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Merge pull request #537 from douliyang/patch-7

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Fix the mm range of guard hole
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0xAX 2017-10-31 13:36:21 +06:00 committed by GitHub
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Theory/Paging.md
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@ -199,7 +199,7 @@ We can see here the memory map for user space, kernel space and the non-canonica
#define __PAGE_OFFSET _AC(0xffff880000000000, UL)
```
Previously this guard hole and `__PAGE_OFFSET` was from `0xffff800000000000` to `0xffff80ffffffffff` to prevent access to non-canonical area, but was later extended by 3 bits for the hypervisor.
Previously this guard hole and `__PAGE_OFFSET` was from `0xffff800000000000` to `0xffff87ffffffffff` to prevent access to non-canonical area, but was later extended by 3 bits for the hypervisor.
Next is the lowest usable address in kernel space - `ffff880000000000`. This virtual memory region is for direct mapping of all the physical memory. After the memory space which maps all the physical addresses, the guard hole. It needs to be between the direct mapping of all the physical memory and the vmalloc area. After the virtual memory map for the first terabyte and the unused hole after it, we can see the `kasan` shadow memory. It was added by [commit](https://github.com/torvalds/linux/commit/ef7f0d6a6ca8c9e4b27d78895af86c2fbfaeedb2) and provides the kernel address sanitizer. After the next unused hole we can see the `esp` fixup stacks (we will talk about it in other parts of this book) and the start of the kernel text mapping from the physical address - `0`. We can find the definition of this address in the same file as the `__PAGE_OFFSET`: