[RFC PATCH 3/3] mm: always steal split buddies in fallback allocations

[Vlastimil Babka]

When allocation falls back to another migratetype, it will steal a page with
highest available order, and (depending on this order and desired migratetype),
it might also steal the rest of free pages from the same pageblock.

Given the preference of highest available order, it is likely that it will be
higher than the desired order, and result in the stolen buddy page being split.
The remaining pages after split are currently stolen only when the rest of the
free pages are stolen. This can however lead to situations where for MOVABLE
allocations we split e.g. order-4 fallback UNMOVABLE page, but steal only
order-0 page. Then on the next MOVABLE allocation (which may be batched to
fill the pcplists) we split another order-3 or higher page, etc. By stealing
all pages that we have split, we can avoid further stealing.

This patch therefore adjust the page stealing so that buddy pages created by
split are always stolen. This has effect only on MOVABLE allocations, as
RECLAIMABLE and UNMOVABLE allocations already always do that in addition to
stealing the rest of free pages from the pageblock.

Note that commit 7118af076f6 ("mm: mmzone: MIGRATE_CMA migration type added")
has already performed this change (unintentinally), but was reverted by commit
0cbef29a7821 ("mm: __rmqueue_fallback() should respect pageblock type").
Neither included evaluation. My evaluation with stress-highalloc from mmtests
shows about 2.5x reduction of page stealing events for MOVABLE allocations,
without affecting the page stealing events for other allocation migratetypes.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a14249c..82096a6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1108,11 +1108,9 @@ static int try_to_steal_freepages(struct zone *zone, struct page *page,
 		if (pages >= (1 << (pageblock_order-1)) ||
 				page_group_by_mobility_disabled)
 			set_pageblock_migratetype(page, start_type);
-
-		return start_type;
 	}
 
-	return fallback_type;
+	return start_type;
 }
 
 /* Remove an element from the buddy allocator from the fallback list */
-- 
2.1.2

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