linux-ext4 - [PATCH] EXT4: Avoid zeroout-ing and writting the same space

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Message-Id: <1464631943-37365-1-git-send-email-jhe@cs.wisc.edu>
Date:	Mon, 30 May 2016 13:12:23 -0500
From:	Jun He <jhe@...wisc.edu>
To:	linux-ext4@...r.kernel.org
Cc:	tytso@....edu, Jun He <jhe@...wisc.edu>
Subject: [PATCH] EXT4: Avoid zeroout-ing and writting the same space

When splitting unwritten extent into two extents, currently ext4
zeroout the whole second extent, which contains the requested
space. The requested space will soon be written with data. The
zeroout of requested space is unncessary, causing more traffic
to disk.

Signed-off-by: Jun He <jhe@...wisc.edu>
---
 fs/ext4/extents.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 2a2eef9..2bba31c 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3597,11 +3597,11 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 	split_map.m_len = map->m_len;

 	if (max_zeroout && (allocated > map->m_len)) {
-		if (allocated <= max_zeroout) {
+		if (allocated - map->m_len <= max_zeroout) {
 			/* case 3 */
 			zero_ex.ee_block =
-					 cpu_to_le32(map->m_lblk);
-			zero_ex.ee_len = cpu_to_le16(allocated);
+			    cpu_to_le32(map->m_lblk + map->m_len);
+			zero_ex.ee_len = cpu_to_le16(allocated - map->m_len);
 			ext4_ext_store_pblock(&zero_ex,
 				ext4_ext_pblock(ex) + map->m_lblk - ee_block);
 			err = ext4_ext_zeroout(inode, &zero_ex);
--
1.9.1

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