diff --git a/fs/xfs/xfs_bmap.h b/fs/xfs/xfs_bmap.h
index 6ff70cd..9f3e3a8 100644
--- a/fs/xfs/xfs_bmap.h
+++ b/fs/xfs/xfs_bmap.h
@@ -54,12 +54,23 @@
 
 /*
  * Header for free extent list.
+ *
+ * xbf_low is used by the allocator to activate the lowspace algorithm -
+ * when free space is running low the extent allocator may choose to
+ * allocate an extent from an AG without leaving sufficient space for
+ * a btree split when inserting the new extent.  In this case the allocator
+ * will enable the lowspace algorithm which is supposed to allow further
+ * allocations (such as btree splits and newroots) to allocate from
+ * sequential AGs.  In order to avoid locking AGs out of order the lowspace
+ * algorithm will start searching for free space from AG 0.  If the correct
+ * transaction reservations have been made then this algorithm will eventually
+ * find all the space it needs.
  */
 typedef	struct xfs_bmap_free
 {
 	xfs_bmap_free_item_t	*xbf_first;	/* list of to-be-free extents */
 	int			xbf_count;	/* count of items on list */
-	int			xbf_low;	/* kludge: alloc in low mode */
+	int			xbf_low;	/* alloc in low mode */
 } xfs_bmap_free_t;
 
 #define	XFS_BMAP_MAX_NMAP	4
diff --git a/fs/xfs/xfs_bmap_btree.c b/fs/xfs/xfs_bmap_btree.c
index 3fc09cd..1140cef 100644
--- a/fs/xfs/xfs_bmap_btree.c
+++ b/fs/xfs/xfs_bmap_btree.c
@@ -1509,7 +1509,9 @@
 		 * block allocation here and corrupt the filesystem.
 		 */
 		args.minleft = xfs_trans_get_block_res(args.tp);
-	} else
+	} else if (cur->bc_private.b.flist->xbf_low)
+		args.type = XFS_ALLOCTYPE_START_BNO;
+	else
 		args.type = XFS_ALLOCTYPE_NEAR_BNO;
 	args.mod = args.alignment = args.total = args.isfl =
 		args.userdata = args.minalignslop = 0;
@@ -2237,7 +2239,9 @@
 #endif
 		args.fsbno = be64_to_cpu(*pp);
 		args.type = XFS_ALLOCTYPE_START_BNO;
-	} else
+	} else if (cur->bc_private.b.flist->xbf_low)
+		args.type = XFS_ALLOCTYPE_START_BNO;
+	else
 		args.type = XFS_ALLOCTYPE_NEAR_BNO;
 	if ((error = xfs_alloc_vextent(&args))) {
 		XFS_BMBT_TRACE_CURSOR(cur, ERROR);