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.

10. High Memory Management

The kernel may only directly address memory for which it has set up a page table entry. In the most common case, the user/kernel address space split of 3GiB/1GiB implies that at best only 896MiB of memory may be directly accessed at any given time on a 32bit machine^10.1 as explained in Section 5.1.

There are many high end 32 bit machines that have more than 1GiB of memory and the inconveniently located memory cannot be simply ignored. The solution Linux uses is to temporarily map pages from high memory into the lower page tables. This will be discussed in Section 10.2.

High memory and IO has a related problem which must be addressed as not all devices are able to address high memory or all the memory available to the CPU in the case of PAE. Indeed some are limited to addresses the size of a signed 32 bit integer or 2GiB. Asking the device to write to memory will fail at best and possibly disrupt the kernel at worst. The solution to this problem is to use a bounce buffer and this will be discussed in Section 10.4.

This chapter begins with a brief description of how the Persistent Kernel Map (PKMap) address space is managed before talking about how pages are mapped and unmapped from high memory. The subsequent section will deal with the case where the mapping must be atomic before discussing bounce buffers in depth. Finally we will talk about how emergency pools are used for when memory is very tight.

Footnotes

... machine^10.1

On 64 bit hardware, this is not really an issue as there is more than enough virtual address space. It is highly unlikely there will be machines running 2.4 kernels with more than terabytes of RAM.