linux-kernel - [PATCH v1 01/14] include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFIG

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Message-Id: <20210313075747.3781593-2-yuzhao@google.com>
Date:   Sat, 13 Mar 2021 00:57:34 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     linux-mm@...ck.org
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Yang Shi <shy828301@...il.com>,
        Ying Huang <ying.huang@...el.com>,
        linux-kernel@...r.kernel.org, page-reclaim@...gle.com,
        Yu Zhao <yuzhao@...gle.com>
Subject: [PATCH v1 01/14] include/linux/memcontrol.h: do not warn in
 page_memcg_rcu() if !CONFIG_MEMCG

We want to make sure the rcu lock is held while using
page_memcg_rcu(). But having a WARN_ON_ONCE() in page_memcg_rcu() when
!CONFIG_MEMCG is superfluous because of the following legit use case:

  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)

This patch removes the WARN_ON_ONCE() from page_memcg_rcu() for the
!CONFIG_MEMCG case.

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

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e6dc793d587d..f325aeb4b4e8 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1079,7 +1079,6 @@ 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;
 }
 
-- 
2.31.0.rc2.261.g7f71774620-goog