Re: [PATCH v2] mm/vmscan: check references from all memcgs for swapbacked memory

[Yu Zhao <yuzhao@google.com>]

On Wed, Oct 5, 2022 at 3:13 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Wed, Oct 5, 2022 at 3:02 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Wed, Oct 5, 2022 at 1:48 PM Yu Zhao <yuzhao@google.com> wrote:
> > >
> > > On Wed, Oct 5, 2022 at 11:37 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> > > >
> > > > During page/folio reclaim, we check if a folio is referenced using
> > > > folio_referenced() to avoid reclaiming folios that have been recently
> > > > accessed (hot memory). The rationale is that this memory is likely to be
> > > > accessed soon, and hence reclaiming it will cause a refault.
> > > >
> > > > For memcg reclaim, we currently only check accesses to the folio from
> > > > processes in the subtree of the target memcg. This behavior was
> > > > originally introduced by commit bed7161a519a ("Memory controller: make
> > > > page_referenced() cgroup aware") a long time ago. Back then, refaulted
> > > > pages would get charged to the memcg of the process that was faulting them
> > > > in. It made sense to only consider accesses coming from processes in the
> > > > subtree of target_mem_cgroup. If a page was charged to memcg A but only
> > > > being accessed by a sibling memcg B, we would reclaim it if memcg A is
> > > > is the reclaim target. memcg B can then fault it back in and get charged
> > > > for it appropriately.
> > > >
> > > > Today, this behavior still makes sense for file pages. However, unlike
> > > > file pages, when swapbacked pages are refaulted they are charged to the
> > > > memcg that was originally charged for them during swapping out. Which
> > > > means that if a swapbacked page is charged to memcg A but only used by
> > > > memcg B, and we reclaim it from memcg A, it would simply be faulted back
> > > > in and charged again to memcg A once memcg B accesses it. In that sense,
> > > > accesses from all memcgs matter equally when considering if a swapbacked
> > > > page/folio is a viable reclaim target.

I just read the entire commit message (sorry for not doing so
previously) to figure out where the confusion came from: the above
claim is wrong for two cases. I'll let you figure out why :)

> > > > Modify folio_referenced() to always consider accesses from all memcgs if
> > > > the folio is swapbacked.
> > >
> > > It seems to me this change can potentially increase the number of
> > > zombie memcgs. Any risk assessment done on this?
> >
> > Do you mind elaborating the case(s) where this could happen? Is this
> > the cgroup v1 case in mem_cgroup_swapout() where we are reclaiming
> > from a zombie memcg and swapping out would let us move the charge to
> > the parent?
>
> The scenario is quite straightforward: for a page charged to memcg A
> and also actively used by memcg B, if we don't ignore the access from
> memcg B, we won't be able to reclaim it after memcg A is deleted.