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Date:   Tue, 13 Apr 2021 00:56:18 -0600
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Subject: [PATCH v2 01/16] include/linux/memcontrol.h: do not warn in
 page_memcg_rcu() if !CONFIG_MEMCG
From:   Yu Zhao <yuzhao@google.com>
To:     linux-mm@kvack.org
Cc:     Alex Shi <alexs@kernel.org>, Andi Kleen <ak@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Manes <ben.manes@gmail.com>,
        Dave Chinner <david@fromorbit.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Michael Larabel <michael@michaellarabel.com>,
        Michal Hocko <mhocko@suse.com>,
        Michel Lespinasse <michel@lespinasse.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Rong Chen <rong.a.chen@intel.com>,
        SeongJae Park <sjpark@amazon.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Yang Shi <shy828301@gmail.com>,
        Ying Huang <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
        linux-kernel@vger.kernel.org, lkp@lists.01.org,
        page-reclaim@google.com, Yu Zhao <yuzhao@google.com>
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Precedence: bulk

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