NOTE: libkvm, w, ps, 'top', and any other utility which depends on struct

      proc or any VM system structure will have to be rebuilt!!!

Much needed overhaul of the VM system. Included in this first round of
changes:

1) Improved pager interfaces: init, alloc, dealloc, getpages, putpages,
   haspage, and sync operations are supported. The haspage interface now
   provides information about clusterability. All pager routines now take
   struct vm_object's instead of "pagers".

2) Improved data structures. In the previous paradigm, there is constant
   confusion caused by pagers being both a data structure ("allocate a
   pager") and a collection of routines. The idea of a pager structure has
   escentially been eliminated. Objects now have types, and this type is
   used to index the appropriate pager. In most cases, items in the pager
   structure were duplicated in the object data structure and thus were
   unnecessary. In the few cases that remained, a un_pager structure union
   was created in the object to contain these items.

3) Because of the cleanup of #1 & #2, a lot of unnecessary layering can now
   be removed. For instance, vm_object_enter(), vm_object_lookup(),
   vm_object_remove(), and the associated object hash list were some of the
   things that were removed.

4) simple_lock's removed. Discussion with several people reveals that the
   SMP locking primitives used in the VM system aren't likely the mechanism
   that we'll be adopting. Even if it were, the locking that was in the code
   was very inadequate and would have to be mostly re-done anyway. The
   locking in a uni-processor kernel was a no-op but went a long way toward
   making the code difficult to read and debug.

5) Places that attempted to kludge-up the fact that we don't have kernel
   thread support have been fixed to reflect the reality that we are really
   dealing with processes, not threads. The VM system didn't have complete
   thread support, so the comments and mis-named routines were just wrong.
   We now use tsleep and wakeup directly in the lock routines, for instance.

6) Where appropriate, the pagers have been improved, especially in the
   pager_alloc routines. Most of the pager_allocs have been rewritten and
   are now faster and easier to maintain.

7) The pagedaemon pageout clustering algorithm has been rewritten and
   now tries harder to output an even number of pages before and after
   the requested page. This is sort of the reverse of the ideal pagein
   algorithm and should provide better overall performance.

8) Unnecessary (incorrect) casts to caddr_t in calls to tsleep & wakeup
   have been removed. Some other unnecessary casts have also been removed.

9) Some almost useless debugging code removed.

10) Terminology of shadow objects vs. backing objects straightened out.
    The fact that the vm_object data structure escentially had this
    backwards really confused things. The use of "shadow" and "backing
    object" throughout the code is now internally consistent and correct
    in the Mach terminology.

11) Several minor bug fixes, including one in the vm daemon that caused
    0 RSS objects to not get purged as intended.

12) A "default pager" has now been created which cleans up the transition
    of objects to the "swap" type. The previous checks throughout the code
    for swp->pg_data != NULL were really ugly. This change also provides
    the rudiments for future backing of "anonymous" memory by something
    other than the swap pager (via the vnode pager, for example), and it
    allows the decision about which of these pagers to use to be made
    dynamically (although will need some additional decision code to do
    this, of course).

13) (dyson) MAP_COPY has been deprecated and the corresponding "copy
    object" code has been removed. MAP_COPY was undocumented and non-
    standard. It was furthermore broken in several ways which caused its
    behavior to degrade to MAP_PRIVATE. Binaries that use MAP_COPY will
    continue to work correctly, but via the slightly different semantics
    of MAP_PRIVATE.

14) (dyson) Sharing maps have been removed. It's marginal usefulness in a
    threads design can be worked around in other ways. Both #12 and #13
    were done to simplify the code and improve readability and maintain-
    ability. (As were most all of these changes)

TODO:

1) Rewrite most of the vnode pager to use VOP_GETPAGES/PUTPAGES. Doing
   this will reduce the vnode pager to a mere fraction of its current size.

2) Rewrite vm_fault and the swap/vnode pagers to use the clustering
   information provided by the new haspage pager interface. This will
   substantially reduce the overhead by eliminating a large number of
   VOP_BMAP() calls. The VOP_BMAP() filesystem interface should be
   improved to provide both a "behind" and "ahead" indication of
   contiguousness.

3) Implement the extended features of pager_haspage in swap_pager_haspage().
   It currently just says 0 pages ahead/behind.

4) Re-implement the swap device (swstrategy) in a more elegant way, perhaps
   via a much more general mechanism that could also be used for disk
   striping of regular filesystems.

