Re: [PATCH net-next 2/2] net/tcp: optimise io_uring zc ubuf refcounting

[Eric Dumazet <edumazet@google.com>]

On Mon, May 15, 2023 at 7:29 PM David Ahern <dsahern@kernel.org> wrote:
>
> On 5/15/23 10:06 AM, Pavel Begunkov wrote:
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 40f591f7fce1..3d18e295bb2f 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -1231,7 +1231,6 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> >       if ((flags & MSG_ZEROCOPY) && size) {
> >               if (msg->msg_ubuf) {
> >                       uarg = msg->msg_ubuf;
> > -                     net_zcopy_get(uarg);
> >                       zc = sk->sk_route_caps & NETIF_F_SG;
> >               } else if (sock_flag(sk, SOCK_ZEROCOPY)) {
> >                       skb = tcp_write_queue_tail(sk);
> > @@ -1458,7 +1457,9 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> >               tcp_push(sk, flags, mss_now, tp->nonagle, size_goal);
> >       }
> >  out_nopush:
> > -     net_zcopy_put(uarg);
> > +     /* msg->msg_ubuf is pinned by the caller so we don't take extra refs */
> > +     if (uarg && !msg->msg_ubuf)
> > +             net_zcopy_put(uarg);
> >       return copied + copied_syn;
> >
> >  do_error:
> > @@ -1467,7 +1468,9 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> >       if (copied + copied_syn)
> >               goto out;
> >  out_err:
> > -     net_zcopy_put_abort(uarg, true);
> > +     /* msg->msg_ubuf is pinned by the caller so we don't take extra refs */
> > +     if (uarg && !msg->msg_ubuf)
> > +             net_zcopy_put_abort(uarg, true);
> >       err = sk_stream_error(sk, flags, err);
> >       /* make sure we wake any epoll edge trigger waiter */
> >       if (unlikely(tcp_rtx_and_write_queues_empty(sk) && err == -EAGAIN)) {
>
> Both net_zcopy_put_abort and net_zcopy_put have an `if (uarg)` check.

Right, but here this might avoid a read of msg->msg_ubuf, which might
be more expensive to fetch.

Compiler will probably remove the second test (uarg) from net_zcopy_put()

Reviewed-by: Eric Dumazet <edumazet@google.com>