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

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Message-ID: <20201029170955.GI827280@carbon.dhcp.thefacebook.com>
Date:   Thu, 29 Oct 2020 10:09:55 -0700
From:   Roman Gushchin <guro@...com>
To:     Shakeel Butt <shakeelb@...gle.com>
CC:     Muchun Song <songmuchun@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 Thu, Oct 29, 2020 at 08:48:45AM -0700, Shakeel Butt wrote:
> 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?

Yes. I'll just change the for loop to include the root_mem_cgroup.

Thanks!