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: <20250528081403.GA28116@system.software.com>
Date: Wed, 28 May 2025 17:14:03 +0900
From: Byungchul Park <byungchul@...com>
To: Pavel Begunkov <asml.silence@...il.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, toke@...hat.com,
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	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()

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@...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?
> 
> 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