Re: [PATCH v3 3/3] mm: vmscan: ignore non-LRU-based reclaim in memcg reclaim

[Yosry Ahmed]

On Fri, Mar 31, 2023 at 12:25 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> On Fri, Mar 31, 2023 at 1:08 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> ...
>
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index a3e38851b34ac..bf9d8e175e92a 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -533,7 +533,35 @@ EXPORT_SYMBOL(mm_account_reclaimed_pages);
> >  static void flush_reclaim_state(struct scan_control *sc,
> >                                 struct reclaim_state *rs)
> >  {
> > -       if (rs) {
> > +       /*
> > +        * Currently, reclaim_state->reclaimed includes three types of pages
> > +        * freed outside of vmscan:
> > +        * (1) Slab pages.
> > +        * (2) Clean file pages from pruned inodes.
> > +        * (3) XFS freed buffer pages.
> > +        *
> > +        * For all of these cases, we have no way of finding out whether these
> > +        * pages were related to the memcg under reclaim. For example, a freed
> > +        * slab page could have had only a single object charged to the memcg
> > +        * under reclaim. Also, populated inodes are not on shrinker LRUs
> > +        * anymore except on highmem systems.
> > +        *
> > +        * Instead of over-reporting the reclaimed pages in a memcg reclaim,
> > +        * only count such pages in system-wide reclaim. This prevents
> > +        * unnecessary retries during memcg charging and false positive from
> > +        * proactive reclaim (memory.reclaim).
>
> What happens when writing to the root memory.reclaim?
>
> > +        *
> > +        * For uncommon cases were the freed pages were actually significantly
> > +        * charged to the memcg under reclaim, and we end up under-reporting, it
> > +        * should be fine. The freed pages will be uncharged anyway, even if
> > +        * they are not reported properly, and we will be able to make forward
> > +        * progress in charging (which is usually in a retry loop).
> > +        *
> > +        * We can go one step further, and report the uncharged objcg pages in
> > +        * memcg reclaim, to make reporting more accurate and reduce
> > +        * under-reporting, but it's probably not worth the complexity for now.
> > +        */
> > +       if (rs && !cgroup_reclaim(sc)) {
>
> To answer the question above, global_reclaim() would be preferred.

Great point, global_reclaim() is fairly recent. I didn't see it
before. Thanks for pointing it out. I will change it for v4 -- will
wait for more feedback before respinning.

Thanks Yu!