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x86/mm/doc: Add missing details in virtual memory layout
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Improve memory layout documentation:

 - Document 4kB guard hole at the end of userspace.
   See TASK_SIZE_MAX definition.

 - Divide the description of the non-canonical hole into two parts:
   userspace and kernel sides.

 - Mention the effect of LAM on the non-canonical range.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20241031084946.2243440-1-kirill.shutemov%40linux.intel.com
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Kirill A. Shutemov authored and Dave Hansen committed Nov 1, 2024
1 parent 42f7652 commit 035c5e2
Showing 1 changed file with 29 additions and 6 deletions.
35 changes: 29 additions & 6 deletions Documentation/arch/x86/x86_64/mm.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -29,15 +29,27 @@ Complete virtual memory map with 4-level page tables
Start addr | Offset | End addr | Size | VM area description
========================================================================================================================
| | | |
0000000000000000 | 0 | 00007fffffffffff | 128 TB | user-space virtual memory, different per mm
0000000000000000 | 0 | 00007fffffffefff | ~128 TB | user-space virtual memory, different per mm
00007ffffffff000 | ~128 TB | 00007fffffffffff | 4 kB | ... guard hole
__________________|____________|__________________|_________|___________________________________________________________
| | | |
0000800000000000 | +128 TB | ffff7fffffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
| | | | virtual memory addresses up to the -128 TB
0000800000000000 | +128 TB | 7fffffffffffffff | ~8 EB | ... huge, almost 63 bits wide hole of non-canonical
| | | | virtual memory addresses up to the -8 EB
| | | | starting offset of kernel mappings.
| | | |
| | | | LAM relaxes canonicallity check allowing to create aliases
| | | | for userspace memory here.
__________________|____________|__________________|_________|___________________________________________________________
|
| Kernel-space virtual memory, shared between all processes:
__________________|____________|__________________|_________|___________________________________________________________
| | | |
8000000000000000 | -8 EB | ffff7fffffffffff | ~8 EB | ... huge, almost 63 bits wide hole of non-canonical
| | | | virtual memory addresses up to the -128 TB
| | | | starting offset of kernel mappings.
| | | |
| | | | LAM_SUP relaxes canonicallity check allowing to create
| | | | aliases for kernel memory here.
____________________________________________________________|___________________________________________________________
| | | |
ffff800000000000 | -128 TB | ffff87ffffffffff | 8 TB | ... guard hole, also reserved for hypervisor
Expand Down Expand Up @@ -88,15 +100,26 @@ Complete virtual memory map with 5-level page tables
Start addr | Offset | End addr | Size | VM area description
========================================================================================================================
| | | |
0000000000000000 | 0 | 00ffffffffffffff | 64 PB | user-space virtual memory, different per mm
0000000000000000 | 0 | 00fffffffffff000 | ~64 PB | user-space virtual memory, different per mm
00fffffffffff000 | ~64 PB | 00ffffffffffffff | 4 kB | ... guard hole
__________________|____________|__________________|_________|___________________________________________________________
| | | |
0100000000000000 | +64 PB | feffffffffffffff | ~16K PB | ... huge, still almost 64 bits wide hole of non-canonical
| | | | virtual memory addresses up to the -64 PB
0100000000000000 | +64 PB | 7fffffffffffffff | ~8 EB | ... huge, almost 63 bits wide hole of non-canonical
| | | | virtual memory addresses up to the -8EB TB
| | | | starting offset of kernel mappings.
| | | |
| | | | LAM relaxes canonicallity check allowing to create aliases
| | | | for userspace memory here.
__________________|____________|__________________|_________|___________________________________________________________
|
| Kernel-space virtual memory, shared between all processes:
____________________________________________________________|___________________________________________________________
8000000000000000 | -8 EB | feffffffffffffff | ~8 EB | ... huge, almost 63 bits wide hole of non-canonical
| | | | virtual memory addresses up to the -64 PB
| | | | starting offset of kernel mappings.
| | | |
| | | | LAM_SUP relaxes canonicallity check allowing to create
| | | | aliases for kernel memory here.
____________________________________________________________|___________________________________________________________
| | | |
ff00000000000000 | -64 PB | ff0fffffffffffff | 4 PB | ... guard hole, also reserved for hypervisor
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