[RFC PATCH 1/3] mm: when stealing freepages, also take pages created by splitting buddy page

[Vlastimil Babka <vbabka@suse.cz>]

When __rmqueue_fallback() is called to allocate a page of order X, it will
find a page of order Y >= X of a fallback migratetype, which is different from
the desired migratetype. With the help of try_to_steal_freepages(), it may
change the migratetype (to the desired one) also of:

1) all currently free pages in the pageblock containing the fallback page
2) the fallback pageblock itself
3) buddy pages created by splitting the fallback page (when Y > X)

These decisions take the order Y into account, as well as the desired
migratetype, with the goal of preventing multiple fallback allocations that
could e.g. distribute UNMOVABLE allocations among multiple pageblocks.

Originally, decision for 1) has implied the decision for 3). Commit
47118af076f6 ("mm: mmzone: MIGRATE_CMA migration type added") changed that
(probably unintentionally) so that the buddy pages in case 3) are always
changed to the desired migratetype, except for CMA pageblocks.

Commit fef903efcf0c ("mm/page_allo.c: restructure free-page stealing code and
fix a bug") did some refactoring and added a comment that the case of 3) is
intended. Commit 0cbef29a7821 ("mm: __rmqueue_fallback() should respect
pageblock type") removed the comment and tried to restore the original behavior
where 1) implies 3), but due to the previous refactoring, the result is instead
that only 2) implies 3) - and the conditions for 2) are less frequently met
than conditions for 1). This may increase fragmentation in situations where the
code decides to steal all free pages from the pageblock (case 1)), but then
gives back the buddy pages produced by splitting.

This patch restores the original intended logic where 1) implies 3). During
testing with stress-highalloc from mmtests, this has shown to decrease the
number of events where UNMOVABLE and RECLAIMABLE allocations steal from MOVABLE
pageblocks, which can lead to permanent fragmentation. It has increased the
number of events when MOVABLE allocations steal from UNMOVABLE or RECLAIMABLE
pageblocks, but these are fixable by sync compaction and thus less harmful.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/page_alloc.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 616a2c9..548b072 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1105,12 +1105,10 @@ static int try_to_steal_freepages(struct zone *zone, struct page *page,
 
 		/* Claim the whole block if over half of it is free */
 		if (pages >= (1 << (pageblock_order-1)) ||
-				page_group_by_mobility_disabled) {
-
+				page_group_by_mobility_disabled)
 			set_pageblock_migratetype(page, start_type);
-			return start_type;
-		}
 
+		return start_type;
 	}
 
 	return fallback_type;
-- 
2.1.2

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