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.

4. Page Table Management

Linux layers the machine independent/dependent layer in an unusual manner in comparison to other operating systems [#!cranor99!#]. Other operating systems have objects which manage the underlying physical pages such as the pmap object in BSD but Linux instead maintains the concept of a three-level page table in the architecture independent code even if the underlying architecture does not support it. While this is relatively easy to understand, it also means that the distinction between different types of pages is very blurry and page types are identified by their flags or what lists they exist on rather than the objects they belong to.

Architectures that manage their MMU differently are expected to emulate the three-level page tables. For example, on the x86 without PAE enabled, only two page table levels are available. The Page Middle Directory (PMD) is defined to be of size 1 and ``folds back'' directly onto the Page Global Directory (PGD) which is optimised out at compile time. Unfortunately, for architectures that do not manage their cache or Translation Lookaside Buffer (TLB) automatically, hooks that are machine dependent have to be explicitly left in the code for when the TLB and CPU caches need to be altered and flushed even if they are null operations on some architectures like the x86. Fortunately, the functions and how they have to be used are very well documented in the cachetlb.txt file in the kernel documentation tree [#!miller00!#].

This chapter will begin by describing how the page table is arranged and what types are used to describe the three separate levels of the page table followed by how a virtual address is broken up into its component parts for navigating the table. Once covered, it will be discussed how the lowest level entry, the Page Table Entry (PTE) and what bits are used by the hardware. After that, the macros used for navigating a page table, setting and checking attributes will be discussed before talking about how the page table is populated and how pages are allocated and freed for the use with page tables. Finally, it will be discussed how the page tables are initialised during boot strapping.