What happens when the kernel memory required exceeds 1 GB

2
votes

I am trying to understand 'Memory Management' in Linux as a part of the course in 'Understanding the Linux Kernel' by Daniel and Marco. Below is my understanding of the Kernel space

  1. On a 32-bit machine, each process has 4GB virtual address space. 3GB - User and 1GB - Kernel space.
  2. The 1 GB is shared among processes and directly mapped to 1 GB of RAM. This space is used to store kernel code, Page tables etc.
  3. The 1 GB cannot be swapped out. Although, it can be freed.

My question is, what if the total kernel space required by processes exceeds 1 GB?

1
memory-managementlinux-kernel

1 Answers

5
votes

First, a correction - the (almost) 1Gb the kernel has mapped in a 1:1 is not used exclusively by the kernel. It's just that the kernel has the easiest access to that memory. it does hold the kernel code and static data.

The kernel virtual space actually has something like 256 Mb (the number is dynamic) at the top of the virtual address space (top of the 1Gb the kernel uses) that is not mapped 1:1 like the rest of the kernel linear addresses, but instead mapped dynamically to various pages - either in to get a virtual continuous region out of non contiguous physical memory by vmalloc, or to map memory mapped IO by ioremap or to get access to higher then 1Gb pages via kmap.

So to sum up - when the kernel needs access to more memory then the (almost) 1Gb it has always mapped in a 1:1 setting it uses dynamic memory just like user space.