Re: [PATCH net-next v10 02/12] netmem: use netmem_desc instead of page to access ->pp in __netmem_get_pp()

[Byungchul Park <byungchul@sk.com>]

On Tue, Jul 15, 2025 at 11:37:45AM +0100, Pavel Begunkov wrote:
> On 7/14/25 20:37, Mina Almasry wrote:
> > On Mon, Jul 14, 2025 at 5:01 AM Byungchul Park <byungchul@sk.com> wrote:
> ...>> +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.
> > 
> > >   /**
> > >    * __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...
> 
> The function is not used, and I don't think the series adds any

That's whay I'd been keeping the patch, 'netmem: remove __netmem_get_pp()'
until v6 [1].

[1] https://lore.kernel.org/all/20250620041224.46646-7-byungchul@sk.com/

However, as the following change log described [2], I excluded the patch
from v7 since __netmem_get_pp() started to be used again by libeth.

[2] https://lore.kernel.org/all/20250625043350.7939-1-byungchul@sk.com/

> new users? It can be killed then. It's a horrible function anyway,
> would be much better to have a variant taking struct page * if
> necessary.
> 
> > 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.
> 
> Glad you liked the diff I suggested :) In either case, seems
> like it's not strictly necessary for this iteration as
> __netmem_get_pp() should be killed, and the rest of patches work
> directly with pages.

Killing __netmem_get_pp() would be the best I think.

	Byungchul
> 
> 
> > 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.
> > 
> 
> --
> Pavel Begunkov