Due to the region-wise ordering of the pages in the buddy allocator's
free lists, whenever we want to delete a free pageblock from a free list
(for ex: when moving blocks of pages from one list to the other), we need
to be able to tell the buddy allocator exactly which migratetype it belongs
to. For that purpose, we can use the page's freepage migratetype (which is
maintained in the page's ->index field).

So, while splitting up higher order pages into smaller ones as part of buddy
operations, keep the new head pages updated with the correct freepage
migratetype information (because we depend on tracking this info accurately,
as outlined above).

Signed-off-by: Srivatsa S. Bhat <redacted>
---

 mm/page_alloc.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d48eb04..e31daf4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -950,6 +950,13 @@ static inline void expand(struct zone *zone, struct page *page,
 		add_to_freelist(&page[size], &area->free_list[migratetype]);
 		area->nr_free++;
 		set_page_order(&page[size], high);
+
+		/*
+		 * Freepage migratetype is tracked using the index field of the
+		 * first page of the block. So we need to update the new first
+		 * page, when changing the page order.
+		 */
+		set_freepage_migratetype(&page[size], migratetype);
 	}
 }
 

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