Linux内存管理 - PAGE_OFFSET理解

Linux内存管理 - PAGE_OFFSET理解

PAGE_OFFSET 代表的是内核空间和用户空间对虚拟地址空间的划分，对不同的体系结构不同。比如在32位系统中3G-4G 属于内核使用的内存空间，所以 PAGE_OFFSET = 0xC0000000。在X86-64架构下是ffff880000000000。可以看到内核程序可以可以访问从PAGE_OFFSET
之后的内存，访问所有的信息（注意页的写保护）。

参考：https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt

Virtual memory map with 4 level page tables:

0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm
hole caused by [48:63] sign extension
ffff800000000000 - ffff87ffffffffff (=43 bits) guard hole, reserved for hypervisor
ffff880000000000 - ffffc7ffffffffff (=64 TB) direct mapping of all phys. memory
ffffc80000000000 - ffffc8ffffffffff (=40 bits) hole
ffffc90000000000 - ffffe8ffffffffff (=45 bits) vmalloc/ioremap space
ffffe90000000000 - ffffe9ffffffffff (=40 bits) hole
ffffea0000000000 - ffffeaffffffffff (=40 bits) virtual memory map (1TB)
... unused hole ...
ffffff0000000000 - ffffff7fffffffff (=39 bits) %esp fixup stacks
... unused hole ...
ffffffff80000000 - ffffffffa0000000 (=512 MB)  kernel text mapping, from phys 0
ffffffffa0000000 - ffffffffff5fffff (=1525 MB) module mapping space
ffffffffff600000 - ffffffffffdfffff (=8 MB) vsyscalls
ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole

The direct mapping covers all memory in the system up to the highest
memory address (this means in some cases it can also include PCI memory
holes).

vmalloc space is lazily synchronized into the different PML4 pages of
the processes using the page fault handler, with init_level4_pgt as
reference.

Current X86-64 implementations only support 40 bits of address space,
but we support up to 46 bits. This expands into MBZ space in the page tables.

->trampoline_pgd:

We map EFI runtime services in the aforementioned PGD in the virtual
range of 64Gb (arbitrarily set, can be raised if needed)

0xffffffef00000000 - 0xffffffff00000000

-Andi Kleen, Jul 2004