linux-kernel - Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <ce5b4b18-9934-41e3-af04-c34653b4b5fa@redhat.com>
Date: Mon, 23 Jun 2025 11:16:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Byungchul Park <byungchul@...com>, willy@...radead.org,
 netdev@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 kernel_team@...ynix.com, kuba@...nel.org, almasrymina@...gle.com,
 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,
 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 v6 9/9] page_pool: access ->pp_magic through
 struct netmem_desc in page_pool_page_is_pp()

On 20.06.25 06:12, Byungchul Park 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 | 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 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 d49ed49d250b..3d1b1dfc9ba5 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -56,6 +56,20 @@ 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 = (struct netmem_desc *)page;
> +
> +	return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> +}
> +#else
> +static inline bool page_pool_page_is_pp(struct page *page)
> +{
> +	return false;
> +}
> +#endif

I wonder how helpful this cleanup is long-term.

page_pool_page_is_pp() is only called from mm/page_alloc.c, right? 
There, we want to make sure that no pagepool page is ever returned to 
the buddy.

How reasonable is this sanity check to have long-term? Wouldn't we be 
able to check that on some higher-level freeing path?

The reason I am commenting is that once we decouple "struct page" from 
"struct netmem_desc", we'd have to lookup here the corresponding "struct 
netmem_desc".

... but at that point here (when we free the actual pages), the "struct 
netmem_desc" would likely already have been freed separately (remember: 
it will be dynamically allocated).

With that in mind:

1) Is there a higher level "struct netmem_desc" freeing path where we 
could check that instead, so we don't have to cast from pages to 
netmem_desc at all.

2) How valuable are these sanity checks deep in the buddy?

-- 
Cheers,

David / dhildenb