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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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
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
in the inline definition. Unless I'm missing something?

-Toke