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

[David Ahern <dsahern@kernel.org>]

On 5/15/23 12:14 PM, Eric Dumazet wrote:
> 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.

agreed.

> 
> Compiler will probably remove the second test (uarg) from net_zcopy_put()
> 
> Reviewed-by: Eric Dumazet <edumazet@google.com>

The one in net_zcopy_put can be removed with the above change. It's
other caller is net_zcopy_put_abort which has already checked uarg is set.