linux-kernel - Re: Kernel Panic with bonding + IPoIB on 3.2.9

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Date:	Mon, 19 Mar 2012 21:30:26 -0700
From:	Jay Vosburgh <fubar@...ibm.com>
To:	Joseph Glanville <joseph.glanville@...onvm.com.au>
cc:	Roland Dreier <roland@...estorage.com>, linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: Kernel Panic with bonding + IPoIB on 3.2.9

Joseph Glanville <joseph.glanville@...onvm.com.au> wrote:

>On 20 March 2012 06:05, Roland Dreier <roland@...estorage.com> wrote:
>> On Sun, Mar 18, 2012 at 1:21 PM, Joseph Glanville
>> <joseph.glanville@...onvm.com.au> wrote:
>>> [  422.047024] kernel BUG at net/core/dev.c:1896!
>>
>> So this line is
>>
>>        BUG_ON(offset >= skb_headlen(skb));
>>
>> right?  No paritcular idea how we hit this, though...
>
>Yep... I have looked through most of /drivers/net/bonding and I can't
>really see why it should be blowing up there.. it really should cause
>the BUG_ON under normal IPoIB if the MTU was the cause - yet I have
>not experienced this.
>The bonding code doesn't seem to do anything special with the MTU
>other than propagating changes to the slaves.

	For IPoIB, though, there is some extra initialization stuff in
bond_setup_by_slave(), and the hard_header_len will end up being set to
something different from the usual Ethernet value.

	In looking at ipoib_setup, I see that hard_header_len appears to
be set to 4 (IPOIB_ENCAP_LEN).  My recollection was that the IPoIB
hard_header_len was quite a bit larger than that; it looks like it
changed very recently from IPOIB_ENCAP_LEN + INFINIBAND_ALEN to what it
is now:

commit afd87adacb5de00768b2e54f0bd851278f2e6179
Author: Roland Dreier <roland@...estorage.com>
Date:   Tue Feb 7 14:51:21 2012 +0000

    IPoIB: Stop lying about hard_header_len and use skb->cb to stash LL addresses
    
    [ Upstream commit 936d7de3d736e0737542641269436f4b5968e9ef ]
    
    Commit a0417fa3a18a ("net: Make qdisc_skb_cb upper size bound
    explicit.") made it possible for a netdev driver to use skb->cb
    between its header_ops.create method and its .ndo_start_xmit
    method.  Use this in ipoib_hard_header() to stash away the LL address
    (GID + QPN), instead of the "ipoib_pseudoheader" hack.  This allows
    IPoIB to stop lying about its hard_header_len, which will let us fix
    the L2 check for GRO.


	I don't know if this change could be causing the problem (it
appears to be new in 3.2.9), but the hard_header_len is one of the few
areas in the TX path of bonding that IPoIB ends up being different from
regular Ethernet.

	-J

---
	-Jay Vosburgh, IBM Linux Technology Center, fubar@...ibm.com

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