2 files changed, 18 insertions, 3 deletions

diff --git a/fs/xfs/xfs_bmap.h b/fs/xfs/xfs_bmap.h
index 6ff70cda451c..9f3e3a836d15 100644
--- a/fs/xfs/xfs_bmap.h
+++ b/fs/xfs/xfs_bmap.h
@@ -54,12 +54,23 @@ typedef struct xfs_bmap_free_item
 
 /*
  * 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 3fc09cd8d517..1140cef4ba99 100644
--- a/fs/xfs/xfs_bmap_btree.c
+++ b/fs/xfs/xfs_bmap_btree.c
@@ -1509,7 +1509,9 @@ xfs_bmbt_split(
                  * 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 @@ xfs_bmbt_newroot(
 #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);