Re: [PATCH 23/24] swap: fix multiple swap leak when after cgroup migrate

[Chris Li <chrisl@kernel.org>]

Hi Kairui,

On Mon, Nov 20, 2023 at 3:17 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> Huang, Ying <ying.huang@intel.com> 于2023年11月20日周一 15:37写道：
> >
> > 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).
>
> Hi Ying, thank you very much for the info!
>
> It is controversial indeed, just the original behavior is kind of
> counter-intuitive.
>
> Image if there are cgroup P1, and its child cgroup C1 C2. If a process
> swapped out some memory in C1 then moved to C2, and C1 is dead.
> On swapoff the charge will be moved out of P1...
>
> And swapoff often happen on some unlimited cgroup or some cgroup for
> management agent.
>
> If P1 have a memory limit, it can breech the limit easily, we will see
> a process that never leave P1 having a much higher RSS that P1/C1/C2's
> limit.
> And if there is a limit for the management agent cgroup, the agent
> will be OOM instead of OOM in P1.

I think I will reply to another similar email.

If you want OOM in P1, you can have an admin program. fork and execute
a new process, add the new process into P1, then swap off from that
new process.

>
> Simply moving a process between the child cgroup of the same parent
> cgroup won't cause such issue, thing get weird when swapoff is
> involved.
>
> Or maybe we should try to be compatible, and introduce a sysctl or
> cmdline for this?

If the above suggestion works, then you don't need to change swap off?

If you still want to change the charging model. I like to see the
bigger picture, what rules it follows and how it works in other
situations.

Chris