SUNRPC: Fix suspicious enobufs issues.

The current test is racy when dealing with fast NICs.
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprtsock.c

@@ -642,6 +642,7 @@ static int xs_tcp_send_request(struct rpc_task *task)
 	struct sock_xprt *transport = container_of(xprt, struct sock_xprt, xprt);
 	struct xdr_buf *xdr = &req->rq_snd_buf;
 	bool zerocopy = true;
+	bool vm_wait = false;
 	int status;
 	int sent;
 
@@ -677,15 +678,33 @@ static int xs_tcp_send_request(struct rpc_task *task)
 			return 0;
 		}
 
+		WARN_ON_ONCE(sent == 0 && status == 0);
+
+		if (status == -EAGAIN ) {
+			/*
+			 * Return EAGAIN if we're sure we're hitting the
+			 * socket send buffer limits.
+			 */
+			if (test_bit(SOCK_NOSPACE, &transport->sock->flags))
+				break;
+			/*
+			 * Did we hit a memory allocation failure?
+			 */
+			if (sent == 0) {
+				status = -ENOBUFS;
+				if (vm_wait)
+					break;
+				/* Retry, knowing now that we're below the
+				 * socket send buffer limit
+				 */
+				vm_wait = true;
+			}
+			continue;
+		}
 		if (status < 0)
 			break;
-		if (sent == 0) {
-			status = -EAGAIN;
-			break;
-		}
+		vm_wait = false;
 	}
-	if (status == -EAGAIN && sk_stream_is_writeable(transport->inet))
-		status = -ENOBUFS;
 
 	switch (status) {
 	case -ENOTSOCK: