1 files changed, 10 insertions, 0 deletions

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 {