Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is_pp()

[Byungchul Park <byungchul@sk.com>]

On Sat, Jul 12, 2025 at 02:58:14PM +0100, Pavel Begunkov wrote:
> On 7/11/25 02:14, Byungchul Park wrote:
> ...>>> +#ifdef CONFIG_PAGE_POOL
> > > > +/* XXX: This would better be moved to mm, once mm gets its way to
> > > > + * identify the type of page for page pool.
> > > > + */
> > > > +static inline bool page_pool_page_is_pp(struct page *page)
> > > > +{
> > > > +       struct netmem_desc *desc = page_to_nmdesc(page);
> > > > +
> > > > +       return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> > > > +}
> > > 
> > > pages can be pp pages (where they have pp fields inside of them) or
> > > non-pp pages (where they don't have pp fields inside them, because
> > > they were never allocated from the page_pool).
> > > 
> > > Casting a page to a netmem_desc, and then checking if the page was a
> > > pp page doesn't makes sense to me on a fundamental level. The
> > > netmem_desc is only valid if the page was a pp page in the first
> > > place. Maybe page_to_nmdesc should reject the cast if the page is not
> > > a pp page or something.
> > 
> > Right, as you already know, the current mainline code already has the
> > same problem but we've been using the werid way so far, in other words,
> > mm code is checking if it's a pp page or not by using ->pp_magic, but
> > it's ->lur, ->buddy_list, or ->pcp_list if it's not a pp page.
> > 
> > Both the mainline code and this patch can make sense *only if* it's
> > actually a pp page.  It's unevitable until mm provides a way to identify
> > the type of page for page pool.  Thoughts?
> Question to mm folks, can we add a new PGTY for page pool and use
> that to filter page pool originated pages? Like in the incomplete
> and untested diff below?
> 
> 
> commit 8fc2347fb3ff4a3fc7929c70a5a21e1128935d4a
> Author: Pavel Begunkov <asml.silence@gmail.com>
> Date:   Sat Jul 12 14:29:52 2025 +0100
> 
>     net/mm: use PGTY for tracking page pool pages
> 
>     Currently, we use page->pp_magic to determine whether a page belongs to
>     a page pool. It's not ideal as the field is aliased with other page
>     types, and thus needs to to rely on elaborated rules to work. Add a new
>     page type for page pool.

Hi Pavel,

I need this work to be done to remove ->pp_magic in struct page.  Will
you let me work on this work?  Or can you please refine and post this
work?  If you let me, I will go for this as a separate patch from this
series.

	Byungchul

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0ef2ba0c667a..975a013f1f17 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4175,7 +4175,7 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>  #ifdef CONFIG_PAGE_POOL
>  static inline bool page_pool_page_is_pp(struct page *page)
>  {
> -       return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> +       return PageNetpp(page);
>  }
>  #else
>  static inline bool page_pool_page_is_pp(struct page *page)
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 4fe5ee67535b..9bd1dfded2fc 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 de1d95f04076..20f5dbb08149 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -113,6 +113,8 @@ static inline bool netmem_is_net_iov(const netmem_ref netmem)
>   */
>  static inline struct page *__netmem_to_page(netmem_ref netmem)
>  {
> +       DEBUG_NET_WARN_ON_ONCE(netmem_is_net_iov(netmem));
> +
>        return (__force struct page *)netmem;
>  }
> 
> diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
> index cd95394399b4..e38c64da1a78 100644
> --- a/net/core/netmem_priv.h
> +++ b/net/core/netmem_priv.h
> @@ -13,16 +13,11 @@ 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;
> +       if (netmem_is_net_iov(netmem))
> +               return true;
> +       return page_pool_page_is_pp(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..52120e2912a6 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -371,6 +371,13 @@ struct page_pool *page_pool_create(const struct page_pool_params *params)
>  }
>  EXPORT_SYMBOL(page_pool_create);
> 
> +static void page_pool_set_page_pp_info(struct page_pool *pool,
> +                                      struct page *page)
> +{
> +       __SetPageNetpp(page);
> +       page_pool_set_pp_info(page_to_netmem(page));
> +}
> +
>  static void page_pool_return_netmem(struct page_pool *pool, netmem_ref netmem);
> 
>  static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
> @@ -534,7 +541,7 @@ static struct page *__page_pool_alloc_page_order(struct page_pool *pool,
>        }
> 
>        alloc_stat_inc(pool, slow_high_order);
> -       page_pool_set_pp_info(pool, page_to_netmem(page));
> +       page_pool_set_page_pp_info(pool, page);
> 
>        /* Track how many pages are held 'in-flight' */
>        pool->pages_state_hold_cnt++;
> @@ -579,7 +586,7 @@ static noinline netmem_ref __page_pool_alloc_netmems_slow(struct page_pool *pool
>                        continue;
>                }
> 
> -               page_pool_set_pp_info(pool, netmem);
> +               page_pool_set_page_pp_info(pool, __netmem_to_page(netmem));
>                pool->alloc.cache[pool->alloc.count++] = netmem;
>                /* Track how many pages are held 'in-flight' */
>                pool->pages_state_hold_cnt++;
> @@ -654,7 +661,6 @@ 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
> @@ -669,7 +675,6 @@ void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
> 
>  void page_pool_clear_pp_info(netmem_ref netmem)
>  {
> -       netmem_clear_pp_magic(netmem);
>        netmem_set_pp(netmem, NULL);
>  }
> 
> @@ -730,8 +735,11 @@ static void page_pool_return_netmem(struct page_pool *pool, netmem_ref netmem)
>        trace_page_pool_state_release(pool, netmem, count);
> 
>        if (put) {
> +               struct page *page = netmem_to_page(netmem);
> +
>                page_pool_clear_pp_info(netmem);
> -               put_page(netmem_to_page(netmem));
> +               __ClearPageNetpp(page);
> +               put_page(page);
>        }
>        /* An optimization would be to call __free_pages(page, pool->p.order)
>         * knowing page is not part of page-cache (thus avoiding a
> 
> --
> Pavel Begunkov