On Mon, May 15, 2023 at 7:29 PM David Ahern <dsahern@xxxxxxxxxx> 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@xxxxxxxxxx>
