netdev - Re: [RFC PATCH v2 15/21] net/tcp: add MSG

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Message-ID: <CANn89iJ5vyx0WqdKTB3uHaWJrG-3jNXqXs6r7PacSqg0jRsRKA@mail.gmail.com>
Date:   Mon, 27 Jul 2020 08:19:43 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, kernel-team <kernel-team@...com>,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        bjorn.topel@...el.com, magnus.karlsson@...el.com,
        borisp@...lanox.com, david@...hat.com
Subject: Re: [RFC PATCH v2 15/21] net/tcp: add MSG_NETDMA flag for sendmsg()

On Mon, Jul 27, 2020 at 12:51 AM Jonathan Lemon
<jonathan.lemon@...il.com> wrote:
>
> This flag indicates that the attached data is a zero-copy send,
> and the pages should be retrieved from the netgpu module.  The
> socket should should already have been attached to a netgpu queue.
>
> Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> ---
>  include/linux/socket.h | 1 +
>  net/ipv4/tcp.c         | 8 ++++++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 04d2bc97f497..63816cc25dee 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -310,6 +310,7 @@ struct ucred {
>                                           */
>
>  #define MSG_ZEROCOPY   0x4000000       /* Use user data in kernel path */
> +#define MSG_NETDMA     0x8000000
>  #define MSG_FASTOPEN   0x20000000      /* Send data in TCP SYN */
>  #define MSG_CMSG_CLOEXEC 0x40000000    /* Set close_on_exec for file
>                                            descriptor received through
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 261c28ccc8f6..340ce319edc9 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1214,6 +1214,14 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                         uarg->zerocopy = 0;
>         }
>
> +       if (flags & MSG_NETDMA && size && sock_flag(sk, SOCK_ZEROCOPY)) {
> +               zc = sk->sk_route_caps & NETIF_F_SG;
> +               if (!zc) {
> +                       err = -EFAULT;
> +                       goto out_err;
> +               }
> +       }
>

Sorry, no, we can not allow adding yet another branch into TCP fast
path for yet another variant of zero copy.

Overall, I think your patch series desperately tries to add changes in
TCP stack, while there is yet no proof
that you have to use TCP transport between the peers.