Re: [PATCH RFC 06/37] mm: page_alloc: Allocate from movable pcp lists only if ALLOC_FROM_METADATA

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Thu, Oct 12, 2023 at 10:25:11AM +0900, Hyesoo Yu wrote:
> On Wed, Aug 23, 2023 at 02:13:19PM +0100, Alexandru Elisei wrote:
> > pcp lists keep MIGRATE_METADATA pages on the MIGRATE_MOVABLE list. Make
> > sure pages from the movable list are allocated only when the
> > ALLOC_FROM_METADATA alloc flag is set, as otherwise the page allocator
> > could end up allocating a metadata page when that page cannot be used.
> > 
> > __alloc_pages_bulk() sidesteps rmqueue() and calls __rmqueue_pcplist()
> > directly. Add a check for the flag before calling __rmqueue_pcplist(), and
> > fallback to __alloc_pages() if the check is false.
> > 
> > Note that CMA isn't a problem for __alloc_pages_bulk(): an allocation can
> > always use CMA pages if the requested migratetype is MIGRATE_MOVABLE, which
> > is not the case with MIGRATE_METADATA pages.
> > 
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> >  mm/page_alloc.c | 21 +++++++++++++++++----
> >  1 file changed, 17 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 829134a4dfa8..a693e23c4733 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2845,11 +2845,16 @@ struct page *rmqueue(struct zone *preferred_zone,
> >  
> >  	if (likely(pcp_allowed_order(order))) {
> >  		/*
> > -		 * MIGRATE_MOVABLE pcplist could have the pages on CMA area and
> > -		 * we need to skip it when CMA area isn't allowed.
> > +		 * PCP lists keep MIGRATE_CMA/MIGRATE_METADATA pages on the same
> > +		 * movable list. Make sure it's allowed to allocate both type of
> > +		 * pages before allocating from the movable list.
> >  		 */
> > -		if (!IS_ENABLED(CONFIG_CMA) || alloc_flags & ALLOC_CMA ||
> > -				migratetype != MIGRATE_MOVABLE) {
> > +		bool movable_allowed = (!IS_ENABLED(CONFIG_CMA) ||
> > +					(alloc_flags & ALLOC_CMA)) &&
> > +				       (!IS_ENABLED(CONFIG_MEMORY_METADATA) ||
> > +					(alloc_flags & ALLOC_FROM_METADATA));
> > +
> > +		if (migratetype != MIGRATE_MOVABLE || movable_allowed) {
> 
> Hi!
> 
> I don't think it would be effcient when the majority of movable pages
> do not use GFP_TAGGED.
> 
> Metadata pages have a low probability of being in the pcp list
> because metadata pages is bypassed when freeing pages.
> 
> The allocation performance of most movable pages is likely to decrease
> if only the request with ALLOC_FROM_METADATA could be allocated.

You're right, I hadn't considered that.

> 
> How about not including metadata pages in the pcp list at all ?

Sounds reasonable, I will keep it in mind for the next iteration of the
series.

Thanks,
Alex

> 
> Thanks,
> Hyesoo Yu.
> 
> >  			page = rmqueue_pcplist(preferred_zone, zone, order,
> >  					migratetype, alloc_flags);
> >  			if (likely(page))
> > @@ -4388,6 +4393,14 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
> >  		goto out;
> >  	gfp = alloc_gfp;
> >  
> > +	/*
> > +	 * pcp lists puts MIGRATE_METADATA on the MIGRATE_MOVABLE list, don't
> > +	 * use pcp if allocating metadata pages is not allowed.
> > +	 */
> > +	if (metadata_storage_enabled() && ac.migratetype == MIGRATE_MOVABLE &&
> > +	    !(alloc_flags & ALLOC_FROM_METADATA))
> > +		goto failed;
> > +
> >  	/* Find an allowed local zone that meets the low watermark. */
> >  	for_each_zone_zonelist_nodemask(zone, z, ac.zonelist, ac.highest_zoneidx, ac.nodemask) {
> >  		unsigned long mark;
> > -- 
> > 2.41.0
> > 
> >