5) Do something to improve the architecture of vm_object_collapse(). The
   fact that it makes calls into the swap pager and knows too much about
   how the swap pager operates really bothers me. It also doesn't allow
   for collapsing of non-swap pager objects ("unnamed" objects backed by
   other pagers).

1 files changed, 166 insertions, 195 deletions

diff --git a/sys/vm/vm_pageout.c b/sys/vm/vm_pageout.c
index 55e7d0e..92e5713 100644
--- a/sys/vm/vm_pageout.c
+++ b/sys/vm/vm_pageout.c
@@ -65,7 +65,7 @@
  * any improvements or extensions that they make and grant Carnegie the
  * rights to redistribute these changes.
  *
- * $Id: vm_pageout.c,v 1.51 1995/05/30 08:16:18 rgrimes Exp $
+ * $Id: vm_pageout.c,v 1.52 1995/07/10 08:53:22 davidg Exp $
  */
 
 /*
@@ -86,8 +86,8 @@
 #include <vm/vm_page.h>
 #include <vm/vm_pageout.h>
 #include <vm/vm_kern.h>
+#include <vm/vm_pager.h>
 #include <vm/swap_pager.h>
-#include <vm/vnode_pager.h>
 
 int vm_pages_needed;		/* Event on which pageout daemon sleeps */
 
@@ -112,181 +112,163 @@ int vm_page_max_wired;		/* XXX max # of wired pages system-wide */
 
 /*
  * vm_pageout_clean:
- * 	cleans a vm_page
+ *
+ * Clean the page and remove it from the laundry.
+ * 
+ * We set the busy bit to cause potential page faults on this page to
+ * block.
+ * 
+ * And we set pageout-in-progress to keep the object from disappearing
+ * during pageout.  This guarantees that the page won't move from the
+ * inactive queue.  (However, any other page on the inactive queue may
+ * move!)
  */
 int
 vm_pageout_clean(m, sync)
-	register vm_page_t m;
+	vm_page_t m;
 	int sync;
 {
-	/*
-	 * Clean the page and remove it from the laundry.
-	 *
-	 * We set the busy bit to cause potential page faults on this page to
-	 * block.
-	 *
-	 * And we set pageout-in-progress to keep the object from disappearing
-	 * during pageout.  This guarantees that the page won't move from the
-	 * inactive queue.  (However, any other page on the inactive queue may
-	 * move!)
-	 */
-
 	register vm_object_t object;
-	register vm_pager_t pager;
 	int pageout_status[VM_PAGEOUT_PAGE_COUNT];
-	vm_page_t ms[VM_PAGEOUT_PAGE_COUNT], mb[VM_PAGEOUT_PAGE_COUNT];
-	int pageout_count, b_pageout_count;
+	vm_page_t mc[2*VM_PAGEOUT_PAGE_COUNT];
+	int pageout_count;
 	int anyok = 0;
-	int i;
+	int i, forward_okay, backward_okay, page_base;
 	vm_offset_t offset = m->offset;
 
 	object = m->object;
-	if (!object) {
-		printf("pager: object missing\n");
-		return 0;
-	}
-	if (!object->pager && (object->flags & OBJ_INTERNAL) == 0) {
-		printf("pager: non internal obj without pager\n");
-	}
+
 	/*
-	 * Try to collapse the object before making a pager for it.  We must
-	 * unlock the page queues first. We try to defer the creation of a
-	 * pager until all shadows are not paging.  This allows
-	 * vm_object_collapse to work better and helps control swap space
-	 * size. (J. Dyson 11 Nov 93)
+	 * If not OBJT_SWAP, additional memory may be needed to do the pageout.
+	 * Try to avoid the deadlock.
 	 */
-
-	if (!object->pager &&
-	    (cnt.v_free_count + cnt.v_cache_count) < cnt.v_pageout_free_min)
+	if ((sync != VM_PAGEOUT_FORCE) &&
+	    (object->type != OBJT_SWAP) &&
+	    ((cnt.v_free_count + cnt.v_cache_count) < cnt.v_pageout_free_min))
 		return 0;
 
+	/*
+	 * Don't mess with the page if it's busy.
+	 */
 	if ((!sync && m->hold_count != 0) ||
 	    ((m->busy != 0) || (m->flags & PG_BUSY)))
 		return 0;
 
