[PATCH v2 2/7] xfs: push rounddown_pow_of_two() to after prealloc throttle

[Brian Foster <bfoster@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 fs/xfs/xfs_iomap.c |   21 +++++++++++++++------
 1 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index bd7c060..b2002a5 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -329,13 +329,14 @@ xfs_iomap_prealloc_size(
                goto check_writeio;
 
        /*
-        * 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.
+        * 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_B_TO_FSB(mp, XFS_ISIZE(ip)) + 1;
-       alloc_blocks = XFS_FILEOFF_MIN(MAXEXTLEN,
-                               rounddown_pow_of_two(alloc_blocks));
+       alloc_blocks = XFS_FILEOFF_MIN(roundup_pow_of_two(MAXEXTLEN),
+                               XFS_B_TO_FSB(mp, XFS_ISIZE(ip)));
 
        xfs_icsb_sync_counters(mp, XFS_ICSB_LAZY_COUNT);
        freesp = mp->m_sb.sb_fdblocks;
@@ -352,6 +353,14 @@ 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;
 
 check_writeio:
        if (alloc_blocks < mp->m_writeio_blocks)
-- 
1.7.7.6