Re: [RFC 02/19] netmem: introduce netmem alloc/put API to wrap page alloc/put API

[Byungchul Park <byungchul@sk.com>]

On Fri, May 09, 2025 at 07:08:23AM -0700, Mina Almasry wrote:
> j
> 
> On Fri, May 9, 2025 at 6:39 AM Mina Almasry <almasrymina@google.com> wrote:
> >
> > On Fri, May 9, 2025 at 4:51 AM Byungchul Park <byungchul@sk.com> wrote:
> > >
> > > To eliminate the use of struct page in page pool, the page pool code
> > > should use netmem descriptor and API instead.
> > >
> > > As part of the work, introduce netmem alloc/put API allowing the code to
> > > use them rather than struct page things.
> > >
> > > Signed-off-by: Byungchul Park <byungchul@sk.com>
> > > ---
> > >  include/net/netmem.h | 18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > >
> > > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > > index 45c209d6cc689..c87a604e980b9 100644
> > > --- a/include/net/netmem.h
> > > +++ b/include/net/netmem.h
> > > @@ -138,6 +138,24 @@ static inline netmem_ref page_to_netmem(struct page *page)
> > >         return (__force netmem_ref)page;
> > >  }
> > >
> > > +static inline netmem_ref alloc_netmems_node(int nid, gfp_t gfp_mask,
> > > +               unsigned int order)
> > > +{
> > > +       return page_to_netmem(alloc_pages_node(nid, gfp_mask, order));
> > > +}
> > > +
> > > +static inline unsigned long alloc_netmems_bulk_node(gfp_t gfp, int nid,
> > > +               unsigned long nr_netmems, netmem_ref *netmem_array)
> > > +{
> > > +       return alloc_pages_bulk_node(gfp, nid, nr_netmems,
> > > +                       (struct page **)netmem_array);
> > > +}
> > > +
> > > +static inline void put_netmem(netmem_ref netmem)
> > > +{
> > > +       put_page(netmem_to_page(netmem));
> > > +}
> >
> > We can't really do this. netmem_ref is an abstraction that can be a
> > struct page or struct net_iov underneath. We can't be sure when
> > put_netmem is called that it is safe to convert to a page via
> > netmem_to_page(). This will crash if put_netmem is called on a
> > netmem_ref that is a net_iov underneath.
> >
> 
> On a closer look, it looks like this put_netmem is only called from
> code paths where you are sure the netmem_ref is a page underneath, so
> this is likely fine for now. There is a net_next series that is adding
> proper put_netmem support [1]. It's probably best to rebase your work
> on top of that, but this should be fine in the meantime.

Indeed.  Hm..  It'd be the best to work on the top of yours.

If it takes too long, I keep working on as it is for now and will adjust
this patch later once your work gets merged.

	Byungchul

> [1] https://lore.kernel.org/netdev/20250508004830.4100853-1-almasrymina@google.com/
> 
> --
> Thanks,
> Mina