linux-kernel - [PATCH v3 2/8] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from

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Message-Id: <20210417043538.9793-3-songmuchun@bytedance.com>
Date:   Sat, 17 Apr 2021 12:35:32 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     guro@...com, hannes@...xchg.org, mhocko@...nel.org,
        akpm@...ux-foundation.org, shakeelb@...gle.com,
        vdavydov.dev@...il.com
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...e.com>
Subject: [PATCH v3 2/8] mm: memcontrol: bail out early when !mm in get_mem_cgroup_from_mm

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@...edance.com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
Acked-by: Roman Gushchin <guro@...com>
Acked-by: Michal Hocko <mhocko@...e.com>
---
 mm/memcontrol.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f229de925aa5..50e3cf1e263e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -901,20 +901,23 @@ 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.
+	 *
+	 * No need to css_get on root memcg as the reference
+	 * counting is disabled on the root level in the
+	 * cgroup core. See CSS_NO_REF.
+	 */
+	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