Re: [PATCH v1 03/15] net: generalise net_iov chunk owners

[Mina Almasry <almasrymina@google.com>]

On Mon, Oct 7, 2024 at 3:16 PM David Wei <dw@davidwei.uk> wrote:
>
> From: Pavel Begunkov <asml.silence@gmail.com>
>
> Currently net_iov stores a pointer to struct dmabuf_genpool_chunk_owner,
> which serves as a useful abstraction to share data and provide a
> context. However, it's too devmem specific, and we want to reuse it for
> other memory providers, and for that we need to decouple net_iov from
> devmem. Make net_iov to point to a new base structure called
> net_iov_area, which dmabuf_genpool_chunk_owner extends.


Similar feeling to Stan initially. I also thought you'd reuse
dmabuf_genpool_chunk_owner. Seems like you're doing that but also
renaming it to net_iov_area almost, which seems fine.

I guess, with this patch, there is no way to tell, given just a
net_iov whether it's dmabuf or something else, right? I wonder if
that's an issue. In my mind when an skb is in tcp_recvmsg() we need to
make sure it's a dmabuf net_iov specifically to call
tcp_recvmsg_dmabuf for example. I'll look deeper here.

...

>
>  static inline struct dmabuf_genpool_chunk_owner *
> -net_iov_owner(const struct net_iov *niov)
> +net_devmem_iov_to_chunk_owner(const struct net_iov *niov)
>  {
> -       return niov->owner;
> -}
> +       struct net_iov_area *owner = net_iov_owner(niov);
>
> -static inline unsigned int net_iov_idx(const struct net_iov *niov)
> -{
> -       return niov - net_iov_owner(niov)->niovs;
> +       return container_of(owner, struct dmabuf_genpool_chunk_owner, area);

Couldn't this end up returning garbage if the net_iov is not actually
a dmabuf one? Is that handled somewhere in a later patch that I
missed?

-- 
Thanks,
Mina