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

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Message-ID: <87sgaesba0.fsf@suse.de>
Date:   Fri, 16 Oct 2020 11:41:43 +0100
From:   Richard Palethorpe <rpalethorpe@...e.de>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Roman Gushchin <guro@...com>, ltp@...ts.linux.it,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from
 charging root

Hello Michal,

Michal Koutný <mkoutny@...e.com> writes:

> Hello.
>
> On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe <rpalethorpe@...e.com> wrote:
>> SLAB objects which outlive their memcg are moved to their parent
>> memcg where they may be uncharged. However if they are moved to the
>> root memcg, uncharging will result in negative page counter values as
>> root has no page counters.
> Why do you think those are reparented objects? If those are originally
> charged in a non-root cgroup, then the charge value should be propagated up the
> hierarchy, including root memcg, so if they're later uncharged in root
> after reparenting, it should still break even. (Or did I miss some stock
> imbalance?)

I traced it and can see they are reparented objects and that the root
groups counters are zero (or negative if I run madvise06 multiple times)
before a drain takes place. I'm guessing this is because the root group
has 'use_hierachy' set to false so that the childs page_counter parents
are set to NULL. However I will check, because I'm not sure about
either.

>
> (But the patch seems justifiable to me as objects (not)charged directly to
> root memcg may be incorrectly uncharged.)
>
> Thanks,
> Michal


-- 
Thank you,
Richard.