Training sessions

Kernel and Embedded Linux

Next training sessions

Linux Kernel: March 16-20
Embedded Linux: May 11-15
and on-site sessions

Bootlin logo

Elixir Cross Referencer

.. SPDX-License-Identifier: GPL-2.0

================
Memory Managment
================

Complete virtual memory map with 4-level page tables
====================================================

.. note::

 - Negative addresses such as "-23 TB" are absolute addresses in bytes, counted down
   from the top of the 64-bit address space. It's easier to understand the layout
   when seen both in absolute addresses and in distance-from-top notation.

   For example 0xffffe90000000000 == -23 TB, it's 23 TB lower than the top of the
   64-bit address space (ffffffffffffffff).

   Note that as we get closer to the top of the address space, the notation changes
   from TB to GB and then MB/KB.

 - "16M TB" might look weird at first sight, but it's an easier to visualize size
   notation than "16 EB", which few will recognize at first sight as 16 exabytes.
   It also shows it nicely how incredibly large 64-bit address space is.

::

  ========================================================================================================================
      Start addr    |   Offset   |     End addr     |  Size   | VM area description
  ========================================================================================================================
                    |            |                  |         |
   0000000000000000 |    0       | 00007fffffffffff |  128 TB | user-space virtual memory, different per mm
  __________________|____________|__________________|_________|___________________________________________________________
                    |            |                  |         |
   0000800000000000 | +128    TB | ffff7fffffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
                    |            |                  |         |     virtual memory addresses up to the -128 TB
                    |            |                  |         |     starting offset of kernel mappings.
  __________________|____________|__________________|_________|___________________________________________________________
                                                              |
                                                              | Kernel-space virtual memory, shared between all processes:
  ____________________________________________________________|___________________________________________________________
                    |            |                  |         |
   ffff800000000000 | -128    TB | ffff87ffffffffff |    8 TB | ... guard hole, also reserved for hypervisor
   ffff880000000000 | -120    TB | ffff887fffffffff |  0.5 TB | LDT remap for PTI
   ffff888000000000 | -119.5  TB | ffffc87fffffffff |   64 TB | direct mapping of all physical memory (page_offset_base)
   ffffc88000000000 |  -55.5  TB | ffffc8ffffffffff |  0.5 TB | ... unused hole
   ffffc90000000000 |  -55    TB | ffffe8ffffffffff |   32 TB | vmalloc/ioremap space (vmalloc_base)
   ffffe90000000000 |  -23    TB | ffffe9ffffffffff |    1 TB | ... unused hole
   ffffea0000000000 |  -22    TB | ffffeaffffffffff |    1 TB | virtual memory map (vmemmap_base)
   ffffeb0000000000 |  -21    TB | ffffebffffffffff |    1 TB | ... unused hole
   ffffec0000000000 |  -20    TB | fffffbffffffffff |   16 TB | KASAN shadow memory
  __________________|____________|__________________|_________|____________________________________________________________
                                                              |
                                                              | Identical layout to the 56-bit one from here on:
  ____________________________________________________________|____________________________________________________________
                    |            |                  |         |
   fffffc0000000000 |   -4    TB | fffffdffffffffff |    2 TB | ... unused hole
                    |            |                  |         | vaddr_end for KASLR
   fffffe0000000000 |   -2    TB | fffffe7fffffffff |  0.5 TB | cpu_entry_area mapping
   fffffe8000000000 |   -1.5  TB | fffffeffffffffff |  0.5 TB | ... unused hole
   ffffff0000000000 |   -1    TB | ffffff7fffffffff |  0.5 TB | %esp fixup stacks
   ffffff8000000000 | -512    GB | ffffffeeffffffff |  444 GB | ... unused hole
   ffffffef00000000 |  -68    GB | fffffffeffffffff |   64 GB | EFI region mapping space
   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | ... unused hole
   ffffffff80000000 |   -2    GB | ffffffff9fffffff |  512 MB | kernel text mapping, mapped to physical address 0
   ffffffff80000000 |-2048    MB |                  |         |
   ffffffffa0000000 |-1536    MB | fffffffffeffffff | 1520 MB | module mapping space
   ffffffffff000000 |  -16    MB |                  |         |
      FIXADDR_START | ~-11    MB | ffffffffff5fffff | ~0.5 MB | kernel-internal fixmap range, variable size and offset
   ffffffffff600000 |  -10    MB | ffffffffff600fff |    4 kB | legacy vsyscall ABI
   ffffffffffe00000 |   -2    MB | ffffffffffffffff |    2 MB | ... unused hole
  __________________|____________|__________________|_________|___________________________________________________________


Complete virtual memory map with 5-level page tables
====================================================

.. note::

 - With 56-bit addresses, user-space memory gets expanded by a factor of 512x,
   from 0.125 PB to 64 PB. All kernel mappings shift down to the -64 PB starting
   offset and many of the regions expand to support the much larger physical
   memory supported.

