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

[David Hildenbrand <david@redhat.com>]

On 21.07.25 07:49, Byungchul Park wrote:
> Hi,
> 
> I focused on converting the existing APIs accessing ->pp_magic field to
> page type APIs.  However, yes.  Additional works would better be
> considered on top like:
> 
>     1. Adjust how to store and retrieve dma index.  Maybe network guys
>        can work better on top.
> 
>     2. Move the sanity check for page pool in mm/page_alloc.c to on free.
> 
>     Byungchul
> 
> ---8<---
>  From 7d207a1b3e9f4ff2a72f5b54b09e3ed0c4aaaca3 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.com>
> Date: Mon, 21 Jul 2025 14:05:20 +0900
> Subject: [PATCH] mm, page_pool: introduce a new page type for page pool in page type
> 
> ->pp_magic field in struct page is current used to identify if a page
> belongs to a page pool.  However, page type e.i. PGTY_netpp can be used
> for that purpose.
> 
> Use the page type APIs e.g. PageNetpp(), __SetPageNetpp(), and
> __ClearPageNetpp() instead, and remove the existing APIs accessing
> ->pp_magic e.g. page_pool_page_is_pp(), netmem_or_pp_magic(), and
> netmem_clear_pp_magic() since they are totally replaced.
> 
> This work was inspired by the following link by Pavel:
> 
> [1] https://lore.kernel.org/all/582f41c0-2742-4400-9c81-0d46bf4e8314@gmail.com/

I'm, sure you saw my comment (including my earlier suggestion for using 
a page type), in particular around ...

> ---
>   .../net/ethernet/mellanox/mlx5/core/en/xdp.c  |  2 +-
>   include/linux/mm.h                            | 28 ++-----------------
>   include/linux/page-flags.h                    |  6 ++++
>   include/net/netmem.h                          |  2 +-
>   mm/page_alloc.c                               |  4 +--
>   net/core/netmem_priv.h                        | 16 ++---------
>   net/core/page_pool.c                          | 10 +++++--
>   7 files changed, 24 insertions(+), 44 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> index 5d51600935a6..def274f5c1ca 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> @@ -707,7 +707,7 @@ static void mlx5e_free_xdpsq_desc(struct mlx5e_xdpsq *sq,
>   				xdpi = mlx5e_xdpi_fifo_pop(xdpi_fifo);
>   				page = xdpi.page.page;
>   
> -				/* No need to check page_pool_page_is_pp() as we
> +				/* No need to check PageNetpp() as we
>   				 * know this is a page_pool page.
>   				 */
>   				page_pool_recycle_direct(pp_page_to_nmdesc(page)->pp,
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ae50c1641bed..736061749535 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4135,10 +4135,9 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>    * DMA mapping IDs for page_pool
>    *
>    * When DMA-mapping a page, page_pool allocates an ID (from an xarray) and
> - * stashes it in the upper bits of page->pp_magic. We always want to be able to
> - * unambiguously identify page pool pages (using page_pool_page_is_pp()). Non-PP
> - * pages can have arbitrary kernel pointers stored in the same field as pp_magic
> - * (since it overlaps with page->lru.next), so we must ensure that we cannot
> + * stashes it in the upper bits of page->pp_magic. Non-PP pages can have
> + * arbitrary kernel pointers stored in the same field as pp_magic (since
> + * it overlaps with page->lru.next), so we must ensure that we cannot
>    * mistake a valid kernel pointer with any of the values we write into this
>    * field.
>    *
> @@ -4168,25 +4167,4 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>   
>   #define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHIFT - 1, \
>   				  PP_DMA_INDEX_SHIFT)
> -
> -/* Mask used for checking in page_pool_page_is_pp() below. page->pp_magic is
> - * OR'ed with PP_SIGNATURE after the allocation in order to preserve bit 0 for
> - * the head page of compound page and bit 1 for pfmemalloc page, as well as the
> - * bits used for the DMA index. page_is_pfmemalloc() is checked in
> - * __page_pool_put_page() to avoid recycling the pfmemalloc page.
> - */
> -#define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
> -
> -#ifdef CONFIG_PAGE_POOL
> -static inline bool page_pool_page_is_pp(const struct page *page)
> -{
> -	return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> -}
> -#else
> -static inline bool page_pool_page_is_pp(const struct page *page)
> -{
> -	return false;
> -}
> -#endif
> -
>   #endif /* _LINUX_MM_H */
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 4fe5ee67535b..906ba7c9e372 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -957,6 +957,7 @@ enum pagetype {
>   	PGTY_zsmalloc		= 0xf6,
>   	PGTY_unaccepted		= 0xf7,
>   	PGTY_large_kmalloc	= 0xf8,
> +	PGTY_netpp		= 0xf9,
>   
>   	PGTY_mapcount_underflow = 0xff
>   };
> @@ -1101,6 +1102,11 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
>   PAGE_TYPE_OPS(Unaccepted, unaccepted, unaccepted)
>   FOLIO_TYPE_OPS(large_kmalloc, large_kmalloc)
>   
> +/*
> + * Marks page_pool allocated pages.
> + */
> +PAGE_TYPE_OPS(Netpp, netpp, netpp)
> +
>   /**
>    * PageHuge - Determine if the page belongs to hugetlbfs
>    * @page: The page to test.
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index f7dacc9e75fd..3667334e16e7 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -298,7 +298,7 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
>    */
>   #define pp_page_to_nmdesc(p)						\
>   ({									\
> -	DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(p));		\
> +	DEBUG_NET_WARN_ON_ONCE(!PageNetpp(p));				\
>   	__pp_page_to_nmdesc(p);						\
>   })
>   
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2ef3c07266b3..71c7666e48a9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -898,7 +898,7 @@ static inline bool page_expected_state(struct page *page,
>   #ifdef CONFIG_MEMCG
>   			page->memcg_data |
>   #endif
> -			page_pool_page_is_pp(page) |
> +			PageNetpp(page) |
>   			(page->flags & check_flags)))
>   		return false;
>   
> @@ -925,7 +925,7 @@ 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)))
> +	if (unlikely(PageNetpp(page)))
>   		bad_reason = "page_pool leak";
>   	return bad_reason;
>   }

^ this

This will not work they way you want it once you rebase on top of 
linux-next, where we have (from mm/mm-stable)


commit 2dfcd1608f3a96364f10de7fcfe28727c0292e5d
Author: David Hildenbrand <david@redhat.com>
Date:   Fri Jul 4 12:24:58 2025 +0200

     mm/page_alloc: let page freeing clear any set page type


I commented what to do already.

-- 
Cheers,

David / dhildenb