Sorry for not having completely answered to this. I initially thought
stop_machine could work when you mentioned it, but I don't think it
can even removing xpmem block-inside-mmu-notifier-method requirements.

For stop_machine to solve this (besides being slower and potentially
not more safe as running stop_machine in a loop isn't nice), we'd need
to prevent preemption in between invalidate_range_start/end.

I think there are two ways:

1) add global lock around mm_lock to remove the sorting

2) remove invalidate_range_start/end, nuke mm_lock as consequence of
   it, and replace all three with invalidate_pages issued inside the
   PT lock, one invalidation for each 512 pte_t modified, so
   serialization against get_user_pages becomes trivial but this will
   be not ok at all for SGI as it increases a lot their invalidation
   frequency

For KVM both ways are almost the same.

I'll implement 1 now then we'll see...
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