netdev - Re: [PATCH net-next v4 11/27] tcp: support externally provided ubufs

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Date:   Thu, 7 Jul 2022 22:06:54 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jens Axboe <axboe@...nel.dk>, kernel-team@...com
Subject: Re: [PATCH net-next v4 11/27] tcp: support externally provided ubufs

On 7/7/22 5:49 AM, Pavel Begunkov wrote:
> Teach ipv4/udp how to use external ubuf_info provided in msghdr and

ipv4/tcp?

> also prepare it for managed frags by sprinkling
> skb_zcopy_downgrade_managed() when it could mix managed and not managed
> frags.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  net/ipv4/tcp.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 390eb3dc53bd..a81f694af5e9 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1223,17 +1223,23 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>  
>  	flags = msg->msg_flags;
>  
> -	if (flags & MSG_ZEROCOPY && size && sock_flag(sk, SOCK_ZEROCOPY)) {
> +	if ((flags & MSG_ZEROCOPY) && size) {
>  		skb = tcp_write_queue_tail(sk);
> -		uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
> -		if (!uarg) {
> -			err = -ENOBUFS;
> -			goto out_err;
> -		}
>  
> -		zc = sk->sk_route_caps & NETIF_F_SG;
> -		if (!zc)
> -			uarg->zerocopy = 0;
> +		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)) {
> +			uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
> +			if (!uarg) {
> +				err = -ENOBUFS;
> +				goto out_err;
> +			}
> +			zc = sk->sk_route_caps & NETIF_F_SG;
> +			if (!zc)
> +				uarg->zerocopy = 0;
> +		}
>  	}
>  
>  	if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect) &&
> @@ -1356,9 +1362,11 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>  
>  			copy = min_t(int, copy, pfrag->size - pfrag->offset);
>  
> -			if (tcp_downgrade_zcopy_pure(sk, skb))
> -				goto wait_for_space;
> -
> +			if (unlikely(skb_zcopy_pure(skb) || skb_zcopy_managed(skb))) {
> +				if (tcp_downgrade_zcopy_pure(sk, skb))
> +					goto wait_for_space;
> +				skb_zcopy_downgrade_managed(skb);
> +			}
>  			copy = tcp_wmem_schedule(sk, copy);
>  			if (!copy)
>  				goto wait_for_space;

You dropped the msg->msg_ubuf checks on jump labels. Removing the one
you had at 'out_nopush' I agree with based on my tests (i.e, it is not
needed).

The one at 'out_err' seems like it is needed - but it has been a few
weeks since I debugged that case. I believe the error path I was hitting
was sk_stream_wait_memory with MSG_DONTWAIT flag set meaning timeout is
0 and it jumps there with EPIPE.