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
