netdev - Re: [bisected] xfrm: TCP connection initiating PMTU discovery stalls on v3.

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Date:	Sat, 13 Dec 2014 01:43:05 +0100
From:	Wolfgang Walter <linux@...m.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Thomas Jarosch <thomas.jarosch@...ra2net.com>,
	netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [bisected] xfrm: TCP connection initiating PMTU discovery stalls on v3.

Am Freitag, 12. Dezember 2014, 16:15:40 schrieb Eric Dumazet:
> On Sat, 2014-12-13 at 00:47 +0100, Wolfgang Walter wrote:
> > I can't disable it as the driver will not allow it:
> > # ethtool -K eth0 tx off
> > Cannot change tx-checksumming
> > Could not change any device features
> 
> Sounds a bug in itself :(
> 

Yes, that is what I meant in my email that there seems also to be bug in the 
GSO path.

I don't understand how tcp_sendmsg() marks gso/non-gso skbs.

tcp_send_mss() calls tcp_xmit_size_goal() which calls tcp_xmit_size_goal() and 
there is a check for GSO ( sk_can_gso(sk) ). If sk_can_gso() returns false 
then tcp_xmit_size_goal() returns mss_now. gso_segs seems to be set to zero. 
In in tcp_output.c gso_segs == 0 seems to allow GSO.

What if mss later shrinks? Couldn't it be that then tcp_set_skb_tso_segs() 
gets called and then there the gso branch is taken?

Regards,
-- 
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts
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