netdev - Re: [PATCH] fix tcp fin memory accounting

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Date:	Thu, 23 Apr 2015 13:12:34 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	johunt@...mai.com, netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCH] fix tcp fin memory accounting

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 23 Apr 2015 09:48:12 -0700

> On Thu, 2015-04-23 at 09:16 -0700, Eric Dumazet wrote:
>> On Thu, 2015-04-23 at 11:54 -0400, David Miller wrote:
>> > From: Eric Dumazet <eric.dumazet@...il.com>
>> > Date: Thu, 23 Apr 2015 07:02:47 -0700
>> > 
>> > > +		if (!tcp_send_head(sk)) {
>> > > +			tp->snd_nxt++;
>> > > +			return;
>> > > +		}
>> > 
>> > I'm not so sure about this.  Why is this needed?
>> > 
>> > Otherwise patch looks fine to me.
>> > 
>> 
>> I guess I need to add a comment then ;)
>> 
>> If we want to pretend FIN was sent, we also need to tweak tp->snd_nxt to
>> match new tskb->end_seq (or tp->write_seq).
>> 
>> I tested following packetdrill script and confirmed that if I do not
>> tweak snd_nxt, last packet sent is incorrect :
>> 
>> 	> . 5001:5001(0) ack 2
>> 
>> This might be because our stack relies that we never coalesce something
>> on one already sent skb (we do this check in tcp_sendmsg() for example)
> 
> Well, real reason is that tp->snd_nxt is not touched in retransmit
> paths, but when new data is sent (tcp_event_new_data_sent())

That makes sense, thanks for explaining.
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