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Update linux-initialization-2.md

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Initialization/linux-initialization-2.md
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@ -444,6 +444,18 @@ And the `_AC` macro is defined in the [include/uapi/linux/const.h](https://elixi
```
So, where `__PAGE_OFFSET` expands to `0xffff880000000000`.
Then, let's dive a little deeper. From Documentation/x86/x86_64/mm.txt:
```
<previous description obsolete, deleted>
Virtual memory map with 4 level page tables:
...
ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys. memory
...
```
As you may see, the virtual address space `ffff880000000000-ffffc7ffffffffff` is direct mapping of all physical memory. When the kernel wants to access all physical memory, it uses direct mapping.
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:
```C