Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

[Jesse Gross <jesse-l0M0P4e3n4LQT0dZR+AlfA@public.gmane.org>]

On Mon, Jan 5, 2015 at 1:02 AM, Fan Du <fengyuleidian0615@gmail.com> wrote:
> 于 2014年12月03日 10:31, Du, Fan 写道:
>
>>
>>
>>> -----Original Message-----
>>> From: Thomas Graf [mailto:tgr@infradead.org] On Behalf Of Thomas Graf
>>> Sent: Wednesday, December 3, 2014 1:42 AM
>>> To: Michael S. Tsirkin
>>> Cc: Du, Fan; 'Jason Wang'; netdev@vger.kernel.org; davem@davemloft.net;
>>> fw@strlen.de; dev@openvswitch.org; jesse@nicira.com; pshelar@nicira.com
>>> Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than
>>> MTU
>>>
>>> On 12/02/14 at 07:34pm, Michael S. Tsirkin wrote:
>>>>
>>>> On Tue, Dec 02, 2014 at 05:09:27PM +0000, Thomas Graf wrote:
>>>>>
>>>>> On 12/02/14 at 01:48pm, Flavio Leitner wrote:
>>>>>>
>>>>>> What about containers or any other virtualization environment that
>>>>>> doesn't use Virtio?
>>>>>
>>>>>
>>>>> The host can dictate the MTU in that case for both veth or OVS
>>>>> internal which would be primary container plumbing techniques.
>>>>
>>>>
>>>> It typically can't do this easily for VMs with emulated devices:
>>>> real ethernet uses a fixed MTU.
>>>>
>>>> IMHO it's confusing to suggest MTU as a fix for this bug, it's an
>>>> unrelated optimization.
>>>> ICMP_DEST_UNREACH/ICMP_FRAG_NEEDED is the right fix here.
>>>
>>>
>>> PMTU discovery only resolves the issue if an actual IP stack is running
>>> inside the
>>> VM. This may not be the case at all.
>>
>>   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> Some thoughts here:
>>
>> Think otherwise, this is indeed what host stack should forge a
>> ICMP_DEST_UNREACH/ICMP_FRAG_NEEDED
>> message with _inner_ skb network and transport header, do whatever type of
>> encapsulation,
>> and thereafter push such packet upward to Guest/Container, which make them
>> feel, the intermediate node
>> or the peer send such message. PMTU should be expected to work correct.
>> And such behavior should be shared by all other encapsulation tech if they
>> are also suffered.
>
>
> Hi David, Jesse and Thomas
>
> As discussed in here:
> https://www.marc.info/?l=linux-netdev&m=141764712631150&w=4 and
> quotes from Jesse:
> My proposal would be something like this:
>  * For L2, reduce the VM MTU to the lowest common denominator on the
> segment.
>  * For L3, use path MTU discovery or fragment inner packet (i.e.
> normal routing behavior).
>  * As a last resort (such as if using an old version of virtio in the
> guest), fragment the tunnel packet.
>
>
> For L2, it's a administrative action
> For L3, PMTU approach looks better, because once the sender is alerted the
> reduced MTU,
> packet size after encapsulation will not exceed physical MTU, so no
> additional fragments
> efforts needed.
> For "As a last resort... fragment the tunnel packet", the original patch:
> https://www.marc.info/?l=linux-netdev&m=141715655024090&w=4 did the job, but
> seems it's
> not welcomed.

This needs to be properly integrated into IP processing if it is to
work correctly. One of the reasons for only doing path MTU discovery
for L3 is that it operates seamlessly as part of normal operation -
there is no need to forge addresses or potentially generate ICMP when
on an L2 network. However, this ignores the IP handling that is going
on (note that in OVS it is possible for L3 to be implemented as a set
of flows coming from a controller).

It also should not be VXLAN specific or duplicate VXLAN encapsulation
code. As this is happening before encapsulation, the generated ICMP
does not need to be encapsulated either if it is created in the right
location.
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