Re: [PATCH 01/18] netmem: introduce struct netmem_desc struct_group_tagged()'ed on struct net_iov

[Byungchul Park <byungchul@sk.com>]

On Tue, May 27, 2025 at 01:03:32PM -0700, Mina Almasry wrote:
> On Mon, May 26, 2025 at 7:50 PM Byungchul Park <byungchul@sk.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@sk.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.

I told you I will fix it.  I will send the updated series shortly for
*review*.  However, it will be for review since we know this work can be
completed once the next works have been done:

   https://lore.kernel.org/all/20250520205920.2134829-2-anthony.l.nguyen@intel.com/
   https://lore.kernel.org/all/1747950086-1246773-9-git-send-email-tariqt@nvidia.com/

> 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

They are going to struct net_iov.  Or I should introduce another struct
mirroring struct page as ptdesc did, that would be the exact same as
struct net_iov.  Do you think I should do that?

> 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

Okay, again.  Thanks.

	Byungchul

> 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