[PATCH] ext4: fix delayed pages writback regression.

["Yan, Zheng" <zheng.z.yan@intel.com>]

From: "Yan, Zheng" <zheng.z.yan@intel.com>

Our Linux Kernel Performance project found that commit 4e7ea81db5
(ext4: restructure writeback path) indroduced a read performance
regression. After the commit, ext4 does not merge adjacent delayed
pages during writeback. The regression is caused by the "buffer
mapped" check in mpage_add_bh_to_extent(), delayed dirty pages are
not mapped.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
---
 fs/ext4/inode.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c79fd7d..f2034cb 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1944,8 +1944,9 @@ static bool mpage_add_bh_to_extent(struct mpage_da_data *mpd, ext4_lblk_t lblk,
 	struct ext4_map_blocks *map = &mpd->map;
 
 	/* Buffer that doesn't need mapping for writeback? */
-	if (!buffer_dirty(bh) || !buffer_mapped(bh) ||
-	    (!buffer_delay(bh) && !buffer_unwritten(bh))) {
+	if (!buffer_dirty(bh) ||
+	    (!buffer_mapped(bh) &&
+	     !buffer_delay(bh) && !buffer_unwritten(bh))) {
 		/* So far no extent to map => we write the buffer right away */
 		if (map->m_len == 0)
 			return true;
-- 
1.8.1.4