netdev - Re: [PATCH net-next v6 07/18] tcp: Support MSG_SPLICE

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Date:   Tue, 11 Apr 2023 19:09:00 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Howells <dhowells@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jeff Layton <jlayton@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Chuck Lever III <chuck.lever@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH net-next v6 07/18] tcp: Support MSG_SPLICE_PAGES

On Tue, Apr 11, 2023 at 6:09 PM David Howells <dhowells@...hat.com> wrote:
>
> Make TCP's sendmsg() support MSG_SPLICE_PAGES.  This causes pages to be
> spliced or copied (if it cannot be spliced) from the source iterator.
>
> This allows ->sendpage() to be replaced by something that can handle
> multiple multipage folios in a single transaction.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Eric Dumazet <edumazet@...gle.com>
> cc: "David S. Miller" <davem@...emloft.net>
> cc: David Ahern <dsahern@...nel.org>
> cc: Jakub Kicinski <kuba@...nel.org>
> cc: Paolo Abeni <pabeni@...hat.com>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: Matthew Wilcox <willy@...radead.org>
> cc: netdev@...r.kernel.org
> ---
>
> Notes:
>     ver #6)
>      - Use common helper.
>
>  net/ipv4/tcp.c | 43 ++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 36 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index fd68d49490f2..0b2213da5aaf 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1221,7 +1221,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>         int flags, err, copied = 0;
>         int mss_now = 0, size_goal, copied_syn = 0;
>         int process_backlog = 0;
> -       bool zc = false;
> +       int zc = 0;
>         long timeo;
>
>         flags = msg->msg_flags;
> @@ -1232,17 +1232,22 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                 if (msg->msg_ubuf) {
>                         uarg = msg->msg_ubuf;
>                         net_zcopy_get(uarg);
> -                       zc = sk->sk_route_caps & NETIF_F_SG;
> +                       if (sk->sk_route_caps & NETIF_F_SG)
> +                               zc = 1;

zc is set to 0, 1, MSG_ZEROCOPY ,   MSG_SPLICE_PAGES

I find this a bit confusing. Maybe use a private enum ?

>                 } 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)
> +                       if (sk->sk_route_caps & NETIF_F_SG)
> +                               zc = MSG_ZEROCOPY;
> +                       else
>                                 uarg_to_msgzc(uarg)->zerocopy = 0;
>                 }
> +       } else if (unlikely(msg->msg_flags & MSG_SPLICE_PAGES) && size) {
> +               if (sk->sk_route_caps & NETIF_F_SG)
> +                       zc = MSG_SPLICE_PAGES;