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From:   Chris Li <chrisl@kernel.org>
Date:   Mon, 20 Nov 2023 12:22:40 -0800
Message-ID: <CAF8kJuNWj_2tQA+0D-hpQgeJwWZRaQWxOCF=qagFnA8+yb3R1Q@mail.gmail.com>
Subject: Re: [PATCH 00/24] Swapin path refactor for optimization and bugfix
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Kairui Song <kasong@tencent.com>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Huang, Ying" <ying.huang@intel.com>,
        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
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Hi Kairui,

On Mon, Nov 20, 2023 at 11:10=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com=
> wrote:
>
> On Sun, Nov 19, 2023 at 11:48=E2=80=AFAM Kairui Song <ryncsn@gmail.com> w=
rote:
> >
> > From: Kairui Song <kasong@tencent.com>
> >
> > This series tries to unify and clean up the swapin path, fixing a few
> > issues with optimizations:
> >
> > 1. Memcg leak issue: when a process that previously swapped out some
> >    migrated to another cgroup, and the origianl cgroup is dead. If we
> >    do a swapoff, swapped in pages will be accounted into the process
> >    doing swapoff instead of the new cgroup. This will allow the process
> >    to use more memory than expect easily.
> >
> >    This can be easily reproduced by:
> >    - Setup a 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.
> >
>
> I guess this only works for anonymous memory and not shmem, right?
>
> I think tying memcg charges to a process is not something we usually
> do. Charging the pages to the memcg of the faulting process if the
> previous owner is dead makes sense, it's essentially recharging the
> memory to the new owner. Swapoff is indeed a special case, since the
> faulting process is not the new owner, but an admin process or so. I
> am guessing charging to the new memcg of the previous owner might make
> sense in this case, but it is a change of behavior.
>

I was looking at this at patch 23 as well. Will ask more questions in
the patch thread.
I would suggest making these two behavior change patches separate out
from the clean up series to give it more exposure and proper
discussion.
Patch 5 and patch 23.

Chris