[PATCH v2] ext4: directly leave out of ext4_find_delalloc_range() if filesystem mount with "nodelalloc"

[Robin Dong <hao.bigrat@gmail.com>]

From: Robin Dong <sanbai@taobao.com>

We found performance regression when using bigalloc with "nodelalloc"  (1MB cluster size):

1. mke2fs -C 1048576 -O ^has_journal,bigalloc /dev/sda
2. mount -o nodelalloc /dev/sda /test/
3. time dd if=/dev/zero of=/test/io bs=1048576 count=1024

The "dd" will cost about 2 seconds to finish, but if we mke2fs without "bigalloc",
"dd" will only cost lesss than 1 second.

The reason is:  when using ext4 with "nodelalloc", it will call ext4_find_delalloc_cluster() nearly
everytime it call ext4_ext_map_blocks(), and ext4_find_delalloc_range() will also scan all pages
in cluster because no buffer is "delayed".
A cluster has 256 pages (1MB cluster), so it will scan 256 * 256k pags when creating a 1G file. That
severely hurts the performance.

Therefore, we return out from ext4_find_delalloc_range() when using "nodelalloc".

Signed-off-by: Robin Dong <sanbai@taobao.com>
---
 fs/ext4/extents.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 61fa9e1..60f5f25 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3282,6 +3282,9 @@ static int ext4_find_delalloc_range(struct inode *inode,
 	ext4_lblk_t i, pg_lblk;
 	pgoff_t index;
 
+	if (!test_opt(inode->i_sb, DELALLOC))
+		return 0;
+
 	/* reverse search wont work if fs block size is less than page size */
 	if (inode->i_blkbits < PAGE_CACHE_SHIFT)
 		search_hint_reverse = 0;
-- 
1.7.4.1