Re: [PATCH v2 rebase as of Jul 28] mm, page_pool: introduce a new page type for page pool in page type

[Mina Almasry <almasrymina@google.com>]

On Mon, Jul 28, 2025 at 1:20 AM Byungchul Park <byungchul@sk.com> wrote:
>
> Rebasing on the latest linux-next/master is required.  Otherwise, a
> build fail is observed.  Sorry about the noise.
>
> 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@redhat.com>.
>
> ---8<---
> From 26c9a731f388b788d6ea972c313bc8da8831412b Mon Sep 17 00:00:00 2001
> From: Byungchul Park <byungchul@sk.com>
> Date: Mon, 28 Jul 2025 17:09:20 +0900
> Subject: [PATCH v2 rebase as of Jul 28] 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.
>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Byungchul Park <byungchul@sk.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                               |  1 +
>  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, 34 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..4cceb97ca26a 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -444,6 +444,7 @@ 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 = NULL;
>         }
>
>         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..0a10f3026faa 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 (netmem_is_net_iov(netmem))
> +               return;
> +       __SetPageNetpp(__netmem_to_page(netmem));

nit:

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

i.e. no need for the early return. I would also move the diff to
replace netmem_or_pp_magic.

>  }
>
>  void page_pool_clear_pp_info(netmem_ref netmem)
>  {
> -       netmem_clear_pp_magic(netmem);
>         netmem_set_pp(netmem, NULL);
> +
> +       if (netmem_is_net_iov(netmem))
> +               return;
> +       __ClearPageNetpp(__netmem_to_page(netmem));

Same here. I would do:

if (!netmem_is_net_iov(netmem))
  __ClearPageNetpp(__netmem_to_page(netmem));

and no need for the early return.

Reviewed-by: Mina Almasry <almasrymina@google.com>


-- 
Thanks,
Mina