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: <20250528093303.GB54984@system.software.com>
Date: Wed, 28 May 2025 18:33:03 +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,
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	akpm@...ux-foundation.org, davem@...emloft.net,
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	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 10:20:29AM +0100, Pavel Begunkov wrote:
> On 5/28/25 10:14, Byungchul Park wrote:
> > On Wed, May 28, 2025 at 10:07:52AM +0100, Pavel Begunkov wrote:
> > > On 5/28/25 09:14, Byungchul Park wrote:
> > > > 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:
> > > 
> > > The question is what is the problem that you need another temporary
> > > solution? If, for example, we go the placeholder way, page_pool_page_is_pp()
> > 
> > I prefer using the place-holder, but Matthew does not.  I explained it:
> > 
> >     https://lore.kernel.org/all/20250528013145.GB2986@system.software.com/
> > 
> > Now, I'm going with the same way as the other approaches e.g. ptdesc.
> 
> Sure, but that doesn't change my point

What's your point?  The other appoaches do not use place-holders.  I
don't get your point.

As I told you, I will introduce a new struct, netmem_desc, instead of
struct_group_tagged() on struct net_iov, and modify the static assert on
the offsets to keep the important fields between struct page and
netmem_desc.

Then, is that following your point?  Or could you explain your point in
more detail?  Did you say other points than these?

	Byungchul
> 
> -- 
> Pavel Begunkov