-	if (!sync && object->shadow) {
+	/*
+	 * Try collapsing before it's too late.
+	 */
+	if (!sync && object->backing_object) {
 		vm_object_collapse(object);
 	}
+	mc[VM_PAGEOUT_PAGE_COUNT] = m;
 	pageout_count = 1;
-	ms[0] = m;
-
-	pager = object->pager;
-	if (pager) {
-		for (i = 1; i < vm_pageout_page_count; i++) {
-			vm_page_t mt;
-
-			ms[i] = mt = vm_page_lookup(object, offset + i * NBPG);
-			if (mt) {
-				if (mt->flags & (PG_BUSY|PG_CACHE) || mt->busy)
-					break;
-				/*
-				 * we can cluster ONLY if: ->> the page is NOT
-				 * busy, and is NOT clean the page is not
-				 * wired, busy, held, or mapped into a buffer.
-				 * and one of the following: 1) The page is
-				 * inactive, or a seldom used active page. 2)
-				 * or we force the issue.
-				 */
-				vm_page_test_dirty(mt);
-				if ((mt->dirty & mt->valid) != 0
-				    && ((mt->flags & PG_INACTIVE) ||
-						(sync == VM_PAGEOUT_FORCE))
-				    && (mt->wire_count == 0)
-				    && (mt->hold_count == 0))
-					pageout_count++;
-				else
-					break;
-			} else
-				break;
-		}
-
-		if ((pageout_count < vm_pageout_page_count) && (offset != 0)) {
-			b_pageout_count = 0;
-			for (i = 0; i < vm_pageout_page_count-pageout_count; i++) {
-				vm_page_t mt;
+	page_base = VM_PAGEOUT_PAGE_COUNT;
+	forward_okay = TRUE;
+	if (offset != 0)
+		backward_okay = TRUE;
+	else
+		backward_okay = FALSE;
+	/*
+	 * Scan object for clusterable pages.
+	 *
+	 * We can cluster ONLY if: ->> the page is NOT
+	 * clean, wired, busy, held, or mapped into a
+	 * buffer, and one of the following:
+	 * 1) The page is inactive, or a seldom used
+	 *    active page.
+	 * -or-
+	 * 2) we force the issue.
+	 */
+	for (i = 1; (i < vm_pageout_page_count) && (forward_okay || backward_okay); i++) {
+		vm_page_t p;
 
-				mt = vm_page_lookup(object, offset - (i + 1) * NBPG);
-				if (mt) {
-					if (mt->flags & (PG_BUSY|PG_CACHE) || mt->busy)
-						break;
-					vm_page_test_dirty(mt);
-					if ((mt->dirty & mt->valid) != 0
-					    && ((mt->flags & PG_INACTIVE) ||
-							(sync == VM_PAGEOUT_FORCE))
-					    && (mt->wire_count == 0)
-					    && (mt->hold_count == 0)) {
-						mb[b_pageout_count] = mt;
-						b_pageout_count++;
-						if ((offset - (i + 1) * NBPG) == 0)
-							break;
-					} else
-						break;
-				} else
-					break;
+		/*
+		 * See if forward page is clusterable.
+		 */
+		if (forward_okay) {
+			/*
+			 * Stop forward scan at end of object.
+			 */
+			if ((offset + i * PAGE_SIZE) > object->size) {
+				forward_okay = FALSE;
+				goto do_backward;
 			}
-			if (b_pageout_count > 0) {
-				for(i=pageout_count - 1;i>=0;--i) {
-					ms[i+b_pageout_count] = ms[i];
+			p = vm_page_lookup(object, offset + i * PAGE_SIZE);
+			if (p) {
+				if ((p->flags & (PG_BUSY|PG_CACHE)) || p->busy) {
+					forward_okay = FALSE;
+					goto do_backward;
 				}
-				for(i=0;i<b_pageout_count;i++) {
-					ms[i] = mb[b_pageout_count - (i + 1)];
+				vm_page_test_dirty(p);
+				if ((p->dirty & p->valid) != 0 &&
+				    ((p->flags & PG_INACTIVE) ||
+				     (sync == VM_PAGEOUT_FORCE)) &&
+				    (p->wire_count == 0) &&
+				    (p->hold_count == 0)) {
+					mc[VM_PAGEOUT_PAGE_COUNT + i] = p;
+					pageout_count++;
+					if (pageout_count == vm_pageout_page_count)
+						break;
+				} else {
+					forward_okay = FALSE;
 				}
-				pageout_count += b_pageout_count;
+			} else {
+				forward_okay = FALSE;
 			}
 		}
-
+do_backward:
 		/*
-		 * we allow reads during pageouts...
+		 * See if backward page is clusterable.
 		 */
-		for (i = 0; i < pageout_count; i++) {
-			ms[i]->flags |= PG_BUSY;
-			vm_page_protect(ms[i], VM_PROT_READ);
-		}
-		object->paging_in_progress += pageout_count;
-	} else {
-
-		m->flags |= PG_BUSY;
-
-		vm_page_protect(m, VM_PROT_READ);
-
-		object->paging_in_progress++;
-
-		pager = vm_pager_allocate(PG_DFLT, 0,
-		    object->size, VM_PROT_ALL, 0);
-		if (pager != NULL) {
-			object->pager = pager;
+		if (backward_okay) {
+			/*
+			 * Stop backward scan at beginning of object.
+			 */
+			if ((offset - i * PAGE_SIZE) == 0) {
+				backward_okay = FALSE;
+			}
+			p = vm_page_lookup(object, offset - i * PAGE_SIZE);
+			if (p) {
+				if ((p->flags & (PG_BUSY|PG_CACHE)) || p->busy) {
+					backward_okay = FALSE;
+					continue;
+				}
+				vm_page_test_dirty(p);
+				if ((p->dirty & p->valid) != 0 &&
+				    ((p->flags & PG_INACTIVE) ||
+				     (sync == VM_PAGEOUT_FORCE)) &&
+				    (p->wire_count == 0) &&
+				    (p->hold_count == 0)) {
+					mc[VM_PAGEOUT_PAGE_COUNT - i] = p;
+					pageout_count++;
+					page_base--;
+					if (pageout_count == vm_pageout_page_count)
+						break;
+				} else {
+					backward_okay = FALSE;
+				}
+			} else {
+				backward_okay = FALSE;
+			}
 		}
 	}
 
