[PATCH 6.6 070/567] ext4: fix e4b bitmap inconsistency reports

Greg Kroah-Hartman
2026-03-23T14:54:45Z

6.6-stable review patch. If anyone has any objections, please let me know.

------------------

From: Yongjian Sun <sunyongjian1@huawei.com>

[ Upstream commit bdc56a9c46b2a99c12313122b9352b619a2e719e ]

A bitmap inconsistency issue was observed during stress tests under
mixed huge-page workloads. Ext4 reported multiple e4b bitmap check
failures like:

ext4_mb_complex_scan_group:2508: group 350, 8179 free clusters as
per group info. But got 8192 blocks

Analysis and experimentation confirmed that the issue is caused by a
race condition between page migration and bitmap modification. Although
this timing window is extremely narrow, it is still hit in practice:

folio_lock ext4_mb_load_buddy
__migrate_folio
check ref count
folio_mc_copy __filemap_get_folio
folio_try_get(folio)
......
mb_mark_used
ext4_mb_unload_buddy
__folio_migrate_mapping
folio_ref_freeze
folio_unlock

The root cause of this issue is that the fast path of load_buddy only
increments the folio's reference count, which is insufficient to prevent
concurrent folio migration. We observed that the folio migration process
acquires the folio lock. Therefore, we can determine whether to take the
fast path in load_buddy by checking the lock status. If the folio is
locked, we opt for the slow path (which acquires the lock) to close this
concurrency window.

Additionally, this change addresses the following issues:

When the DOUBLE_CHECK macro is enabled to inspect bitmap-related
issues, the following error may be triggered:

corruption in group 324 at byte 784(6272): f in copy != ff on
disk/prealloc

Analysis reveals that this is a false positive. There is a specific race
window where the bitmap and the group descriptor become momentarily
inconsistent, leading to this error report:

ext4_mb_load_buddy ext4_mb_load_buddy
__filemap_get_folio(create|lock)
folio_lock
ext4_mb_init_cache
folio_mark_uptodate
__filemap_get_folio(no lock)
......
mb_mark_used
mb_mark_used_double
mb_cmp_bitmaps
mb_set_bits(e4b->bd_bitmap)
folio_unlock

The original logic assumed that since mb_cmp_bitmaps is called when the
bitmap is newly loaded from disk, the folio lock would be sufficient to
prevent concurrent access. However, this overlooks a specific race
condition: if another process attempts to load buddy and finds the folio
is already in an uptodate state, it will immediately begin using it without
holding folio lock.

Signed-off-by: Yongjian Sun <sunyongjian1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260106090820.836242-1-sunyongjian@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ext4/mballoc.c | 21 +++++++++++----------
1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 877b336c651f7..d0f4e5905bf12 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1634,16 +1634,17 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,

/* Avoid locking the folio in the fast path ... */
folio = __filemap_get_folio(inode->i_mapping, pnum, FGP_ACCESSED, 0);
- if (IS_ERR(folio) || !folio_test_uptodate(folio)) {
+ if (IS_ERR(folio) || !folio_test_uptodate(folio) || folio_test_locked(folio)) {
+ /*
+ * folio_test_locked is employed to detect ongoing folio
+ * migrations, since concurrent migrations can lead to
+ * bitmap inconsistency. And if we are not uptodate that
+ * implies somebody just created the folio but is yet to
+ * initialize it. We can drop the folio reference and
+ * try to get the folio with lock in both cases to avoid
+ * concurrency.
+ */
if (!IS_ERR(folio))
- /*
- * drop the folio reference and try
- * to get the folio with lock. If we
- * are not uptodate that implies
- * somebody just created the folio but
- * is yet to initialize it. So
- * wait for it to initialize.
- */
folio_put(folio);
folio = __filemap_get_folio(inode->i_mapping, pnum,
FGP_LOCK | FGP_ACCESSED | FGP_CREAT, gfp);
@@ -1685,7 +1686,7 @@ ext4_mb_load_buddy_gfp(struct super_block *sb, ext4_group_t group,
poff = block % blocks_per_page;

folio = __filemap_get_folio(inode->i_mapping, pnum, FGP_ACCESSED, 0);
- if (IS_ERR(folio) || !folio_test_uptodate(folio)) {
+ if (IS_ERR(folio) || !folio_test_uptodate(folio) || folio_test_locked(folio)) {
if (!IS_ERR(folio))
folio_put(folio);
folio = __filemap_get_folio(inode->i_mapping, pnum,
--
2.51.0