[PATCH V3] xfs: use roundup_pow_of_two instead of ffs during xlog_find_tail

[Wang Jianchao]

In our production environment, we find that mounting a 500M /boot
which is umount cleanly needs ~6s. One cause is that ffs() is
used by xlog_write_log_records() to decide the buffer size. It
can cause a lot of small IO easily when xlog_clear_stale_blocks()
needs to wrap around the end of log area and log head block is
not power of two. Things are similar in xlog_find_verify_cycle().

The code is able to handed bigger buffer very well, we can use
roundup_pow_of_two() to replace ffs() directly to avoid small
and sychronous IOs.

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
Signed-off-by: Wang Jianchao <wangjc136@xxxxxxxxx>
---

Changes in V2:
 - Move change log below "---"
 - Add reviewed-by Dave Chinner tag

Changes in V1:
 - Also replace the ffs in xlog_find_verify_cycle()

 fs/xfs/xfs_log_recover.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 82c81d20459d..13b94d2e605b 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -329,7 +329,7 @@ xlog_find_verify_cycle(
 	 * try a smaller size.  We need to be able to read at least
 	 * a log sector, or we're out of luck.
 	 */
-	bufblks = 1 << ffs(nbblks);
+	bufblks = roundup_pow_of_two(nbblks);
 	while (bufblks > log->l_logBBsize)
 		bufblks >>= 1;
 	while (!(buffer = xlog_alloc_buffer(log, bufblks))) {
@@ -1528,7 +1528,7 @@ xlog_write_log_records(
 	 * a smaller size.  We need to be able to write at least a
 	 * log sector, or we're out of luck.
 	 */
-	bufblks = 1 << ffs(blocks);
+	bufblks = roundup_pow_of_two(blocks);
 	while (bufblks > log->l_logBBsize)
 		bufblks >>= 1;
 	while (!(buffer = xlog_alloc_buffer(log, bufblks))) {
-- 
2.34.1