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

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/15/23 19:40, David Ahern wrote:
> 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.

I put it there to avoid one extra check in the non-zerocopy path.
msg->msg_ubuf is null there, the conditional will pass and it'll
still have to test uarg.


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

Thank you for reviews!


> 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.

Ah yes, do you want me to fold it in?

-- 
Pavel Begunkov