Declare MSG_SPLICE_PAGES, an internal sendmsg() flag, that hints to a
network protocol that it should splice pages from the source iterator
rather than copying the data if it can.  This is set in msg->msg_kflags,
not msg->msg_flags, thereby isolating it from the UAPI.

This is intended as a replacement for the ->sendpage() op, allowing a way
to splice in several multipage folios in one go.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
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
---
 include/linux/socket.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/socket.h b/include/linux/socket.h
index 13c3a237b9c9..229f54484d3c 100644
--- a/include/linux/socket.h
+++ b/include/linux/socket.h
@@ -72,6 +72,7 @@ struct msghdr {
 	bool		msg_control_is_user : 1;
 	bool		msg_get_inq : 1;/* return INQ after receive */
 	unsigned int	msg_flags;	/* flags on received message */
+	unsigned int	msg_kflags;	/* Kernel internal flags */
 	__kernel_size_t	msg_controllen;	/* ancillary data buffer length */
 	struct kiocb	*msg_iocb;	/* ptr to iocb for async requests */
 	struct ubuf_info *msg_ubuf;
@@ -337,6 +338,8 @@ struct ucred {
 #define MSG_CMSG_COMPAT	0		/* We never have 32 bit fixups */
 #endif
 
+/* Flags for msghdr::msg_kflags (all internal to the kernel) */
+#define MSG_SPLICE_PAGES 0x00000001	/* Splice the pages from the iterator in sendmsg() */
 
 /* Setsockoptions(2) level. Thanks to BSD these must match IPPROTO_xxx */
 #define SOL_IP		0