freebsd-skq/sys/vm/vm_object.c

/*
 * 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 the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	from: @(#)vm_object.c	8.5 (Berkeley) 3/22/94
 *
 *
 * Copyright (c) 1987, 1990 Carnegie-Mellon University.
 * All rights reserved.
 *
 * Authors: Avadis Tevanian, Jr., Michael Wayne Young
 *
 * Permission to use, copy, modify and distribute this software and
 * its documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 *
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 *
 * Carnegie Mellon requests users of this software to return to
 *
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 *
 * any improvements or extensions that they make and grant Carnegie the
 * rights to redistribute these changes.
 *
 * $Id: vm_object.c,v 1.17 1995/01/11 20:19:20 davidg Exp $
 */

/*
 *	Virtual memory object module.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>		/* for curproc, pageproc */
#include <sys/malloc.h>
#include <sys/vnode.h>
#include <sys/mount.h>

#include <vm/vm.h>
#include <vm/vm_page.h>
#include <vm/vm_pageout.h>
#include <vm/vm_pager.h>
#include <vm/swap_pager.h>
#include <vm/vnode_pager.h>

static void _vm_object_allocate(vm_size_t, vm_object_t);
static void vm_object_rcollapse(vm_object_t, vm_object_t);

/*
 *	Virtual memory objects maintain the actual data
 *	associated with allocated virtual memory.  A given
 *	page of memory exists within exactly one object.
 *
 *	An object is only deallocated when all "references"
 *	are given up.  Only one "reference" to a given
 *	region of an object should be writeable.
 *
 *	Associated with each object is a list of all resident
 *	memory pages belonging to that object; this list is
 *	maintained by the "vm_page" module, and locked by the object's
 *	lock.
 *
 *	Each object also records a "pager" routine which is
 *	used to retrieve (and store) pages to the proper backing
 *	storage.  In addition, objects may be backed by other
 *	objects from which they were virtual-copied.
 *
 *	The only items within the object structure which are
 *	modified after time of creation are:
 *		reference count		locked by object's lock
 *		pager routine		locked by object's lock
 *
 */


struct vm_object kernel_object_store;
struct vm_object kmem_object_store;

int vm_object_cache_max;

#define	VM_OBJECT_HASH_COUNT	509

struct vm_object_hash_head vm_object_hashtable[VM_OBJECT_HASH_COUNT];

long object_collapses = 0;
long object_bypasses = 0;

static void
_vm_object_allocate(size, object)
	vm_size_t size;
	register vm_object_t object;
{
	bzero(object, sizeof *object);
	TAILQ_INIT(&object->memq);
	TAILQ_INIT(&object->reverse_shadow_head);
	vm_object_lock_init(object);
	object->ref_count = 1;
	object->resident_page_count = 0;
	object->size = size;
	object->flags = OBJ_INTERNAL;	/* vm_allocate_with_pager will reset */
	object->paging_in_progress = 0;
	object->copy = NULL;

	/*
	 * Object starts out read-write, with no pager.
	 */

	object->pager = NULL;
	object->paging_offset = 0;
	object->shadow = NULL;
	object->shadow_offset = (vm_offset_t) 0;

	simple_lock(&vm_object_list_lock);
	TAILQ_INSERT_TAIL(&vm_object_list, object, object_list);
	vm_object_count++;
	cnt.v_nzfod += atop(size);
	simple_unlock(&vm_object_list_lock);
}

/*
 *	vm_object_init:
 *
 *	Initialize the VM objects module.
 */
void
vm_object_init(vm_offset_t nothing)
{
	register int i;

	TAILQ_INIT(&vm_object_cached_list);
	TAILQ_INIT(&vm_object_list);
	vm_object_count = 0;
	simple_lock_init(&vm_cache_lock);
	simple_lock_init(&vm_object_list_lock);
	vm_object_cache_max = (cnt.v_page_count - 500) / 8;

	for (i = 0; i < VM_OBJECT_HASH_COUNT; i++)
		TAILQ_INIT(&vm_object_hashtable[i]);

	kernel_object = &kernel_object_store;
	_vm_object_allocate(VM_MAX_KERNEL_ADDRESS - VM_MIN_KERNEL_ADDRESS,
	    kernel_object);

	kmem_object = &kmem_object_store;
	_vm_object_allocate(VM_MAX_KERNEL_ADDRESS - VM_MIN_KERNEL_ADDRESS,
	    kmem_object);
}

/*
 *	vm_object_allocate:
 *
 *	Returns a new object with the given size.
 */

vm_object_t
vm_object_allocate(size)
	vm_size_t size;
{
	register vm_object_t result;

	result = (vm_object_t)
	    malloc((u_long) sizeof *result, M_VMOBJ, M_WAITOK);


	_vm_object_allocate(size, result);

	return (result);
}


/*
 *	vm_object_reference:
 *
 *	Gets another reference to the given object.
 */
inline void
vm_object_reference(object)
	register vm_object_t object;
{
	if (object == NULL)
		return;

	vm_object_lock(object);
	object->ref_count++;
	vm_object_unlock(object);
}

/*
 *	vm_object_deallocate:
 *
 *	Release a reference to the specified object,
 *	gained either through a vm_object_allocate
 *	or a vm_object_reference call.  When all references
 *	are gone, storage associated with this object
 *	may be relinquished.
 *
 *	No object may be locked.
 */
void
vm_object_deallocate(object)
	vm_object_t object;
{
	vm_object_t temp;

	while (object != NULL) {

		/*
		 * The cache holds a reference (uncounted) to the object; we
		 * must lock it before removing the object.
		 */

		vm_object_cache_lock();

