[TCP]: Do not round window to MSS if window scaling.

Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@redhat.com>

net/ipv4/tcp_output.c

@@ -682,17 +682,32 @@ u32 __tcp_select_window(struct sock *sk)
 	if (free_space > tp->rcv_ssthresh)
 		free_space = tp->rcv_ssthresh;
 
-	/* Get the largest window that is a nice multiple of mss.
-	 * Window clamp already applied above.
-	 * If our current window offering is within 1 mss of the
-	 * free space we just keep it. This prevents the divide
-	 * and multiply from happening most of the time.
-	 * We also don't do any window rounding when the free space
-	 * is too small.
+	/* Don't do rounding if we are using window scaling, since the
+	 * scaled window will not line up with the MSS boundary anyway.
 	 */
 	window = tp->rcv_wnd;
-	if (window <= free_space - mss || window > free_space)
-		window = (free_space/mss)*mss;
+	if (tp->rcv_wscale) {
+		window = free_space;
+
+		/* Advertise enough space so that it won't get scaled away.
+		 * Import case: prevent zero window announcement if
+		 * 1<<rcv_wscale > mss.
+		 */
+		if (((window >> tp->rcv_wscale) << tp->rcv_wscale) != window)
+			window = (((window >> tp->rcv_wscale) + 1)
+				  << tp->rcv_wscale);
+	} else {
+		/* Get the largest window that is a nice multiple of mss.
+		 * Window clamp already applied above.
+		 * If our current window offering is within 1 mss of the
+		 * free space we just keep it. This prevents the divide
+		 * and multiply from happening most of the time.
+		 * We also don't do any window rounding when the free space
+		 * is too small.
+		 */
+		if (window <= free_space - mss || window > free_space)
+			window = (free_space/mss)*mss;
+	}
 
 	return window;
 }