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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 20 Oct 2020 09:56:51 -0700
Message-ID: <CALvZod45mAzyo9LNR4YtX_3J0gZJDagYTNv8NbJAuXzwK5A2DA@mail.gmail.com>
Subject: Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from
 charging root
To:     rpalethorpe@suse.de
Cc:     Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        LTP List <ltp@lists.linux.it>, Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Michal Hocko <mhocko@kernel.org>, Tejun Heo <tj@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>
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On Tue, Oct 20, 2020 at 6:49 AM Richard Palethorpe <rpalethorpe@suse.de> wrote:
>
> Hello,
>
> Richard Palethorpe <rpalethorpe@suse.de> writes:
>
> > Hello Shakeel,
> >
> > Shakeel Butt <shakeelb@google.com> 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.