netdev - Re: [RFC v3 18/18] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <20250530011621.GB3093@system.software.com>
Date: Fri, 30 May 2025 10:16:21 +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,
	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,
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	linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
	vishal.moola@...il.com
Subject: Re: [RFC v3 18/18] page_pool: access ->pp_magic through struct
 netmem_desc in page_pool_page_is_pp()

On Thu, May 29, 2025 at 12:54:31PM -0700, Mina Almasry wrote:
> On Wed, May 28, 2025 at 8:11 PM Byungchul Park <byungchul@...com> wrote:
> >
> > To simplify struct page, the effort to separate 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 directly accessing page pool
> > members of struct page.
> >
> > Access ->pp_magic through struct netmem_desc instead of directly
> > accessing it through struct page in page_pool_page_is_pp().  Plus, move
> > page_pool_page_is_pp() from mm.h to netmem.h to use struct netmem_desc
> > without header dependency issue.
> >
> > Signed-off-by: Byungchul Park <byungchul@...com>
> > ---
> >  include/linux/mm.h   | 12 ------------
> >  include/net/netmem.h | 14 ++++++++++++++
> >  mm/page_alloc.c      |  1 +
> >  3 files changed, 15 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 8dc012e84033..de10ad386592 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4311,16 +4311,4 @@ 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;
> > -}
> > -#else
> > -static inline bool page_pool_page_is_pp(struct page *page)
> > -{
> > -       return false;
> > -}
> > -#endif
> > -
> >  #endif /* _LINUX_MM_H */
> > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > index f05a8b008d00..9e4ed3530788 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -53,6 +53,20 @@ NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
> >   */
> >  static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount));
> >
> > +#ifdef CONFIG_PAGE_POOL
> > +static inline bool page_pool_page_is_pp(struct page *page)
> > +{
> > +       struct netmem_desc *desc = (__force struct netmem_desc *)page;
> > +
> 
> Is it expected that page can be cast to netmem_desc freely? I know it
> works now since netmem_desc and page have the same layout, but how is
> it going to continue to work when page is shrunk and no longer has

This should be updated once struct netmem_desc has its own instance from
slab.  As Pavel mentioned, that should be done another way.

> 'pp_magic' inside of it? Is that series going to fixup all the places
> where casts are done?
> 
> Is it also allowed that we can static cast netmem_desc to page?
> 
> Consider creating netmem_desc_page helper like ptdesc_page.

Do we need casting netmem_desc to page?

> 
> I'm not sure the __force is needed too.

Ah, I will remove it.

	Byungchul
> 
> -- 
> Thanks,
> Mina