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Merge pull request #755 from Mutated1994/patch-4

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feat: Diving a little deeper about the PAGE_OFFSET
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Sebastian Fricke 2020-11-26 06:12:10 +01:00 committed by GitHub
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Initialization/linux-initialization-2.md
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@ -442,9 +442,17 @@ And the `_AC` macro is defined in the [include/uapi/linux/const.h](https://elixi
#define _AC(X,Y) __AC(X,Y)
#endif
```
So, where `__PAGE_OFFSET` expands to `0xffff880000000000`.
Where `__PAGE_OFFSET` expands to `0xffff888000000000`. But, why is it possible to translate a virtual address to a physical address by subtracting `__PAGE_OFFSET`? The answer is in the [Documentation/x86/x86_64/mm.rst](https://elixir.bootlin.com/linux/v5.10-rc5/source/Documentation/x86/x86_64/mm.rst#L45) documentation:
We initialize `pmd` and pass it to the `__early_make_pgtable` function along with `address`. The `__early_make_pgtable` function is defined in the same file as the `early_make_pgtable` function as the following:
```
...
ffff888000000000 | -119.5 TB | ffffc87fffffffff | 64 TB | direct mapping of all physical memory (page_offset_base)
...
```
As explained above, the virtual address space `ffff888000000000-ffffc87fffffffff` is direct mapping of all physical memory. When the kernel wants to access all physical memory, it uses direct mapping.
Okay, let's get back to discussing `early_make_pgtable`. We initialize `pmd` and pass it to the `__early_make_pgtable` function along with `address`. The `__early_make_pgtable` function is defined in the same file as the `early_make_pgtable` function as follows:
```C
int __init __early_make_pgtable(unsigned long address, pmdval_t pmd)