After this documentation was released in July 2003, I was approached by Prentice Hall and asked to write a book on the Linux VM under the Bruce Peren's Open Book Series.

The book is available and called simply "Understanding The Linux Virtual Memory Manager". There is a lot of additional material in the book that is not available here, including details on later 2.4 kernels, introductions to 2.6, a whole new chapter on the shared memory filesystem, coverage of TLB management, a lot more code commentary, countless other additions and clarifications and a CD with lots of cool stuff on it. This material (although now dated and lacking in comparison to the book) will remain available although I obviously encourge you to buy the book from your favourite book store :-) . As the book is under the Bruce Perens Open Book Series, it will be available 90 days after appearing on the book shelves which means it is not available right now. When it is available, it will be downloadable from http://www.phptr.com/perens so check there for more information.

To be fully clear, this webpage is not the actual book.

2.2 Getting Started

When a new developer or researcher asks how to begin reading the code, they are often recommended to start with the initialisation code and work from there. I do not believe that this is the best approach as initialisation is quite architecture dependent and requires a detailed hardware knowledge to decipher it. It also does not give much information about how a subsystem like the VM works as it is only in the late stages of initialisation that memory is set up in the way the running system sees it.

The best starting point for kernel documentation is first and foremost the Documentation/ tree. It is very loosely organised but contains much Linux specific information that will be unavailable elsewhere. The second visiting point is the Kernel Newbies website at http://www.kernelnewbies.org which is a site dedicated to people starting kernel development and includes a Frequently Asked Questions (FAQ) section and a recommended reading list.

The best starting point to understanding the VM, I believe, is now this document and the companion code commentary. It describes a VM that is reasonably comprehensive without being overly complicated. Later VMs are more complex but are essentially extensions of the one described here rather than totally new so understanding the 2.4.20 VM is an important starting point.

For when the code has to be approached afresh with a later VM, it is always best to start in an isolated region that has the minimum number of dependencies. In the case of the VM, the best starting point is the Out Of Memory (OOM) manager in mm/oom_kill.c. It is a very gentle introduction to one corner of the VM where a process is selected to be killed in the event that memory in the system is low. The second subsystem to then examine is the non-contiguous memory allocator located in mm/vmalloc.c and discussed in Chapter 8 as it is reasonably contained within one file. The third system should be physical page allocator located in mm/page_alloc.c and discussed in Chapter 7 for similar reasons. The fourth system of interest is the creation of VMAs and memory areas for processes discussed in Chapter 5. Between these systems, they have the bulk of the code patterns that are prevalent throughout the rest of the kernel code making the deciphering of more complex systems such as the page replacement policy or the buffer IO much easier to comprehend.

The second recommendation that is given by experienced developers is to benchmark and test but unfortunately the VM is difficult to test accurately and benchmarking is just a shade above vague handwaving at timing figures. A tool called VM Regress was developed during the course of research and is available at http://www.csn.ul.ie/$\sim$mel/vmregress that lays the foundation required to build a fully fledged testing, regression and benchmarking tool for the VM. It uses a combination of kernel modules and userspace tools to test small parts of the VM in a reproducible manner and has one benchmark for testing the page replacement policy using a large reference string. It is intended as a framework for the development of a testing utility and has a number of Perl libraries and helper kernel modules to do much of the work but is in the early stages of development at time of writing.