[RFC net-next v3 13/29] net: let callers provide extra ubuf_info refs

[Pavel Begunkov <asml.silence@gmail.com>]

Subsystems providing external ubufs to the net layer, i.e. ->msg_ubuf,
might have a better way to refcount it. For instance, io_uring can
ammortise ref allocation.

Add a way to pass one extra ref to ->msg_ubuf into the network stack by
setting struct msghdr::msg_ubuf_ref bit. Whoever consumes the ref should
clear the flat. If not consumed, it's the responsibility of the caller
to put it. Make __ip{,6}_append_data() to use it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 include/linux/socket.h | 1 +
 net/ipv4/ip_output.c   | 3 +++
 net/ipv6/ip6_output.c  | 3 +++
 3 files changed, 7 insertions(+)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index ba84ee614d5a..ae869dee82de 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -72,6 +72,7 @@ struct msghdr {
 	 * to be non-NULL.
 	 */
 	bool		msg_managed_data : 1;
+	bool		msg_ubuf_ref : 1;
 	unsigned int	msg_flags;	/* flags on received message */
 	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
 	struct kiocb	*msg_iocb;	/* ptr to iocb for async requests */
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 3fd1bf675598..d73ec0a73bd2 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1032,6 +1032,9 @@ static int __ip_append_data(struct sock *sk,
 				paged = true;
 				zc = true;
 				uarg = msg->msg_ubuf;
+				/* we might've been given a free ref */
+				extra_uref = msg->msg_ubuf_ref;
+				msg->msg_ubuf_ref = false;
 			}
 		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
 			uarg = msg_zerocopy_realloc(sk, length, skb_zcopy(skb));
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index f4138ce6eda3..90bbaab21dbc 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1557,6 +1557,9 @@ static int __ip6_append_data(struct sock *sk,
 				paged = true;
 				zc = true;
 				uarg = msg->msg_ubuf;
+				/* we might've been given a free ref */
+				extra_uref = msg->msg_ubuf_ref;
+				msg->msg_ubuf_ref = false;
 			}
 		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
 			uarg = msg_zerocopy_realloc(sk, length, skb_zcopy(skb));
-- 
2.36.1