Message-ID: <20250526011539.GB74632@system.software.com>
Date: Mon, 26 May 2025 10:15:39 +0900
From: Byungchul Park <byungchul@...com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: willy@...radead.org, netdev@...r.kernel.org,
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Subject: Re: [PATCH 01/18] netmem: introduce struct netmem_desc
 struct_group_tagged()'ed on struct net_iov

On Fri, May 23, 2025 at 10:00:55AM -0700, Mina Almasry wrote:
> On Thu, May 22, 2025 at 8:26 PM Byungchul Park <byungchul@...com> 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;
> 
> Like Toke says, moving this to the beginning of this struct is not
> allowed. The first 3 bits of pp_magic are overlaid with page->lru so
> the pp makes sure not to use them. _pp_mapping_pad is overlaid with
> page->mapping, so the pp makes sure not to use it. AFAICT, this moving
> of _pp_mapping_pad is not necessary for this patch. I think just drop
> it.

Sure, I will.  Thanks.

> >                         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;
> > +        *    };
> > +        */
> 
> I'm unclear on why moving to this format is a TODO for the future. Why
> isn't this state in the comment the state in the code? I think I gave
> the same code snippet on the RFC, but here again:
> 
> struct netmem_desc {
>                         /**
>                          * @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;
> };
> 
> (Roughly):
> 
> struct page {
>    ...
>   struct {        /* page_pool used by netstack */
>      struct netmem_desc;

This is unnecessary since it will be removed shortly.

>   };
>   ...
> };
> 
> struct net_iov {
>     enum net_iov_type type;
>     struct netmem_desc;

This requires a huge change in a single commit since all the code
referring to any of the page pool fields, struct net_iov, and maybe
io_uring(?) should be altered at once.

Plus, much more changes are required since struct netmem_desc would not
overlay on struct page with what you suggest, which breaks the
assumption that struct netmem_desc overlays on struct page in the
current code.

So at least, the work should be started once the code doesn't need the
assumption.

Thoughts?

	Byungchul

>     struct net_iov_area *owner;
> }
> 
> AFAICT, this should work..?
> 
> -- 
> Thanks,
> Mina

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