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Date:   Fri, 24 Mar 2023 18:14:11 +0100
From:   Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>
To:     Cai Xinchen <caixinchen1@huawei.com>
Cc:     songmuchun@bytedance.com, akpm@linux-foundation.org,
        hannes@cmpxchg.org, longman@redhat.com, mhocko@kernel.org,
        roman.gushchin@linux.dev, shakeelb@google.com,
        cgroups@vger.kernel.org, duanxiongchun@bytedance.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        yosryahmed@google.com, mpenttil@redhat.com
Subject: Re: [PATCH 0/1] Fix vmstat_percpu incorrect subtraction after
 reparent
Message-ID: <20230324171411.t2romq3ydphoonu2@blackpad>
References: <20230320030648.50663-1-caixinchen1@huawei.com>
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On Mon, Mar 20, 2023 at 03:06:47AM +0000, Cai Xinchen <caixinchen1@huawei.c=
om> wrote:
> There are two problems left:
>=20
>      root
>      /  \
>     A    B
>    / \    \
>   C   E    D
>=20
> 1. In some case of reparent, some page cache may be used by other memcg
> D but it charges to the parent memcg A of dying memcg E. D is getting
> away with using the page for free while A is taxed.

Note that A is (effectively) taxed even before E is removed due to
hierarchical nature of charging. Then what you describe transforms into
"well-known" problem of shared charging (with not well-known solution
:-/).

HTH,
Michal

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