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);
 		}
@@ -1041,35 +1042,6 @@
 	return (0);
 }
 
-static bool
-swp_pager_xfer_source(vm_object_t srcobject, vm_object_t dstobject,
-    vm_pindex_t pindex, daddr_t addr)
-{
-	daddr_t dstaddr __diagused;
-
-	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);
-	}
-
-	/*
-	 * Destination has no swapblk and is not resident, transfer source.
-	 * swp_pager_meta_build() can sleep.
-	 */
-	VM_OBJECT_WUNLOCK(srcobject);
-	dstaddr = swp_pager_meta_build(dstobject, pindex, addr);
-	KASSERT(dstaddr == SWAPBLK_NONE,
-	    ("Unexpected destination swapblk"));
-	VM_OBJECT_WLOCK(srcobject);
-
-	return (true);
-}
-
 /*
  * SWAP_PAGER_COPY() -  copy blocks from source pager to destination pager
  *			and destroy the source.
@@ -1548,7 +1520,7 @@
 			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);
@@ -2052,16 +2024,19 @@
 		uma_zfree(swblk_zone, sb);
 	}
 }
-   
+
 /*
  * 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;
@@ -2088,6 +2063,8 @@
 					printf("swblk zone ok\n");
 				break;
 			}
+			if (nowait)
+				return (swapblk);
 			VM_OBJECT_WUNLOCK(object);
 			if (uma_zone_exhausted(swblk_zone)) {
 				if (atomic_cmpset_int(&swblk_zone_exhausted,
@@ -2118,6 +2095,10 @@
 					printf("swpctrie zone ok\n");
 				break;
 			}
+			if (nowait) {
+				uma_zfree(swblk_zone, sb);
+				return (swapblk);
+			}
 			VM_OBJECT_WUNLOCK(object);
 			if (uma_zone_exhausted(swpctrie_zone)) {
 				if (atomic_cmpset_int(&swpctrie_zone_exhausted,
@@ -2144,14 +2125,16 @@
 	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);
 }
 
@@ -2168,6 +2151,7 @@
 {
 	struct page_range range;
 	struct swblk *sb;
+	daddr_t blk;
 	vm_page_t m;
 	vm_pindex_t offset, last;
 	vm_size_t mc;
@@ -2193,12 +2177,25 @@
 		limit = last - sb->p < SWAP_META_PAGES ? last - sb->p :
 		    SWAP_META_PAGES;
 		for (i = start; i < limit; i++) {
-			if (sb->d[i] == SWAPBLK_NONE)
+			blk = sb->d[i];
+			if (blk == SWAPBLK_NONE)
 				continue;
 			if (dstobject == NULL ||
-			    !swp_pager_xfer_source(srcobject, dstobject, 
-			    sb->p + i - offset, sb->d[i])) {
+			    (blk = swp_pager_meta_build(dstobject,
+			    sb->p + i - offset, blk, true),
+			    blk != sb->d[i] && blk != SWAPBLK_NONE))
 				swp_pager_update_freerange(&range, sb->d[i]);
+			else if (blk == sb->d[i]) {
+				/*
+				 * Destination has no swapblk and is not
+				 * resident, so transfer source.
+				 * swp_pager_meta_build() failed memory
+				 * allocation already, likely to sleep in retry.
+				 */
+				VM_OBJECT_WUNLOCK(srcobject);
+				swp_pager_meta_build(dstobject,
+				    sb->p + i - offset, blk, false);
+				VM_OBJECT_WLOCK(srcobject);
 			}
 			if (moved != NULL) {
 				if (m != NULL && m->pindex != pindex + i - 1)