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

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Date:	Thu, 23 Apr 2015 09:48:12 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	johunt@...mai.com, netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCH] fix tcp fin memory accounting

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())




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