[XFS] use minleft when allocating in xfs_bmbt_split()

The bmap btree split code relies on a previous data extent allocation
(from xfs_bmap_btalloc()) to find an AG that has sufficient space to
perform a full btree split, when inserting the extent. When converting
unwritten extents we don't allocate a data extent so a btree split will be
the first allocation. In this case we need to set minleft so the allocator
will pick an AG that has space to complete the split(s).

SGI-PV: 983338

SGI-Modid: xfs-linux-melb:xfs-kern:31357a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>

2 files changed, 24 insertions, 1 deletions

diff --git a/fs/xfs/xfs_bmap_btree.c b/fs/xfs/xfs_bmap_btree.c
index 4aa2f11ba563..3fc09cd8d517 100644
--- a/fs/xfs/xfs_bmap_btree.c
+++ b/fs/xfs/xfs_bmap_btree.c
@@ -1493,12 +1493,25 @@ xfs_bmbt_split(
         left = XFS_BUF_TO_BMBT_BLOCK(lbp);
         args.fsbno = cur->bc_private.b.firstblock;
         args.firstblock = args.fsbno;
+        args.minleft = 0;
         if (args.fsbno == NULLFSBLOCK) {
                 args.fsbno = lbno;
                 args.type = XFS_ALLOCTYPE_START_BNO;
+                /*
+                 * Make sure there is sufficient room left in the AG to
+                 * complete a full tree split for an extent insert.  If
+                 * we are converting the middle part of an extent then
+                 * we may need space for two tree splits.
+                 *
+                 * We are relying on the caller to make the correct block
+                 * reservation for this operation to succeed.  If the
+                 * reservation amount is insufficient then we may fail a
+                 * block allocation here and corrupt the filesystem.
+                 */
+                args.minleft = xfs_trans_get_block_res(args.tp);
         } else
                 args.type = XFS_ALLOCTYPE_NEAR_BNO;
-        args.mod = args.minleft = args.alignment = args.total = args.isfl =
+        args.mod = args.alignment = args.total = args.isfl =
                 args.userdata = args.minalignslop = 0;
         args.minlen = args.maxlen = args.prod = 1;
         args.wasdel = cur->bc_private.b.flags & XFS_BTCUR_BPRV_WASDEL;
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 7edcde691d1a..67f22b2b44b3 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -889,6 +889,16 @@ xfs_iomap_write_unwritten(
         count_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + count);
         count_fsb = (xfs_filblks_t)(count_fsb - offset_fsb);
 
+        /*
+         * Reserve enough blocks in this transaction for two complete extent
+         * btree splits.  We may be converting the middle part of an unwritten
+         * extent and in this case we will insert two new extents in the btree
+         * each of which could cause a full split.
+         *
+         * This reservation amount will be used in the first call to
+         * xfs_bmbt_split() to select an AG with enough space to satisfy the
+         * rest of the operation.
+         */
         resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;
 
         do {