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

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Message-Id: <20100420.180253.159346294.davem@davemloft.net>
Date:	Tue, 20 Apr 2010 18:02:53 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mpatocka@...hat.com
Cc:	netdev@...r.kernel.org, kaber@...sh.net
Subject: Re: crash with bridge and inconsistent handling of NETDEV_TX_OK

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.
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