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

David Ahern
2022-07-08T04:07:06Z

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@gmail.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.