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: <20250526022307.GA27145@system.software.com>
Date: Mon, 26 May 2025 11:23:07 +0900
From: Byungchul Park <byungchul@...com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: willy@...radead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH 12/18] page_pool: use netmem APIs to access
 page->pp_magic in page_pool_page_is_pp()

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.

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