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>

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 344557eef9288cc5d36460d8558331657a86fc27..9072794e3a42fe0c7dab8135a3bdae8a4d101e3e 100644 (file)
--- 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