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

Byungchul Park
2025-05-26T02:23:13Z

On Fri, May 23, 2025 at 10:21:17AM -0700, Mina Almasry wrote:
> 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.

Once the page pool fields are removed from struct page, struct page will
have neither struct netmem_desc nor the fields..

So it's unevitable to cast it to netmem_desc in order to refer to
pp_magic. Again, pp_magic is no longer associated to struct page.

Thoughts?

Byungchul

> 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