sunrpc: flag transports as having congestion control

NFSv4 requires a transport protocol with congestion control in most
cases.

On an IP network, that means that NFSv4 over UDP should be forbidden.

The situation with RDMA is a bit more nuanced, but most RDMA transports
are suitable for this. For now, we assume that all RDMA transports are
suitable, but we may need to revise that at some point.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/linux/sunrpc/svc_xprt.h

@@ -67,6 +67,7 @@ struct svc_xprt {
 #define XPT_CACHE_AUTH	11		/* cache auth info */
 #define XPT_LOCAL	12		/* connection from loopback interface */
 #define XPT_KILL_TEMP   13		/* call xpo_kill_temp_xprt before closing */
+#define XPT_CONG_CTRL	14		/* has congestion control */
 
 	struct svc_serv		*xpt_server;	/* service for transport */
 	atomic_t    	    	xpt_reserved;	/* space on outq that is rsvd */

net/sunrpc/svcsock.c

@@ -1306,6 +1306,7 @@ static void svc_tcp_init(struct svc_sock *svsk, struct svc_serv *serv)
 	svc_xprt_init(sock_net(svsk->sk_sock->sk), &svc_tcp_class,
 		      &svsk->sk_xprt, serv);
 	set_bit(XPT_CACHE_AUTH, &svsk->sk_xprt.xpt_flags);
+	set_bit(XPT_CONG_CTRL, &svsk->sk_xprt.xpt_flags);
 	if (sk->sk_state == TCP_LISTEN) {
 		dprintk("setting up TCP socket for listening\n");
 		set_bit(XPT_LISTENER, &svsk->sk_xprt.xpt_flags);

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -569,6 +569,14 @@ static struct svcxprt_rdma *rdma_create_xprt(struct svc_serv *serv,
 	spin_lock_init(&cma_xprt->sc_ctxt_lock);
 	spin_lock_init(&cma_xprt->sc_map_lock);
 
+	/*
+	 * Note that this implies that the underlying transport support
+	 * has some form of congestion control (see RFC 7530 section 3.1
+	 * paragraph 2). For now, we assume that all supported RDMA
+	 * transports are suitable here.
+	 */
+	set_bit(XPT_CONG_CTRL, &cma_xprt->sc_xprt.xpt_flags);
+
 	if (listener)
 		set_bit(XPT_LISTENER, &cma_xprt->sc_xprt.xpt_flags);