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

[Byungchul Park]

On Mon, May 26, 2025 at 11:23:07AM +0900, Byungchul Park wrote:
> 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@xxxxxx> 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@xxxxxx>
> > > ---
> > >  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.

Options that come across my mind are:

   1. use lru field of struct page instead, with appropriate comment but
      looks so ugly.
   2. instead of a full word for the magic, use a bit of flags or use
      the private field for that purpose.
   3. do not check magic number for page pool.
   4. more?

	Byungchul
> 
> 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