Commit a3d2e9f8 authored Jun 27, 2016 by Eric Dumazet Committed by David S. Miller Jun 29, 2016

tcp: do not send too big packets at retransmit time



Arjun reported a bug in TCP stack and bisected it to a recent commit.

In case where we process SACK, we can coalesce multiple skbs
into fat ones (tcp_shift_skb_data()), to lower write queue
overhead, because we do not expect to retransmit these packets.

However, SACK reneging can happen, forcing the sender to retransmit
all these packets. If skb->len is above 64KB, we then send buggy
IP packets that could hang TSO engine on cxgb4.

Neal suggested to use tcp_tso_autosize() instead of tp->gso_segs
so that we cook packets of optimal size vs TCP/pacing.

Thanks to Arjun for reporting the bug and running the tests !

Fixes: 10d3be56 ("tcp-tso: do not split TSO packets at retransmit time")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Arjun V <arjun@chelsio.com>
Tested-by: Arjun V <arjun@chelsio.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 96183182

net/ipv4/tcp_output.c

+6 −1

Original line number	Diff line number	Diff line
		@@ -2751,7 +2751,7 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
		struct tcp_sock *tp = tcp_sk(sk);
		struct sk_buff *skb;
		struct sk_buff *hole = NULL;
		u32 last_lost;
		u32 max_segs, last_lost;
		int mib_idx;
		int fwd_rexmitting = 0;

		@@ -2771,6 +2771,7 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
		last_lost = tp->snd_una;
		}

		max_segs = tcp_tso_autosize(sk, tcp_current_mss(sk));
		tcp_for_write_queue_from(skb, sk) {
		__u8 sacked = TCP_SKB_CB(skb)->sacked;
		int segs;
		@@ -2784,6 +2785,10 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
		segs = tp->snd_cwnd - tcp_packets_in_flight(tp);
		if (segs <= 0)
		return;
		/* In case tcp_shift_skb_data() have aggregated large skbs,
		* we need to make sure not sending too bigs TSO packets
		*/
		segs = min_t(int, segs, max_segs);

		if (fwd_rexmitting) {
		begin_fwd: