Re: [PATCH net-next v5 05/19] tcp: Support MSG_SPLICE_PAGES

[Willem de Bruijn]

On Thu, Apr 6, 2023 at 5:43 AM David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Make TCP's sendmsg() support MSG_SPLICE_PAGES.  This causes pages to 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@xxxxxxxxxx>
> cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> cc: David Ahern <dsahern@xxxxxxxxxx>
> cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> cc: Jens Axboe <axboe@xxxxxxxxx>
> cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> cc: netdev@xxxxxxxxxxxxxxx
> ---
>  net/ipv4/tcp.c | 67 ++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 60 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index fd68d49490f2..510bacc7ce7b 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;
>                 } 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 = 1;
> +                       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 = 2;
>         }
>
>         if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect) &&
> @@ -1305,7 +1310,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                 goto do_error;
>
>         while (msg_data_left(msg)) {
> -               int copy = 0;
> +               ssize_t copy = 0;
>
>                 skb = tcp_write_queue_tail(sk);
>                 if (skb)
> @@ -1346,7 +1351,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                 if (copy > msg_data_left(msg))
>                         copy = msg_data_left(msg);
>
> -               if (!zc) {
> +               if (zc == 0) {
>                         bool merge = true;
>                         int i = skb_shinfo(skb)->nr_frags;
>                         struct page_frag *pfrag = sk_page_frag(sk);
> @@ -1391,7 +1396,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                                 page_ref_inc(pfrag->page);
>                         }
>                         pfrag->offset += copy;
> -               } else {
> +               } else if (zc == 1)  {

Instead of 1 and 2, MSG_ZEROCOPY and MSG_SPLICE_PAGES make the code
more self-documenting.

>                         /* First append to a fragless skb builds initial
>                          * pure zerocopy skb
>                          */
> @@ -1412,6 +1417,54 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
>                         if (err < 0)
>                                 goto do_error;
>                         copy = err;
> +               } else if (zc == 2) {
> +                       /* Splice in data. */
> +                       struct page *page = NULL, **pages = &page;
> +                       size_t off = 0, part;
> +                       bool can_coalesce;
> +                       int i = skb_shinfo(skb)->nr_frags;
> +
> +                       copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
> +                                                     copy, 1, 0, &off);
> +                       if (copy <= 0) {
> +                               err = copy ?: -EIO;
> +                               goto do_error;
> +                       }
> +
> +                       can_coalesce = skb_can_coalesce(skb, i, page, off);
> +                       if (!can_coalesce && i >= READ_ONCE(sysctl_max_skb_frags)) {
> +                               tcp_mark_push(tp, skb);
> +                               iov_iter_revert(&msg->msg_iter, copy);
> +                               goto new_segment;
> +                       }
> +                       if (tcp_downgrade_zcopy_pure(sk, skb)) {
> +                               iov_iter_revert(&msg->msg_iter, copy);
> +                               goto wait_for_space;
> +                       }
> +
> +                       part = tcp_wmem_schedule(sk, copy);
> +                       iov_iter_revert(&msg->msg_iter, copy - part);
> +                       if (!part)
> +                               goto wait_for_space;
> +                       copy = part;
> +
> +                       if (can_coalesce) {
> +                               skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
> +                       } else {
> +                               get_page(page);
> +                               skb_fill_page_desc_noacc(skb, i, page, off, copy);
> +                       }
> +                       page = NULL;
> +
> +                       if (!(flags & MSG_NO_SHARED_FRAGS))
> +                               skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
> +
> +                       skb->len += copy;
> +                       skb->data_len += copy;
> +                       skb->truesize += copy;
> +                       sk_wmem_queued_add(sk, copy);
> +                       sk_mem_charge(sk, copy);
> +

Similar to udp, perhaps in a helper?