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!
