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

["Huang, Ying" <ying.huang@intel.com>]

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