Understanding the mapping of physical memory to kernel address space

[Sunny Shah]

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 at citrix.com> wrote:

> -----Original Message-----
> From: kernelnewbies-bounces at kernelnewbies.org [mailto:
> kernelnewbies-bounces at 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 at 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
>
>
>
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