162 lines
11 KiB
ReStructuredText
162 lines
11 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=================
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Memory Management
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=================
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Complete virtual memory map with 4-level page tables
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====================================================
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.. note::
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- Negative addresses such as "-23 TB" are absolute addresses in bytes, counted down
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from the top of the 64-bit address space. It's easier to understand the layout
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when seen both in absolute addresses and in distance-from-top notation.
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For example 0xffffe90000000000 == -23 TB, it's 23 TB lower than the top of the
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64-bit address space (ffffffffffffffff).
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Note that as we get closer to the top of the address space, the notation changes
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from TB to GB and then MB/KB.
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- "16M TB" might look weird at first sight, but it's an easier way to visualize size
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notation than "16 EB", which few will recognize at first sight as 16 exabytes.
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It also shows it nicely how incredibly large 64-bit address space is.
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::
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========================================================================================================================
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Start addr | Offset | End addr | Size | VM area description
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========================================================================================================================
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0000000000000000 | 0 | 00007fffffffffff | 128 TB | user-space virtual memory, different per mm
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__________________|____________|__________________|_________|___________________________________________________________
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0000800000000000 | +128 TB | ffff7fffffffffff | ~16M TB | ... huge, almost 64 bits wide hole of non-canonical
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| | | | virtual memory addresses up to the -128 TB
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| | | | starting offset of kernel mappings.
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__________________|____________|__________________|_________|___________________________________________________________
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| Kernel-space virtual memory, shared between all processes:
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____________________________________________________________|___________________________________________________________
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ffff800000000000 | -128 TB | ffff87ffffffffff | 8 TB | ... guard hole, also reserved for hypervisor
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ffff880000000000 | -120 TB | ffff887fffffffff | 0.5 TB | LDT remap for PTI
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ffff888000000000 | -119.5 TB | ffffc87fffffffff | 64 TB | direct mapping of all physical memory (page_offset_base)
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ffffc88000000000 | -55.5 TB | ffffc8ffffffffff | 0.5 TB | ... unused hole
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ffffc90000000000 | -55 TB | ffffe8ffffffffff | 32 TB | vmalloc/ioremap space (vmalloc_base)
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ffffe90000000000 | -23 TB | ffffe9ffffffffff | 1 TB | ... unused hole
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ffffea0000000000 | -22 TB | ffffeaffffffffff | 1 TB | virtual memory map (vmemmap_base)
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ffffeb0000000000 | -21 TB | ffffebffffffffff | 1 TB | ... unused hole
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ffffec0000000000 | -20 TB | fffffbffffffffff | 16 TB | KASAN shadow memory
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__________________|____________|__________________|_________|____________________________________________________________
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| Identical layout to the 56-bit one from here on:
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____________________________________________________________|____________________________________________________________
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fffffc0000000000 | -4 TB | fffffdffffffffff | 2 TB | ... unused hole
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| | | | vaddr_end for KASLR
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fffffe0000000000 | -2 TB | fffffe7fffffffff | 0.5 TB | cpu_entry_area mapping
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fffffe8000000000 | -1.5 TB | fffffeffffffffff | 0.5 TB | ... unused hole
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ffffff0000000000 | -1 TB | ffffff7fffffffff | 0.5 TB | %esp fixup stacks
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ffffff8000000000 | -512 GB | ffffffeeffffffff | 444 GB | ... unused hole
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ffffffef00000000 | -68 GB | fffffffeffffffff | 64 GB | EFI region mapping space
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ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | ... unused hole
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ffffffff80000000 | -2 GB | ffffffff9fffffff | 512 MB | kernel text mapping, mapped to physical address 0
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ffffffff80000000 |-2048 MB | | |
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ffffffffa0000000 |-1536 MB | fffffffffeffffff | 1520 MB | module mapping space
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ffffffffff000000 | -16 MB | | |
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FIXADDR_START | ~-11 MB | ffffffffff5fffff | ~0.5 MB | kernel-internal fixmap range, variable size and offset
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ffffffffff600000 | -10 MB | ffffffffff600fff | 4 kB | legacy vsyscall ABI
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ffffffffffe00000 | -2 MB | ffffffffffffffff | 2 MB | ... unused hole
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__________________|____________|__________________|_________|___________________________________________________________
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Complete virtual memory map with 5-level page tables
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====================================================
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.. note::
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- With 56-bit addresses, user-space memory gets expanded by a factor of 512x,
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from 0.125 PB to 64 PB. All kernel mappings shift down to the -64 PB starting
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offset and many of the regions expand to support the much larger physical
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memory supported.
