[RFC PATCH 2/5] mm/page_alloc: count a merged buddy's pages under the merged type

[Rik van Riel <riel@surriel.com>]

__free_one_page() merges a freed page with a free buddy of another
mergeable type at pageblock order and above, and retypes the buddy to
match. The buddy's pages stay counted under the old type: nothing moves
them to the merged type in the free counts.

That is unobservable today. NR_FREE_PAGES does not separate types, so the
missing move nets to zero there.  Any future per-type free count would
undercount on a merge from the other side, and allocating the merged
run later could wrap it.

Move the buddy's pages to the merged type in the count too.

Assisted-by: LLM
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 mm/page_alloc.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4658af97be010..ae38e25971559 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -993,6 +993,11 @@ static inline void __free_one_page(struct page *page,
 			account_freepages(zone, 1 << order, migratetype);
 		} else {
 			__del_page_from_free_list(buddy, zone, order, buddy_mt);
+			/* The buddy's free pages join the merged block's type. */
+			if (unlikely(buddy_mt != migratetype)) {
+				account_freepages(zone, -(1 << order), buddy_mt);
+				account_freepages(zone, 1 << order, migratetype);
+			}
 		}
 
 		if (unlikely(buddy_mt != migratetype)) {
-- 
2.53.0-Meta