Index: sys/vm/swap_pager.c
===================================================================
--- sys/vm/swap_pager.c
+++ sys/vm/swap_pager.c
@@ -486,7 +486,8 @@
 /*
  * Metadata functions
  */
-static daddr_t swp_pager_meta_build(vm_object_t, vm_pindex_t, daddr_t);
+static daddr_t swp_pager_meta_build(vm_object_t, vm_pindex_t, daddr_t,
+	bool);
 static void swp_pager_meta_free(vm_object_t, vm_pindex_t, vm_pindex_t,
     vm_size_t *);
 static void swp_pager_meta_transfer(vm_object_t src, vm_object_t dst,
@@ -1031,7 +1032,7 @@
 		}
 		for (j = 0; j < n; ++j) {
 			addr = swp_pager_meta_build(object,
-			    start + i + j, blk + j);
+			    start + i + j, blk + j, false);
 			if (addr != SWAPBLK_NONE)
 				swp_pager_update_freerange(&range, addr);
 		}
@@ -1045,16 +1046,17 @@
 swp_pager_xfer_source(vm_object_t srcobject, vm_object_t dstobject,
     vm_pindex_t pindex, daddr_t addr)
 {
-	daddr_t dstaddr __diagused;
+	daddr_t dstaddr;
 
 	KASSERT((srcobject->flags & OBJ_SWAP) != 0,
 	    ("%s: srcobject not swappable", __func__));
 	KASSERT((dstobject->flags & OBJ_SWAP) != 0,
 	    ("%s: dstobject not swappable", __func__));
 
-	if (swp_pager_meta_lookup(dstobject, pindex) != SWAPBLK_NONE) {
-		/* Caller should destroy the source block. */
-		return (false);
+	dstaddr = swp_pager_meta_build(dstobject, pindex, addr, true);
+	if (dstaddr != addr) {
+		/* Should caller destroy the source block? */
+		return (dstaddr == SWAPBLK_NONE);
 	}
 
 	/*
@@ -1062,7 +1064,7 @@
 	 * swp_pager_meta_build() can sleep.
 	 */
 	VM_OBJECT_WUNLOCK(srcobject);
-	dstaddr = swp_pager_meta_build(dstobject, pindex, addr);
+	dstaddr = swp_pager_meta_build(dstobject, pindex, addr, false);
 	KASSERT(dstaddr == SWAPBLK_NONE,
 	    ("Unexpected destination swapblk"));
 	VM_OBJECT_WLOCK(srcobject);
@@ -1548,7 +1550,7 @@
 			mreq = ma[i + j];
 			vm_page_aflag_clear(mreq, PGA_SWAP_FREE);
 			addr = swp_pager_meta_build(mreq->object, mreq->pindex,
-			    blk + j);
+			    blk + j, false);
 			if (addr != SWAPBLK_NONE)
 				swp_pager_update_freerange(&range, addr);
 			MPASS(mreq->dirty == VM_PAGE_BITS_ALL);
@@ -2052,106 +2054,105 @@
 		uma_zfree(swblk_zone, sb);
 	}
 }
-   
+
+/*
+ * SWP_PAGER_ALLOC_WAIT() -	wait before retrying allocation from a zone.
+ */
+static void
+swp_pager_alloc_wait(vm_object_t object, uma_zone_t zone,
+    volatile int *zone_exhausted, const char *errmsg, const char *pausemsg)
+{
+	VM_OBJECT_WUNLOCK(object);
+	if (uma_zone_exhausted(zone)) {
+		if (atomic_cmpset_int(zone_exhausted, 0, 1))
+			printf("swap %s zone exhausted, "
+			    "increase kern.maxswzone\n", errmsg);
+		vm_pageout_oom(VM_OOM_SWAPZ);
+		pause(pausemsg, 10);
+	} else
+		uma_zwait(zone);
+	VM_OBJECT_WLOCK(object);
+}
+
 /*
  * SWP_PAGER_META_BUILD() -	add swap block to swap meta data for object
  *
- *	The specified swapblk is added to the object's swap metadata.  If
- *	the swapblk is not valid, it is freed instead.  Any previously
- *	assigned swapblk is returned.
+ *	Try to add the specified swapblk to the object's swap metadata.  If
+ *	nowait and memory allocation fails, return the specified swapblk
+ *	immediately to indicate failure.  If nowait, add the specified swapblk
+ *	only if there is no previously assigned swapblk.  Return the previously
+ *	assigned swapblk, if any.
  */
 static daddr_t
-swp_pager_meta_build(vm_object_t object, vm_pindex_t pindex, daddr_t swapblk)
+swp_pager_meta_build(vm_object_t object, vm_pindex_t pindex, daddr_t swapblk,
+    bool nowait)
 {
 	static volatile int swblk_zone_exhausted, swpctrie_zone_exhausted;
 	struct swblk *sb, *sb1;
 	vm_pindex_t modpi, rdpi;
 	daddr_t prev_swapblk;
-	int error, i;
+	int i;
 
