linux-kernel - Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <20250623101622.GB3199@system.software.com>
Date: Mon, 23 Jun 2025 19:16:22 +0900
From: Byungchul Park <byungchul@...com>
To: David Hildenbrand <david@...hat.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, almasrymina@...gle.com,
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	akpm@...ux-foundation.org, davem@...emloft.net,
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	jackmanb@...gle.com
Subject: Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through
 struct netmem_desc in page_pool_page_is_pp()

On Mon, Jun 23, 2025 at 11:16:43AM +0200, David Hildenbrand wrote:
> On 20.06.25 06:12, Byungchul Park 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>
> > Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> > Reviewed-by: Pavel Begunkov <asml.silence@...il.com>
> > Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> > Acked-by: Harry Yoo <harry.yoo@...cle.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 0ef2ba0c667a..0b7f7f998085 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4172,16 +4172,4 @@ 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 d49ed49d250b..3d1b1dfc9ba5 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
> 
> I wonder how helpful this cleanup is long-term.
> 
> page_pool_page_is_pp() is only called from mm/page_alloc.c, right?

Yes.

> There, we want to make sure that no pagepool page is ever returned to
> the buddy.
> 
> How reasonable is this sanity check to have long-term? Wouldn't we be
> able to check that on some higher-level freeing path?
> 
> The reason I am commenting is that once we decouple "struct page" from
> "struct netmem_desc", we'd have to lookup here the corresponding "struct
> netmem_desc".
> 
> ... but at that point here (when we free the actual pages), the "struct
> netmem_desc" would likely already have been freed separately (remember:
> it will be dynamically allocated).
> 
> With that in mind:
> 
> 1) Is there a higher level "struct netmem_desc" freeing path where we
> could check that instead, so we don't have to cast from pages to
> netmem_desc at all.

I also thought it's too paranoiac.  However, I thought it's other issue
than this work.  That's why I left the API as is for now, it can be gone
once we get convinced the check is unnecessary in deep buddy.  Wrong?

> 2) How valuable are these sanity checks deep in the buddy?

That was also what I felt weird on.

	Byungchul

> --
> Cheers,
> 
> David / dhildenb