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

[Roman Gushchin <guro@fb.com>]

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.

Thanks!