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

[Shakeel Butt <shakeelb@google.com>]

On Thu, Oct 29, 2020 at 10:10 AM Roman Gushchin <guro@fb.com> wrote:
>
> 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@bytedance.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@bytedance.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.
>

Then do we really need this patch since it's not tagged for stable?