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From:   Muchun Song <songmuchun@bytedance.com>
To:     guro@fb.com, hannes@cmpxchg.org, mhocko@kernel.org,
        akpm@linux-foundation.org, shakeelb@google.com,
        vdavydov.dev@gmail.com
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, fam.zheng@bytedance.com,
        bsingharora@gmail.com, shy828301@gmail.com,
        alex.shi@linux.alibaba.com, Muchun Song <songmuchun@bytedance.com>
Subject: [RFC PATCH v2 02/18] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm
Date:   Fri,  9 Apr 2021 20:29:43 +0800
Message-Id: <20210409122959.82264-3-songmuchun@bytedance.com>
In-Reply-To: <20210409122959.82264-1-songmuchun@bytedance.com>
References: <20210409122959.82264-1-songmuchun@bytedance.com>
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When mm is NULL, we do not need to hold rcu lock and call css_tryget for
the root memcg. And we also do not need to check !mm in every loop of
while. So bail out early when !mm.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f229de925aa5..9cbfff59b171 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -901,20 +901,19 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 	if (mem_cgroup_disabled())
 		return NULL;
 
+	/*
+	 * Page cache insertions can happen without an
+	 * actual mm context, e.g. during disk probing
+	 * on boot, loopback IO, acct() writes etc.
+	 */
+	if (unlikely(!mm))
+		return root_mem_cgroup;
+
 	rcu_read_lock();
 	do {
-		/*
-		 * Page cache insertions can happen without an
-		 * actual mm context, e.g. during disk probing
-		 * on boot, loopback IO, acct() writes etc.
-		 */
-		if (unlikely(!mm))
+		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
+		if (unlikely(!memcg))
 			memcg = root_mem_cgroup;
-		else {
-			memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
-			if (unlikely(!memcg))
-				memcg = root_mem_cgroup;
-		}
 	} while (!css_tryget(&memcg->css));
 	rcu_read_unlock();
 	return memcg;
-- 
2.11.0