--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h
@@ -544,6 +544,7 @@
 extern void zio_add_child(zio_t *pio, zio_t *cio);
 
 extern void *zio_buf_alloc(size_t size);
+extern void *zio_buf_alloc_flags(size_t size, int aflags);
 extern void zio_buf_free(void *buf, size_t size);
 extern void *zio_data_buf_alloc(size_t size);
 extern void zio_data_buf_free(void *buf, size_t size);
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio.c
@@ -288,7 +288,7 @@
  * excess / transient data in-core during a crashdump.
  */
 void *
-zio_buf_alloc(size_t size)
+zio_buf_alloc_flags(size_t size, int aflags)
 {
 	size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
 	int flags = zio_exclude_metadata ? KM_NODEBUG : 0;
@@ -298,7 +298,14 @@
 	if (zio_use_uma)
 		return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
 	else
-		return (kmem_alloc(size, KM_SLEEP|flags));
+		return (kmem_alloc(size, aflags | flags));
+}
+
+void *
+zio_buf_alloc(size_t size)
+{
+
+	return (zio_buf_alloc_flags(size, KM_SLEEP));
 }
 
 /*
--- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c
+++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_queue.c
@@ -211,10 +211,10 @@
 {
 	zio_t *fio, *lio, *aio, *dio, *nio, *mio;
 	avl_tree_t *t;
-	int flags;
 	uint64_t maxspan = zfs_vdev_aggregation_limit;
-	uint64_t maxgap;
-	int stretch;
+	uint64_t maxgap, size;
+	int flags, stretch;
+	void *buf;
 
 again:
 	ASSERT(MUTEX_HELD(&vq->vq_lock));
@@ -302,25 +302,39 @@
 			}
 		}
 
-		if (stretch) {
-			/* This may be a no-op. */
-			VERIFY((dio = AVL_NEXT(t, lio)) != NULL);
-			dio->io_flags &= ~ZIO_FLAG_OPTIONAL;
-		} else {
+		if (!stretch) {
 			while (lio != mio && lio != fio) {
 				ASSERT(lio->io_flags & ZIO_FLAG_OPTIONAL);
 				lio = AVL_PREV(t, lio);
 				ASSERT(lio != NULL);
 			}
 		}
-	}
+
+		if (fio == lio) {
+			/* Nothing to aggregate. */
+			goto out;
+		}
 
-	if (fio != lio) {
-		uint64_t size = IO_SPAN(fio, lio);
+		size = IO_SPAN(fio, lio);
 		ASSERT(size <= zfs_vdev_aggregation_limit);
 
-		aio = zio_vdev_delegated_io(fio->io_vd, fio->io_offset,
-		    zio_buf_alloc(size), size, fio->io_type, ZIO_PRIORITY_AGG,
+		buf = zio_buf_alloc_flags(size, KM_NOSLEEP);
+		if (buf == NULL) {
+			/*
+			 * Not enough memory? Skip aggregation to avoid
+			 * possible deadlock with the pageout daemon.
+			 */
+			goto out;
+		}
+
+		if (stretch) {
+			/* This may be a no-op. */
+			VERIFY((dio = AVL_NEXT(t, lio)) != NULL);
+			dio->io_flags &= ~ZIO_FLAG_OPTIONAL;
+		}
+
+		aio = zio_vdev_delegated_io(fio->io_vd, fio->io_offset, buf,
+		    size, fio->io_type, ZIO_PRIORITY_AGG,
 		    flags | ZIO_FLAG_DONT_CACHE | ZIO_FLAG_DONT_QUEUE,
 		    vdev_queue_agg_io_done, NULL);
 		aio->io_timestamp = fio->io_timestamp;
@@ -353,6 +367,7 @@
 		return (aio);
 	}
 
+out:
 	ASSERT(fio->io_vdev_tree == t);
 	vdev_queue_io_remove(vq, fio);