[PATCH v3 01/14] include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFIG_MEMCG

[Yu Zhao]

page_memcg_rcu() warns on !rcu_read_lock_held() regardless of
CONFIG_MEMCG. The following legit code trips 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@xxxxxxxxxx>
Tested-by: Konstantin Kharlamov <Hi-Angel@xxxxxxxxx>
---
 include/linux/memcontrol.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index c193be760709..6bcac3d91dd1 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1131,7 +1131,6 @@ 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;
 }
 
-- 
2.31.1.751.gd2f1c929bd-goog