From: Krister Johansen <redacted>
Date: Fri, 30 Dec 2016 20:10:58 -0800

The ipvlan code already knows how to detect when a duplicate address is
about to be assigned to an ipvlan device.  However, that failure is not
propogated outward and leads to a silent failure.  This teaches the ip
address addition functions how to report this error to the user
applications so that a notifier chain failure during ip address addition
will not appear to succeed when it actually has not.

This can be especially useful if it is necessary to provision many
ipvlans in containers.  The provisioning software (or operator) can use
this to detect situations where an ip address is unexpectedly in use.

Signed-off-by: Krister Johansen <redacted>

Your patch isn't handling the case of primary address promotions,
which also issue NETDEV_UP events on these notifier chains.

But on a more basic level, it's extremely important that once you
start using the notifier_{from,to}_errno() handling for a notifier,
you must start doing so for all such cases of that notifier.