Re: [PATCH linux-next v3] mm, page_pool: introduce a new page type for page pool in page type

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 29 Jul 2025 20:02:10 +0900 Byungchul Park wrote:
> [PATCH linux-next v3] mm, page_pool: introduce a new page type for page pool in page type

linux-next does not accept patches. This has to go either via networking or MM.

> -	if (unlikely(page_has_type(page)))
> +	if (unlikely(page_has_type(page))) {

Maybe add :

		/* 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;
> +	}

>  static inline bool netmem_is_pp(netmem_ref netmem)
>  {
> -	return (netmem_get_pp_magic(netmem) & PP_MAGIC_MASK) == PP_SIGNATURE;
> +	/* Use ->pp for net_iov to identify if it's pp, 

Please try to use precise language, this code is confusing as is.
net_iov may _belong_ to a page pool.

	* which requires that non-pp net_iov should have ->pp NULL'd.

I don't think this adds any information.

> +	 */
> +	if (netmem_is_net_iov(netmem))
> +		return !!__netmem_clear_lsb(netmem)->pp;
> +
> +	/* For system memory, page type bit in struct page can be used

"page type bit" -> "page type", it's not a bit.

> +	 * to identify if it's pp.

... to identify pages which belong to a page pool.

> +	 */
> +	return PageNetpp(__netmem_to_page(netmem));
>  }
>  
>  static inline void netmem_set_pp(netmem_ref netmem, struct page_pool *pool)
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 05e2e22a8f7c..37eeab76c41c 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -654,7 +654,6 @@ s32 page_pool_inflight(const struct page_pool *pool, bool strict)
>  void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
>  {
>  	netmem_set_pp(netmem, pool);
> -	netmem_or_pp_magic(netmem, PP_SIGNATURE);
>  
>  	/* Ensuring all pages have been split into one fragment initially:
>  	 * page_pool_set_pp_info() is only called once for every page when it
> @@ -665,12 +664,19 @@ void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
>  	page_pool_fragment_netmem(netmem, 1);
>  	if (pool->has_init_callback)
>  		pool->slow.init_callback(netmem, pool->slow.init_arg);
> +
> +	/* If it's page-backed */

Please don't add obvious comments.

> +	if (!netmem_is_net_iov(netmem))
> +		__SetPageNetpp(__netmem_to_page(netmem));