linux-ext4 - [PATCH] fs/mbcache: make sure mb_cache_count() not return negative value.

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Message-Id: <1515376842-66291-1-git-send-email-jiang.biao2@zte.com.cn>
Date:   Mon, 8 Jan 2018 10:00:42 +0800
From:   Jiang Biao <jiang.biao2@....com.cn>
To:     linux-fsdevel@...r.kernel.org
Cc:     linux-ext4@...r.kernel.org, tytso@....edu, ebiggers@...gle.com,
        akpm@...ux-foundation.org, jack@...e.cz, jiang.biao2@....com.cn,
        zhong.weidong@....com.cn
Subject: [PATCH] fs/mbcache: make sure mb_cache_count() not return negative value.

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
....

Trace info 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() not return a negative value,
which makes the mbcache shrinker more robust.

Signed-off-by: Jiang Biao <jiang.biao2@....com.cn>
CC: "Theodore Ts'o" <tytso@....edu>
CC: Eric Biggers <ebiggers@...gle.com>
CC: Andrew Morton <akpm@...ux-foundation.org>
CC: Jan Kara <jack@...e.cz>
---
 fs/mbcache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index b8b8b9c..c758458 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -238,7 +238,9 @@ void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
 			spin_lock(&cache->c_list_lock);
 			if (!list_empty(&entry->e_list)) {
 				list_del_init(&entry->e_list);
-				cache->c_entry_count--;
+				/*Make sure c_entry_count is not zero before dec*/
+				if (cache->c_entry_count != 0)
+					cache->c_entry_count--;
 				atomic_dec(&entry->e_refcnt);
 			}
 			spin_unlock(&cache->c_list_lock);
-- 
2.7.4