linux-kernel - Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <CAHS8izMXkyGvYmf1u6r_kMY_QGSOoSCECkF0QJC4pdKx+DOq0A@mail.gmail.com>
Date: Thu, 10 Jul 2025 11:19:53 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Byungchul Park <byungchul@...com>
Cc: willy@...radead.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, kernel_team@...ynix.com, kuba@...nel.org, 
	ilias.apalodimas@...aro.org, harry.yoo@...cle.com, hawk@...nel.org, 
	akpm@...ux-foundation.org, davem@...emloft.net, john.fastabend@...il.com, 
	andrew+netdev@...n.ch, asml.silence@...il.com, toke@...hat.com, 
	tariqt@...dia.com, edumazet@...gle.com, pabeni@...hat.com, saeedm@...dia.com, 
	leon@...nel.org, ast@...nel.org, daniel@...earbox.net, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz, 
	rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, horms@...nel.org, 
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com, 
	hannes@...xchg.org, ziy@...dia.com, jackmanb@...gle.com
Subject: Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through
 struct netmem_desc in page_pool_page_is_pp()

On Thu, Jul 10, 2025 at 1:28 AM Byungchul Park <byungchul@...com> wrote:
>
> To simplify struct page, the effort to separate its own descriptor from
> struct page is required and the work for page pool is on going.
>
> To achieve that, all the code should avoid directly accessing page pool
> members of struct page.
>
> Access ->pp_magic through struct netmem_desc instead of directly
> accessing it through struct page in page_pool_page_is_pp().  Plus, move
> page_pool_page_is_pp() from mm.h to netmem.h to use struct netmem_desc
> without header dependency issue.
>
> Signed-off-by: Byungchul Park <byungchul@...com>
> Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> Reviewed-by: Pavel Begunkov <asml.silence@...il.com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Acked-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  include/linux/mm.h   | 12 ------------
>  include/net/netmem.h | 17 +++++++++++++++++
>  mm/page_alloc.c      |  1 +
>  3 files changed, 18 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 0ef2ba0c667a..0b7f7f998085 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4172,16 +4172,4 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>   */
>  #define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
>
> -#ifdef CONFIG_PAGE_POOL
> -static inline bool page_pool_page_is_pp(struct page *page)
> -{
> -       return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> -}
> -#else
> -static inline bool page_pool_page_is_pp(struct page *page)
> -{
> -       return false;
> -}
> -#endif
> -
>  #endif /* _LINUX_MM_H */
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index ad9444be229a..11e9de45efcb 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -355,6 +355,23 @@ static inline void *nmdesc_address(struct netmem_desc *nmdesc)
>         return page_address(nmdesc_to_page(nmdesc));
>  }
>
> +#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.

-- 
Thanks,
Mina