Index: sys/vm/vm_object.c
===================================================================
--- sys/vm/vm_object.c
+++ sys/vm/vm_object.c
@@ -1426,10 +1426,10 @@
 static int
 vm_object_backing_scan(vm_object_t object, int op)
 {
-	int r = 1;
 	vm_page_t p;
 	vm_object_t backing_object;
 	vm_pindex_t backing_offset_index;
+	int r = 1, pbusy;
 
 	VM_OBJECT_ASSERT_WLOCKED(object);
 	VM_OBJECT_ASSERT_WLOCKED(object->backing_object);
@@ -1566,14 +1566,7 @@
 			}
 
 			pp = vm_page_lookup(object, new_pindex);
-			if (
-			    (op & OBSC_COLLAPSE_NOWAIT) != 0 &&
-			    (pp != NULL && pp->valid == 0)
-			) {
-				if (backing_object->type == OBJT_SWAP)
-					swap_pager_freespace(backing_object, 
-					    p->pindex, 1);
-
+			if (pp != NULL && pp->valid == 0) {
 				/*
 				 * The page in the parent is not (yet) valid.
 				 * We don't know anything about the state of
@@ -1585,7 +1578,30 @@
 				 * page before we can (re)lock the parent.
 				 * Hence we can get here.
 				 */
-				p = next;
+				if ((op & OBSC_COLLAPSE_NOWAIT) != 0) {
+					p = next;
+					continue;
+				}
+
+				/*
+				 * Same thing goes for the WAIT case, except we
+				 * must not skip any pages in the backing
+				 * object.  Relock and try again from the top,
+				 * draining busy status (if any) on the parent
+				 * page.
+				 */
+				pbusy = vm_page_busied(pp);
+				if (pbusy)
+					vm_page_lock(pp);
+				VM_OBJECT_WUNLOCK(object);
+				VM_OBJECT_WUNLOCK(backing_object);
+
+				if (pbusy)
+					vm_page_busy_sleep(pp, "vmocolp");
+
+				VM_OBJECT_WLOCK(object);
+				VM_OBJECT_WLOCK(backing_object);
+				p = TAILQ_FIRST(&backing_object->memq);
 				continue;
 			}
 			if (