linux-kernel - Re: [PATCH v2] mm: memcg/slab: Fix return child memcg objcg for root memcg

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Message-ID: <CALvZod6p_y2fTEK5fzAL=JfPsguqYbttgWC4_GPc=rF1PsN6TQ@mail.gmail.com>
Date:   Thu, 29 Oct 2020 08:48:45 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Yafang Shao <laoar.shao@...il.com>,
        Chris Down <chris@...isdown.name>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>, esyr@...hat.com,
        Suren Baghdasaryan <surenb@...gle.com>, areber@...hat.com,
        Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>, Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm: memcg/slab: Fix return child memcg objcg for root memcg

On Tue, Oct 27, 2020 at 8:50 PM Muchun Song <songmuchun@...edance.com> wrote:
>
> Consider the following memcg hierarchy.
>
>                     root
>                    /    \
>                   A      B
>
> If we get the objcg of memcg A failed,

Please fix the above statement.

> the get_obj_cgroup_from_current
> can return the wrong objcg for the root memcg.
>
> Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  changelog in v2:
>  1. Do not use a comparison with the root_mem_cgroup
>
>  mm/memcontrol.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1337775b04f3..8c8b4c3ed5a0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2961,6 +2961,7 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
>                 objcg = rcu_dereference(memcg->objcg);
>                 if (objcg && obj_cgroup_tryget(objcg))
>                         break;
> +               objcg = NULL;

Roman, in your cleanup, are you planning to have objcg for root memcg as well?

>         }
>         rcu_read_unlock();
>
> --
> 2.20.1
>