linux-kernel - Re: [PATCH net-next 9/9] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <20250610014500.GB65598@system.software.com>
Date: Tue, 10 Jun 2025 10:45:00 +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,
	asml.silence@...il.com, toke@...hat.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 net-next 9/9] page_pool: access ->pp_magic through struct
 netmem_desc in page_pool_page_is_pp()

On Mon, Jun 09, 2025 at 10:39:06AM -0700, Mina Almasry wrote:
> On Sun, Jun 8, 2025 at 9:32 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>
> > Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.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 e51dba8398f7..f23560853447 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 d84ab624b489..8f354ae7d5c3 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -56,6 +56,20 @@ 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 = (struct netmem_desc *)page;
> > +
> > +       return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> > +}
> > +#else
> > +static inline bool page_pool_page_is_pp(struct page *page)
> > +{
> > +       return false;
> > +}
> > +#endif
> > +
> >  /* net_iov */
> >
> >  DECLARE_STATIC_KEY_FALSE(page_pool_mem_providers);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 4f29e393f6af..be0752c0ac92 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -55,6 +55,7 @@
> >  #include <linux/delayacct.h>
> >  #include <linux/cacheinfo.h>
> >  #include <linux/pgalloc_tag.h>
> > +#include <net/netmem.h>
> 
> mm files starting to include netmem.h is a bit interesting. I did not
> expect/want dependencies outside of net. If anything the netmem stuff
> include linux/mm.h

That's what I also concerned.  However, now that there are no way to
check the type of memory in a general way but require to use one of pp
fields, page_pool_page_is_pp() should be served by pp code e.i. network
subsystem.

This should be changed once either 1) mm provides a general way to check
the type or 2) pp code is moved to mm code.  I think this approach
should acceptable until then.

> But I don't have a butter suggestion here and I don't see any huge
			^
			lol

	Byungchul

> problems with this off the top of my head, so
> 
> Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> 
> Lets see if Jakub objects though. To be fair, we did put the netmem
> private stuff in net/core/netmem_priv.h, so technically
> include/net/netmem.h should be exportable indeed.
> 
> -- 
> Thanks,
> Mina