<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 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

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