[PATCH] ext4: return correct wbc.nr_to_write in ext4_da_writepages

Richard Kennedy
2009-12-17T15:17:54Z

ext4: always re-base nr_to_write in ext4_da_writepages

When ext4_da_writepages increases the nr_to_write in writeback_control
then it must always re-base the return value.

Without this change, when wb_writeback calculates how many pages were
actually written it can get a negative value and loop more times than
necessary. In tests I have seen nearly all the dirty pages pushed out to
writeback due to this issue.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>

----

patch against 2.6.32
tested on x86_64

wb_writeback calculates (MAX_WRITE_PAGES - nr_to_write) & cannot know
that the value got changed.

I'm not sure what the test I removed was for.
Perhaps
if (nr_to_writebump)
wbc->nr_to_write -= nr_to_writebump;
was intended?

regards
Richard

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2c8caa5..52a573c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2999,8 +2999,7 @@ retry:
out_writepages:
if (!no_nrwrite_index_update)
wbc->no_nrwrite_index_update = 0;
- if (wbc->nr_to_write > nr_to_writebump)
- wbc->nr_to_write -= nr_to_writebump;
+ wbc->nr_to_write -= nr_to_writebump;
wbc->range_start = range_start;
trace_ext4_da_writepages_result(inode, wbc, ret, pages_written);
return ret;