net: Add MSG_BATCH flag

Add a new msg flag called MSG_BATCH. This flag is used in sendmsg to
indicate that more messages will follow (i.e. a batch of messages is
being sent). This is similar to MSG_MORE except that the following
messages are not merged into one packet, they are sent individually.
sendmmsg is updated so that each contained message except for the
last one is marked as MSG_BATCH.

MSG_BATCH is a performance optimization in cases where a socket
implementation can benefit by transmitting packets in a batch.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/socket.h

@@ -274,6 +274,7 @@ struct ucred {
 #define MSG_MORE	0x8000	/* Sender will send more */
 #define MSG_WAITFORONE	0x10000	/* recvmmsg(): block until 1+ packets avail */
 #define MSG_SENDPAGE_NOTLAST 0x20000 /* sendpage() internal : not the last page */
+#define MSG_BATCH	0x40000 /* sendmmsg(): more messages coming */
 #define MSG_EOF         MSG_FIN
 
 #define MSG_FASTOPEN	0x20000000	/* Send data in TCP SYN */

net/socket.c

@@ -2008,6 +2008,7 @@ int __sys_sendmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 	struct compat_mmsghdr __user *compat_entry;
 	struct msghdr msg_sys;
 	struct used_address used_address;
+	unsigned int oflags = flags;
 
 	if (vlen > UIO_MAXIOV)
 		vlen = UIO_MAXIOV;
@@ -2022,8 +2023,12 @@ int __sys_sendmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 	entry = mmsg;
 	compat_entry = (struct compat_mmsghdr __user *)mmsg;
 	err = 0;
+	flags |= MSG_BATCH;
 
 	while (datagrams < vlen) {
+		if (datagrams == vlen - 1)
+			flags = oflags;
+
 		if (MSG_CMSG_COMPAT & flags) {
 			err = ___sys_sendmsg(sock, (struct user_msghdr __user *)compat_entry,
 					     &msg_sys, flags, &used_address, MSG_EOR);