[PATCH v1 04/29] mm/page_alloc: let page freeing clear any set page type

[Lorenzo Stoakes]

On Mon, Jun 30, 2025 at 02:59:45PM +0200, David Hildenbrand wrote:
> Currently, any user of page types must clear that type before freeing
> a page back to the buddy, otherwise we'll run into mapcount related
> sanity checks (because the page type currently overlays the page
> mapcount).
>
> Let's allow for not clearing the page type by page type users by letting
> the buddy handle it instead.
>
> We'll focus on having a page type set on the first page of a larger
> allocation only.
>
> With this change, we can reliably identify typed folios even though
> they might be in the process of getting freed, which will come in handy
> in migration code (at least in the transition phase).
>
> In the future we might want to warn on some page types. Instead of
> having an "allow list", let's rather wait until we know about once that
> should go on such a "disallow list".

Is the idea here to get this to show up on folio dumps or?

>
> Reviewed-by: Zi Yan <ziy at nvidia.com>
> Acked-by: Harry Yoo <harry.yoo at oracle.com>
> Signed-off-by: David Hildenbrand <david at redhat.com>
> ---
>  mm/page_alloc.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 858bc17653af9..44e56d31cfeb1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1380,6 +1380,9 @@ __always_inline bool free_pages_prepare(struct page *page,
>  			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
>  		page->mapping = NULL;
>  	}
> +	if (unlikely(page_has_type(page)))
> +		page->page_type = UINT_MAX;

Feels like this could do with a comment!

> +
>  	if (is_check_pages_enabled()) {
>  		if (free_page_is_bad(page))
>  			bad++;
> --
> 2.49.0
>