Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

Huang, Ying
2022-10-27T06:05:19Z

Feng Tang <feng.tang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> writes:

> On Thu, Oct 27, 2022 at 01:13:30PM +0800, Huang, Ying wrote:
>> Feng Tang <feng.tang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> writes:
>>
>> > In page reclaim path, memory could be demoted from faster memory tier
>> > to slower memory tier. Currently, there is no check about cpuset's
>> > memory policy, that even if the target demotion node is not allowd
>> > by cpuset, the demotion will still happen, which breaks the cpuset
>> > semantics.
>> >
>> > So add cpuset policy check in the demotion path and skip demotion
>> > if the demotion targets are not allowed by cpuset.
>> >
>> > Signed-off-by: Feng Tang <feng.tang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
>> > ---
> [...]
>> > index 18f6497994ec..c205d98283bc 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1537,9 +1537,21 @@ static struct page *alloc_demote_page(struct page *page, unsigned long private)
>> > {
>> > struct page *target_page;
>> > nodemask_t *allowed_mask;
>> > - struct migration_target_control *mtc;
>> > + struct migration_target_control *mtc = (void *)private;
>> >
>> > - mtc = (struct migration_target_control *)private;
>>
>> I think we should avoid (void *) conversion here.
>
> OK, will change back.
>
>> > +#if IS_ENABLED(CONFIG_MEMCG) && IS_ENABLED(CONFIG_CPUSETS)
>> > + struct mem_cgroup *memcg;
>> > + nodemask_t cpuset_nmask;
>> > +
>> > + memcg = page_memcg(page);
>> > + cpuset_get_allowed_mem_nodes(memcg->css.cgroup, &cpuset_nmask);
>> > +
>> > + if (!node_isset(mtc->nid, cpuset_nmask)) {
>> > + if (mtc->nmask)
>> > + nodes_and(*mtc->nmask, *mtc->nmask, cpuset_nmask);
>> > + return alloc_migration_target(page, (unsigned long)mtc);
>> > + }
>>
>> If node_isset(mtc->nid, cpuset_nmask) == true, we should keep the
>> original 2 steps allocation and apply nodes_and() on node mask.
>
> Good catch! Yes, the nodes_and() call should be taken out from this
> check and done before calling node_isset().
>
>> > +#endif
>> >
>> > allowed_mask = mtc->nmask;
>> > /*
>> > @@ -1649,6 +1661,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>> > enum folio_references references = FOLIOREF_RECLAIM;
>> > bool dirty, writeback;
>> > unsigned int nr_pages;
>> > + bool skip_this_demotion = false;
>> >
>> > cond_resched();
>> >
>> > @@ -1658,6 +1671,22 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>> > if (!folio_trylock(folio))
>> > goto keep;
>> >
>> > +#if IS_ENABLED(CONFIG_MEMCG) && IS_ENABLED(CONFIG_CPUSETS)
>> > + if (do_demote_pass) {
>> > + struct mem_cgroup *memcg;
>> > + nodemask_t nmask, nmask1;
>> > +
>> > + node_get_allowed_targets(pgdat, &nmask);
>>
>> pgdat will not change in the loop, so we can move this out of the loop?
>
> Yes
>
>> > + memcg = folio_memcg(folio);
>> > + if (memcg)
>> > + cpuset_get_allowed_mem_nodes(memcg->css.cgroup,
>> > + &nmask1);
>> > +
>> > + if (!nodes_intersects(nmask, nmask1))
>> > + skip_this_demotion = true;
>> > + }
>>
>> If nodes_intersects() == true, we will call
>> cpuset_get_allowed_mem_nodes() twice. Better to pass the intersecting
>> mask to demote_folio_list()?
>
> The pages in the loop may come from different mem control group, and
> the cpuset's nodemask could be different, I don't know how to save
> this per-page info to be used later in demote_folio_list.

Yes. You are right. We cannot do that.

Best Regards,
Huang, Ying

>
>> > +#endif
>> > +
>> > VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
>> >
>> > nr_pages = folio_nr_pages(folio);
>> > @@ -1799,7 +1828,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>> > * Before reclaiming the folio, try to relocate
>> > * its contents to another node.
>> > */
>> > - if (do_demote_pass &&
>> > + if (do_demote_pass && !skip_this_demotion &&
>> > (thp_migration_supported() || !folio_test_large(folio))) {
>> > list_add(&folio->lru, &demote_folios);
>> > folio_unlock(folio);
>>
>> Best Regards,
>> Huang, Ying