 	/*
-	 * If there is no pager for the page, use the default pager.  If
-	 * there's no place to put the page at the moment, leave it in the
-	 * laundry and hope that there will be paging space later.
+	 * we allow reads during pageouts...
 	 */
-
-	if ((pager && pager->pg_type == PG_SWAP) ||
-	    (cnt.v_free_count + cnt.v_cache_count) >= cnt.v_pageout_free_min) {
-		if (pageout_count == 1) {
-			pageout_status[0] = pager ?
-			    vm_pager_put(pager, m,
-			    ((sync || (object == kernel_object)) ? TRUE : FALSE)) :
-			    VM_PAGER_FAIL;
-		} else {
-			if (!pager) {
-				for (i = 0; i < pageout_count; i++)
-					pageout_status[i] = VM_PAGER_FAIL;
-			} else {
-				vm_pager_put_pages(pager, ms, pageout_count,
-				    ((sync || (object == kernel_object)) ? TRUE : FALSE),
-				    pageout_status);
-			}
-		}
-	} else {
-		for (i = 0; i < pageout_count; i++)
-			pageout_status[i] = VM_PAGER_FAIL;
+	for (i = page_base; i < (page_base + pageout_count); i++) {
+		mc[i]->flags |= PG_BUSY;
+		vm_page_protect(mc[i], VM_PROT_READ);
 	}
+	object->paging_in_progress += pageout_count;
+
+	vm_pager_put_pages(object, &mc[page_base], pageout_count,
+	    ((sync || (object == kernel_object)) ? TRUE : FALSE),
+	    pageout_status);
 
