netdev - Re: [RFC v3 18/18] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <CAHS8izNyXM_KQiySAw4hZQ+FU8yxAZmcqvjsO7P3pM0HNy0STA@mail.gmail.com>
Date: Thu, 29 May 2025 12:54:31 -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, 
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	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
Subject: Re: [RFC v3 18/18] page_pool: access ->pp_magic through struct
 netmem_desc in page_pool_page_is_pp()

On Wed, May 28, 2025 at 8:11 PM 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>
> ---
>  include/linux/mm.h   | 12 ------------
>  include/net/netmem.h | 14 ++++++++++++++
>  mm/page_alloc.c      |  1 +
>  3 files changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8dc012e84033..de10ad386592 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4311,16 +4311,4 @@ 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 f05a8b008d00..9e4ed3530788 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -53,6 +53,20 @@ NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
>   */
>  static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount));
>
> +#ifdef CONFIG_PAGE_POOL
> +static inline bool page_pool_page_is_pp(struct page *page)
> +{
> +       struct netmem_desc *desc = (__force struct netmem_desc *)page;
> +

Is it expected that page can be cast to netmem_desc freely? I know it
works now since netmem_desc and page have the same layout, but how is
it going to continue to work when page is shrunk and no longer has
'pp_magic' inside of it? Is that series going to fixup all the places
where casts are done?

Is it also allowed that we can static cast netmem_desc to page?

Consider creating netmem_desc_page helper like ptdesc_page.

I'm not sure the __force is needed too.

-- 
Thanks,
Mina