ext4: Turn off multiple page-io submission by default

Jon Nelson has found a test case which causes postgresql to fail with
the error:

psql:t.sql:4: ERROR: invalid page header in block 38269 of relation base/16384/16581

Under memory pressure, it looks like part of a file can end up getting
replaced by zero's.  Until we can figure out the cause, we'll roll
back the change and use block_write_full_page() instead of
ext4_bio_write_page().  The new, more efficient writing function can
be used via the mount option mblk_io_submit, so we can test and fix
the new page I/O code.

To reproduce the problem, install postgres 8.4 or 9.0, and pin enough
memory such that the system just at the end of triggering writeback
before running the following sql script:

begin;
create temporary table foo as select x as a, ARRAY[x] as b FROM
generate_series(1, 10000000 ) AS x;
create index foo_a_idx on foo (a);
create index foo_b_idx on foo USING GIN (b);
rollback;

If the temporary table is created on a hard drive partition which is
encrypted using dm_crypt, then under memory pressure, approximately
30-40% of the time, pgsql will issue the above failure.

This patch should fix this problem, and the problem will come back if
the file system is mounted with the mblk_io_submit mount option.

Reported-by: Jon Nelson <jnelson@jamponi.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 4 insertions, 1 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index bdbe699..e659597 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2125,9 +2125,12 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
 			 */
 			if (unlikely(journal_data && PageChecked(page)))
 				err = __ext4_journalled_writepage(page, len);
-			else
+			else if (test_opt(inode->i_sb, MBLK_IO_SUBMIT))
 				err = ext4_bio_write_page(&io_submit, page,
 							  len, mpd->wbc);
+			else
+				err = block_write_full_page(page,
+					noalloc_get_block_write, mpd->wbc);
 
 			if (!err)
 				mpd->pages_written++;