[PATCH AUTOSEL for 4.14 077/100] fs/mbcache.c: make count_objects() more robust

From: Sasha Levin
Date: Tue Jan 23 2018 - 23:46:58 EST


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 d818fd236787..b8b8b9ced9f8 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