Thank you guys!

I have two more questions from your replies:

I thought I had understood HIGH_MEM and LOW_MEM, but it appears I was wrong. Does the concept of high memory/low memory correspond to physical address space or virtual address space? Also, does LOW_MEM always have to be 1 GiB maximum?
For a RAM of 896 MiB - 4096 MiB, the book says:
"In this case, the RAM cannot be mapped entirely into the kernel linear address space. The best Linux can do during the initialization phase is to map a RAM window of size 896 MB into the kernel linear address space."

Why is there a need to map the whole RAM into the kernel space (the usage of the word "entirely") ? Shouldn't it be only LOW_MEM ? Or am I confusing the two things here ?

I believe all doubts are pointing to the concepts of LOW_MEM and HIGH_MEM, but I'm still not being able to wrap my head around them.

Thanks,

Sunny

On Thu, Mar 12, 2015 at 11:49 PM, Jeff Haran <Jeff.Haran@citrix.com> wrote:

-----Original Message-----
From: kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies-bounces@kernelnewbies.org] On Behalf Of Arun KS
Sent: Thursday, March 12, 2015 11:03 AM
To: Sunny Shah
Cc: kernelnewbies
Subject: Re: Understanding the mapping of physical memory to kernel address space

Hello Sunny,

On Thu, Mar 12, 2015 at 10:32 PM, Sunny Shah <shahsunny715@gmail.com> wrote:
> Hello,
>
> This is my first mail on this list, so please let me know if I'm erring.
>
> I'm reading Bovet and Cesati's "Understanding the Linux Kernel",
> specifically the chapter "Memory Addressing", sub-section "Kernel Page
> Tables". Here they describe how Linux initializes its page tables for
> various RAM sizes and how much of the physical address space is mapped
> onto the kernel virtual address space.
>
> I have several questions from my reading:
>
> My understanding is that the 32 bit virtual address space of a process
> is split into 2 parts - the first 3 GiB for the user space and the
> remaining 1GiB for the kernel (with the same kernel mapping being used
> for all processes. However, although the kernel is mapped into the
> higher portion of the address space, it resides in the lower 1 GiB of RAM. Is this correct?
Yes. Incase of 3:1 mapping, kernel virtual address starts at 0xc0000000. You can also have 2:2 mappings aswell. It is a configurable option

Just an FYI, I've seen 1:3 mapping too. We had to do that with the kernels we built
when I was at one company because we needed 3GB of virtual address space to map all
of the memory mapped registers on their ASICs.

There's lots of options here.

Jeff Haran