Since this code is so rather, um, custom, I was going to reimplement 
stop_machine in the module.

that sounds like a big mistake. I assume you want your VMI module to be
part of mainline for one.

And this is the sort of thing that if we want to support it, we better
support it inside the main kernel, eg provide an api to modules to use
it, rather than having each module hack their own....



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