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.

11.5 Swapping Out Process Pages

When max_mapped pages have been found in the page cache, swap_out() is called to start swapping out process pages. Starting from the mm_struct pointed to by swap_mm and the address mm$\rightarrow$swap_address, the page tables are searched forward until nr_pages have been freed.

Figure 11.5: Call Graph: swap_out()

All process mapped pages are examined regardless of where they are in the lists or when they were last referenced but pages which are part of the active_list or have been recently referenced will be skipped over. The examination of hot pages is a bit costly but insignificant in comparison to linearly searching all processes for the PTEs that reference a particular struct page.

Once it has been decided to swap out pages from a process, an attempt will be made to swap out at least SWAP_CLUSTER number of pages and the full list of mm_structs will only be examined once to avoid constant looping when no pages are available. Writing out the pages in bulk increases the chance that pages close together in the process address space will be written out to adjacent slots on disk.

swap_mm is initialised to point to init_mm and the swap_address is initialised to 0 the first time it is used. A task has been fully searched when the swap_address is equal to TASK_SIZE. Once a task has been selected to swap pages from, the reference count to the mm_struct is incremented so that it will not be freed early and swap_out_mm() is called with the selected mm_struct as a parameter. This function walks each VMA the process holds and calls swap_out_vma() for it. This is to avoid having to walk the entire page table which will be largely sparse. swap_out_pgd() and swap_out_pmd() walk the page tables for given VMA until finally try_to_swap_out() is called on the actual page and PTE.

try_to_swap_out() first checks to make sure the page is not part of the active_list, been recently referenced or part of a zone that we are not interested in. Once it has been established this is a page to be swapped out, it is removed from the page tables of the process and further work is performed. It is at this point the PTE is checked to see if it is dirty. If it is, the struct page flags will be updated to reflect that so that it will get laundered. Pages with buffers are not handled further as they can not be swapped out to backing storage so the PTE for the process is simply established again and the page will be flushed later.

The process of allocating space in the backing storage and swapping pages out is discussed further in Chapter 12.