Re: What's the deal with 1/3G separation between kernel address space and user address space?

["Roar Bjørgum Rotvik" <roarbr@tihlde.org>]

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
--
To unsubscribe from this list: send the line "unsubscribe linux-newbie" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.linux-learn.org/faqs