xfs: push rounddown_pow_of_two() to after prealloc throttle

The round down occurs towards the beginning of the function. Push
it down after throttling has occurred. This is to support adding
further transformations to 'alloc_blocks' that might not preserve
power-of-two alignment (and thus could lead to rounding down
multiple times).

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 344557eef928..9072794e3a42 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -389,8 +389,15 @@ xfs_iomap_prealloc_size(
         if (!alloc_blocks)
                 goto check_writeio;
 
-        alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
-                                rounddown_pow_of_two(alloc_blocks));
+        /*
+         * MAXEXTLEN is not a power of two value but we round the prealloc down
+         * to the nearest power of two value after throttling. To prevent the
+         * round down from unconditionally reducing the maximum supported prealloc
+         * size, we round up first, apply appropriate throttling, round down and
+         * cap the value to MAXEXTLEN.
+         */
+        alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
+                                       alloc_blocks);
 
         xfs_icsb_sync_counters(mp, XFS_ICSB_LAZY_COUNT);
         freesp = mp->m_sb.sb_fdblocks;
@@ -407,6 +414,14 @@ xfs_iomap_prealloc_size(
         }
         if (shift)
                 alloc_blocks >>= shift;
+        /*
+         * rounddown_pow_of_two() returns an undefined result if we pass in
+         * alloc_blocks = 0.
+         */
+        if (alloc_blocks)
+                alloc_blocks = rounddown_pow_of_two(alloc_blocks);
+        if (alloc_blocks > MAXEXTLEN)
+                alloc_blocks = MAXEXTLEN;
 
         /*
          * If we are still trying to allocate more space than is