		/*
		 * Lose the reference
		 */
		vm_object_lock(object);
		if (--(object->ref_count) != 0) {
			if (object->ref_count == 1) {
				if (object->reverse_shadow_head.tqh_first) {
					++object->reverse_shadow_head.tqh_first->ref_count;
					if (vm_object_lock_try(object->reverse_shadow_head.tqh_first)) {
						vm_object_rcollapse(object->reverse_shadow_head.tqh_first, object);
						vm_object_unlock(object->reverse_shadow_head.tqh_first);
					}
					vm_object_deallocate(object->reverse_shadow_head.tqh_first);
				}
			}
			vm_object_unlock(object);
			/*
			 * If there are still references, then we are done.
			 */
			vm_object_cache_unlock();
			return;
		}
		/*
		 * See if this object can persist.  If so, enter it in the
		 * cache, then deactivate all of its pages.
		 */

		if (object->flags & OBJ_CANPERSIST) {

			TAILQ_INSERT_TAIL(&vm_object_cached_list, object,
			    cached_list);
			vm_object_cached++;
			vm_object_cache_unlock();

			vm_object_unlock(object);

			vm_object_cache_trim();
			return;
		}
		/*
		 * Make sure no one can look us up now.
		 */
		object->flags |= OBJ_DEAD;
		vm_object_remove(object->pager);
		vm_object_cache_unlock();

		temp = object->shadow;
		if (temp)
			TAILQ_REMOVE(&temp->reverse_shadow_head, object, reverse_shadow_list);
		vm_object_terminate(object);
		/* unlocks and deallocates object */
		object = temp;
	}
}

/*
 *	vm_object_terminate actually destroys the specified object, freeing
 *	up all previously used resources.
 *
 *	The object must be locked.
 */
void
vm_object_terminate(object)
	register vm_object_t object;
{
	register vm_page_t p, next;
	vm_object_t shadow_object;
	int s;
	struct vnode *vp = NULL;

	/*
	 * Detach the object from its shadow if we are the shadow's copy.
	 */
	if ((shadow_object = object->shadow) != NULL) {
		vm_object_lock(shadow_object);
		if (shadow_object->copy == object)
			shadow_object->copy = NULL;
/*
		else if (shadow_object->copy != NULL)
			panic("vm_object_terminate: copy/shadow inconsistency");
*/
		vm_object_unlock(shadow_object);
	}
	if (object->pager && (object->pager->pg_type == PG_VNODE)) {
		vn_pager_t vnp = object->pager->pg_data;

		vp = vnp->vnp_vp;
		VOP_FSYNC(vp, NOCRED, MNT_WAIT, NULL);
		vinvalbuf(vp, 0, NOCRED, NULL, 0, 0);
	}
	/*
	 * Wait until the pageout daemon is through with the object.
	 */

	s = splhigh();
	while (object->paging_in_progress) {
		vm_object_unlock(object);
		tsleep((caddr_t) object, PVM, "objtrm", 0);
		vm_object_lock(object);
	}
	splx(s);

	/*
	 * While the paging system is locked, pull the object's pages off the
	 * active and inactive queues.  This keeps the pageout daemon from
	 * playing with them during vm_pager_deallocate.
	 * 
	 * We can't free the pages yet, because the object's pager may have to
	 * write them out before deallocating the paging space.
	 */

	for (p = object->memq.tqh_first; p; p = next) {
		VM_PAGE_CHECK(p);
		next = p->listq.tqe_next;

		vm_page_lock_queues();
		if (p->flags & PG_CACHE)
			vm_page_free(p);
		else
			vm_page_unqueue(p);
		vm_page_unlock_queues();
		p = next;
	}

	if (object->paging_in_progress != 0)
		panic("vm_object_deallocate: pageout in progress");

	/*
	 * Clean and free the pages, as appropriate. All references to the
	 * object are gone, so we don't need to lock it.
	 */

	if ((object->flags & OBJ_INTERNAL) == 0) {
		(void) vm_object_page_clean(object, 0, 0, TRUE, TRUE);
	}
	/*
	 * one last time -- get rid of buffers that might have been created
	 * for the vm_object_page_clean
	 */
	if (vp != NULL) {
		vm_object_unlock(object);
		vinvalbuf(vp, 0, NOCRED, NULL, 0, 0);
		vm_object_lock(object);
	}
	/*
	 * Now free the pages. For internal objects, this also removes them
	 * from paging queues.
	 */
	while ((p = object->memq.tqh_first) != NULL) {
		VM_PAGE_CHECK(p);
		vm_page_lock_queues();
		PAGE_WAKEUP(p);
		vm_page_free(p);
		cnt.v_pfree++;
		vm_page_unlock_queues();
	}
	vm_object_unlock(object);

	/*
	 * Let the pager know object is dead.
	 */
	if (object->pager != NULL)
		vm_pager_deallocate(object->pager);

	simple_lock(&vm_object_list_lock);
	TAILQ_REMOVE(&vm_object_list, object, object_list);
	vm_object_count--;
	simple_unlock(&vm_object_list_lock);

	/*
	 * Free the space for the object.
	 */
	free((caddr_t) object, M_VMOBJ);
}

/*
 *	vm_object_page_clean
 *
 *	Clean all dirty pages in the specified range of object.
 *	Leaves page on whatever queue it is currently on.
 *
 *	Odd semantics: if start == end, we clean everything.
 *
 *	The object must be locked.
 */
#if 1
boolean_t
vm_object_page_clean(object, start, end, syncio, de_queue)
	register vm_object_t object;
	register vm_offset_t start;
	register vm_offset_t end;
	boolean_t syncio;
	boolean_t de_queue;
{
	register vm_page_t p, nextp;
	int size;

	if (object->pager == NULL)
		return 1;

	if (start != end) {
		start = trunc_page(start);
		end = round_page(end);
	}
	size = end - start;

again:
	/*
	 * Wait until the pageout daemon is through with the object.
	 */
	while (object->paging_in_progress) {
		tsleep(object, PVM, "objpcw", 0);
	}

	nextp = object->memq.tqh_first;
	while ((p = nextp) && ((start == end) || (size != 0))) {
		nextp = p->listq.tqe_next;
		if (start == end || (p->offset >= start && p->offset < end)) {
			if ((p->flags & PG_BUSY) || p->busy) {
				int s = splhigh();

				p->flags |= PG_WANTED;
				tsleep(p, PVM, "objpcn", 0);
				splx(s);
				goto again;
			}
			size -= PAGE_SIZE;

			vm_page_test_dirty(p);

			if ((p->dirty & p->valid) != 0) {
				vm_pageout_clean(p, VM_PAGEOUT_FORCE);
				goto again;
			}
		}
	}
	wakeup((caddr_t) object);
	return 1;
}
#endif
/*
 *	vm_object_page_clean
 *
 *	Clean all dirty pages in the specified range of object.
 *	If syncio is TRUE, page cleaning is done synchronously.
 *	If de_queue is TRUE, pages are removed from any paging queue
 *	they were on, otherwise they are left on whatever queue they
 *	were on before the cleaning operation began.
 *
 *	Odd semantics: if start == end, we clean everything.
 *
 *	The object must be locked.
 *
 *	Returns TRUE if all was well, FALSE if there was a pager error
 *	somewhere.  We attempt to clean (and dequeue) all pages regardless
 *	of where an error occurs.
 */
#if 0
boolean_t
vm_object_page_clean(object, start, end, syncio, de_queue)
	register vm_object_t object;
	register vm_offset_t start;
	register vm_offset_t end;
	boolean_t syncio;
	boolean_t de_queue;
{
	register vm_page_t p;
	int onqueue;
	boolean_t noerror = TRUE;

	if (object == NULL)
		return (TRUE);

