The Linux Virtual Memory System: The Linux Address Space
Get an overview of the Linux address space and the two types of kernel addresses present in Linux.
We'll cover the following
We’ll now discuss some of the more interesting aspects of the Linux VM system. Linux development has been driven forward by real engineers solving real problems encountered in production, and thus a large number of features have slowly been incorporated into what is now a fully functional, feature-filled virtual memory system.
While we won’t be able to discuss every aspect of Linux VM, we’ll touch on the most important ones, especially where it has gone beyond what is found in classic VM systems such as VAX/VMS. We’ll also try to highlight commonalities between Linux and older systems.
For this discussion, we’ll focus on Linux for Intel x86. While Linux can and does run on many different processor architectures, Linux on x86 is its most dominant and important deployment, and thus the focus of our attention.
Overview
Much like other modern operating systems, and also like VAX/VMS,
In classic 32-bit Linux (i.e., Linux with a 32-bit virtual address space), the split between user and kernel portions of the address space takes place at address 0xC0000000
, or three-quarters of the way through the address space. Thus, virtual addresses 0
through 0xBFFFFFFF
are user virtual addresses; the remaining virtual addresses (0xC0000000
through 0xFFFFFFFF
) are in the kernel’s virtual address space. 64-bit Linux has a similar split but at slightly different points. The figure below shows a depiction of a typical (simplified) address space.
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