[PATCH AUTOSEL for 4.9 38/55] fs/mbcache.c: make count_objects() more robust

[Sasha Levin]

From: Jiang Biao <jiang.biao2@xxxxxxxxxx>

[ Upstream commit d5dabd633922ac5ee5bcc67748f7defb8b211469 ]

When running ltp stress test for 7*24 hours, vmscan occasionally emits
the following warning continuously:

  mb_cache_scan+0x0/0x3f0 negative objects to delete
  nr=-9232265467809300450
  ...

Tracing shows the freeable(mb_cache_count returns) is -1, which causes
the continuous accumulation and overflow of total_scan.

This patch makes sure that mb_cache_count() cannot return a negative
value, which makes the mbcache shrinker more robust.

Link: http://lkml.kernel.org/r/1511753419-52328-1-git-send-email-jiang.biao2@xxxxxxxxxx
Signed-off-by: Jiang Biao <jiang.biao2@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: <zhong.weidong@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 fs/mbcache.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index c5bd19ffa326..35ab4187bfe1 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -269,6 +269,9 @@ static unsigned long mb_cache_count(struct shrinker *shrink,
 	struct mb_cache *cache = container_of(shrink, struct mb_cache,
 					      c_shrink);
 
+	/* Unlikely, but not impossible */
+	if (unlikely(cache->c_entry_count < 0))
+		return 0;
 	return cache->c_entry_count;
 }
 
-- 
2.11.0