Re: [PATCH 3/6] mm/page_alloc: Explicitly record high-order atomic allocations in alloc_flags

[Mel Gorman <mgorman@techsingularity.net>]

First off, sorry for the long delay getting back to you. I was sick for
a few weeks and still catching up. I'm still not 100%.

On Thu, Dec 08, 2022 at 05:51:11PM +0100, Vlastimil Babka wrote:
> On 11/29/22 16:16, Mel Gorman wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index da746e9eb2cf..e2b65767dda0 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3710,7 +3710,7 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
> >  		 * reserved for high-order atomic allocation, so order-0
> >  		 * request should skip it.
> >  		 */
> > -		if (order > 0 && alloc_flags & ALLOC_HARDER)
> > +		if (alloc_flags & ALLOC_HIGHATOMIC)
> >  			page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
> >  		if (!page) {
> >  			page = __rmqueue(zone, order, migratetype, alloc_flags);
> > @@ -4028,8 +4028,10 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
> >  			return true;
> >  		}
> >  #endif
> > -		if (alloc_harder && !free_area_empty(area, MIGRATE_HIGHATOMIC))
> > +		if ((alloc_flags & ALLOC_HIGHATOMIC) &&
> > +		    !free_area_empty(area, MIGRATE_HIGHATOMIC)) {
> >  			return true;
> 
> alloc_harder is defined as
> 	(alloc_flags & (ALLOC_HARDER|ALLOC_OOM));
> AFAICS this means we no longer allow ALLOC_OOM to use the highatomic
> reserve. Isn't that a risk?
> 

Yes, it is. I intend to apply the patch below on top. I didn't alter the
first check for ALLOC_HIGHATOMIC as I wanted OOM handling to only use the
high-order reserves if there was no other option. While this is a change
in behaviour, it should be a harmless one. I'll add a note in the changelog.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 50fc1e7cb154..0ef4f3236a5a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3710,6 +3710,16 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
 			page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
 		if (!page) {
 			page = __rmqueue(zone, order, migratetype, alloc_flags);
+
+			/*
+			 * If the allocation fails, allow OOM handling access
+			 * to HIGHATOMIC reserves as failing now is worse than
+			 * failing a high-order atomic allocation in the
+			 * future.
+			 */
+			if (!page && (alloc_flags & ALLOC_OOM))
+				page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
+
 			if (!page) {
 				spin_unlock_irqrestore(&zone->lock, flags);
 				return NULL;
@@ -4023,7 +4033,7 @@ bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 			return true;
 		}
 #endif
-		if ((alloc_flags & ALLOC_HIGHATOMIC) &&
+		if ((alloc_flags & (ALLOC_HIGHATOMIC|ALLOC_OOM)) &&
 		    !free_area_empty(area, MIGRATE_HIGHATOMIC)) {
 			return true;
 		}
-- 
Mel Gorman
SUSE Labs