Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from charging root

[Shakeel Butt]

On Tue, Oct 20, 2020 at 6:49 AM Richard Palethorpe <rpalethorpe@xxxxxxx> wrote:
>
> Hello,
>
> Richard Palethorpe <rpalethorpe@xxxxxxx> writes:
>
> > Hello Shakeel,
> >
> > Shakeel Butt <shakeelb@xxxxxxxxxx> writes:
> >>>
> >>> V3: Handle common case where use_hierarchy=1 and update description.
> >>>
> >>>  mm/memcontrol.c | 7 +++++--
> >>>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >>> index 6877c765b8d0..34b8c4a66853 100644
> >>> --- a/mm/memcontrol.c
> >>> +++ b/mm/memcontrol.c
> >>> @@ -291,7 +291,7 @@ static void obj_cgroup_release(struct percpu_ref *ref)
> >>>
> >>>         spin_lock_irqsave(&css_set_lock, flags);
> >>>         memcg = obj_cgroup_memcg(objcg);
> >>> -       if (nr_pages)
> >>> +       if (nr_pages && (!mem_cgroup_is_root(memcg) || memcg->use_hierarchy))
> >>
> >> If we have non-root memcg with use_hierarchy as 0 and this objcg was
> >> reparented then this __memcg_kmem_uncharge() can potentially underflow
> >> the page counter and give the same warning.
> >
> > Yes, although the kernel considers such a config to be broken, and
> > prints a warning to the log, it does allow it.
>
> Actually this can not happen because if use_hierarchy=0 then the objcg
> will be reparented to root.
>

Yup, you are right. I do wonder if we should completely deprecate
use_hierarchy=0.