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

["Huang, Ying" <ying.huang@intel.com>]

Feng Tang <feng.tang@intel.com> writes:

> On Thu, Oct 27, 2022 at 01:13:30PM +0800, Huang, Ying wrote:
>> Feng Tang <feng.tang@intel.com> 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@intel.com>
>> > ---
> [...]
>> > 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