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From:   Muchun Song <songmuchun@bytedance.com>
To:     hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com,
        akpm@linux-foundation.org
Cc:     cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Muchun Song <songmuchun@bytedance.com>
Subject: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page
Date:   Fri,  5 Feb 2021 14:27:19 +0800
Message-Id: <20210205062719.74431-1-songmuchun@bytedance.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The get_mem_cgroup_from_page() is called under page lock, so the page
memcg cannot be changed under us. Also, css_get is enough because page
has a reference to the memcg.

If we really want to make the get_mem_cgroup_from_page() suitable for
arbitrary page, we should use page_memcg_rcu() instead of page_memcg()
and call it after rcu_read_lock().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memcontrol.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 87f01bc05d1f..6c7f1ea3955e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1063,16 +1063,15 @@ EXPORT_SYMBOL(get_mem_cgroup_from_mm);
  */
 struct mem_cgroup *get_mem_cgroup_from_page(struct page *page)
 {
-	struct mem_cgroup *memcg = page_memcg(page);
+	struct mem_cgroup *memcg;
 
 	if (mem_cgroup_disabled())
 		return NULL;
 
-	rcu_read_lock();
 	/* Page should not get uncharged and freed memcg under us. */
-	if (!memcg || WARN_ON_ONCE(!css_tryget(&memcg->css)))
-		memcg = root_mem_cgroup;
-	rcu_read_unlock();
+	memcg = page_memcg(page) ? : root_mem_cgroup;
+	css_get(&memcg->css);
+
 	return memcg;
 }
 EXPORT_SYMBOL(get_mem_cgroup_from_page);
-- 
2.11.0