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

[Shakeel Butt <shakeelb@google.com>]

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?

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