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.

12.5 Activating a Swap Area

As it has now been covered what swap areas are, how they are represented and how pages are tracked, it is time to see how they all tie together to activate an area. Activating an area is conceptually quite simple; Open the file, load the header information from disk, populate a swap_info_struct and add it to the swap list.

The function responsible for the activation of a swap area is sys_swapon() and it takes two parameters, the path to the special file for the swap area and a set of flags. While swap is been activated, the Big Kernel Lock (BKL) is held which prevents any application entering kernel space while this operation is been performed. The function is quite large but can be broken down into the following simple steps;

• Find a free swap_info_struct in the swap_info array an initialise it with default values

• Call user_path_walk() which traverses the directory tree for the supplied specialfile and populates a namidata structure with the available data on the file, such as the dentry and the filesystem information for where it is stored (vfsmount)

• Populate swap_info_struct fields pertaining to the dimensions of the swap area and how to find it. If the swap area is a partition, the block size will be configured to the PAGE_SIZE before calculating the size. If it is a file, the information is obtained directly from the inode

• Ensure the area is not already activated. If not, allocate a page from memory and read the first page sized slot from the swap area. This page contains information such as the number of good slots and how to populate the swap_info_struct$\rightarrow$swap_map with the bad entries

• Allocate memory with vmalloc() for swap_info_struct$\rightarrow$swap_map and initialise each entry with 0 for good slots and SWAP_MAP_BAD otherwise. Ideally the header information will be a version 2 file format as version 1 was limited to swap areas of just under 128MiB for architectures with 4KiB page sizes like the x86^12.3

• After ensuring the information indicated in the header matches the actual swap area, fill in the remaining information in the swap_info_struct such as the maximum number of pages and the available good pages. Update the global statistics for nr_swap_pages and total_swap_pages

• The swap area is now fully active and initialised and so it is inserted into the swap list in the correct position based on priority of the newly activated area

At the end of the function, the BKL is released and the system now has a new swap area available for paging to.

Footnotes

... x86^12.3

See the Code Commentary for the comprehensive reason for this.