Re: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic in page_pool_page_is_pp()

Mina Almasry
2025-05-23T17:21:30Z

On Thu, May 22, 2025 at 8:26 PM Byungchul Park <byungchul@sk.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@sk.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