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Re MMU use by garbage collectors:
One certainly could use Mill protection for this purpose, but there’s a better way.
Mill protection has byte granularity, so the GC would need only one region descriptor for the whole of any one kind of space. In a generational GC for example, you might use one descriptor per generation (typical GCs use three generations). This would be an easy port, just replacing the small amount of code that manages the page tables with similar code that manages the region descriptors.
However, there’s a better way, one that uses the GC support “event bits” in the pointer format. With these, the GC can work at the granularity of single objects rather than pages or regions, and would be expected to have sharply reduced overheads. Porting a GC to use these would probably be a bit more work, because the model changes and that requires a bit of thought. The actual code changes should be near trivial though, mostly involving taking stuff out.