Re: [net-next v1 13/16] tcp: RX path for devmem TCP

[Mina Almasry]

On Fri, Dec 8, 2023 at 9:55 AM David Ahern <dsahern@xxxxxxxxxx> wrote:
>
> On 12/7/23 5:52 PM, Mina Almasry wrote:
> > In tcp_recvmsg_locked(), detect if the skb being received by the user
> > is a devmem skb. In this case - if the user provided the MSG_SOCK_DEVMEM
> > flag - pass it to tcp_recvmsg_devmem() for custom handling.
> >
> > tcp_recvmsg_devmem() copies any data in the skb header to the linear
> > buffer, and returns a cmsg to the user indicating the number of bytes
> > returned in the linear buffer.
> >
> > tcp_recvmsg_devmem() then loops over the unaccessible devmem skb frags,
> > and returns to the user a cmsg_devmem indicating the location of the
> > data in the dmabuf device memory. cmsg_devmem contains this information:
> >
> > 1. the offset into the dmabuf where the payload starts. 'frag_offset'.
> > 2. the size of the frag. 'frag_size'.
> > 3. an opaque token 'frag_token' to return to the kernel when the buffer
> > is to be released.
> >
> > The pages awaiting freeing are stored in the newly added
> > sk->sk_user_pages, and each page passed to userspace is get_page()'d.
> > This reference is dropped once the userspace indicates that it is
> > done reading this page.  All pages are released when the socket is
> > destroyed.
> >
> > Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx>
> > Signed-off-by: Kaiyuan Zhang <kaiyuanz@xxxxxxxxxx>
> > Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
> >
> > ---
> >
> > Changes in v1:
> > - Added dmabuf_id to dmabuf_cmsg (David/Stan).
> > - Devmem -> dmabuf (David).
> > - Change tcp_recvmsg_dmabuf() check to skb->dmabuf (Paolo).
> > - Use __skb_frag_ref() & napi_pp_put_page() for refcounting (Yunsheng).
> >
> > RFC v3:
> > - Fixed issue with put_cmsg() failing silently.
> >
>
> What happens if a retransmitted packet is received or an rx window is
> closed and a probe is received where the kernel drops the skb - is the
> iov reference(s) in the skb returned to the pool by the stack and ready
> for use again?

When an skb is dropped, skb_frag_unref() is called on the frags, which
calls napi_pp_put_page(), drops the references, and the iov is
recycled, yes.

-- 
Thanks,
Mina