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

[Feng Tang]

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

Thanks,
Feng

> > +#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