Virtual memory map with 4 level page tables: 0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm hole caused by [48:63] sign extension ffff800000000000 - ffff87ffffffffff (=43 bits) guard hole, reserved for hypervisor ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys. memory ffffc80000000000 - ffffc8ffffffffff (=40 bits) hole ffffc90000000000 - ffffe8ffffffffff (=45 bits) vmalloc/ioremap space ffffe90000000000 - ffffe9ffffffffff (=40 bits) hole ffffea0000000000 - ffffeaffffffffff (=40 bits) virtual memory map (1TB) ... unused hole ... ffffec0000000000 - fffffbffffffffff (=44 bits) kasan shadow memory (16TB) ... unused hole ... ffffff0000000000 - ffffff7fffffffff (=39 bits) %esp fixup stacks ... unused hole ... ffffffef00000000 - fffffffeffffffff (=64 GB) EFI region mapping space ... unused hole ... ffffffff80000000 - ffffffff9fffffff (=512 MB) kernel text mapping, from phys 0 ffffffffa0000000 - ffffffffff5fffff (=1526 MB) module mapping space ffffffffff600000 - ffffffffffdfffff (=8 MB) vsyscalls ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole The direct mapping covers all memory in the system up to 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 support up to 46 bits of address space (64 TB), which is our current limit. This expands into MBZ space in the page tables. We map EFI runtime services in the 'efi_pgd' PGD in a 64Gb large virtual memory window (this size is arbitrary, it can be raised later if needed). The mappings are not part of any other kernel PGD and are only available during EFI runtime calls. Note that if CONFIG_RANDOMIZE_MEMORY is enabled, the direct mapping of all physical memory, vmalloc/ioremap space and virtual memory map are randomized. Their order is preserved but their base will be offset early at boot time. -Andi Kleen, Jul 2004