While going through the "Output of dmesg" I could see a list of values which i am not able to understand properly.

Memory: 2047804k/2086248k available (3179k kernel code, 37232k reserved, 1935k data, 436k init, 1176944k highmem)
virtual kernel memory layout:
    fixmap  : 0xffc57000 - 0xfffff000   (3744 kB)
    pkmap   : 0xff800000 - 0xffa00000   (2048 kB)
    vmalloc : 0xf7ffe000 - 0xff7fe000   ( 120 MB)
    lowmem  : 0xc0000000 - 0xf77fe000   ( 887 MB)
      .init : 0xc0906000 - 0xc0973000   ( 436 kB)
      .data : 0xc071ae6a - 0xc08feb78   (1935 kB)
      .text : 0xc0400000 - 0xc071ae6a   (3179 kB)

From the values i understand that i have 2GB RAM(Physical memory). But rest of the things seems to be Magic Numbers for me.

I would like to know about each one (fixmap, pkmap,.. etc.) in brief(if more doubts, I will post each one as a separate Question)?

Could someone explain that to me?

1 Answer 1


First off, a 32 bit system has 0xffffffff (4'294'967'295) linear addresses to access a physical location ontop of the RAM.
The kernel divides these addresses into user and kernel space.

User space (high memory) can be accessed by the user and, if necessary, also by the kernel.
The address range in hex and dec notation:

0x00000000 - 0xbfffffff
0 - 3'221'225'471

Kernel space (low memory) can only be accessed by the kernel.
The address range in hex and dec notation:

0xc0000000 - 0xffffffff
3'221'225'472 - 4'294'967'295

Like this:

0x00000000             0xc0000000  0xffffffff 
    |                        |          |
    |  User                  |  Kernel  |
    |  space                 |  space   |

Thus, the memory layout you saw in dmesg corresponds to the mapping of linear addresses in kernel space.

First, the .text, .data and .init sequences which provide the initialization of the kernel's own page tables (translate linear into physical addresses).

.text : 0xc0400000 - 0xc071ae6a   (3179 kB)

The range where the kernel code resides.

.data : 0xc071ae6a - 0xc08feb78   (1935 kB)

The range where the kernel data segments reside.

.init : 0xc0906000 - 0xc0973000   ( 436 kB)

The range where the kernel's initial page tables reside.

(and another 128 kB for some dynamic data structures.)

This minimal address space is just large enough to install the kernel in the RAM and to initialize its core data structures.

Their used size is shown inside the parenthesis, take for example the kernel code:

0xc071ae6a - 0xc0400000 = 31AE6A

In decimal notation, that's 3'255'914 (3179 kB).

Second, the usage of kernel space after initialization

lowmem  : 0xc0000000 - 0xf77fe000   ( 887 MB)

The lowmem range can be used by the kernel to directly access physical addresses.
This is not the full 1 GB, because the kernel always requires at least 128 MB of linear addresses to implement noncontiguous memory allocation and fix-mapped linear addresses.

vmalloc : 0xf7ffe000 - 0xff7fe000   ( 120 MB)

Virtual memory allocation can allocate page frames based on a noncontiguous scheme. The main advantage of this schema is to avoid external fragmentation, this is used for swap areas, kernel modules or allocation of buffers to some I/O devices.

pkmap   : 0xff800000 - 0xffa00000   (2048 kB)

The permanent kernel mapping allows the kernel to establish long-lasting mappings of high-memory page frames into the kernel address space. When a HIGHMEM page is mapped using kmap(), virtual addresses are assigned from here.

fixmap  : 0xffc57000 - 0xfffff000   (3744 kB)

These are fix-mapped linear addresses which can refer to any physical address in the RAM, not just the last 1 GB like the lowmem addresses. Fix-mapped linear addresses are a bit more efficient than their lowmem and pkmap colleagues. There are dedicated page table descriptors assigned for fixed mapping, and mappings of HIGHMEM pages using kmap_atomic are allocated from here.

If you want to dive deeper into the rabbit hole:
Understanding the Linux Kernel

  • Thanks for this great answer. I would like to know why low mem is not full 1GB and more about the next part of the sentence "because the kernel always requires at least 128 MB of linear addresses to implement noncontiguous memory allocation and fix-mapped linear addresses." Commented Dec 26, 2010 at 7:27
  • The kernel has to access high-memory code from time to time (f.e. BIOS and ACPI information reside on the first MB of the RAM), he cannot do that directly (like the lowmem area), therefore he needs to map low-memory to high-memory linear addresses, the 128MB is reserved only for this purpose. The vmalloc area is mostly temporary mapped onto some high-memory areas and get's remapped quite fast.
    – wag
    Commented Dec 26, 2010 at 13:09
  • So the pages set up by kernel for virtual system call are also a part of fixmap??? I ran into this question becaues I want to know what exactly lies in the page with address fffb5000, fffa1000 etc ... I am getting overhead in my virtual Machine record replay because many virtual CPUs access this page a LOT.... how do I know what exactly is in this address ... GREAT ANSWER by the way :) Commented Dec 27, 2012 at 5:13

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