linux-kernel - [PATCH] mm: memcontrol: Don't css_get() on root_mem_cgroup in get_mem_cgroup_from

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Message-Id: <20231012161504.3445042-1-longman@redhat.com>
Date:   Thu, 12 Oct 2023 12:15:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Waiman Long <longman@...hat.com>
Subject: [PATCH] mm: memcontrol: Don't css_get() on root_mem_cgroup in get_mem_cgroup_from_mm()

As reference counting in the root memcg is disabled, there is no need
to get a reference if root memcg is to be returned.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 mm/memcontrol.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 5b009b233ab8..2b3864194042 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1060,8 +1060,10 @@ struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
 	rcu_read_lock();
 	do {
 		memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
-		if (unlikely(!memcg))
+		if (unlikely(!memcg)) {
 			memcg = root_mem_cgroup;
+			break;
+		}
 	} while (!css_tryget(&memcg->css));
 	rcu_read_unlock();
 	return memcg;
-- 
2.39.3