linux-kernel - [PATCH] mm: list_lru: fix the return value of list_lru_count

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Message-Id: <20211025124910.56433-1-songmuchun@bytedance.com>
Date:   Mon, 25 Oct 2021 20:49:10 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     akpm@...ux-foundation.org, mhocko@...nel.org, shakeelb@...gle.com,
        willy@...radead.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: [PATCH] mm: list_lru: fix the return value of list_lru_count_one()

Since commit 2788cf0c401c ("memcg: reparent list_lrus and free
kmemcg_id on css offline"), ->nr_items can be negative during
memory cgroup reparenting. In this case, list_lru_count_one()
will return an unusual and huge value, which can surprise
users. At least for now it hasn’t affected any users. But it
is better to let list_lru_count_ont() returns zero when ->nr_items
is negative.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 mm/list_lru.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index a6031f1c5bd7..2bba1cd68bb3 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -176,13 +176,16 @@ unsigned long list_lru_count_one(struct list_lru *lru,
 {
 	struct list_lru_node *nlru = &lru->node[nid];
 	struct list_lru_one *l;
-	unsigned long count;
+	long count;
 
 	rcu_read_lock();
 	l = list_lru_from_memcg_idx(nlru, memcg_cache_id(memcg));
 	count = READ_ONCE(l->nr_items);
 	rcu_read_unlock();
 
+	if (unlikely(count < 0))
+		count = 0;
+
 	return count;
 }
 EXPORT_SYMBOL_GPL(list_lru_count_one);
-- 
2.11.0