Commit 2884b6b7 authored Jun 28, 2021 by Muchun Song Committed by Linus Torvalds Jun 29, 2021

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>

parent 8dc87c7d

mm/memcontrol.c

+14 −11

Original line number	Diff line number	Diff line
		@@ -919,20 +919,23 @@ struct mem_cgroup get_mem_cgroup_from_mm(struct mm_struct mm)
		if (mem_cgroup_disabled())
		return NULL;

		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.
		*
		* 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))
		memcg = root_mem_cgroup;
		else {
		return root_mem_cgroup;

		rcu_read_lock();
		do {
		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;