Re: [PATCH v3] mm: don't be stuck to rmap lock on reclaim path

[Minchan Kim]

On Tue, May 10, 2022 at 02:16:07PM -0400, Johannes Weiner wrote:
> Hi Minchan,
> 
> The patch looks reasonable to me, but one thing stands out:
> 
> On Tue, May 10, 2022 at 10:11:00AM -0700, Minchan Kim wrote:
> > @@ -1391,6 +1391,10 @@ static enum page_references folio_check_references(struct folio *folio,
> >  	if (vm_flags & VM_LOCKED)
> >  		return PAGEREF_ACTIVATE;
> >  
> > +	/* page_referenced didn't work due to lock contention */
> > +	if (referenced_ptes == -1)
> > +		return PAGEREF_KEEP;
> > +
> >  	if (referenced_ptes) {
> >  		/*
> >  		 * All mapped folios start out with page table
> 
> This means contended inactive pages get rotated.
> 
> > @@ -2492,7 +2496,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
> >  		}
> >  
> >  		if (folio_referenced(folio, 0, sc->target_mem_cgroup,
> > -				     &vm_flags)) {
> > +				     &vm_flags) > 0) {
> >  			/*
> >  			 * Identify referenced, file-backed active pages and
> >  			 * give them one more trip around the active list. So
> 
> This means contended active pages do NOT get rotated.
> 
> It's a bit of an arbitrary choice what to do with reclaim candidates
> for which you can get no reference information, but staying consistent
> is likely a good idea. My preference would be to just rotate contended
> pages throughout rather than risk dropping the workingset on the floor.
> 
> 		/* Referenced or rmap lock contention: rotate */
> 		if (folio_referenced() != 0) {
> 			...
> 
> What do you think?

Sure, it's good start before adding more heuristic later.

Thanks for the review. I will post v4.