netdev - Re: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic in page_pool_page_is

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Message-ID: <CAHS8izN6QAcAr-qkFSYAy0JaTU+hdM56r-ug-AWDGGqLvHkNuQ@mail.gmail.com>
Date: Fri, 23 May 2025 10:21:17 -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, 
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	akpm@...ux-foundation.org, davem@...emloft.net, john.fastabend@...il.com, 
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	linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com
Subject: Re: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic
 in page_pool_page_is_pp()

On Thu, May 22, 2025 at 8:26 PM Byungchul Park <byungchul@...com> wrote:
>
> To simplify struct page, the effort to seperate 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 accessing page pool members
> of struct page directly, but use safe APIs for the purpose.
>
> Use netmem_is_pp() instead of directly accessing page->pp_magic in
> page_pool_page_is_pp().
>
> Signed-off-by: Byungchul Park <byungchul@...com>
> ---
>  include/linux/mm.h   | 5 +----
>  net/core/page_pool.c | 5 +++++
>  2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8dc012e84033..3f7c80fb73ce 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4312,10 +4312,7 @@ 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;
> -}

I vote for keeping this function as-is (do not convert it to netmem),
and instead modify it to access page->netmem_desc->pp_magic.

The reason is that page_pool_is_pp() is today only called from code
paths we have a page and not a netmem. Casting the page to a netmem
which will cast it back to a page pretty much is a waste of cpu
cycles. The page_pool is a place where we count cycles and we have
benchmarks to verify performance (I pointed you to
page_pool_bench_simple on the RFC).

So lets avoid the cpu cycles if possible.

-- 
Thanks,
Mina