tcp: allow one skb to be received per socket under memory pressure

While testing tight tcp_mem settings, I found tcp sessions could be
stuck because we do not allow even one skb to be received on them.

By allowing one skb to be received, we introduce fairness and
eventuallu force memory hogs to release their allocation.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -4507,10 +4507,12 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
 
 		if (eaten <= 0) {
 queue_and_out:
-			if (eaten < 0 &&
-			    tcp_try_rmem_schedule(sk, skb, skb->truesize))
-				goto drop;
-
+			if (eaten < 0) {
+				if (skb_queue_len(&sk->sk_receive_queue) == 0)
+					sk_forced_mem_schedule(sk, skb->truesize);
+				else if (tcp_try_rmem_schedule(sk, skb, skb->truesize))
+					goto drop;
+			}
 			eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
 		}
 		tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);