When there are two huge page mappings, like the two in the example
	below, first one at the end of PGDIR_SIZE, and second one starts at
	next PGDIR_SIZE (64GB with 16K page size):

	8000000ff0000000-8000001000000000 rw-s
	8000001000000000-8000001010000000 rw-s

	Unmapping the first vma would trick free_pgtable to think it
	can remove one set of pgd indexed at 0x400, and it went ahead
	purge the entire pmd/pte that are still in use by the second
	mapping. Now any subsequent access to pmd/pte for the second
	active mapping will trigger the bug.  We've seen hard kernel
	hang on some platform, some other platform will generate MCA,
	plus all kinds of unpleasant result.