linux-kernel - Re: [PATCH v3] mm: memcg/slab: Stop reparented obj

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Date:   Tue, 20 Oct 2020 14:49:47 +0100
From:   Richard Palethorpe <rpalethorpe@...e.de>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        "LTP List" <ltp@...ts.linux.it>, Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Christoph Lameter" <cl@...ux.com>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from
 charging root

Hello,

Richard Palethorpe <rpalethorpe@...e.de> writes:

> Hello Shakeel,
>
> Shakeel Butt <shakeelb@...gle.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.

>
>>
>> We never set root_mem_cgroup->objcg, so, no need to check for root
>
> I don't think that is relevant as we get the memcg from objcg->memcg
> which is set during reparenting. I suppose however, we can determine if
> the objcg was reparented by inspecting memcg->objcg.
>
>> here. I think checking just memcg->use_hierarchy should be sufficient.
>
> If we just check use_hierarchy then objects directly charged to the
> memcg where use_hierarchy=0 will not be uncharged. However, maybe it is
> better to check if it was reparented and if use_hierarchy=0.

-- 
Thank you,
Richard.