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

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Message-ID: <CAHS8izP3knY42632AcfTHcpgpSz49gP0j6CnyswUoHW6JtC3=w@mail.gmail.com>
Date: Fri, 9 May 2025 07:08:23 -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, 
	linux-mm@...ck.org, kernel_team@...ynix.com, kuba@...nel.org, 
	ilias.apalodimas@...aro.org, harry.yoo@...cle.com, hawk@...nel.org, 
	akpm@...ux-foundation.org, ast@...nel.org, daniel@...earbox.net, 
	davem@...emloft.net, john.fastabend@...il.com, andrew+netdev@...n.ch, 
	edumazet@...gle.com, pabeni@...hat.com, vishal.moola@...il.com
Subject: Re: [RFC 02/19] netmem: introduce netmem alloc/put API to wrap page
 alloc/put API

j

On Fri, May 9, 2025 at 6:39 AM Mina Almasry <almasrymina@...gle.com> wrote:
>
> On Fri, May 9, 2025 at 4:51 AM Byungchul Park <byungchul@...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@...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.

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

--
Thanks,
Mina