linux-kernel - [RFC PATCH 3/3] net: Declare MSG_SPLICE

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Message-Id: <20230322135612.3265850-4-dhowells@redhat.com>
Date:   Wed, 22 Mar 2023 13:56:12 +0000
From:   David Howells <dhowells@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     David Howells <dhowells@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: [RFC PATCH 3/3] net: Declare MSG_SPLICE_PAGES internal sendmsg() flag

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@...hat.com>
cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>
cc: "David S. Miller" <davem@...emloft.net>
cc: Eric Dumazet <edumazet@...gle.com>
cc: Jakub Kicinski <kuba@...nel.org>
cc: Paolo Abeni <pabeni@...hat.com>
cc: Jens Axboe <axboe@...nel.dk>
cc: Matthew Wilcox <willy@...radead.org>
cc: netdev@...r.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