kcm: Support MSG_SPLICE_PAGES

Make AF_KCM sendmsg() support MSG_SPLICE_PAGES.  This causes pages to be
spliced from the source iterator if possible.

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: Tom Herbert <tom@herbertland.com>
cc: Tom Herbert <tom@quantonium.net>
cc: Cong Wang <cong.wang@bytedance.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/kcm/kcmsock.c

@@ -989,9 +989,29 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 			merge = false;
 		}
 
+		if (msg->msg_flags & MSG_SPLICE_PAGES) {
+			copy = msg_data_left(msg);
+			if (!sk_wmem_schedule(sk, copy))
+				goto wait_for_memory;
+
+			err = skb_splice_from_iter(skb, &msg->msg_iter, copy,
+						   sk->sk_allocation);
+			if (err < 0) {
+				if (err == -EMSGSIZE)
+					goto wait_for_memory;
+				goto out_error;
+			}
+
+			copy = err;
+			skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
+			sk_wmem_queued_add(sk, copy);
+			sk_mem_charge(sk, copy);
+
+			if (head != skb)
+				head->truesize += copy;
+		} else {
 			copy = min_t(int, msg_data_left(msg),
 				     pfrag->size - pfrag->offset);
-
 			if (!sk_wmem_schedule(sk, copy))
 				goto wait_for_memory;
 
@@ -1004,7 +1024,8 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 
 			/* Update the skb. */
 			if (merge) {
-			skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
+				skb_frag_size_add(
+					&skb_shinfo(skb)->frags[i - 1], copy);
 			} else {
 				skb_fill_page_desc(skb, i, pfrag->page,
 						   pfrag->offset, copy);
@@ -1012,6 +1033,8 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 			}
 
 			pfrag->offset += copy;
+		}
+
 		copied += copy;
 		if (head != skb) {
 			head->len += copy;