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.

7.1 Managing Free Blocks

As stated, the allocator maintains blocks of free pages where each block is a power of two number of pages. The exponent for the power of two sized block is referred to as the order. An array of free_area_t structures are maintained for each order that points to a linked list of blocks of pages that are free as indicated by Figure 7.1.

Figure 7.1: Free page block management

Hence, the 0th element of the array will point to a list of free page blocks of size $2^0$ or 1 page, the 1st element will be a list of $2^1$ (2) pages up to $2^{\mathrm{MAX\_ORDER}-1}$ number of pages, where the MAX_ORDER is currently defined as 10. This eliminates the chance that a larger block will be split to satisfy a request where a smaller block would have sufficed. The page blocks are maintained on a linear linked list via page$\rightarrow$list.

Each zone has a free_area_t struct array called free_area[MAX_ORDER]. It is declared in $<$linux/mm.h$>$ as follows:

 22 typedef struct free_area_struct {
 23         struct list_head        free_list;
 24         unsigned long           *map;
 25 } free_area_t;

The fields in this struct are simply:

free_list A linked list of free page blocks;

map A bitmap representing the state of a pair of buddies.

Linux saves memory by only using one bit instead of two to represent each pair of buddies. Each time a buddy is allocated or freed, the bit representing the pair of buddies is toggled so that the bit is zero if the pair of pages are both free or both full and 1 if only one buddy is in use. To toggle the correct bit, the macro MARK_USED() in page_alloc.c is used which is declared as follows:

164 #define MARK_USED(index, order, area) \
165         __change_bit((index) >> (1+(order)), (area)->map)

index is the index of the page within the global mem_map array. By shifting it right by 1+order bits, the bit within map representing the pair of buddies is revealed.