[PATCH 5.15 083/913] mm: fs: fix lru_cache_disabled race in bh_lru

[Greg Kroah-Hartman]

From: Minchan Kim <minchan@xxxxxxxxxx>

commit c0226eb8bde854e016a594a16f5c0d98aca426fa upstream.

Check lru_cache_disabled under bh_lru_lock.  Otherwise, it could introduce
race below and it fails to migrate pages containing buffer_head.

   CPU 0					CPU 1

bh_lru_install
                                       lru_cache_disable
  lru_cache_disabled = false
                                       atomic_inc(&lru_disable_count);
				       invalidate_bh_lrus_cpu of CPU 0
				       bh_lru_lock
				       __invalidate_bh_lrus
				       bh_lru_unlock
  bh_lru_lock
  install the bh
  bh_lru_unlock

WHen this race happens a CMA allocation fails, which is critical for
the workload which depends on CMA.

Link: https://lkml.kernel.org/r/20220308180709.2017638-1-minchan@xxxxxxxxxx
Fixes: 8cc621d2f45d ("mm: fs: invalidate BH LRU during page migration")
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Chris Goldsworthy <cgoldswo@xxxxxxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Cc: John Dias <joaodias@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/buffer.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1235,16 +1235,18 @@ static void bh_lru_install(struct buffer
 	int i;
 
 	check_irqs_on();
+	bh_lru_lock();
+
 	/*
 	 * the refcount of buffer_head in bh_lru prevents dropping the
 	 * attached page(i.e., try_to_free_buffers) so it could cause
 	 * failing page migration.
 	 * Skip putting upcoming bh into bh_lru until migration is done.
 	 */
-	if (lru_cache_disabled())
+	if (lru_cache_disabled()) {
+		bh_lru_unlock();
 		return;
-
-	bh_lru_lock();
+	}
 
 	b = this_cpu_ptr(&bh_lrus);
 	for (i = 0; i < BH_LRU_SIZE; i++) {