First off: unregister the region from MemoryManager before unmapping it.
The order of operations here was a bit strange, presumably to avoid a
situation where a fault would happen while unmapping, and the fault
handler would find the MemoryManager region list in an invalid state.
Unregistering it before unmapping sidesteps the whole problem, and
allows us to easily fix another problem: a deadlock could occur due
to inconsistent acquisition order (PageDirectory must come before MM.)
It may happen that CPU A manages to page in from the same inode
while we're just entering the same page fault handler on CPU B.
Handle it gracefully by checking if the data has already been paged in
(instead of VERIFY'ing that it hasn't) and then remap the page if that's
the case.
When booting AP's, we identity map a region at 0x8000 while doing the
initial bringup sequence. This is the only thing in the kernel that
requires an identity mapping, yet we had a bunch of generic API's and a
dedicated VirtualRangeAllocator in every PageDirectory for this purpose.
This patch simplifies the situation by moving the identity mapping logic
to the AP boot code and removing the generic API's.
...and also RangeAllocator => VirtualRangeAllocator.
This clarifies that the ranges we're dealing with are *virtual* memory
ranges and not anything else.
