On Tue, May 05, 2015 at 12:31:12PM -0400, Tejun Heo wrote:

What I don't want to happen is controllers failing migrations
willy-nilly for random reasons leaving users baffled, which we've
actually been doing unfortunately.  Maybe we need to deal with this
fixed resource arbitration as a separate class and allow them to fail
migration w/ -EBUSY.

Ah, _that_ was the problem.

Which is something created by this co-mounting of controllers.

You could of course store the ss-id of the failing operation in
task_struct and have a file reporting the name of the ss-id.

That way, there is a simple way to find out which controller failed the
migrate.