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.
Link: https://lkml.kernel.org/r/20210417043538.9793-3-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 239f69e..babbaf4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -919,20 +919,23 @@ 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;
