netdev - Re: [PATCH v1 03/15] net: generalise net

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Message-ID: <CAHS8izMyOur0_SCrh8CJet2xeW8T39CC8b9K3hzfn5fh7hVB6Q@mail.gmail.com>
Date: Wed, 9 Oct 2024 13:44:00 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: David Wei <dw@...idwei.uk>
Cc: io-uring@...r.kernel.org, netdev@...r.kernel.org, 
	Jens Axboe <axboe@...nel.dk>, Pavel Begunkov <asml.silence@...il.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v1 03/15] net: generalise net_iov chunk owners

On Mon, Oct 7, 2024 at 3:16 PM David Wei <dw@...idwei.uk> wrote:
>
> From: Pavel Begunkov <asml.silence@...il.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