xfs: speculative delayed allocation uses rounddown_power_of_2 badly

rounddown_power_of_2() returns an undefined result when passed a
value of zero. The specualtive delayed allocation code is doing this
when the inode is zero length. Hence occasionally the preallocation
is much, much larger than is necessary (e.g. 8GB for a 270 _byte_
file). Ensure we don't even pass a zero value to this function so
the result of preallocation is always the desired size.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 55582bd66659140ed9592a85ec0189187bcfd4ac..8a0f044750c3207eff174c4137bcd77580465894 100644 (file)
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -337,7 +337,12 @@ xfs_iomap_prealloc_size(
                int shift = 0;
                int64_t freesp;
 
-               alloc_blocks = XFS_B_TO_FSB(mp, ip->i_size);
+               /*
+                * rounddown_pow_of_two() returns an undefined result
+                * if we pass in alloc_blocks = 0. Hence the "+ 1" to
+                * ensure we always pass in a non-zero value.
+                */
+               alloc_blocks = XFS_B_TO_FSB(mp, ip->i_size) + 1;
                alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
                                        rounddown_pow_of_two(alloc_blocks));