[ovs-dev] Odd gre_sys MTU setting (2.6.x vs 2.8.x)

Christian Ehrhardt christian.ehrhardt at canonical.com
Mon Jan 15 16:48:36 UTC 2018


> But that does not disable the central check.
> And this only relies on the max_mtu values in the device struct

gre's own check would be called by
dev.c:dev_set_mtu
  * core check here*
  -> __dev_set_mtu
    -> ops->ndo_change_mtu

But the check that refuses us is before the call to gre so it never gets there.

> I still have to look what gro actually sets on initial setup, but atm
> I'd assume it could do some default which might be 1500 or so and due
> to that limits it later on our try to increase the value.

gre uses ether_setup to set up the device which sets sane ethernet defaults.
But in this case this will be
»···dev->mtu»···»···= ETH_DATA_LEN;
»···dev->min_mtu»···»···= ETH_MIN_MTU;
»···dev->max_mtu»···»···= ETH_DATA_LEN;

so max = initial mtu, and that is not big enough to increase later.


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