linux-kernel - Re: [PATCH 0/1] Fix vmstat_percpu incorrect subtraction after reparent

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Message-ID: <20230324171411.t2romq3ydphoonu2@blackpad>
Date:   Fri, 24 Mar 2023 18:14:11 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Cai Xinchen <caixinchen1@...wei.com>
Cc:     songmuchun@...edance.com, akpm@...ux-foundation.org,
        hannes@...xchg.org, longman@...hat.com, mhocko@...nel.org,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        cgroups@...r.kernel.org, duanxiongchun@...edance.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        yosryahmed@...gle.com, mpenttil@...hat.com
Subject: Re: [PATCH 0/1] Fix vmstat_percpu incorrect subtraction after
 reparent

On Mon, Mar 20, 2023 at 03:06:47AM +0000, Cai Xinchen <caixinchen1@...wei.com> wrote:
> There are two problems left:
> 
>      root
>      /  \
>     A    B
>    / \    \
>   C   E    D
> 
> 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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