[PATCH] fs/mbcache: make count_objects more robust.

[Jiang Biao]

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

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

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

This patch make sure the mb_cache_count not return negative value,
which make the mbcache shrinker more robust.

Signed-off-by: Jiang Biao <jiang.biao2@xxxxxxxxxx>
---
 fs/mbcache.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index d818fd2..b8b8b9c 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;
 }
 
-- 
2.7.4