Message-ID: <CAHS8izNbE+sb8U2Ws2_0C9H6Tf2DzJjh2beu04uyzxk7xFw4ng@mail.gmail.com>
Date: Mon, 14 Jul 2025 12:09:51 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Byungchul Park <byungchul@...com>
Cc: willy@...radead.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
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Subject: Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through
 struct netmem_desc in page_pool_page_is_pp()

On Thu, Jul 10, 2025 at 6:14 PM Byungchul Park <byungchul@...com> wrote:
>
> On Thu, Jul 10, 2025 at 11:19:53AM -0700, Mina Almasry wrote:
> > On Thu, Jul 10, 2025 at 1:28 AM 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>
> > > 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 | 17 +++++++++++++++++
> > >  mm/page_alloc.c      |  1 +
> > >  3 files changed, 18 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 ad9444be229a..11e9de45efcb 100644
> > > --- a/include/net/netmem.h
> > > +++ b/include/net/netmem.h
> > > @@ -355,6 +355,23 @@ static inline void *nmdesc_address(struct netmem_desc *nmdesc)
> > >         return page_address(nmdesc_to_page(nmdesc));
> > >  }
> > >
> > > +#ifdef CONFIG_PAGE_POOL
> > > +/* XXX: This would better be moved to mm, once mm gets its way to
> > > + * identify the type of page for page pool.
> > > + */
> > > +static inline bool page_pool_page_is_pp(struct page *page)
> > > +{
> > > +       struct netmem_desc *desc = page_to_nmdesc(page);
> > > +
> > > +       return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> > > +}
> >
> > pages can be pp pages (where they have pp fields inside of them) or
> > non-pp pages (where they don't have pp fields inside them, because
> > they were never allocated from the page_pool).
> >
> > Casting a page to a netmem_desc, and then checking if the page was a
> > pp page doesn't makes sense to me on a fundamental level. The
> > netmem_desc is only valid if the page was a pp page in the first
> > place. Maybe page_to_nmdesc should reject the cast if the page is not
> > a pp page or something.
>
> Right, as you already know, the current mainline code already has the
> same problem but we've been using the werid way so far, in other words,
> mm code is checking if it's a pp page or not by using ->pp_magic, but
> it's ->lur, ->buddy_list, or ->pcp_list if it's not a pp page.
>
> Both the mainline code and this patch can make sense *only if* it's
> actually a pp page.  It's unevitable until mm provides a way to identify
> the type of page for page pool.  Thoughts?

I don't see mainline having a problem. Mainline checks that the page
is a pp page via the magic before using any of the pp fields. This is
because a page* can be a pp page or a non-pp page.

With netmem_desc, having a netmem_desc* should imply that the
underlying memory is a pp page. Having a netmem_desc* that is not
valid because the pp_magic is not correct complicates the code for no
reason. Every user of netmem_desc has to check pp_magic before
actually using the fields. page_to_nmdesc should just refuse to return
a netmem_desc* if the page is not a pp page.

Also, this patch has my Reviewed-by, even though I honestly don't see
it as acceptable and I clearly have feedback (and Pavel seems too?).

__please__, when you make significant changes to a patch, you have to
reset the Reviewed-by tags.

-- 
Thanks,
Mina

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