 	VM_OBJECT_ASSERT_WLOCKED(object);
 
 	rdpi = rounddown(pindex, SWAP_META_PAGES);
 	sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks, rdpi);
-	if (sb == NULL) {
-		if (swapblk == SWAPBLK_NONE)
-			return (SWAPBLK_NONE);
-		for (;;) {
-			sb = uma_zalloc(swblk_zone, M_NOWAIT | (curproc ==
-			    pageproc ? M_USE_RESERVE : 0));
-			if (sb != NULL) {
-				sb->p = rdpi;
-				for (i = 0; i < SWAP_META_PAGES; i++)
-					sb->d[i] = SWAPBLK_NONE;
-				if (atomic_cmpset_int(&swblk_zone_exhausted,
-				    1, 0))
-					printf("swblk zone ok\n");
-				break;
-			}
-			VM_OBJECT_WUNLOCK(object);
-			if (uma_zone_exhausted(swblk_zone)) {
-				if (atomic_cmpset_int(&swblk_zone_exhausted,
-				    0, 1))
-					printf("swap blk zone exhausted, "
-					    "increase kern.maxswzone\n");
-				vm_pageout_oom(VM_OOM_SWAPZ);
-				pause("swzonxb", 10);
-			} else
-				uma_zwait(swblk_zone);
-			VM_OBJECT_WLOCK(object);
-			sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks,
-			    rdpi);
-			if (sb != NULL)
-				/*
-				 * Somebody swapped out a nearby page,
-				 * allocating swblk at the rdpi index,
-				 * while we dropped the object lock.
-				 */
-				goto allocated;
+	if (sb != NULL)
+		goto allocated;
+	if (swapblk == SWAPBLK_NONE)
+		return (SWAPBLK_NONE);
+	while ((sb = uma_zalloc(swblk_zone,
+	    M_NOWAIT | (curproc == pageproc ? M_USE_RESERVE : 0))) == NULL) {
+		if (nowait)
+			return (swapblk);
+		swp_pager_alloc_wait(object, swblk_zone,
+		    &swblk_zone_exhausted, "blk", "swzonxb");
+		sb = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks, rdpi);
+		if (sb != NULL)
+			/*
+			 * Somebody swapped out a nearby page, allocating swblk
+			 * at the rdpi index, while we dropped the object lock.
+			 */
+			goto allocated;
+	}
+	sb->p = rdpi;
+	for (i = 0; i < SWAP_META_PAGES; i++)
+		sb->d[i] = SWAPBLK_NONE;
+	if (atomic_cmpset_int(&swblk_zone_exhausted, 1, 0))
+		printf("swblk zone ok\n");
+	while (SWAP_PCTRIE_INSERT(&object->un_pager.swp.swp_blks, sb) != 0) {
+		if (nowait) {
+			uma_zfree(swblk_zone, sb);
+			return (swapblk);
 		}
-		for (;;) {
-			error = SWAP_PCTRIE_INSERT(
-			    &object->un_pager.swp.swp_blks, sb);
-			if (error == 0) {
-				if (atomic_cmpset_int(&swpctrie_zone_exhausted,
-				    1, 0))
-					printf("swpctrie zone ok\n");
-				break;
-			}
-			VM_OBJECT_WUNLOCK(object);
-			if (uma_zone_exhausted(swpctrie_zone)) {
-				if (atomic_cmpset_int(&swpctrie_zone_exhausted,
-				    0, 1))
-					printf("swap pctrie zone exhausted, "
-					    "increase kern.maxswzone\n");
-				vm_pageout_oom(VM_OOM_SWAPZ);
-				pause("swzonxp", 10);
-			} else
-				uma_zwait(swpctrie_zone);
-			VM_OBJECT_WLOCK(object);
-			sb1 = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks,
-			    rdpi);
-			if (sb1 != NULL) {
-				uma_zfree(swblk_zone, sb);
-				sb = sb1;
-				goto allocated;
-			}
+		swp_pager_alloc_wait(object, swpctrie_zone,
+		    &swpctrie_zone_exhausted, "pctrie", "swzonxp");
+		sb1 = SWAP_PCTRIE_LOOKUP(&object->un_pager.swp.swp_blks, rdpi);
+		if (sb1 != NULL) {
+			uma_zfree(swblk_zone, sb);
+			sb = sb1;
+			goto allocated;
 		}
 	}
+	if (atomic_cmpset_int(&swpctrie_zone_exhausted, 1, 0))
+		printf("swpctrie zone ok\n");
+
 allocated:
 	MPASS(sb->p == rdpi);
 
 	modpi = pindex % SWAP_META_PAGES;
 	/* Return prior contents of metadata. */
 	prev_swapblk = sb->d[modpi];
-	/* Enter block into metadata. */
-	sb->d[modpi] = swapblk;
+	if (!nowait || prev_swapblk == SWAPBLK_NONE) {
+		/* Enter block into metadata. */
+		sb->d[modpi] = swapblk;
 
-	/*
-	 * Free the swblk if we end up with the empty page run.
-	 */
-	if (swapblk == SWAPBLK_NONE)
-		swp_pager_free_empty_swblk(object, sb);
+		/*
+		 * Free the swblk if we end up with the empty page run.
+		 */
+		if (swapblk == SWAPBLK_NONE)
+			swp_pager_free_empty_swblk(object, sb);
+	}
 	return (prev_swapblk);
 }