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::
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========================================================================================================================
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Start addr | Offset | End addr | Size | VM area description
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========================================================================================================================
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0000000000000000 | 0 | 00ffffffffffffff | 64 PB | user-space virtual memory, different per mm
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__________________|____________|__________________|_________|___________________________________________________________
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0100000000000000 | +64 PB | feffffffffffffff | ~16K PB | ... huge, still almost 64 bits wide hole of non-canonical
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| | | | virtual memory addresses up to the -64 PB
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| | | | starting offset of kernel mappings.
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__________________|____________|__________________|_________|___________________________________________________________
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| Kernel-space virtual memory, shared between all processes:
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____________________________________________________________|___________________________________________________________
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ff00000000000000 | -64 PB | ff0fffffffffffff | 4 PB | ... guard hole, also reserved for hypervisor
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ff10000000000000 | -60 PB | ff10ffffffffffff | 0.25 PB | LDT remap for PTI
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ff11000000000000 | -59.75 PB | ff90ffffffffffff | 32 PB | direct mapping of all physical memory (page_offset_base)
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ff91000000000000 | -27.75 PB | ff9fffffffffffff | 3.75 PB | ... unused hole
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ffa0000000000000 | -24 PB | ffd1ffffffffffff | 12.5 PB | vmalloc/ioremap space (vmalloc_base)
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ffd2000000000000 | -11.5 PB | ffd3ffffffffffff | 0.5 PB | ... unused hole
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ffd4000000000000 | -11 PB | ffd5ffffffffffff | 0.5 PB | virtual memory map (vmemmap_base)
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ffd6000000000000 | -10.5 PB | ffdeffffffffffff | 2.25 PB | ... unused hole
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ffdf000000000000 | -8.25 PB | fffffbffffffffff | ~8 PB | KASAN shadow memory
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__________________|____________|__________________|_________|____________________________________________________________
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| Identical layout to the 47-bit one from here on:
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____________________________________________________________|____________________________________________________________
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fffffc0000000000 | -4 TB | fffffdffffffffff | 2 TB | ... unused hole
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| | | | vaddr_end for KASLR
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fffffe0000000000 | -2 TB | fffffe7fffffffff | 0.5 TB | cpu_entry_area mapping
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fffffe8000000000 | -1.5 TB | fffffeffffffffff | 0.5 TB | ... unused hole
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ffffff0000000000 | -1 TB | ffffff7fffffffff | 0.5 TB | %esp fixup stacks
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ffffff8000000000 | -512 GB | ffffffeeffffffff | 444 GB | ... unused hole
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ffffffef00000000 | -68 GB | fffffffeffffffff | 64 GB | EFI region mapping space
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ffffffff00000000 | -4 GB | ffffffff7fffffff | 2 GB | ... unused hole
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ffffffff80000000 | -2 GB | ffffffff9fffffff | 512 MB | kernel text mapping, mapped to physical address 0
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ffffffff80000000 |-2048 MB | | |
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ffffffffa0000000 |-1536 MB | fffffffffeffffff | 1520 MB | module mapping space
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ffffffffff000000 | -16 MB | | |
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FIXADDR_START | ~-11 MB | ffffffffff5fffff | ~0.5 MB | kernel-internal fixmap range, variable size and offset
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ffffffffff600000 | -10 MB | ffffffffff600fff | 4 kB | legacy vsyscall ABI
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ffffffffffe00000 | -2 MB | ffffffffffffffff | 2 MB | ... unused hole
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__________________|____________|__________________|_________|___________________________________________________________
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Architecture defines a 64-bit virtual address. Implementations can support
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less. Currently supported are 48- and 57-bit virtual addresses. Bits 63
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through to the most-significant implemented bit are sign extended.
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This causes hole between user space and kernel addresses if you interpret them
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as unsigned.
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The direct mapping covers all memory in the system up to the highest
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memory address (this means in some cases it can also include PCI memory
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holes).
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vmalloc space is lazily synchronized into the different PML4/PML5 pages of
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the processes using the page fault handler, with init_top_pgt as
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reference.
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We map EFI runtime services in the 'efi_pgd' PGD in a 64Gb large virtual
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memory window (this size is arbitrary, it can be raised later if needed).
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The mappings are not part of any other kernel PGD and are only available
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during EFI runtime calls.
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Note that if CONFIG_RANDOMIZE_MEMORY is enabled, the direct mapping of all
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physical memory, vmalloc/ioremap space and virtual memory map are randomized.
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Their order is preserved but their base will be offset early at boot time.
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Be very careful vs. KASLR when changing anything here. The KASLR address
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range must not overlap with anything except the KASAN shadow area, which is
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correct as KASAN disables KASLR.
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For both 4- and 5-level layouts, the STACKLEAK_POISON value in the last 2MB
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hole: ffffffffffff4111
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