Re: [PATCH net-next v9 1/8] netmem: introduce struct netmem_desc mirroring struct page

[Mina Almasry <almasrymina@google.com>]

On Mon, Jul 14, 2025 at 4:28 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 7/14/25 05:23, Byungchul Park wrote:
> > On Sat, Jul 12, 2025 at 03:39:59PM +0100, Pavel Begunkov wrote:
> >> On 7/10/25 09:28, 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, and make it union'ed with the existing fields in struct
> >>> net_iov, allowing to organize the fields of struct net_iov.
> >>
> >> FWIW, regardless of memdesc business, I think it'd be great to have
> >> this patch, as it'll help with some of the netmem casting ugliness and
> >> shed some cycles as well. For example, we have a bunch of
> >> niov -> netmem -> niov casts in various places.
> >
> > If Jakub agrees with this, I will re-post this as a separate patch so
> > that works that require this base can go ahead.
>
> I think it'd be a good idea. It's needed to clean up netmem handling,
> and I'll convert io_uring and get rid of the union in niov.
>
> The diff below should give a rough idea of what I want to use it for.
> It kills __netmem_clear_lsb() to avoid casting struct page * to niov.
> And saves some masking for zcrx, see page_pool_get_dma_addr_nmdesc(),
> and there are more places like that.
>
>
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index 535cf17b9134..41f3a3fd6b6c 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -247,6 +247,8 @@ static inline unsigned long netmem_pfn_trace(netmem_ref netmem)
>         return page_to_pfn(netmem_to_page(netmem));
>   }
>
> +#define pp_page_to_nmdesc(page)        ((struct netmem_desc *)(page))
> +
>   /* __netmem_clear_lsb - convert netmem_ref to struct net_iov * for access to
>    * common fields.
>    * @netmem: netmem reference to extract as net_iov.
> @@ -262,11 +264,18 @@ static inline unsigned long netmem_pfn_trace(netmem_ref netmem)
>    *
>    * Return: the netmem_ref cast to net_iov* regardless of its underlying type.
>    */
> -static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> +static inline struct net_iov *__netmem_to_niov(netmem_ref netmem)
>   {
>         return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
>   }
>
> +static inline struct netmem_desc *netmem_to_nmdesc(netmem_ref netmem)
> +{
> +       if (netmem_is_net_iov(netmem))
> +               return &__netmem_to_niov(netmem)->desc;
> +       return pp_page_to_nmdesc(__netmem_to_page(netmem));
> +}
> +

I think instead of netmem_to_nmdesc, you want __netmem_clear_lsb to
return a netmem_desc instead of net_iov.

__netmem_clear_lsb returning a net_iov was always a bit of a hack. The
return value of __netmem_clear_lsb is clearly not a net_iov, but we
needed to access the pp fields, and net_iov encapsulates the pp
fields.

-- 
Thanks,
Mina