__PAGE_OFFSET 早已经不是固定的0xC0000000了。
问题起源， 看lguest （2.6.23）发现要求page_offset的值，很是困惑，一查才发现。
2.6.34有如下
#define __PAGE_OFFSET _AC(CONFIG_PAGE_OFFSET, UL)
查可知CONFIG_PAGE_OFFSET从2.6.16就有了。
2.6.34
arch/x86/Kconfig有如下内容
choice
prompt "High Memory Support"
default HIGHMEM4G if !X86_NUMAQ
default HIGHMEM64G if X86_NUMAQ
depends on X86_32
config NOHIGHMEM
bool "off"
depends on !X86_NUMAQ
---help---
Linux can use up to 64 Gigabytes of physical memory on x86 systems.
However, the address space of 32-bit x86 processors is only 4
Gigabytes large. That means that, if you have a large amount of
physical memory, not all of it can be "permanently mapped" by the
kernel. The physical memory that's not permanently mapped is called
"high memory".
If you are compiling a kernel which will never run on a machine with
more than 1 Gigabyte total physical RAM, answer "off" here (default
choice and suitable for most users). This will result in a "3GB/1GB"
split: 3GB are mapped so that each process sees a 3GB virtual memory
space and the remaining part of the 4GB virtual memory space is used
by the kernel to permanently map as much physical memory as
possible.
If the machine has between 1 and 4 Gigabytes physical RAM, then
answer "4GB" here.
If more than 4 Gigabytes is used then answer "64GB" here. This
selection turns Intel PAE (Physical Address Extension) mode on.
PAE implements 3-level paging on IA32 processors. PAE is fully
supported by Linux, PAE mode is implemented on all recent Intel
processors (Pentium Pro and better). NOTE: If you say "64GB" here,
then the kernel will not boot on CPUs that don't support PAE!
The actual amount of total physical memory will either be
auto detected or can be forced by using a kernel command line option
such as "mem=256M". (Try "man bootparam" or see the documentation of
your boot loader (lilo or loadlin) about how to pass options to the
kernel at boot time.)
If unsure, say "off".
config HIGHMEM4G
bool "4GB"
depends on !X86_NUMAQ
---help---
Select this if you have a 32-bit processor and between 1 and 4
gigabytes of physical RAM.
config HIGHMEM64G
bool "64GB"
depends on !M386 && !M486
select X86_PAE
---help---
Select this if you have a 32-bit processor and more than 4
gigabytes of physical RAM.
endchoice
choice
depends on EXPERIMENTAL
prompt "Memory split" if EMBEDDED
default VMSPLIT_3G
depends on X86_32
---help---
Select the desired split between kernel and user memory.
If the address range available to the kernel is less than the
physical memory installed, the remaining memory will be available
as "high memory". Accessing high memory is a little more costly
than low memory, as it needs to be mapped into the kernel first.
Note that increasing the kernel address space limits the range
available to user programs, making the address space there
tighter. Selecting anything other than the default 3G/1G split
will also likely make your kernel incompatible with binary-only
kernel modules.
If you are not absolutely sure what you are doing, leave this
option alone!
config VMSPLIT_3G
bool "3G/1G user/kernel split"
config VMSPLIT_3G_OPT
depends on !X86_PAE
bool "3G/1G user/kernel split (for full 1G low memory)"
config VMSPLIT_2G
bool "2G/2G user/kernel split"
config VMSPLIT_2G_OPT
depends on !X86_PAE
bool "2G/2G user/kernel split (for full 2G low memory)"
config VMSPLIT_1G
bool "1G/3G user/kernel split"
endchoice
config PAGE_OFFSET
hex
default 0xB0000000 if VMSPLIT_3G_OPT
default 0x80000000 if VMSPLIT_2G
default 0x78000000 if VMSPLIT_2G_OPT
default 0x40000000 if VMSPLIT_1G
default 0xC0000000
depends on X86_32
config HIGHMEM
def_bool y
depends on X86_32 && (HIGHMEM64G || HIGHMEM4G)
config X86_PAE
bool "PAE (Physical Address Extension) Support"
depends on X86_32 && !HIGHMEM4G
---help---
PAE is required for NX support, and furthermore enables
larger swapspace support for non-overcommit purposes. It
has the cost of more pagetable lookup overhead, and also
consumes more pagetable space per process.
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