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

[Michal Hocko <mhocko@suse.com>]

On Tue 28-11-23 08:53:56, Nhat Pham wrote:
> 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).

This should be a part of the changelog along with an explanation why
this cannot be handled on the caller level? You have a pin on the memcg,
you can check it is online and scratch it if not, right? Why do we need
to make a rather convoluted iterator interface more complex when most
users simply do not require that?

-- 
Michal Hocko
SUSE Labs