	/*
	 * If it is an internal object and there is no pager, attempt to
	 * allocate one.  Note that vm_object_collapse may relocate one from a
	 * collapsed object so we must recheck afterward.
	 */
	if ((object->flags & OBJ_INTERNAL) && object->pager == NULL) {
		vm_object_collapse(object);
		if (object->pager == NULL) {
			vm_pager_t pager;

			vm_object_unlock(object);
			pager = vm_pager_allocate(PG_DFLT, (caddr_t) 0,
			    object->size, VM_PROT_ALL,
			    (vm_offset_t) 0);
			if (pager)
				vm_object_setpager(object, pager, 0, FALSE);
			vm_object_lock(object);
		}
	}
	if (object->pager == NULL)
		return (FALSE);

again:
	/*
	 * Wait until the pageout daemon is through with the object.
	 */
	while (object->paging_in_progress) {
		vm_object_sleep((int) object, object, FALSE);
		vm_object_lock(object);
	}
	/*
	 * Loop through the object page list cleaning as necessary.
	 */
	for (p = object->memq.tqh_first; p != NULL; p = p->listq.tqe_next) {
		onqueue = 0;
		if ((start == end || p->offset >= start && p->offset < end) &&
		    !(p->flags & PG_FICTITIOUS)) {
			vm_page_test_dirty(p);
			/*
			 * Remove the page from any paging queue. This needs
			 * to be done if either we have been explicitly asked
			 * to do so or it is about to be cleaned (see comment
			 * below).
			 */
			if (de_queue || (p->dirty & p->valid)) {
				vm_page_lock_queues();
				if (p->flags & PG_ACTIVE) {
					TAILQ_REMOVE(&vm_page_queue_active,
					    p, pageq);
					p->flags &= ~PG_ACTIVE;
					cnt.v_active_count--;
					onqueue = 1;
				} else if (p->flags & PG_INACTIVE) {
					TAILQ_REMOVE(&vm_page_queue_inactive,
					    p, pageq);
					p->flags &= ~PG_INACTIVE;
					cnt.v_inactive_count--;
					onqueue = -1;
				} else
					onqueue = 0;
				vm_page_unlock_queues();
			}
			/*
			 * To ensure the state of the page doesn't change
			 * during the clean operation we do two things. First
			 * we set the busy bit and write-protect all mappings
			 * to ensure that write accesses to the page block (in
			 * vm_fault).  Second, we remove the page from any
			 * paging queue to foil the pageout daemon
			 * (vm_pageout_scan).
			 */
			pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_READ);
			if (p->dirty & p->valid) {
				p->flags |= PG_BUSY;
				object->paging_in_progress++;
				vm_object_unlock(object);
				/*
				 * XXX if put fails we mark the page as clean
				 * to avoid an infinite loop. Will loose
				 * changes to the page.
				 */
				if (vm_pager_put(object->pager, p, syncio)) {
					printf("%s: pager_put error\n",
					    "vm_object_page_clean");
					p->dirty = 0;
					noerror = FALSE;
				}
				vm_object_lock(object);
				object->paging_in_progress--;
				if (!de_queue && onqueue) {
					vm_page_lock_queues();
					if (onqueue > 0)
						vm_page_activate(p);
					else
						vm_page_deactivate(p);
					vm_page_unlock_queues();
				}
				PAGE_WAKEUP(p);
				goto again;
			}
		}
	}
	return (noerror);
}
#endif

/*
 *	vm_object_deactivate_pages
 *
 *	Deactivate all pages in the specified object.  (Keep its pages
 *	in memory even though it is no longer referenced.)
 *
 *	The object must be locked.
 */
void
vm_object_deactivate_pages(object)
	register vm_object_t object;
{
	register vm_page_t p, next;

	for (p = object->memq.tqh_first; p != NULL; p = next) {
		next = p->listq.tqe_next;
		vm_page_lock_queues();
		vm_page_deactivate(p);
		vm_page_unlock_queues();
	}
}

/*
 *	Trim the object cache to size.
 */
void
vm_object_cache_trim()
{
	register vm_object_t object;

	vm_object_cache_lock();
	while (vm_object_cached > vm_object_cache_max) {
		object = vm_object_cached_list.tqh_first;
		vm_object_cache_unlock();

		if (object != vm_object_lookup(object->pager))
			panic("vm_object_cache_trim: I'm sooo confused.");

		pager_cache(object, FALSE);

		vm_object_cache_lock();
	}
	vm_object_cache_unlock();
}


/*
 *	vm_object_pmap_copy:
 *
 *	Makes all physical pages in the specified
 *	object range copy-on-write.  No writeable
 *	references to these pages should remain.
 *
 *	The object must *not* be locked.
 */
void 
vm_object_pmap_copy(object, start, end)
	register vm_object_t object;
	register vm_offset_t start;
	register vm_offset_t end;
{
	register vm_page_t p;

	if (object == NULL)
		return;

	vm_object_lock(object);
	for (p = object->memq.tqh_first; p != NULL; p = p->listq.tqe_next) {
		if ((start <= p->offset) && (p->offset < end)) {
			pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_READ);
			p->flags |= PG_COPYONWRITE;
		}
	}
	vm_object_unlock(object);
}

