netdev - Re: [PATCH] vxlan: Fix error that was resulting in VXLAN MTU size being 10 bytes too large

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Message-ID: <CAOzFzEg0F4f4i_Z-468jdw2hTETtL93-WasBY1VJvOeoiAg6nw@mail.gmail.com>
Date:	Tue, 20 Nov 2012 10:37:39 +1100
From:	Joseph Glanville <joseph.glanville@...onvm.com.au>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	David Miller <davem@...emloft.net>, alexander.h.duyck@...el.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH] vxlan: Fix error that was resulting in VXLAN MTU size
 being 10 bytes too large

On 20 November 2012 03:03, Stephen Hemminger <shemminger@...tta.com> wrote:
> On Mon, 19 Nov 2012 22:33:50 +1100
> Joseph Glanville <joseph.glanville@...onvm.com.au> wrote:
>
>> On 14 November 2012 08:33, Stephen Hemminger <shemminger@...tta.com> wrote:
>> > On Tue, 13 Nov 2012 14:37:19 -0500 (EST)
>> > David Miller <davem@...emloft.net> wrote:
>> >
>> >> From: Alexander Duyck <alexander.h.duyck@...el.com>
>> >> Date: Fri, 09 Nov 2012 15:35:24 -0800
>> >>
>> >> > This change fixes an issue I found where VXLAN frames were fragmented when
>> >> > they were up to the VXLAN MTU size.  I root caused the issue to the fact that
>> >> > the headroom was 4 + 20 + 8 + 8.  This math doesn't appear to be correct
>> >> > because we are not inserting a VLAN header, but instead a 2nd Ethernet header.
>> >> > As such the math for the overhead should be 20 + 8 + 8 + 14 to account for the
>> >> > extra headers that are inserted for VXLAN.
>> >> >
>> >> > Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
>> >>
>> >> Applied, thanks for the detailed commit message.
>> >
>> > Probably need smarter code there to look at header length requirement
>> > of underlying device as well, maybe someone will be perverse and runn
>> > vxlan over a tunnel or IPoIB.
>>
>> Forgive my ignorance but why would running VXLAN on IPoIB require
>> special header handling? (and would it work or behave strangely?)
>>
>> I was planning on giving this a go when 3.7 is released but I might do
>> that sooner if problems are anticipated.
>>
>> > --
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>>
>> Joseph.
>>
>
> Some lower layers require bigger (or smaller headers). As it was, vxlan
> was only allocating skb with a fixed amount of headroom. This would lead to
> lower layers having to copy the skb.
>
> My suggestion has already been addressed by a later patch.

Thankyou for the clarification, I found the patch about the lower_dev
hard_header_len!

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