tcp: use TCP_DEFAULT_INIT_RCVWND in tcp_fixup_rcvbuf()

Since commit 356f0398 (TCP: increase default initial receive
window.), we allow sender to send 10 (TCP_DEFAULT_INIT_RCVWND) segments.

Change tcp_fixup_rcvbuf() to reflect this change, even if no real change
is expected, since sysctl_tcp_rmem[1] = 87380 and this value
is bigger than tcp_fixup_rcvbuf() computed rcvmem (~23720)

Note: Since commit 356f0398 limited default window to maximum of
10*1460 and 2*MSS, we use same heuristic in this patch.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -345,17 +345,24 @@ static void tcp_grow_window(struct sock *sk, struct sk_buff *skb)
 
 static void tcp_fixup_rcvbuf(struct sock *sk)
 {
-	struct tcp_sock *tp = tcp_sk(sk);
-	int rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
+	u32 mss = tcp_sk(sk)->advmss;
+	u32 icwnd = TCP_DEFAULT_INIT_RCVWND;
+	int rcvmem;
 
-	/* Try to select rcvbuf so that 4 mss-sized segments
-	 * will fit to window and corresponding skbs will fit to our rcvbuf.
-	 * (was 3; 4 is minimum to allow fast retransmit to work.)
+	/* Limit to 10 segments if mss <= 1460,
+	 * or 14600/mss segments, with a minimum of two segments.
 	 */
-	while (tcp_win_from_space(rcvmem) < tp->advmss)
+	if (mss > 1460)
+		icwnd = max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);
+
+	rcvmem = SKB_TRUESIZE(mss + MAX_TCP_HEADER);
+	while (tcp_win_from_space(rcvmem) < mss)
 		rcvmem += 128;
-	if (sk->sk_rcvbuf < 4 * rcvmem)
-		sk->sk_rcvbuf = min(4 * rcvmem, sysctl_tcp_rmem[2]);
+
+	rcvmem *= icwnd;
+
+	if (sk->sk_rcvbuf < rcvmem)
+		sk->sk_rcvbuf = min(rcvmem, sysctl_tcp_rmem[2]);
 }
 
 /* 4. Try to fixup all. It is made immediately after connection enters