Message-ID: <CAHS8izNh7aCJOb1WKTx7CXNDPv_UBqFyq2XEHHhqHH=5JPmJCQ@mail.gmail.com>
Date: Mon, 14 Jul 2025 12:58:15 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Byungchul Park <byungchul@...com>
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Subject: Re: [PATCH net-next v10 02/12] netmem: use netmem_desc instead of
 page to access ->pp in __netmem_get_pp()

On Mon, Jul 14, 2025 at 12:37 PM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Mon, Jul 14, 2025 at 5:01 AM Byungchul Park <byungchul@...com> wrote:
> >
> > To eliminate the use of the page pool fields in struct page, the page
> > pool code should use netmem descriptor and APIs instead.
> >
> > However, __netmem_get_pp() still accesses ->pp via struct page.  So
> > change it to use struct netmem_desc instead, since ->pp no longer will
> > be available in struct page.
> >
> > While at it, add a helper, pp_page_to_nmdesc(), that can be used to
> > extract netmem_desc from page only if it's pp page.  For now that
> > netmem_desc overlays on page, it can be achieved by just casting.
> >
> > Signed-off-by: Byungchul Park <byungchul@...com>
> > ---
> >  include/net/netmem.h | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > index 535cf17b9134..2b8a7b51ac99 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -267,6 +267,17 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> >         return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
> >  }
> >
> > +static inline struct netmem_desc *pp_page_to_nmdesc(struct page *page)
> > +{
> > +       DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(page));
> > +
> > +       /* XXX: How to extract netmem_desc from page must be changed,
> > +        * once netmem_desc no longer overlays on page and will be
> > +        * allocated through slab.
> > +        */
> > +       return (struct netmem_desc *)page;
> > +}
> > +
>
> Same thing. Do not create a generic looking pp_page_to_nmdesc helper
> which does not check that the page is the correct type. The
> DEBUG_NET... is not good enough.
>
> You don't need to add a generic helper here. There is only one call
> site. Open code this in the callsite. The one callsite is marked as
> unsafe, only called by code that knows that the netmem is specifically
> a pp page. Open code this in the unsafe callsite, instead of creating
> a generic looking unsafe helper and not even documenting it's unsafe.
>

On second read through the series, I actually now think this is a
great idea :-) Adding this helper has simplified the series greatly. I
did not realize you were converting entire drivers to netmem just to
get rid of page->pp accesses. Adding a pp_page_to_nmdesc helper makes
the entire series simpler.

You're also calling it only from code paths like drivers that already
assumed that the page is a pp page and did page->pp deference without
a check, so this should be safe.

Only thing I would change is add a comment explaining that the calling
code needs to check the page is pp page or know it's a pp page (like a
driver that supports pp).


> >  /**
> >   * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
> >   * @netmem: netmem reference to get the pointer from
> > @@ -280,7 +291,7 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> >   */
> >  static inline struct page_pool *__netmem_get_pp(netmem_ref netmem)
> >  {
> > -       return __netmem_to_page(netmem)->pp;
> > +       return pp_page_to_nmdesc(__netmem_to_page(netmem))->pp;
> >  }
>
> This makes me very sad. Casting from netmem -> page -> nmdesc...
>
> Instead, we should be able to go from netmem directly to nmdesc. I
> would suggest rename __netmem_clear_lsb to netmem_to_nmdesc and have
> it return netmem_desc instead of net_iov. Then use it here.
>
> We could have an unsafe version of netmem_to_nmdesc which converts the
> netmem to netmem_desc without clearing the lsb and mark it unsafe.
>

This, I think, we should address to keep some sanity in the code and
reduce the casts and make it a bit more maintainable.

-- 
Thanks,
Mina

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