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.

6.4 Freeing Memory

In contrast to the allocation functions, only two free function are provided which are free_bootmem() for UMA and free_bootmem_node() for NUMA. They both call free_bootmem_core() with the only difference being that a pgdat is supplied with NUMA.

The core function is relatively simple in comparison to the rest of the allocator. For each full page affected by the free, the corresponding bit in the bitmap is set to 0. If it already was 0, BUG() is called to signal a double-free.

An important restriction with the free functions is that only full pages may be freed. It is never recorded when a page is partially allocated so if only partially freed, the full page remains reserved. This is not as major a problem as it appears as the allocations always persist for the lifetime of the system; However, it is still an important restriction for developers during boot time.