netdev - Re: crash with bridge and inconsistent handling of NETDEV_TX

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Date:	Tue, 20 Apr 2010 21:10:04 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	David Miller <davem@...emloft.net>
cc:	netdev@...r.kernel.org, kaber@...sh.net
Subject: Re: crash with bridge and inconsistent handling of NETDEV_TX_OK

On Tue, 20 Apr 2010, David Miller wrote:

> From: Mikulas Patocka <mpatocka@...hat.com>
> Date: Tue, 20 Apr 2010 20:23:57 -0400 (EDT)
> 
> > I have two NICs, each with 1500 MTU. The stack trace indicates that a 
> > packet was received by one NIC and bridged to the other. The stack trace 
> > also indicates that it went through GSO code path. The question is why? 
> > How could a forwarded packet be so large to use GSO?
> 
> GRO automatically accumulates packets together, accumulating them into
> larger super-packets.  This is done regardless of input device feeding
> it.
> 
> Can you understand this now?  In software, we accumulate all incoming
> packets for a TCP stream into larger super-packets.  Just because it's
> a bridging scenerio doesn't mean we disable GRO.
> 
> These super-packets are being bridged, then forwarded out your
> destination device and GSO has to de-segment these GRO frames.
> 
> GRO is done unconditionally, all the time, for all packets.

I see, but GRO is turned off on my interfaces, according to ethtool.

Mikulas
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