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

Mina Almasry
2025-05-27T20:03:45Z

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.

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