<previous description obsolete, deleted>
  
  Virtual memory map with 4 level page tables:
  
  0000000000000000 - 00007fffffffffff (=47bits) user space, different per mm
  hole caused by [48:63] sign extension
  ffff800000000000 - ffff80ffffffffff (=40bits) guard hole

  ffff810000000000 - ffffc0ffffffffff (=46bits) direct mapping of all phys. memory

  ffffc10000000000 - ffffc1ffffffffff (=40bits) hole
  ffffc20000000000 - ffffe1ffffffffff (=45bits) vmalloc/ioremap space
  ... unused hole ...
  ffffffff80000000 - ffffffff82800000 (=40MB)   kernel text mapping, from phys 0
  ... unused hole ...
  ffffffff88000000 - fffffffffff00000 (=1919MB) module mapping space

  The direct mapping covers all memory in the system upto the highest
  memory address (this means in some cases it can also include PCI memory
  holes)

  vmalloc space is lazily synchronized into the different PML4 pages of
  the processes using the page fault handler, with init_level4_pgt as
  reference.
  
  Current X86-64 implementations only support 40 bit of address space,
  but we support upto 46bits. This expands into MBZ space in the page tables.
  
  -Andi Kleen, Jul 2004