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From:   Richard Palethorpe <rpalethorpe@suse.de>
To:     Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>
Cc:     Roman Gushchin <guro@fb.com>, ltp@lists.linux.it,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Christoph Lameter <cl@linux.com>,
        Michal Hocko <mhocko@kernel.org>, Tejun Heo <tj@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from
 charging root
Reply-To: rpalethorpe@suse.de
In-reply-to: <20201016094702.GA95052@blackbook>
Date:   Fri, 16 Oct 2020 11:41:43 +0100
Message-ID: <87sgaesba0.fsf@suse.de>
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Hello Michal,

Michal Koutný <mkoutny@suse.com> writes:

> Hello.
>
> On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe <rpalethorpe@suse.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.