/*
 *	vm_object_pmap_remove:
 *
 *	Removes all physical pages in the specified
 *	object range from all physical maps.
 *
 *	The object must *not* be locked.
 */
void
vm_object_pmap_remove(object, start, end)
	register vm_object_t object;
	register vm_offset_t start;
	register vm_offset_t end;
{
	register vm_page_t p;
	int s;

	if (object == NULL)
		return;
	++object->paging_in_progress;

	vm_object_lock(object);
again:
	for (p = object->memq.tqh_first; p != NULL; p = p->listq.tqe_next) {
		if ((start <= p->offset) && (p->offset < end)) {
			s = splhigh();
			if ((p->flags & PG_BUSY) || p->busy) {
				p->flags |= PG_WANTED;
				tsleep((caddr_t) p, PVM, "vmopmr", 0);
				splx(s);
				goto again;
			}
			splx(s);
			pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
		}
	}
	vm_object_unlock(object);
	--object->paging_in_progress;
	if (object->paging_in_progress == 0)
		wakeup((caddr_t) object);
}

/*
 *	vm_object_copy:
 *
 *	Create a new object which is a copy of an existing
 *	object, and mark all of the pages in the existing
 *	object 'copy-on-write'.  The new object has one reference.
 *	Returns the new object.
 *
 *	May defer the copy until later if the object is not backed
 *	up by a non-default pager.
 */
void 
vm_object_copy(src_object, src_offset, size,
    dst_object, dst_offset, src_needs_copy)
	register vm_object_t src_object;
	vm_offset_t src_offset;
	vm_size_t size;
	vm_object_t *dst_object;/* OUT */
	vm_offset_t *dst_offset;/* OUT */
	boolean_t *src_needs_copy;	/* OUT */
{
	register vm_object_t new_copy;
	register vm_object_t old_copy;
	vm_offset_t new_start, new_end;

	register vm_page_t p;

	if (src_object == NULL) {
		/*
		 * Nothing to copy
		 */
		*dst_object = NULL;
		*dst_offset = 0;
		*src_needs_copy = FALSE;
		return;
	}
	/*
	 * If the object's pager is null_pager or the default pager, we don't
	 * have to make a copy of it.  Instead, we set the needs copy flag and
	 * make a shadow later.
	 */

	vm_object_lock(src_object);

	/*
	 * Try to collapse the object before copying it.
	 */

	vm_object_collapse(src_object);

	if (src_object->pager == NULL ||
	    src_object->pager->pg_type == PG_SWAP ||
	    (src_object->flags & OBJ_INTERNAL)) {

		/*
		 * Make another reference to the object
		 */
		src_object->ref_count++;

		/*
		 * Mark all of the pages copy-on-write.
		 */
		for (p = src_object->memq.tqh_first; p; p = p->listq.tqe_next)
			if (src_offset <= p->offset &&
			    p->offset < src_offset + size)
				p->flags |= PG_COPYONWRITE;
		vm_object_unlock(src_object);

		*dst_object = src_object;
		*dst_offset = src_offset;

		/*
		 * Must make a shadow when write is desired
		 */
		*src_needs_copy = TRUE;
		return;
	}
	/*
	 * If the object has a pager, the pager wants to see all of the
	 * changes.  We need a copy-object for the changed pages.
	 * 
	 * If there is a copy-object, and it is empty, no changes have been made
	 * to the object since the copy-object was made.  We can use the same
	 * copy- object.
	 */

Retry1:
	old_copy = src_object->copy;
	if (old_copy != NULL) {
		/*
		 * Try to get the locks (out of order)
		 */
		if (!vm_object_lock_try(old_copy)) {
			vm_object_unlock(src_object);

			/* should spin a bit here... */
			tsleep((caddr_t) old_copy, PVM, "cpylck", 1);
			vm_object_lock(src_object);
			goto Retry1;
		}
		if (old_copy->resident_page_count == 0 &&
		    old_copy->pager == NULL) {
			/*
			 * Return another reference to the existing
			 * copy-object.
			 */
			old_copy->ref_count++;
			vm_object_unlock(old_copy);
			vm_object_unlock(src_object);
			*dst_object = old_copy;
			*dst_offset = src_offset;
			*src_needs_copy = FALSE;
			return;
		}
		vm_object_unlock(old_copy);
	}
	vm_object_unlock(src_object);

	/*
	 * If the object has a pager, the pager wants to see all of the
	 * changes.  We must make a copy-object and put the changed pages
	 * there.
	 * 
	 * The copy-object is always made large enough to completely shadow the
	 * original object, since it may have several users who want to shadow
	 * the original object at different points.
	 */

	new_copy = vm_object_allocate(src_object->size);

Retry2:
	vm_object_lock(src_object);
	/*
	 * Copy object may have changed while we were unlocked
	 */
	old_copy = src_object->copy;
	if (old_copy != NULL) {
		/*
		 * Try to get the locks (out of order)
		 */
		if (!vm_object_lock_try(old_copy)) {
			vm_object_unlock(src_object);
			tsleep((caddr_t) old_copy, PVM, "cpylck", 1);
			goto Retry2;
		}
		/*
		 * Consistency check
		 */
		if (old_copy->shadow != src_object ||
		    old_copy->shadow_offset != (vm_offset_t) 0)
			panic("vm_object_copy: copy/shadow inconsistency");

		/*
		 * Make the old copy-object shadow the new one. It will
		 * receive no more pages from the original object.
		 */

		src_object->ref_count--;	/* remove ref. from old_copy */
		if (old_copy->shadow)
			TAILQ_REMOVE(&old_copy->shadow->reverse_shadow_head, old_copy, reverse_shadow_list);
		old_copy->shadow = new_copy;
		TAILQ_INSERT_TAIL(&old_copy->shadow->reverse_shadow_head, old_copy, reverse_shadow_list);
		new_copy->ref_count++;	/* locking not needed - we have the
					 * only pointer */
		vm_object_unlock(old_copy);	/* done with old_copy */
	}
	new_start = (vm_offset_t) 0;	/* always shadow original at 0 */
	new_end = (vm_offset_t) new_copy->size;	/* for the whole object */

	/*
	 * Point the new copy at the existing object.
	 */

	new_copy->shadow = src_object;
	TAILQ_INSERT_TAIL(&new_copy->shadow->reverse_shadow_head, new_copy, reverse_shadow_list);
	new_copy->shadow_offset = new_start;
	src_object->ref_count++;
	src_object->copy = new_copy;

