dlm: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage

When transmitting data, call down a layer using a single sendmsg with
MSG_SPLICE_PAGES to indicate that content should be spliced rather using
sendpage.  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: Christine Caulfield <ccaulfie@redhat.com>
cc: David Teigland <teigland@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: cluster-devel@redhat.com
Link: https://lore.kernel.org/r/20230623225513.2732256-7-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

fs/dlm/lowcomms.c

@@ -1395,8 +1395,11 @@ int dlm_lowcomms_resend_msg(struct dlm_msg *msg)
 /* Send a message */
 static int send_to_sock(struct connection *con)
 {
-	const int msg_flags = MSG_DONTWAIT | MSG_NOSIGNAL;
 	struct writequeue_entry *e;
+	struct bio_vec bvec;
+	struct msghdr msg = {
+		.msg_flags = MSG_SPLICE_PAGES | MSG_DONTWAIT | MSG_NOSIGNAL,
+	};
 	int len, offset, ret;
 
 	spin_lock_bh(&con->writequeue_lock);
@@ -1412,8 +1415,9 @@ static int send_to_sock(struct connection *con)
 	WARN_ON_ONCE(len == 0 && e->users == 0);
 	spin_unlock_bh(&con->writequeue_lock);
 
-	ret = kernel_sendpage(con->sock, e->page, offset, len,
-			      msg_flags);
+	bvec_set_page(&bvec, e->page, len, offset);
+	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, len);
+	ret = sock_sendmsg(con->sock, &msg);
 	trace_dlm_send(con->nodeid, ret);
 	if (ret == -EAGAIN || ret == 0) {
 		lock_sock(con->sock->sk);