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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Kairui Song <ryncsn@gmail.com>
Cc:     linux-mm@kvack.org, Kairui Song <kasong@tencent.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 23/24] swap: fix multiple swap leak when after cgroup
 migrate
In-Reply-To: <20231119194740.94101-24-ryncsn@gmail.com> (Kairui Song's message
        of "Mon, 20 Nov 2023 03:47:39 +0800")
References: <20231119194740.94101-1-ryncsn@gmail.com>
        <20231119194740.94101-24-ryncsn@gmail.com>
Date:   Mon, 20 Nov 2023 15:35:05 +0800
Message-ID: <87msv8c1xy.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Kairui Song <ryncsn@gmail.com> writes:

> From: Kairui Song <kasong@tencent.com>
>
> When a process which previously swapped some memory was moved to
> another cgroup, and the cgroup it previous in is dead, then swapped in
> pages will be leaked into rootcg. Previous commits fixed the bug for
> no readahead path, this commit fix the same issue for readahead path.
>
> This can be easily reproduced by:
> - Setup a SSD or HDD swap.
> - Create memory cgroup A, B and C.
> - Spawn process P1 in cgroup A and make it swap out some pages.
> - Move process P1 to memory cgroup B.
> - Destroy cgroup A.
> - Do a swapoff in cgroup C
> - Swapped in pages is accounted into cgroup C.
>
> This patch will fix it make the swapped in pages accounted in cgroup B.

Accroding to "Memory Ownership" section of
Documentation/admin-guide/cgroup-v2.rst,

"
A memory area is charged to the cgroup which instantiated it and stays
charged to the cgroup until the area is released.  Migrating a process
to a different cgroup doesn't move the memory usages that it
instantiated while in the previous cgroup to the new cgroup.
"

Because we don't move the charge when we move a task from one cgroup to
another.  It's controversial which cgroup should be charged to.
According to the above document, it's acceptable to charge to the cgroup
C (cgroup where swapoff happens).

--
Best Regards,
Huang, Ying