	/*
	 * Mark all the affected pages of the existing object copy-on-write.
	 */
	for (p = src_object->memq.tqh_first; p != NULL; p = p->listq.tqe_next)
		if ((new_start <= p->offset) && (p->offset < new_end))
			p->flags |= PG_COPYONWRITE;

	vm_object_unlock(src_object);

	*dst_object = new_copy;
	*dst_offset = src_offset - new_start;
	*src_needs_copy = FALSE;
}

/*
 *	vm_object_shadow:
 *
 *	Create a new object which is backed by the
 *	specified existing object range.  The source
 *	object reference is deallocated.
 *
 *	The new object and offset into that object
 *	are returned in the source parameters.
 */

void
vm_object_shadow(object, offset, length)
	vm_object_t *object;	/* IN/OUT */
	vm_offset_t *offset;	/* IN/OUT */
	vm_size_t length;
{
	register vm_object_t source;
	register vm_object_t result;

	source = *object;

	/*
	 * Allocate a new object with the given length
	 */

	if ((result = vm_object_allocate(length)) == NULL)
		panic("vm_object_shadow: no object for shadowing");

	/*
	 * The new object shadows the source object, adding a reference to it.
	 * Our caller changes his reference to point to the new object,
	 * removing a reference to the source object.  Net result: no change
	 * of reference count.
	 */
	result->shadow = source;
	if (source)
		TAILQ_INSERT_TAIL(&result->shadow->reverse_shadow_head, result, reverse_shadow_list);

	/*
	 * Store the offset into the source object, and fix up the offset into
	 * the new object.
	 */

	result->shadow_offset = *offset;

	/*
	 * Return the new things
	 */

	*offset = 0;
	*object = result;
}

/*
 *	Set the specified object's pager to the specified pager.
 */

void
vm_object_setpager(object, pager, paging_offset,
    read_only)
	vm_object_t object;
	vm_pager_t pager;
	vm_offset_t paging_offset;
	boolean_t read_only;
{
	vm_object_lock(object);	/* XXX ? */
	if (object->pager && object->pager != pager) {
		panic("!!!pager already allocated!!!\n");
	}
	object->pager = pager;
	object->paging_offset = paging_offset;
	vm_object_unlock(object);	/* XXX ? */
}

/*
 *	vm_object_hash hashes the pager/id pair.
 */

#define vm_object_hash(pager) \
	(((unsigned)pager >> 5)%VM_OBJECT_HASH_COUNT)

/*
 *	vm_object_lookup looks in the object cache for an object with the
 *	specified pager and paging id.
 */

vm_object_t 
vm_object_lookup(pager)
	vm_pager_t pager;
{
	register vm_object_hash_entry_t entry;
	vm_object_t object;

	cnt.v_lookups++;
	vm_object_cache_lock();

	for (entry = vm_object_hashtable[vm_object_hash(pager)].tqh_first;
	    entry != NULL;
	    entry = entry->hash_links.tqe_next) {
		object = entry->object;
		if (object->pager == pager) {
			vm_object_lock(object);
			if (object->ref_count == 0) {
				TAILQ_REMOVE(&vm_object_cached_list, object,
				    cached_list);
				vm_object_cached--;
			}
			object->ref_count++;
			vm_object_unlock(object);
			vm_object_cache_unlock();
			cnt.v_hits++;
			return (object);
		}
	}

	vm_object_cache_unlock();
	return (NULL);
}

/*
 *	vm_object_enter enters the specified object/pager/id into
 *	the hash table.
 */

void 
vm_object_enter(object, pager)
	vm_object_t object;
	vm_pager_t pager;
{
	struct vm_object_hash_head *bucket;
	register vm_object_hash_entry_t entry;

	/*
	 * We don't cache null objects, and we can't cache objects with the
	 * null pager.
	 */

	if (object == NULL)
		return;
	if (pager == NULL)
		return;

	bucket = &vm_object_hashtable[vm_object_hash(pager)];
	entry = (vm_object_hash_entry_t)
	    malloc((u_long) sizeof *entry, M_VMOBJHASH, M_WAITOK);
	entry->object = object;
	object->flags |= OBJ_CANPERSIST;

	vm_object_cache_lock();
	TAILQ_INSERT_TAIL(bucket, entry, hash_links);
	vm_object_cache_unlock();
}

/*
 *	vm_object_remove:
 *
 *	Remove the pager from the hash table.
 *	Note:  This assumes that the object cache
 *	is locked.  XXX this should be fixed
 *	by reorganizing vm_object_deallocate.
 */
void
vm_object_remove(pager)
	register vm_pager_t pager;
{
	struct vm_object_hash_head *bucket;
	register vm_object_hash_entry_t entry;
	register vm_object_t object;

	bucket = &vm_object_hashtable[vm_object_hash(pager)];

	for (entry = bucket->tqh_first;
	    entry != NULL;
	    entry = entry->hash_links.tqe_next) {
		object = entry->object;
		if (object->pager == pager) {
			TAILQ_REMOVE(bucket, entry, hash_links);
			free((caddr_t) entry, M_VMOBJHASH);
			break;
		}
	}
}

static void
vm_object_rcollapse(object, sobject)
	register vm_object_t object, sobject;
{
	register vm_object_t backing_object;
	register vm_offset_t backing_offset, new_offset;
	register vm_page_t p, pp;
	register vm_size_t size;
	int s;

	if (!object)
		return;
	backing_object = object->shadow;
	if (backing_object != sobject) {
		printf("backing obj != sobject!!!\n");
		return;
	}
	if (!backing_object)
		return;
	if ((backing_object->flags & OBJ_INTERNAL) == 0)
		return;
	if (backing_object->shadow != NULL &&
	    backing_object->shadow->copy == backing_object)
		return;
	if (backing_object->ref_count != 1)
		return;

	object->ref_count += 2;
	backing_object->ref_count += 2;

	backing_offset = object->shadow_offset;
	size = object->size;

again:
	s = splbio();
	/* XXX what about object->paging_in_progress? */
	while (backing_object->paging_in_progress) {
		tsleep(backing_object, PVM, "rcolpp", 0);
	}
	splx(s);

	p = backing_object->memq.tqh_first;
	while (p) {
		vm_page_t next;

		next = p->listq.tqe_next;

