[PATCH v2 01/16] include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFIG_MEMCG

Yu Zhao
2021-04-13T06:56:52Z

page_memcg_rcu() warns on !rcu_read_lock_held() regardless of
CONFIG_MEMCG. The following code is legit, but it triggers the warning
when !CONFIG_MEMCG, since lock_page_memcg() and unlock_page_memcg()
are empty for this config.

memcg = lock_page_memcg(page1)
(rcu_read_lock() if CONFIG_MEMCG=y)

do something to page1

if (page_memcg_rcu(page2) == memcg)
do something to page2 too as it cannot be migrated away from the
memcg either.

unlock_page_memcg(page1)
(rcu_read_unlock() if CONFIG_MEMCG=y)

Locking/unlocking rcu consistently for both configs is rigorous but it
also forces unnecessary locking upon users who have no interest in
CONFIG_MEMCG.

This patch removes the assertion for !CONFIG_MEMCG, because
page_memcg_rcu() has a few callers and there are no concerns regarding
their correctness at the moment.

Signed-off-by: Yu Zhao <yuzhao@google.com>
---
include/linux/memcontrol.h | 1 -
1 file changed, 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 0c04d39a7967..f13dc02cf277 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1077,7 +1077,6 @@ static inline struct mem_cgroup *page_memcg(struct page *page)

static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
{
- WARN_ON_ONCE(!rcu_read_lock_held());
return NULL;
}

--
2.31.1.295.g9ea45b61b8-goog