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 09:35:03AM +0200, Toke Høiland-Jørgensen wrote:
> Byungchul Park <byungchul@sk.com> writes:
> 
> > On Mon, May 26, 2025 at 11:54:33AM +0200, Toke Høiland-Jørgensen wrote:
> >> Byungchul Park <byungchul@sk.com> writes:
> >> 
> >> > On Mon, May 26, 2025 at 10:40:30AM +0200, Toke Høiland-Jørgensen wrote:
> >> >> Byungchul Park <byungchul@sk.com> writes:
> >> >> 
> >> >> > 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@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.
> >> >> >
> >> >> > 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?
> >> >> 
> >> >> I'm not sure I understand Mina's concern about CPU cycles from casting.
> >> >> The casting is a compile-time thing, which shouldn't affect run-time
> >> >
> >> > I didn't mention it but yes.
> >> >
> >> >> performance as long as the check is kept as an inline function. So it's
> >> >> "just" a matter of exposing struct netmem_desc to mm.h so it can use it
> >> >
> >> > Then.. we should expose net_iov as well, but I'm afraid it looks weird.
> >> > Do you think it's okay?
> >> 
> >> Well, it'll be ugly, I grant you that :)
> >> 
> >> Hmm, so another idea could be to add the pp_magic field to the inner
> >> union that the lru field is in, and keep the page_pool_page_is_pp()
> >> as-is. Then add an assert for offsetof(struct page, pp_magic) ==
> >> offsetof(netmem_desc, pp_magic) on the netmem side, which can be removed
> >> once the two structs no longer shadow each other?
> >> 
> >> That way you can still get rid of the embedded page_pool struct in
> >> struct page, and the pp_magic field will just be a transition thing
> >> until things are completely separated...
> >
> > Or what about to do that as mm folks did in page_is_pfmemalloc()?
> >
> > static inline bool page_pool_page_is_pp(struct page *page)
> > {
> > 	/*
> > 	 * XXX: The space of page->lru.next is used as pp_magic in
> > 	 * struct netmem_desc overlaying on struct page temporarily.
> > 	 * This API will be unneeded shortly.  Let's use the ugly but
> > 	 * temporal way to access pp_magic until struct netmem_desc has
> > 	 * its own instance.
> > 	 */
> >         return (((unsigned long)page->lru.next) & PP_MAGIC_MASK) == PP_SIGNATURE;
> > }
> 
> Sure, that can work as a temporary solution (maybe with a static assert
> somewhere that pp_magic and lru have the same offsetof())?

Sure.  I will do that as I posted in the cover letter:

   https://lore.kernel.org/all/20250528022911.73453-1-byungchul@sk.com/

	Byungchul
> 
> -Toke