include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFI…

…G_MEMCG

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

@@ -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;
 }