netdev - Re: [PATCH net] tcp: fix tcp_fastretrans

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Date:   Fri, 10 Nov 2017 18:11:08 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     ycheng@...gle.com
Cc:     netdev@...r.kernel.org, edumazet@...gle.com, ncardwell@...gle.com,
        guro@...com, alexei.starovoitov@...il.com
Subject: Re: [PATCH net] tcp: fix tcp_fastretrans_alert warning

From: Yuchung Cheng <ycheng@...gle.com>
Date: Tue,  7 Nov 2017 15:33:43 -0800

> This patch fixes the cause of an WARNING indicatng TCP has pending
> retransmission in Open state in tcp_fastretrans_alert().
> 
> The root cause is a bad interaction between path mtu probing,
> if enabled, and the RACK loss detection. Upong receiving a SACK
> above the sequence of the MTU probing packet, RACK could mark the
> probe packet lost in tcp_fastretrans_alert(), prior to calling
> tcp_simple_retransmit().
> 
> tcp_simple_retransmit() only enters Loss state if it newly marks
> the probe packet lost. If the probe packet is already identified as
> lost by RACK, the sender remains in Open state with some packets
> marked lost and retransmitted. Then the next SACK would trigger
> the warning. The likely scenario is that the probe packet was
> lost due to its size or network congestion. The actual impact of
> this warning is small by potentially entering fast recovery an
> ACK later.
> 
> The simple fix is always entering recovery (Loss) state if some
> packet is marked lost during path MTU probing.
> 
> Fixes: a0370b3f3f2c ("tcp: enable RACK loss detection to trigger recovery")
> Reported-by: Oleksandr Natalenko <oleksandr@...alenko.name>
> Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Reported-by: Roman Gushchin <guro@...com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>

Applied and queued up for -stable, thank you.