Message-ID: <20250813060901.GA9086@system.software.com>
Date: Wed, 13 Aug 2025 15:09:01 +0900
From: Byungchul Park <byungchul@...com>
To: akpm@...ux-foundation.org, kuba@...nel.org
Cc: linux-kernel@...r.kernel.org, kernel_team@...ynix.com,
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Subject: Re: [PATCH linux-next v3] mm, page_pool: introduce a new page type
 for page pool in page type

On Tue, Jul 29, 2025 at 08:02:10PM +0900, Byungchul Park wrote:
> Changes from v2:
> 	1. Rebase on linux-next as of Jul 29.
> 	2. Skip 'niov->pp = NULL' when it's allocated using __GFP_ZERO.
> 	3. Change trivial coding style. (feedbacked by Mina)
> 	4. Add Co-developed-by, Acked-by, and Reviewed-by properly.
> 	   Thanks to all.
> 
> Changes from v1:
> 	1. Rebase on linux-next.
> 	2. Initialize net_iov->pp = NULL when allocating net_iov in
> 	   net_devmem_bind_dmabuf() and io_zcrx_create_area().
> 	3. Use ->pp for net_iov to identify if it's pp rather than
> 	   always consider net_iov as pp.
> 	4. Add Suggested-by: David Hildenbrand <david@...hat.com>.
> 
> ---8<---
> >From 88bcb9907a0cef65a9c0adf35e144f9eb67e0542 Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@...com>
> Date: Tue, 29 Jul 2025 19:49:44 +0900
> Subject: [PATCH linux-next v3] 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, ->pp_magic will be removed and page
> type bit in struct page e.i. PGTY_netpp can be used for that purpose.
> 
> Introduce and 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().
> 
> For net_iov, use ->pp to identify if it's pp, with making sure that ->pp
> is NULL for non-pp net_iov.
> 
> This work was inspired by the following link:
> 
> [1] https://lore.kernel.org/all/582f41c0-2742-4400-9c81-0d46bf4e8314@gmail.com/
> 
> While at it, move the sanity check for page pool to on free.

Hi, Andrew and Jakub

I will spin the next one with some modified, once the following patch,
[1], gets merged.

   [1] https://lore.kernel.org/all/a8643abedd208138d3d550db71631d5a2e4168d1.1754929026.git.asml.silence@gmail.com/

This is about both mm and network.  I have no idea which tree should I
aim at between mm tree and network tree?  I prefer the network tree tho.

However, it's totally fine regardless of what it would be.  Suggestion?

	Byungchul

> Suggested-by: David Hildenbrand <david@...hat.com>
> Co-developed-by: Pavel Begunkov <asml.silence@...il.com>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> Signed-off-by: Byungchul Park <byungchul@...com>
> Acked-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> ---
>  .../net/ethernet/mellanox/mlx5/core/en/xdp.c  |  2 +-
>  include/linux/mm.h                            | 27 +++----------------
>  include/linux/page-flags.h                    |  6 +++++
>  include/net/netmem.h                          |  2 +-
>  io_uring/zcrx.c                               |  4 +++
>  mm/page_alloc.c                               |  7 +++--
>  net/core/devmem.c                             |  1 +
>  net/core/netmem_priv.h                        | 23 +++++++---------
>  net/core/page_pool.c                          | 10 +++++--
>  9 files changed, 37 insertions(+), 45 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 0d4ee569aa6b..d01b296e7184 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4171,10 +4171,9 @@ 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.
>   *
> @@ -4205,26 +4204,6 @@ 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
> -
>  #define PAGE_SNAPSHOT_FAITHFUL (1 << 0)
>  #define PAGE_SNAPSHOT_PG_BUDDY (1 << 1)
>  #define PAGE_SNAPSHOT_PG_IDLE  (1 << 2)
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 8d3fa3a91ce4..84247e39e9e7 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -933,6 +933,7 @@ enum pagetype {
>  	PGTY_zsmalloc		= 0xf6,
>  	PGTY_unaccepted		= 0xf7,
>  	PGTY_large_kmalloc	= 0xf8,
> +	PGTY_netpp		= 0xf9,
>  
>  	PGTY_mapcount_underflow = 0xff
>  };
> @@ -1077,6 +1078,11 @@ 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/io_uring/zcrx.c b/io_uring/zcrx.c
> index e5ff49f3425e..f771bb3e756d 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -444,6 +444,10 @@ static int io_zcrx_create_area(struct io_zcrx_ifq *ifq,
>  		area->freelist[i] = i;
>  		atomic_set(&area->user_refs[i], 0);
>  		niov->type = NET_IOV_IOURING;
> +
> +		/* niov->pp is already initialized to NULL by
> +		 * kvmalloc_array(__GFP_ZERO).
> +		 */
>  	}
>  
>  	area->free_count = nr_iovs;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d1d037f97c5f..2f6a55fab942 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1042,7 +1042,6 @@ static inline bool page_expected_state(struct page *page,
>  #ifdef CONFIG_MEMCG
>  			page->memcg_data |
>  #endif
> -			page_pool_page_is_pp(page) |
>  			(page->flags & check_flags)))
>  		return false;
>  
> @@ -1069,8 +1068,6 @@ 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;
>  }
>  
> @@ -1379,9 +1376,11 @@ __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))) {
> +		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))
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index b3a62ca0df65..40e7a4ec9009 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -285,6 +285,7 @@ net_devmem_bind_dmabuf(struct net_device *dev,
>  			niov = &owner->area.niovs[i];
>  			niov->type = NET_IOV_DMABUF;
>  			niov->owner = &owner->area;
> +			niov->pp = NULL;
>  			page_pool_set_dma_addr_netmem(net_iov_to_netmem(niov),
>  						      net_devmem_get_dma_addr(niov));
>  			if (direction == DMA_TO_DEVICE)
> diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
> index cd95394399b4..4b90332d6c64 100644
> --- a/net/core/netmem_priv.h
> +++ b/net/core/netmem_priv.h
> @@ -8,21 +8,18 @@ static inline unsigned long netmem_get_pp_magic(netmem_ref netmem)
>  	return __netmem_clear_lsb(netmem)->pp_magic & ~PP_DMA_INDEX_MASK;
>  }
>  
> -static inline void netmem_or_pp_magic(netmem_ref netmem, unsigned long pp_magic)
> -{
> -	__netmem_clear_lsb(netmem)->pp_magic |= pp_magic;
> -}
> -
> -static inline void netmem_clear_pp_magic(netmem_ref netmem)
> -{
> -	WARN_ON_ONCE(__netmem_clear_lsb(netmem)->pp_magic & PP_DMA_INDEX_MASK);
> -
> -	__netmem_clear_lsb(netmem)->pp_magic = 0;
> -}
> -
>  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, which requires
> +	 * that non-pp net_iov should have ->pp NULL'd.
> +	 */
> +	if (netmem_is_net_iov(netmem))
> +		return !!__netmem_clear_lsb(netmem)->pp;
> +
> +	/* For system memory, page type bit in struct page can be used
> +	 * to identify if it's pp.
> +	 */
> +	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 */
> +	if (!netmem_is_net_iov(netmem))
> +		__SetPageNetpp(__netmem_to_page(netmem));
>  }
>  
>  void page_pool_clear_pp_info(netmem_ref netmem)
>  {
> -	netmem_clear_pp_magic(netmem);
>  	netmem_set_pp(netmem, NULL);
> +
> +	/* If it's page-backed */
> +	if (!netmem_is_net_iov(netmem))
> +		__ClearPageNetpp(__netmem_to_page(netmem));
>  }
>  
>  static __always_inline void __page_pool_release_netmem_dma(struct page_pool *pool,
> 
> base-commit: 54efec8782214652b331c50646013f8526570e8d
> -- 
> 2.17.1

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