 	for (i = 0; i < pageout_count; i++) {
+		vm_page_t mt = mc[page_base + i];
+
 		switch (pageout_status[i]) {
 		case VM_PAGER_OK:
 			++anyok;
@@ -300,8 +282,8 @@ vm_pageout_clean(m, sync)
 			 * essentially lose the changes by pretending it
 			 * worked.
 			 */
-			pmap_clear_modify(VM_PAGE_TO_PHYS(ms[i]));
-			ms[i]->dirty = 0;
+			pmap_clear_modify(VM_PAGE_TO_PHYS(mt));
+			mt->dirty = 0;
 			break;
 		case VM_PAGER_ERROR:
 		case VM_PAGER_FAIL:
@@ -310,8 +292,8 @@ vm_pageout_clean(m, sync)
 			 * page so it doesn't clog the inactive list.  (We
 			 * will try paging out it again later).
 			 */
-			if (ms[i]->flags & PG_INACTIVE)
-				vm_page_activate(ms[i]);
+			if (mt->flags & PG_INACTIVE)
+				vm_page_activate(mt);
 			break;
 		case VM_PAGER_AGAIN:
 			break;
@@ -326,14 +308,14 @@ vm_pageout_clean(m, sync)
 		 */
 		if (pageout_status[i] != VM_PAGER_PEND) {
 			vm_object_pip_wakeup(object);
-			if ((ms[i]->flags & (PG_REFERENCED|PG_WANTED)) ||
-			    pmap_is_referenced(VM_PAGE_TO_PHYS(ms[i]))) {
-				pmap_clear_reference(VM_PAGE_TO_PHYS(ms[i]));
-				ms[i]->flags &= ~PG_REFERENCED;
-				if (ms[i]->flags & PG_INACTIVE)
-					vm_page_activate(ms[i]);
+			if ((mt->flags & (PG_REFERENCED|PG_WANTED)) ||
+			    pmap_is_referenced(VM_PAGE_TO_PHYS(mt))) {
+				pmap_clear_reference(VM_PAGE_TO_PHYS(mt));
+				mt->flags &= ~PG_REFERENCED;
+				if (mt->flags & PG_INACTIVE)
+					vm_page_activate(mt);
 			}
-			PAGE_WAKEUP(ms[i]);
+			PAGE_WAKEUP(mt);
 		}
 	}
 	return anyok;
@@ -345,7 +327,7 @@ vm_pageout_clean(m, sync)
  *	deactivate enough pages to satisfy the inactive target
  *	requirements or if vm_page_proc_limit is set, then
  *	deactivate all of the pages in the object and its
- *	shadows.
+ *	backing_objects.
  *
  *	The object and map must be locked.
  */
@@ -364,16 +346,18 @@ vm_pageout_object_deactivate_pages(map, object, count, map_remove_only)
 	if (count == 0)
 		count = 1;
 
-	if (object->pager && (object->pager->pg_type == PG_DEVICE))
+	if (object->type == OBJT_DEVICE)
 		return 0;
 
-	if (object->shadow) {
-		if (object->shadow->ref_count == 1)
-			dcount += vm_pageout_object_deactivate_pages(map, object->shadow, count / 2 + 1, map_remove_only);
+	if (object->backing_object) {
+		if (object->backing_object->ref_count == 1)
+			dcount += vm_pageout_object_deactivate_pages(map,
+			    object->backing_object, count / 2 + 1, map_remove_only);
 		else
-			vm_pageout_object_deactivate_pages(map, object->shadow, count, 1);
+			vm_pageout_object_deactivate_pages(map,
+			    object->backing_object, count, 1);
 	}
-	if (object->paging_in_progress || !vm_object_lock_try(object))
+	if (object->paging_in_progress)
 		return dcount;
 
 	/*
@@ -384,7 +368,6 @@ vm_pageout_object_deactivate_pages(map, object, count, map_remove_only)
 	while (p && (rcount-- > 0)) {
 		next = p->listq.tqe_next;
 		cnt.v_pdpages++;
-		vm_page_lock_queues();
 		if (p->wire_count != 0 ||
 		    p->hold_count != 0 ||
 		    p->busy != 0 ||
@@ -427,8 +410,6 @@ vm_pageout_object_deactivate_pages(map, object, count, map_remove_only)
 					++dcount;
 					if (count <= 0 &&
 					    cnt.v_inactive_count > cnt.v_inactive_target) {
-						vm_page_unlock_queues();
-						vm_object_unlock(object);
 						return dcount;
 					}
 				}
@@ -447,10 +428,8 @@ vm_pageout_object_deactivate_pages(map, object, count, map_remove_only)
 		} else if ((p->flags & (PG_INACTIVE | PG_BUSY)) == PG_INACTIVE) {
 			vm_page_protect(p, VM_PROT_NONE);
 		}
-		vm_page_unlock_queues();
 		p = next;
 	}
-	vm_object_unlock(object);
 	return dcount;
 }
 
@@ -505,7 +484,7 @@ vm_req_vmdaemon()
 	static int lastrun = 0;
 
 	if ((ticks > (lastrun + hz / 10)) || (ticks < lastrun)) {
-		wakeup((caddr_t) &vm_daemon_needed);
+		wakeup(&vm_daemon_needed);
 		lastrun = ticks;
 	}
 }
@@ -602,15 +581,14 @@ rescan1:
 			struct vnode *vp = NULL;
 
 			object = m->object;
-			if ((object->flags & OBJ_DEAD) || !vm_object_lock_try(object)) {
+			if (object->flags & OBJ_DEAD) {
 				m = next;
 				continue;
 			}
 
