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

[Byungchul Park <byungchul@sk.com>]

On Wed, May 28, 2025 at 08:51:47AM +0100, Pavel Begunkov wrote:
> On 5/26/25 03:23, 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@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?
> 
> Once the indirection / page shrinking is realized, the page is
> supposed to have a type field, isn't it? And all pp_magic trickery
> will be replaced with something like
> 
> page_pool_page_is_pp() { return page->type == PAGE_TYPE_PP; }

Agree, but we need a temporary solution until then.  I will use the
following way for now:

   https://lore.kernel.org/all/20250528051452.GB59539@system.software.com/

	Byungchul
> 
> 
> -- 
> Pavel Begunkov