[patch 061/262] mm: list_lru: fix the return value of list_lru_count_one()

Andrew Morton
2021-11-05T20:37:53Z

From: Muchun Song <songmuchun@bytedance.com>
Subject: 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.

Link: https://lkml.kernel.org/r/20211025124910.56433-1-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/list_lru.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/list_lru.c~mm-list_lru-fix-the-return-value-of-list_lru_count_one
+++ a/mm/list_lru.c
@@ -176,13 +176,16 @@ unsigned long list_lru_count_one(struct
{
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);
_