svcrdma: Ignore source port when computing DRC hash

The DRC appears to be effectively empty after an RPC/RDMA transport
reconnect. The problem is that each connection uses a different
source port, which defeats the DRC hash.

Clients always have to disconnect before they send retransmissions
to reset the connection's credit accounting, thus every retransmit
on NFS/RDMA will miss the DRC.

An NFS/RDMA client's IP source port is meaningless for RDMA
transports. The transport layer typically sets the source port value
on the connection to a random ephemeral port. The server already
ignores it for the "secure port" check. See commit 16e4d93f
("NFSD: Ignore client's source port on RDMA transports").

The Linux NFS server's DRC resolves XID collisions from the same
source IP address by using the checksum of the first 200 bytes of
the RPC call header.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: stable@vger.kernel.org # v4.14+
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -211,9 +211,14 @@ static void handle_connect_req(struct rdma_cm_id *new_cma_id,
 	/* Save client advertised inbound read limit for use later in accept. */
 	newxprt->sc_ord = param->initiator_depth;
 
-	/* Set the local and remote addresses in the transport */
 	sa = (struct sockaddr *)&newxprt->sc_cm_id->route.addr.dst_addr;
 	svc_xprt_set_remote(&newxprt->sc_xprt, sa, svc_addr_len(sa));
+	/* The remote port is arbitrary and not under the control of the
+	 * client ULP. Set it to a fixed value so that the DRC continues
+	 * to be effective after a reconnect.
+	 */
+	rpc_set_port((struct sockaddr *)&newxprt->sc_xprt.xpt_remote, 0);
+
 	sa = (struct sockaddr *)&newxprt->sc_cm_id->route.addr.src_addr;
 	svc_xprt_set_local(&newxprt->sc_xprt, sa, svc_addr_len(sa));