		if ((p->flags & (PG_BUSY | PG_FICTITIOUS | PG_CACHE)) ||
		    !p->valid || p->hold_count || p->wire_count || p->busy || p->bmapped) {
			p = next;
			continue;
		}
		pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
		new_offset = (p->offset - backing_offset);
		if (p->offset < backing_offset ||
		    new_offset >= size) {
			if (backing_object->pager)
				swap_pager_freespace(backing_object->pager,
				    backing_object->paging_offset + p->offset, PAGE_SIZE);
			vm_page_lock_queues();
			vm_page_free(p);
			vm_page_unlock_queues();
		} else {
			pp = vm_page_lookup(object, new_offset);
			if (pp != NULL ||
			    (object->pager &&
			    vm_pager_has_page(object->pager, object->paging_offset + new_offset))) {
				if (backing_object->pager)
					swap_pager_freespace(backing_object->pager,
					    backing_object->paging_offset + p->offset, PAGE_SIZE);
				vm_page_lock_queues();
				vm_page_free(p);
				vm_page_unlock_queues();
			} else {
				if (!backing_object->pager ||
				    !vm_pager_has_page(backing_object->pager, backing_object->paging_offset + p->offset))
					vm_page_rename(p, object, new_offset);
			}
		}
		p = next;
	}
	backing_object->ref_count -= 2;
	object->ref_count -= 2;
}

/*
 * this version of collapse allows the operation to occur earlier and
 * when paging_in_progress is true for an object...  This is not a complete
 * operation, but should plug 99.9% of the rest of the leaks.
 */
static void
vm_object_qcollapse(object)
	register vm_object_t object;
{
	register vm_object_t backing_object;
	register vm_offset_t backing_offset, new_offset;
	register vm_page_t p, pp;
	register vm_size_t size;

	backing_object = object->shadow;
	if (!backing_object)
		return;
	if ((backing_object->flags & OBJ_INTERNAL) == 0)
		return;
	if (backing_object->shadow != NULL &&
	    backing_object->shadow->copy == backing_object)
		return;
	if (backing_object->ref_count != 1)
		return;

	backing_object->ref_count += 2;

	backing_offset = object->shadow_offset;
	size = object->size;
	p = backing_object->memq.tqh_first;
	while (p) {
		vm_page_t next;

		next = p->listq.tqe_next;
		if ((p->flags & (PG_BUSY | PG_FICTITIOUS | PG_CACHE)) ||
		    !p->valid || p->hold_count || p->wire_count || p->busy || p->bmapped) {
			p = next;
			continue;
		}
		pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
		new_offset = (p->offset - backing_offset);
		if (p->offset < backing_offset ||
		    new_offset >= size) {
			if (backing_object->pager)
				swap_pager_freespace(backing_object->pager,
				    backing_object->paging_offset + p->offset, PAGE_SIZE);
			vm_page_lock_queues();
			vm_page_free(p);
			vm_page_unlock_queues();
		} else {
			pp = vm_page_lookup(object, new_offset);
			if (pp != NULL || (object->pager && vm_pager_has_page(object->pager,
				    object->paging_offset + new_offset))) {
				if (backing_object->pager)
					swap_pager_freespace(backing_object->pager,
					    backing_object->paging_offset + p->offset, PAGE_SIZE);
				vm_page_lock_queues();
				vm_page_free(p);
				vm_page_unlock_queues();
			} else {
				if (!backing_object->pager ||
				    !vm_pager_has_page(backing_object->pager, backing_object->paging_offset + p->offset))
					vm_page_rename(p, object, new_offset);
			}
		}
		p = next;
	}
	backing_object->ref_count -= 2;
}

boolean_t vm_object_collapse_allowed = TRUE;

/*
 *	vm_object_collapse:
 *
 *	Collapse an object with the object backing it.
 *	Pages in the backing object are moved into the
 *	parent, and the backing object is deallocated.
 *
 *	Requires that the object be locked and the page
 *	queues be unlocked.
 *
 *	This routine has significant changes by John S. Dyson
 *	to fix some swap memory leaks.  18 Dec 93
 *
 */
void
vm_object_collapse(object)
	register vm_object_t object;

{
	register vm_object_t backing_object;
	register vm_offset_t backing_offset;
	register vm_size_t size;
	register vm_offset_t new_offset;
	register vm_page_t p, pp;

	if (!vm_object_collapse_allowed)
		return;

	while (TRUE) {
		/*
		 * Verify that the conditions are right for collapse:
		 * 
		 * The object exists and no pages in it are currently being paged
		 * out.
		 */
		if (object == NULL)
			return;

		/*
		 * Make sure there is a backing object.
		 */
		if ((backing_object = object->shadow) == NULL)
			return;

		if (object->paging_in_progress != 0) {
			if (backing_object) {
				if (vm_object_lock_try(backing_object)) {
					vm_object_qcollapse(object);
					vm_object_unlock(backing_object);
				}
			}
			return;
		}

		vm_object_lock(backing_object);
		/*
		 * ... The backing object is not read_only, and no pages in
		 * the backing object are currently being paged out. The
		 * backing object is internal.
		 */

		if ((backing_object->flags & OBJ_INTERNAL) == 0 ||
		    backing_object->paging_in_progress != 0) {
			vm_object_unlock(backing_object);
			vm_object_qcollapse(object);
			return;
		}
		/*
		 * The backing object can't be a copy-object: the
		 * shadow_offset for the copy-object must stay as 0.
		 * Furthermore (for the 'we have all the pages' case), if we
		 * bypass backing_object and just shadow the next object in
		 * the chain, old pages from that object would then have to be
		 * copied BOTH into the (former) backing_object and into the
		 * parent object.
		 */
		if (backing_object->shadow != NULL &&
		    backing_object->shadow->copy == backing_object) {
			vm_object_unlock(backing_object);
			return;
		}
		/*
		 * we can deal only with the swap pager
		 */
		if ((object->pager &&
			object->pager->pg_type != PG_SWAP) ||
		    (backing_object->pager &&
			backing_object->pager->pg_type != PG_SWAP)) {
			vm_object_unlock(backing_object);
			return;
		}
		/*
		 * We know that we can either collapse the backing object (if
		 * the parent is the only reference to it) or (perhaps) remove
		 * the parent's reference to it.
		 */

		backing_offset = object->shadow_offset;
		size = object->size;

