netdev - Re: [PATCH] net: Convert skb->csum_(start|offset) integrity BUG_ON() to WARN

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Date:	Wed, 13 Feb 2013 23:48:43 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] net: Convert skb->csum_(start|offset) integrity BUG_ON()
 to WARN_ON() & drop

On 02/13/13 at 11:40pm, Thomas Graf wrote:
> They have been hit with IPoIB which uses a 64K MTU. If a TCP
> retransmission gets partially ACKed and collapsed multiple times
> it is possible for the headroom to grow beyond 64K which will
> overflow the 16bit skb->csum_start.

On the subject of fixing this, I considered:

 a) Reallocate the headroom in tcp_trim_head() if it would
    overflow. I disregarded this idea because replacing the
    old skb with the new skb on the write queue for such a
    rare situation seems overkill.

 b) No longer collapse if the new skb would result in a
    a headroom + data that exceeds 64K. This seems to be the
    most trivial fix.

 c) Increase size of csum_start or store checksum_start_offset
    differently.

Other ideas?
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