{
	/*
	 *	We just paused to let the pagers catch up.
	 *	If vm_page_laundry_count is still high,
	 *	then we aren't waiting long enough.
	 *	If we have paused some vm_pageout_pause_max times without
	 *	adjusting vm_pageout_burst_wait, it might be too big,
	 *	so we decrease it.
	 */

	vm_page_lock_queues();
	if (vm_page_laundry_count > vm_pageout_burst_min) {
		vm_pageout_burst_wait++;
		vm_pageout_pause_count = 0;
	} else if (++vm_pageout_pause_count > vm_pageout_pause_max) {
		vm_pageout_burst_wait = (vm_pageout_burst_wait * 3) / 4;
		if (vm_pageout_burst_wait < 1)
			vm_pageout_burst_wait = 1;
		vm_pageout_pause_count = 0;
	}
	vm_page_unlock_queues();

#ifdef	CONTINUATIONS
	vm_pageout_continue();
	/*NOTREACHED*/
#endif	/* CONTINUATIONS */
}

/*
 *	vm_pageout is the high level pageout daemon.
 */

void vm_pageout_continue()
{
	/*
	 *	The pageout daemon is never done, so loop forever.
	 *	We should call vm_pageout_scan at least once each
	 *	time we are woken, even if vm_page_free_wanted is
	 *	zero, to check vm_page_free_target and
	 *	vm_page_inactive_target.
	 */

	for (;;) {
		vm_pageout_scan();
		/* we hold vm_page_queue_free_lock now */
		assert(vm_page_free_wanted == 0);

		assert_wait(&vm_page_free_wanted, FALSE);
		simple_unlock(&vm_page_queue_free_lock);
		counter(c_vm_pageout_block++);
		thread_block(vm_pageout_continue);
	}
}

void vm_pageout()
{
	int		free_after_reserve;

	current_thread()->vm_privilege = TRUE;
	stack_privilege(current_thread());

	/*
	 *	Initialize some paging parameters.
	 */

	if (vm_pageout_burst_max == 0)
		vm_pageout_burst_max = VM_PAGEOUT_BURST_MAX;

	if (vm_pageout_burst_min == 0)
		vm_pageout_burst_min = VM_PAGEOUT_BURST_MIN;

	if (vm_pageout_burst_wait == 0)
		vm_pageout_burst_wait = VM_PAGEOUT_BURST_WAIT;

	if (vm_pageout_empty_wait == 0)
		vm_pageout_empty_wait = VM_PAGEOUT_EMPTY_WAIT;

	if (vm_page_free_reserved == 0)
		vm_page_free_reserved = VM_PAGE_FREE_RESERVED;

	if (vm_pageout_pause_max == 0)
		vm_pageout_pause_max = VM_PAGEOUT_PAUSE_MAX;

	if (vm_pageout_reserved_internal == 0)
		vm_pageout_reserved_internal =
			VM_PAGEOUT_RESERVED_INTERNAL(vm_page_free_reserved);

	if (vm_pageout_reserved_really == 0)
		vm_pageout_reserved_really =
			VM_PAGEOUT_RESERVED_REALLY(vm_page_free_reserved);

	free_after_reserve = vm_page_free_count - vm_page_free_reserved;

	if (vm_page_external_limit == 0)
	        vm_page_external_limit = 
			VM_PAGE_EXTERNAL_LIMIT (free_after_reserve);

	if (vm_page_external_target == 0)
	        vm_page_external_target = 
			VM_PAGE_EXTERNAL_TARGET (free_after_reserve);

	if (vm_page_free_min == 0)
		vm_page_free_min = vm_page_free_reserved +
			VM_PAGE_FREE_MIN(free_after_reserve);

	if (vm_page_free_target == 0)
		vm_page_free_target = vm_page_free_reserved +
			VM_PAGE_FREE_TARGET(free_after_reserve);

	if (vm_page_free_target < vm_page_free_min + 5)
		vm_page_free_target = vm_page_free_min + 5;

	/*
	 *	vm_pageout_scan will set vm_page_inactive_target.
	 */

	vm_pageout_continue();
	/*NOTREACHED*/
}