		/*
		 * If there is exactly one reference to the backing object, we
		 * can collapse it into the parent.
		 */

		if (backing_object->ref_count == 1) {

			/*
			 * We can collapse the backing object.
			 * 
			 * Move all in-memory pages from backing_object to the
			 * parent.  Pages that have been paged out will be
			 * overwritten by any of the parent's pages that
			 * shadow them.
			 */

			while ((p = backing_object->memq.tqh_first) != 0) {

				new_offset = (p->offset - backing_offset);

				/*
				 * If the parent has a page here, or if this
				 * page falls outside the parent, dispose of
				 * it.
				 * 
				 * Otherwise, move it as planned.
				 */

				if (p->offset < backing_offset ||
				    new_offset >= size) {
					vm_page_lock_queues();
					pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
					PAGE_WAKEUP(p);
					vm_page_free(p);
					vm_page_unlock_queues();
				} else {
					pp = vm_page_lookup(object, new_offset);
					if (pp != NULL || (object->pager && vm_pager_has_page(object->pager,
					    object->paging_offset + new_offset))) {
						vm_page_lock_queues();
						pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
						PAGE_WAKEUP(p);
						vm_page_free(p);
						vm_page_unlock_queues();
					} else {
						vm_page_rename(p, object, new_offset);
					}
				}
			}

			/*
			 * Move the pager from backing_object to object.
			 */

			if (backing_object->pager) {
				backing_object->paging_in_progress++;
				if (object->pager) {
					vm_pager_t bopager;

					object->paging_in_progress++;
					/*
					 * copy shadow object pages into ours
					 * and destroy unneeded pages in
					 * shadow object.
					 */
					bopager = backing_object->pager;
					backing_object->pager = NULL;
					vm_object_remove(backing_object->pager);
					swap_pager_copy(
					    bopager, backing_object->paging_offset,
					    object->pager, object->paging_offset,
					    object->shadow_offset);
					object->paging_in_progress--;
					if (object->paging_in_progress == 0)
						wakeup((caddr_t) object);
				} else {
					object->paging_in_progress++;
					/*
					 * grab the shadow objects pager
					 */
					object->pager = backing_object->pager;
					object->paging_offset = backing_object->paging_offset + backing_offset;
					vm_object_remove(backing_object->pager);
					backing_object->pager = NULL;
					/*
					 * free unnecessary blocks
					 */
					swap_pager_freespace(object->pager, 0, object->paging_offset);
					object->paging_in_progress--;
					if (object->paging_in_progress == 0)
						wakeup((caddr_t) object);
				}
				backing_object->paging_in_progress--;
				if (backing_object->paging_in_progress == 0)
					wakeup((caddr_t) backing_object);
			}
			/*
			 * Object now shadows whatever backing_object did.
			 * Note that the reference to backing_object->shadow
			 * moves from within backing_object to within object.
			 */

			TAILQ_REMOVE(&object->shadow->reverse_shadow_head, object,
			    reverse_shadow_list);
			if (backing_object->shadow)
				TAILQ_REMOVE(&backing_object->shadow->reverse_shadow_head,
				    backing_object, reverse_shadow_list);
			object->shadow = backing_object->shadow;
			if (object->shadow)
				TAILQ_INSERT_TAIL(&object->shadow->reverse_shadow_head,
				    object, reverse_shadow_list);

			object->shadow_offset += backing_object->shadow_offset;
			if (object->shadow != NULL &&
			    object->shadow->copy != NULL) {
				panic("vm_object_collapse: we collapsed a copy-object!");
			}
			/*
			 * Discard backing_object.
			 * 
			 * Since the backing object has no pages, no pager left,
			 * and no object references within it, all that is
			 * necessary is to dispose of it.
			 */

			vm_object_unlock(backing_object);

			simple_lock(&vm_object_list_lock);
			TAILQ_REMOVE(&vm_object_list, backing_object,
			    object_list);
			vm_object_count--;
			simple_unlock(&vm_object_list_lock);

			free((caddr_t) backing_object, M_VMOBJ);

			object_collapses++;
		} else {
			/*
			 * If all of the pages in the backing object are
			 * shadowed by the parent object, the parent object no
			 * longer has to shadow the backing object; it can
			 * shadow the next one in the chain.
			 * 
			 * The backing object must not be paged out - we'd have
			 * to check all of the paged-out pages, as well.
			 */

			if (backing_object->pager != NULL) {
				vm_object_unlock(backing_object);
				return;
			}
			/*
			 * Should have a check for a 'small' number of pages
			 * here.
			 */

			for (p = backing_object->memq.tqh_first; p; p = p->listq.tqe_next) {
				new_offset = (p->offset - backing_offset);

				/*
				 * If the parent has a page here, or if this
				 * page falls outside the parent, keep going.
				 * 
				 * Otherwise, the backing_object must be left in
				 * the chain.
				 */

				if (p->offset >= backing_offset &&
				    new_offset <= size &&
				    ((pp = vm_page_lookup(object, new_offset)) == NULL ||
					!pp->valid) &&
				    (!object->pager || !vm_pager_has_page(object->pager, object->paging_offset + new_offset))) {
					/*
					 * Page still needed. Can't go any
					 * further.
					 */
					vm_object_unlock(backing_object);
					return;
				}
			}

			/*
			 * Make the parent shadow the next object in the
			 * chain.  Deallocating backing_object will not remove
			 * it, since its reference count is at least 2.
			 */

			TAILQ_REMOVE(&object->shadow->reverse_shadow_head,
			    object, reverse_shadow_list);
			vm_object_reference(object->shadow = backing_object->shadow);
			if (object->shadow)
				TAILQ_INSERT_TAIL(&object->shadow->reverse_shadow_head,
				    object, reverse_shadow_list);
			object->shadow_offset += backing_object->shadow_offset;

			/*
			 * Backing object might have had a copy pointer to us.
			 * If it did, clear it.
			 */
			if (backing_object->copy == object) {
				backing_object->copy = NULL;
			}
			/*
			 * Drop the reference count on backing_object. Since
			 * its ref_count was at least 2, it will not vanish;
			 * so we don't need to call vm_object_deallocate.
			 */
			if (backing_object->ref_count == 1)
				printf("should have called obj deallocate\n");
			backing_object->ref_count--;
			vm_object_unlock(backing_object);

			object_bypasses++;

