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

[David Ahern <dsahern@kernel.org>]

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@google.com>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@google.com>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
> 
> 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?