[PATCH AUTOSEL 4.9 2/3] ext4: improve write performance with disabled delalloc

[Sasha Levin]

From: Jan Kara <jack@xxxxxxx>

[ Upstream commit 8d5459c11f548131ce48b2fbf45cccc5c382558f ]

When delayed allocation is disabled (either through mount option or
because we are running low on free space), ext4_write_begin() allocates
blocks with EXT4_GET_BLOCKS_IO_CREATE_EXT flag. With this flag extent
merging is disabled and since ext4_write_begin() is called for each page
separately, we end up with a *lot* of 1 block extents in the extent tree
and following writeback is writing 1 block at a time which results in
very poor write throughput (4 MB/s instead of 200 MB/s). These days when
ext4_get_block_unwritten() is used only by ext4_write_begin(),
ext4_page_mkwrite() and inline data conversion, we can safely allow
extent merging to happen from these paths since following writeback will
happen on different boundaries anyway. So use
EXT4_GET_BLOCKS_CREATE_UNRIT_EXT instead which restores the performance.

Signed-off-by: Jan Kara <jack@xxxxxxx>
Link: https://lore.kernel.org/r/20220520111402.4252-1-jack@xxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/ext4/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 754b33828853..9aebd9bb68a4 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -791,7 +791,7 @@ int ext4_get_block_unwritten(struct inode *inode, sector_t iblock,
 	ext4_debug("ext4_get_block_unwritten: inode %lu, create flag %d\n",
 		   inode->i_ino, create);
 	return _ext4_get_block(inode, iblock, bh_result,
-			       EXT4_GET_BLOCKS_IO_CREATE_EXT);
+			       EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT);
 }
 
 /* Maximum number of blocks we map for direct IO at once. */
-- 
2.35.1