		}

		/*
		 * Try again with this object's new backing object.
		 */
	}
}

/*
 *	vm_object_page_remove: [internal]
 *
 *	Removes all physical pages in the specified
 *	object range from the object's list of pages.
 *
 *	The object must be locked.
 */
void
vm_object_page_remove(object, start, end)
	register vm_object_t object;
	register vm_offset_t start;
	register vm_offset_t end;
{
	register vm_page_t p, next;
	vm_offset_t size;
	int s;

	if (object == NULL)
		return;

	object->paging_in_progress++;
	start = trunc_page(start);
	end = round_page(end);
again:
	size = end - start;
	if (size > 4 * PAGE_SIZE || size >= object->size / 4) {
		for (p = object->memq.tqh_first; p != NULL; p = next) {
			next = p->listq.tqe_next;
			if ((start <= p->offset) && (p->offset < end)) {
				s = splhigh();
				if (p->bmapped) {
					splx(s);
					continue;
				}
				if ((p->flags & PG_BUSY) || p->busy) {
					p->flags |= PG_WANTED;
					tsleep((caddr_t) p, PVM, "vmopar", 0);
					splx(s);
					goto again;
				}
				splx(s);
				pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
				vm_page_lock_queues();
				PAGE_WAKEUP(p);
				vm_page_free(p);
				vm_page_unlock_queues();
			}
		}
	} else {
		while (size > 0) {
			while ((p = vm_page_lookup(object, start)) != 0) {
				s = splhigh();
				if (p->bmapped) {
					splx(s);
					break;
				}
				if ((p->flags & PG_BUSY) || p->busy) {
					p->flags |= PG_WANTED;
					tsleep((caddr_t) p, PVM, "vmopar", 0);
					splx(s);
					goto again;
				}
				splx(s);
				pmap_page_protect(VM_PAGE_TO_PHYS(p), VM_PROT_NONE);
				vm_page_lock_queues();
				PAGE_WAKEUP(p);
				vm_page_free(p);
				vm_page_unlock_queues();
			}
			start += PAGE_SIZE;
			size -= PAGE_SIZE;
		}
	}
	--object->paging_in_progress;
	if (object->paging_in_progress == 0)
		wakeup((caddr_t) object);
}

/*
 *	Routine:	vm_object_coalesce
 *	Function:	Coalesces two objects backing up adjoining
 *			regions of memory into a single object.
 *
 *	returns TRUE if objects were combined.
 *
 *	NOTE:	Only works at the moment if the second object is NULL -
 *		if it's not, which object do we lock first?
 *
 *	Parameters:
 *		prev_object	First object to coalesce
 *		prev_offset	Offset into prev_object
 *		next_object	Second object into coalesce
 *		next_offset	Offset into next_object
 *
 *		prev_size	Size of reference to prev_object
 *		next_size	Size of reference to next_object
 *
 *	Conditions:
 *	The object must *not* be locked.
 */
boolean_t 
vm_object_coalesce(prev_object, next_object,
    prev_offset, next_offset,
    prev_size, next_size)
	register vm_object_t prev_object;
	vm_object_t next_object;
	vm_offset_t prev_offset, next_offset;
	vm_size_t prev_size, next_size;
{
	vm_size_t newsize;

	if (next_object != NULL) {
		return (FALSE);
	}
	if (prev_object == NULL) {
		return (TRUE);
	}
	vm_object_lock(prev_object);

	/*
	 * Try to collapse the object first
	 */
	vm_object_collapse(prev_object);

	/*
	 * Can't coalesce if: . more than one reference . paged out . shadows
	 * another object . has a copy elsewhere (any of which mean that the
	 * pages not mapped to prev_entry may be in use anyway)
	 */

	if (prev_object->ref_count > 1 ||
	    prev_object->pager != NULL ||
	    prev_object->shadow != NULL ||
	    prev_object->copy != NULL) {
		vm_object_unlock(prev_object);
		return (FALSE);
	}
	/*
	 * Remove any pages that may still be in the object from a previous
	 * deallocation.
	 */

	vm_object_page_remove(prev_object,
	    prev_offset + prev_size,
	    prev_offset + prev_size + next_size);

	/*
	 * Extend the object if necessary.
	 */
	newsize = prev_offset + prev_size + next_size;
	if (newsize > prev_object->size)
		prev_object->size = newsize;

	vm_object_unlock(prev_object);
	return (TRUE);
}

/*
 * returns page after looking up in shadow chain
 */

vm_page_t
vm_object_page_lookup(object, offset)
	vm_object_t object;
	vm_offset_t offset;
{
	vm_page_t m;

	if (!(m = vm_page_lookup(object, offset))) {
		if (!object->shadow)
			return 0;
		else
			return vm_object_page_lookup(object->shadow, offset + object->shadow_offset);
	}
	return m;
}

#define DEBUG
#if defined(DEBUG) || defined(DDB)
/*
 *	vm_object_print:	[ debug ]
 */
void 
vm_object_print(object, full)
	vm_object_t object;
	boolean_t full;
{
	register vm_page_t p;
	extern indent;

	register int count;

	if (object == NULL)
		return;

	iprintf("Object 0x%x: size=0x%x, res=%d, ref=%d, ",
	    (int) object, (int) object->size,
	    object->resident_page_count, object->ref_count);
	printf("pager=0x%x+0x%x, shadow=(0x%x)+0x%x\n",
	    (int) object->pager, (int) object->paging_offset,
	    (int) object->shadow, (int) object->shadow_offset);
	printf("cache: next=%p, prev=%p\n",
	    object->cached_list.tqe_next, object->cached_list.tqe_prev);

	if (!full)
		return;

	indent += 2;
	count = 0;
	for (p = object->memq.tqh_first; p != NULL; p = p->listq.tqe_next) {
		if (count == 0)
			iprintf("memory:=");
		else if (count == 6) {
			printf("\n");
			iprintf(" ...");
			count = 0;
		} else
			printf(",");
		count++;

		printf("(off=0x%lx,page=0x%lx)",
		    (u_long) p->offset, (u_long) VM_PAGE_TO_PHYS(p));
	}
	if (count != 0)
		printf("\n");
	indent -= 2;
}
#endif				/* defined(DEBUG) || defined(DDB) */