[PATCH 5.15 385/570] ext4: always drain queued discard work in ext4_mb_release()

Greg Kroah-Hartman
2026-04-13T16:34:36Z

From: Theodore Ts'o <tytso@mit.edu>

commit 9ee29d20aab228adfb02ca93f87fb53c56c2f3af upstream.

While reviewing recent ext4 patch[1], Sashiko raised the following
concern[2]:

> If the filesystem is initially mounted with the discard option,
> deleting files will populate sbi->s_discard_list and queue
> s_discard_work. If it is then remounted with nodiscard, the
> EXT4_MOUNT_DISCARD flag is cleared, but the pending s_discard_work is
> neither cancelled nor flushed.

[1] https://lore.kernel.org/r/20260319094545.19291-1-qiang.zhang@linux.dev/
[2] https://sashiko.dev/#/patchset/20260319094545.19291-1-qiang.zhang%40linux.dev

The concern was valid, but it had nothing to do with the patch[1].
One of the problems with Sashiko in its current (early) form is that
it will detect pre-existing issues and report it as a problem with the
patch that it is reviewing.

In practice, it would be hard to hit deliberately (unless you are a
malicious syzkaller fuzzer), since it would involve mounting the file
system with -o discard, and then deleting a large number of files,
remounting the file system with -o nodiscard, and then immediately
unmounting the file system before the queued discard work has a change
to drain on its own.

Fix it because it's a real bug, and to avoid Sashiko from raising this
concern when analyzing future patches to mballoc.c.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Fixes: 55cdd0af2bc5 ("ext4: get discard out of jbd2 commit kthread contex")
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/ext4/mballoc.c | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)

--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3628,13 +3628,11 @@ int ext4_mb_release(struct super_block *
struct kmem_cache *cachep = get_groupinfo_cache(sb->s_blocksize_bits);
int count;

- if (test_opt(sb, DISCARD)) {
- /*
- * wait the discard work to drain all of ext4_free_data
- */
- flush_work(&sbi->s_discard_work);
- WARN_ON_ONCE(!list_empty(&sbi->s_discard_list));
- }
+ /*
+ * wait the discard work to drain all of ext4_free_data
+ */
+ flush_work(&sbi->s_discard_work);
+ WARN_ON_ONCE(!list_empty(&sbi->s_discard_list));

if (sbi->s_group_info) {
for (i = 0; i < ngroups; i++) {