linux-kernel - Re: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic in page_pool_page_is

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Message-ID: <87ldqjae92.fsf@toke.dk>
Date: Mon, 26 May 2025 11:54:33 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Byungchul Park <byungchul@...com>
Cc: Mina Almasry <almasrymina@...gle.com>, willy@...radead.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 kernel_team@...ynix.com, kuba@...nel.org, ilias.apalodimas@...aro.org,
 harry.yoo@...cle.com, hawk@...nel.org, akpm@...ux-foundation.org,
 davem@...emloft.net, john.fastabend@...il.com, andrew+netdev@...n.ch,
 asml.silence@...il.com, tariqt@...dia.com, edumazet@...gle.com,
 pabeni@...hat.com, saeedm@...dia.com, leon@...nel.org, ast@...nel.org,
 daniel@...earbox.net, david@...hat.com, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
 surenb@...gle.com, mhocko@...e.com, horms@...nel.org,
 linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com
Subject: Re: [PATCH 12/18] page_pool: use netmem APIs to access
 page->pp_magic in page_pool_page_is_pp()

Byungchul Park <byungchul@...com> writes:

> On Mon, May 26, 2025 at 10:40:30AM +0200, Toke Høiland-Jørgensen wrote:
>> Byungchul Park <byungchul@...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@...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@...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...

-Toke