Commit 035c5e21 authored by Kiryl Shutsemau's avatar Kiryl Shutsemau Committed by Dave Hansen
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x86/mm/doc: Add missing details in virtual memory layout 



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: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20241031084946.2243440-1-kirill.shutemov%40linux.intel.com
parent 42f7652d

Documentation/arch/x86/x86_64/mm.rst

+29 −6
Original line number Diff line number Diff line
@@ -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
@@ -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