Re: [RFC mm v6] mm: introduce a new page type for page pool in page type

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 17/11/2025 06.20, Byungchul Park wrote:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 600d9e981c23..01dd14123065 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1041,7 +1041,6 @@ static inline bool page_expected_state(struct page *page,
>   #ifdef CONFIG_MEMCG
>   			page->memcg_data |
>   #endif
> -			page_pool_page_is_pp(page) |
>   			(page->flags.f & check_flags)))
>   		return false;
>   
> @@ -1068,8 +1067,6 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
>   	if (unlikely(page->memcg_data))
>   		bad_reason = "page still charged to cgroup";
>   #endif
> -	if (unlikely(page_pool_page_is_pp(page)))
> -		bad_reason = "page_pool leak";
>   	return bad_reason;
>   }

This code have helped us catch leaks in the past.
When this happens the result is that the page is marked as a bad page.

>   
> @@ -1378,9 +1375,12 @@ __always_inline bool free_pages_prepare(struct page *page,
>   		mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
>   		folio->mapping = NULL;
>   	}
> -	if (unlikely(page_has_type(page)))
> +	if (unlikely(page_has_type(page))) {
> +		/* networking expects to clear its page type before releasing */
> +		WARN_ON_ONCE(PageNetpp(page));
>   		/* Reset the page_type (which overlays _mapcount) */
>   		page->page_type = UINT_MAX;
> +	}
>   
>   	if (is_check_pages_enabled()) {
>   		if (free_page_is_bad(page))

What happens to the page? ... when it gets marked with:
   page->page_type = UINT_MAX

Will it get freed and allowed to be used by others?
- if so it can result in other hard-to-catch bugs

--Jesper