Antino Kim wrote:
Hi all,
I was looking into Linux kernel documentation, and I ran into one very
fundamental question: Why does Linux divide virtual address space into
1/3G between kernel and user address space?
Please note that I'm not asking "why 1/3G? why not 2/2G?". I was able
to find some discussion online about this matter. However, my question
is more basic - naive, some may say.
Why is there a division at all? As I understand, there occurs a
context switch between user-level process and the kernel when syscall
is made. Then why are we reserving 1G for kernel? Why isn't it
possible to give full 4G address space to the user process, and kernel
gets full 4G space when switched in?
I'm no expert on this either, but will try to explain it shortly.
Since userspace applications calls kernel functions (read(), open() and so on), those
kernel functions used must be mapped in the address space that an application may see,
i.e. the userspace and kernel space must share the address range. For 32-bit this gives
4GB address space in total.
For linux the split is 1GB for kernel and 3GB for userspace as you noticed. In those 1GB
the kernel code and data structures must be contained, including the virtual mappings for
physical memory.
Please see chapter "Memory mapping and DMA" from Linux Device Drivers (LDD) version 2 or
3, available online.
http://lwn.net/Kernel/LDD2/ (HTML + PFD versions of chapters)
http://lwn.net/Kernel/LDD3/ (only PDF version of chapters)
Here is a link to the pfd version from LDD3: http://lwn.net/images/pdf/LDD3/ch15.pdf
This chapter explains memory management in linux, including this memory split, address
types, high and low memory, page tables. I think it should cover most of your questions,
and perhaps other experts may give additional info.
--
Roar Bjørgum Rotvik
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