linux-kernel - Re: [PATCH 01/18] netmem: introduce struct netmem_desc struct_group_tagged()'ed on struct net

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Message-ID: <CAHS8izOJ6BEhiY6ApKuUkKw8+_R_pZ7kKwE9NqzCyC=g_2JGcA@mail.gmail.com>
Date: Tue, 27 May 2025 13:03:32 -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, davem@...emloft.net, john.fastabend@...il.com, 
	andrew+netdev@...n.ch, asml.silence@...il.com, toke@...hat.com, 
	tariqt@...dia.com, edumazet@...gle.com, pabeni@...hat.com, saeedm@...dia.com, 
	leon@...nel.org, ast@...nel.org, daniel@...earbox.net, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz, 
	rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, horms@...nel.org, 
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com
Subject: Re: [PATCH 01/18] netmem: introduce struct netmem_desc
 struct_group_tagged()'ed on struct net_iov

On Mon, May 26, 2025 at 7:50 PM Byungchul Park <byungchul@...com> wrote:
>
> On Fri, May 23, 2025 at 12:25:52PM +0900, Byungchul Park wrote:
> > To simplify struct page, the page pool members of struct page should be
> > moved to other, allowing these members to be removed from struct page.
> >
> > Introduce a network memory descriptor to store the members, struct
> > netmem_desc, reusing struct net_iov that already mirrored struct page.
> >
> > While at it, relocate _pp_mapping_pad to group struct net_iov's fields.
> >
> > Signed-off-by: Byungchul Park <byungchul@...com>
> > ---
> >  include/linux/mm_types.h |  2 +-
> >  include/net/netmem.h     | 43 +++++++++++++++++++++++++++++++++-------
> >  2 files changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 56d07edd01f9..873e820e1521 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -120,13 +120,13 @@ struct page {
> >                       unsigned long private;
> >               };
> >               struct {        /* page_pool used by netstack */
> > +                     unsigned long _pp_mapping_pad;
> >                       /**
> >                        * @pp_magic: magic value to avoid recycling non
> >                        * page_pool allocated pages.
> >                        */
> >                       unsigned long pp_magic;
> >                       struct page_pool *pp;
> > -                     unsigned long _pp_mapping_pad;
> >                       unsigned long dma_addr;
> >                       atomic_long_t pp_ref_count;
> >               };
> > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > index 386164fb9c18..08e9d76cdf14 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -31,12 +31,41 @@ enum net_iov_type {
> >  };
> >
> >  struct net_iov {
> > -     enum net_iov_type type;
> > -     unsigned long pp_magic;
> > -     struct page_pool *pp;
> > -     struct net_iov_area *owner;
> > -     unsigned long dma_addr;
> > -     atomic_long_t pp_ref_count;
> > +     /*
> > +      * XXX: Now that struct netmem_desc overlays on struct page,
> > +      * struct_group_tagged() should cover all of them.  However,
> > +      * a separate struct netmem_desc should be declared and embedded,
> > +      * once struct netmem_desc is no longer overlayed but it has its
> > +      * own instance from slab.  The final form should be:
> > +      *
> > +      *    struct netmem_desc {
> > +      *         unsigned long pp_magic;
> > +      *         struct page_pool *pp;
> > +      *         unsigned long dma_addr;
> > +      *         atomic_long_t pp_ref_count;
> > +      *    };
> > +      *
> > +      *    struct net_iov {
> > +      *         enum net_iov_type type;
> > +      *         struct net_iov_area *owner;
> > +      *         struct netmem_desc;
> > +      *    };
> > +      */
> > +     struct_group_tagged(netmem_desc, desc,
>
> So..  For now, this is the best option we can pick.  We can do all that
> you told me once struct netmem_desc has it own instance from slab.
>
> Again, it's because the page pool fields (or netmem things) from struct
> page will be gone by this series.
>
> Mina, thoughts?
>

Can you please post an updated series with the approach you have in
mind? I think this series as-is seems broken vis-a-vie the
_pp_padding_map param move that looks incorrect. Pavel and I have also
commented on patch 18 that removing the ASSERTS seems incorrect as
it's breaking the symmetry between struct page and struct net_iov.

It's not clear to me if the fields are being removed from struct page,
where are they going... the approach ptdesc for example has taken is
to create a mirror of struct page, then show via asserts that the
mirror is equivalent to struct page, AFAIU:

https://elixir.bootlin.com/linux/v6.14.3/source/include/linux/mm_types.h#L437

Also the same approach for zpdesc:

https://elixir.bootlin.com/linux/v6.14.3/source/mm/zpdesc.h#L29

In this series you're removing the entries from struct page, I'm not
really sure where they went, and you're removing the asserts that we
have between net_iov and struct page so we're not even sure that those
are in sync anymore. I would suggest for me at least reposting with
the new types you have in mind and with clear asserts showing what is
meant to be in sync (and overlay) what.

-- 
Thanks,
Mina