-			if (object->pager && object->pager->pg_type == PG_VNODE) {
-				vp = ((vn_pager_t) object->pager->pg_data)->vnp_vp;
+			if (object->type == OBJT_VNODE) {
+				vp = object->handle;
 				if (VOP_ISLOCKED(vp) || vget(vp, 1)) {
-					vm_object_unlock(object);
 					if (object->flags & OBJ_WRITEABLE)
 						++vnodes_skipped;
 					m = next;
@@ -629,8 +607,6 @@ rescan1:
 			if (vp)
 				vput(vp);
 
-			vm_object_unlock(object);
-
 			if (!next) {
 				break;
 			}
@@ -744,7 +720,7 @@ rescan1:
 		    (cnt.v_cache_count + cnt.v_free_count) < cnt.v_free_min) {
 			if (!vfs_update_wakeup) {
 				vfs_update_wakeup = 1;
-				wakeup((caddr_t) &vfs_update_wakeup);
+				wakeup(&vfs_update_wakeup);
 			}
 		}
 		/*
@@ -804,7 +780,7 @@ rescan1:
 			bigproc->p_estcpu = 0;
 			bigproc->p_nice = PRIO_MIN;
 			resetpriority(bigproc);
-			wakeup((caddr_t) &cnt.v_free_count);
+			wakeup(&cnt.v_free_count);
 		}
 	}
 	return force_wakeup;
@@ -853,7 +829,7 @@ vm_pageout()
 		vm_page_max_wired = cnt.v_free_count / 3;
 
 
-	(void) swap_pager_alloc(0, 0, 0, 0);
+	swap_pager_swap_init();
 	/*
 	 * The pageout daemon is never done, so loop forever.
 	 */
@@ -864,7 +840,7 @@ vm_pageout()
 			((cnt.v_free_count >= cnt.v_free_reserved) &&
 			 (cnt.v_free_count + cnt.v_cache_count >= cnt.v_free_min))) {
 			vm_pages_needed = 0;
-			tsleep((caddr_t) &vm_pages_needed, PVM, "psleep", 0);
+			tsleep(&vm_pages_needed, PVM, "psleep", 0);
 		}
 		vm_pages_needed = 0;
 		splx(s);
@@ -872,8 +848,8 @@ vm_pageout()
 		vm_pager_sync();
 		vm_pageout_scan();
 		vm_pager_sync();
-		wakeup((caddr_t) &cnt.v_free_count);
-		wakeup((caddr_t) kmem_map);
+		wakeup(&cnt.v_free_count);
+		wakeup(kmem_map);
 	}
 }
 
@@ -884,8 +860,8 @@ vm_daemon()
 	struct proc *p;
 
 	while (TRUE) {
-		tsleep((caddr_t) &vm_daemon_needed, PUSER, "psleep", 0);
-		if( vm_pageout_req_swapout) {
+		tsleep(&vm_daemon_needed, PUSER, "psleep", 0);
+		if (vm_pageout_req_swapout) {
 			swapout_procs();
 			vm_pageout_req_swapout = 0;
 		}
@@ -934,27 +910,22 @@ vm_daemon()
 				    (vm_map_entry_t) 0, &overage, vm_pageout_object_deactivate_pages);
 			}
 		}
-	}
 
-	/*
-	 * we remove cached objects that have no RSS...
-	 */
-restart:
-	vm_object_cache_lock();
-	object = vm_object_cached_list.tqh_first;
-	while (object) {
-		vm_object_cache_unlock();
 		/*
-		 * if there are no resident pages -- get rid of the object
+		 * we remove cached objects that have no RSS...
 		 */
-		if (object->resident_page_count == 0) {
-			if (object != vm_object_lookup(object->pager))
-				panic("vm_object_cache_trim: I'm sooo confused.");
-			pager_cache(object, FALSE);
-			goto restart;
+restart:
+		object = vm_object_cached_list.tqh_first;
+		while (object) {
+			/*
+			 * if there are no resident pages -- get rid of the object
+			 */
+			if (object->resident_page_count == 0) {
+				vm_object_reference(object);
+				pager_cache(object, FALSE);
+				goto restart;
+			}
+			object = object->cached_list.tqe_next;
 		}
-		object = object->cached_list.tqe_next;
-		vm_object_cache_lock();
 	}
-	vm_object_cache_unlock();
 }