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

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Oct 06, 2026 at 10:12:47PM -0400, Rik van Riel wrote:
> __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);

Reasonable on one hand, but I think at that point it would be simpler
and more robust to drop that 'single account_freepages() + __add/__del
operations' optimization, and just use the accounting add/del variants
directly instead.