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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 3/4] mm: memcontrol: bail out early when id is zero
Date:   Sat, 13 Feb 2021 01:01:58 +0800
Message-Id: <20210212170159.32153-3-songmuchun@bytedance.com>
In-Reply-To: <20210212170159.32153-1-songmuchun@bytedance.com>
References: <20210212170159.32153-1-songmuchun@bytedance.com>
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The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id,
so idr_find() is pointless and wastes CPU cycles.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/memcontrol.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a3f26522765a..68ed4b297c13 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg)
 struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
 {
 	WARN_ON_ONCE(!rcu_read_lock_held());
+	/* The memcg ID cannot be zero. */
+	if (id == 0)
+		return NULL;
 	return idr_find(&mem_cgroup_idr, id);
 }
 
-- 
2.11.0