From 2884b6b7eed4fc14c0630fb16e56a4c66c786d33 Mon Sep 17 00:00:00 2001 From: Muchun Song Date: Mon, 28 Jun 2021 19:37:50 -0700 Subject: [PATCH] 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 Acked-by: Johannes Weiner Reviewed-by: Shakeel Butt Acked-by: Roman Gushchin Acked-by: Michal Hocko Cc: Vladimir Davydov Cc: Xiongchun Duan Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 239f69ed1ac1d..babbaf49ee368 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; -- 2.30.2