Unmapping unused frames in page tables

[vinaychandra]

In mapper, the current unmap method removes and returns a PhysFrame. This has the side affect that there might be P1 entries that have no further entries.
It would be more ergonomic to provide an option for unmap to even recursively unmap any empty layers all the layers and return to the user. Currently, the only way to do this afaik is to unmap each of the layer by iterating manually and checking if they exist or not.

[phil-opp]

The problem is that we don't know if there are still other active entries without iterating over each of the 512 entries. Doing this on every unmap would considerably decrease performance. Also, there are valid use cases where you want to keep the empty table, e.g. if you know that it'll be needed again soon. So I don't think that combining this with unmap is a good idea.

However, I'd be fine with adding a separate method to clean up empty page tables (with documentation about the performance impact). It should also be easier to integrate your own cleanup logic, e.g. by managing state in separate data structures.