[PATCH 2/6] mm: page_alloc: fix up block types when merging compatible blocks

[Johannes Weiner]

The buddy allocator coalesces compatible blocks during freeing, but it
doesn't update the types of the subblocks to match. When an allocation
later breaks the chunk down again, its pieces will be put on freelists
of the wrong type. This encourages incompatible page mixing (ask for
one type, get another), and thus long-term fragmentation.

Update the subblocks when merging a larger chunk, such that a later
expand() will maintain freelist type hygiene.

v2:
- remove spurious change_pageblock_range() move (Zi Yan)

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/page_alloc.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e3f1c777feed..3db405414174 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -783,10 +783,17 @@ static inline void __free_one_page(struct page *page,
 			 */
 			int buddy_mt = get_pfnblock_migratetype(buddy, buddy_pfn);
 
-			if (migratetype != buddy_mt
-					&& (!migratetype_is_mergeable(migratetype) ||
-						!migratetype_is_mergeable(buddy_mt)))
-				goto done_merging;
+			if (migratetype != buddy_mt) {
+				if (!migratetype_is_mergeable(migratetype) ||
+				    !migratetype_is_mergeable(buddy_mt))
+					goto done_merging;
+				/*
+				 * Match buddy type. This ensures that
+				 * an expand() down the line puts the
+				 * sub-blocks on the right freelists.
+				 */
+				set_pageblock_migratetype(buddy, migratetype);
+			}
 		}
 
 		/*
-- 
2.42.0