::

  ========================================================================================================================
      Start addr    |   Offset   |     End addr     |  Size   | VM area description
  ========================================================================================================================
                    |            |                  |         |
   0000000000000000 |    0       | 00ffffffffffffff |   64 PB | user-space virtual memory, different per mm
  __________________|____________|__________________|_________|___________________________________________________________
                    |            |                  |         |
   0100000000000000 |  +64    PB | feffffffffffffff | ~16K PB | ... huge, still almost 64 bits wide hole of non-canonical
                    |            |                  |         |     virtual memory addresses up to the -64 PB
                    |            |                  |         |     starting offset of kernel mappings.
  __________________|____________|__________________|_________|___________________________________________________________
                                                              |
                                                              | Kernel-space virtual memory, shared between all processes:
  ____________________________________________________________|___________________________________________________________
                    |            |                  |         |
   ff00000000000000 |  -64    PB | ff0fffffffffffff |    4 PB | ... guard hole, also reserved for hypervisor
   ff10000000000000 |  -60    PB | ff10ffffffffffff | 0.25 PB | LDT remap for PTI
   ff11000000000000 |  -59.75 PB | ff90ffffffffffff |   32 PB | direct mapping of all physical memory (page_offset_base)
   ff91000000000000 |  -27.75 PB | ff9fffffffffffff | 3.75 PB | ... unused hole
   ffa0000000000000 |  -24    PB | ffd1ffffffffffff | 12.5 PB | vmalloc/ioremap space (vmalloc_base)
   ffd2000000000000 |  -11.5  PB | ffd3ffffffffffff |  0.5 PB | ... unused hole
   ffd4000000000000 |  -11    PB | ffd5ffffffffffff |  0.5 PB | virtual memory map (vmemmap_base)
   ffd6000000000000 |  -10.5  PB | ffdeffffffffffff | 2.25 PB | ... unused hole
   ffdf000000000000 |   -8.25 PB | fffffbffffffffff |   ~8 PB | KASAN shadow memory
  __________________|____________|__________________|_________|____________________________________________________________
                                                              |
                                                              | Identical layout to the 47-bit one from here on:
  ____________________________________________________________|____________________________________________________________
                    |            |                  |         |
   fffffc0000000000 |   -4    TB | fffffdffffffffff |    2 TB | ... unused hole
                    |            |                  |         | vaddr_end for KASLR
   fffffe0000000000 |   -2    TB | fffffe7fffffffff |  0.5 TB | cpu_entry_area mapping
   fffffe8000000000 |   -1.5  TB | fffffeffffffffff |  0.5 TB | ... unused hole
   ffffff0000000000 |   -1    TB | ffffff7fffffffff |  0.5 TB | %esp fixup stacks
   ffffff8000000000 | -512    GB | ffffffeeffffffff |  444 GB | ... unused hole
   ffffffef00000000 |  -68    GB | fffffffeffffffff |   64 GB | EFI region mapping space
   ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | ... unused hole
   ffffffff80000000 |   -2    GB | ffffffff9fffffff |  512 MB | kernel text mapping, mapped to physical address 0
   ffffffff80000000 |-2048    MB |                  |         |
   ffffffffa0000000 |-1536    MB | fffffffffeffffff | 1520 MB | module mapping space
   ffffffffff000000 |  -16    MB |                  |         |
      FIXADDR_START | ~-11    MB | ffffffffff5fffff | ~0.5 MB | kernel-internal fixmap range, variable size and offset
   ffffffffff600000 |  -10    MB | ffffffffff600fff |    4 kB | legacy vsyscall ABI
   ffffffffffe00000 |   -2    MB | ffffffffffffffff |    2 MB | ... unused hole
  __________________|____________|__________________|_________|___________________________________________________________

Architecture defines a 64-bit virtual address. Implementations can support
less. Currently supported are 48- and 57-bit virtual addresses. Bits 63
through to the most-significant implemented bit are sign extended.
This causes hole between user space and kernel addresses if you interpret them
as unsigned.

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/PML5 pages of
the processes using the page fault handler, with init_top_pgt as
reference.

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.

Be very careful vs. KASLR when changing anything here. The KASLR address
range must not overlap with anything except the KASAN shadow area, which is
correct as KASAN disables KASLR.

For both 4- and 5-level layouts, the STACKLEAK_POISON value in the last 2MB
hole: ffffffffffff4111