Re: [RFC PATCH 1/3] zram: charge the compressed RAM to the page's memcgroup

[Fabian Deutsch]

On Thu, Jun 15, 2023 at 6:59 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> On Wed, Jun 14, 2023 at 9:48 PM Zhongkun He
> <hezhongkun.hzk@xxxxxxxxxxxxx> wrote:
> >
> > The compressed RAM is currently charged to kernel, not to
> > any memory cgroup, which is not satisfy our usage scenario.
> > if the memory of a task is limited by memcgroup, it will
> > swap out the memory to zram swap device when the memory
> > is insufficient. In that case, the memory limit will have
> > no effect.
> >
> > So, it should makes sense to charge the compressed RAM to
> > the page's memory cgroup.

While looking at this in the past weeks, I believe that there are two
distinct problems:
1. Direct zram usage by process within a cg ie. a process writing to a
zram device
2. Indirect zram usage by a process within a cg via swap (described above)

Both of them probably require different solutions.
In order to fix #1, accounting a zram device should be accounted
towards a cgroup. IMHO this is something that should be fixed.

Yu Zhao and Yosry are probably much more familiar with the solution to #2.
WRT per-cgrou-swapfile, to me this is addressing #2, but I agree with
Yu Zhao, that there are probably better solutions to this.

Lastly, this patchset, while it will possibly not address the swap
issue (#2) completely, is it satisfying the needs of #1?

- fabian

> We used to do this a long time ago, but we had per-memcg swapfiles [1[
> to prevent compressed pages from different memcgs from sharing the
> same zspage.
>
> Does this patchset alone suffer from the same problem, i.e., memcgs
> sharing zspages?
>
> [1] https://lwn.net/Articles/592923/
>