Re: [PATCH net] vxlan: Restore initial MTU setting based on lower device

On Thu, 14 Dec 2017 00:57:32 +0100
Matthias Schiffer [off-list ref] wrote:

As you note, there is another occurrence of this calculation in
vxlan_config_apply():


[...]
        if (lowerdev) {
[...]
                max_mtu = lowerdev->mtu - (use_ipv6 ? VXLAN6_HEADROOM :
                                           VXLAN_HEADROOM);
        }

        if (dev->mtu > max_mtu)
                dev->mtu = max_mtu;
[...]


Unless I'm overlooking something, this should already do the same thing and
your patch is redundant.

The code above sets max_mtu, and only if dev->mtu exceeds that, the
latter is then clamped.

What my patch does is to actually set dev->mtu to that value, no matter
what's the previous value set by ether_setup() (only on creation, and
only if lowerdev is there), just like the previous behaviour used to be.

Let's consider these two cases, on the existing code:

1. lowerdev->mtu is 1500:
   - ether_setup(), called by vxlan_setup(), sets dev->mtu to 1500
   - here max_mtu is 1450
   - we enter the second if clause above (dev->mtu > max_mtu)
   - at the end of vxlan_config_apply(), dev->mtu will be 1450

which is consistent with the previous behaviour.

2. lowerdev->mtu is 9000:
   - ether_setup(), called by vxlan_setup(), sets dev->mtu to 1500
   - here max_mtu is 8950
   - we do not enter the second if clause above (dev->mtu < max_mtu)
   - at the end of vxlan_config_apply(), dev->mtu will still be 1500

which is not consistent with the previous behaviour, where it used to
be 8950 instead.

-- 
Stefano