vm-region supports creating nested regions of virtual memory, each with their
own memory protection restrictions.
vm-objects can be mapped into a vm-region, making the underlying memory
accessible. all mappings are lazy: page tables are not updated until the
mapped memory is accessed.
vm-object can be used to demand-allocate non-contiguous physical memory, and
will provide an api for userspace programs to do the same. unless a vm-object
is created in-place (i.e. to represent a specific area of physical memory),
its memory pages are only allocated when the object is mapped AND someone
attempts to access the memory.
if kmalloc is called with count=N before vm_bootstrap is finished,
the request will be fulfilled using memblock_alloc.
if N is a power of 2, the returned pointer will be aligned to
an N-byte boundary.
