On Tue, Jun 30, 2015 at 04:31:15PM +0100, Chris Wilson wrote:

That suggests MAP_FIXED is special in invalidating the range unlike a
normal mmap().  This is arguing that we must always make any access after
invalidate_range be EFAULT. The danger here is that I am not sure if there
are any circumstances where invalidate_range is called and the vma
survives. Looking again at mm/, I can't see any place where we can
legally be expecting to reuse the same address for a userptr after the
invalidate.

Sigh, changing page protections (e.g. after a fork) if I am not mistaken
also generates an invalidate_range. Back to the drawing board here I am
afraid, as I think I need a worker to do cancel_userptr with all the
complications of coordinating between the multiple workers.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre