[PATCH net-next v6 15/18] ip6, udp6: Support MSG_SPLICE_PAGES

[David Howells <dhowells@redhat.com>]

Make IP6/UDP6 sendmsg() support MSG_SPLICE_PAGES.  This causes pages to be
spliced from the source iterator if possible, copying the data if not.

This allows ->sendpage() to be replaced by something that can handle
multiple multipage folios in a single transaction.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
cc: David Ahern <dsahern@kernel.org>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: netdev@vger.kernel.org
---

Notes:
    ver #6)
     - Use common helper.

 net/ipv6/ip6_output.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 0b6140f0179d..4709abee5966 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1589,6 +1589,14 @@ static int __ip6_append_data(struct sock *sk,
 				skb_zcopy_set(skb, uarg, &extra_uref);
 			}
 		}
+	} else if ((flags & MSG_SPLICE_PAGES) && length) {
+		if (inet_sk(sk)->hdrincl)
+			return -EPERM;
+		if (rt->dst.dev->features & NETIF_F_SG)
+			/* We need an empty buffer to attach stuff to */
+			paged = true;
+		else
+			flags &= ~MSG_SPLICE_PAGES;
 	}
 
 	/*
@@ -1778,6 +1786,15 @@ static int __ip6_append_data(struct sock *sk,
 				err = -EFAULT;
 				goto error;
 			}
+		} else if (flags & MSG_SPLICE_PAGES) {
+			struct msghdr *msg = from;
+
+			err = skb_splice_from_iter(skb, &msg->msg_iter, copy,
+						   sk->sk_allocation);
+			if (err < 0)
+				goto error;
+			copy = err;
+			wmem_alloc_delta += copy;
 		} else if (!zc) {
 			int i = skb_shinfo(skb)->nr_frags;