Re: [PATCH v6 2/6] memcontrol: allows mem_cgroup_iter() to check for onlineness

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Nov 28, 2023 at 1:38 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 27-11-23 11:36:59, Nhat Pham wrote:
> > The new zswap writeback scheme requires an online-only memcg hierarchy
> > traversal. Add a new parameter to mem_cgroup_iter() to check for
> > onlineness before returning.
>
> Why is this needed?

For context, in patch 3 of this series, Domenico and I are adding
cgroup-aware LRU to zswap, so that we can perform workload-specific
zswap writeback. When the reclaim happens due to the global zswap
limit being hit, a cgroup is selected by the mem_cgroup_iter(), and
the last one selected is saved in the zswap pool (so that the
iteration can follow from there next time the limit is hit).

However, one problem with this scheme is we will be pinning the
reference to that saved memcg until the next global reclaim attempt,
which could prevent it from being killed for quite some time after it
has been offlined. Johannes, Yosry, and I discussed a couple of
approaches for a while, and decided to add a callback that would
release the reference held by the zswap pool when the memcg is
offlined, and the zswap pool will obtain the reference to the next
online memcg in the traversal (or at least one that has not had the
zswap-memcg-release-callback run on it yet).

> --
> Michal Hocko
> SUSE Labs