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.

10.1 Managing the PKMap Address Space

Space is reserved at the top of the kernel page tables from PKMAP_BASE to FIXADDR_START for a PKMap. The size of the space reserved varies slightly. On the x86, PKMAP_BASE is at 0xFE000000 and the address of FIXADDR_START is a compile time constant that varies with configure options but is typically only a few pages. This means that there is slightly below 32MiB of page table space for mapping pages from high memory into usable space.

For mapping pages, a single page set of PTEs is stored at the beginning of the PKMap area to allow 1024 high pages to be mapped into low memory for short periods with the function kmap() and unmapped with kunmap(). The pool seems very small but the page is only mapped by kmap() for a very short time. Comments in the code indicate that there was a plan to allocate contiguous page table entries to expand this area but it has remained just that, comments in the code so a large portion of the PKMap is unused.

The page table entry for use with kmap() is called pkmap_page_table which is located at PKMAP_BASE and set up during system initialisation^10.2. The pages for the PGD and PMD entries are allocated by the boot memory allocator to ensure they exist.

The current state of the page table entries is managed by a simple array called called pkmap_count which has LAST_PKMAP entries in it. On an x86 system without PAE, this is 1024 and with PAE, it is 512. More accurately, albeit not expressed in code, the LAST_PKMAP variable is equivalent to PTRS_PER_PTE.

Each element is not exactly a reference count but it is very close. If the entry is 0, the page is free and has not been used since the last TLB flush. If it is 1, the slot is unused but a page is still mapped there waiting for a TLB flush. Flushes are delayed until every slot has been used at least once as a global flush is required for all CPUs when the global page tables are modified and is extremely expensive. Any higher value is a reference count of n-1 users of the page.

Footnotes

... initialisation^10.2

On the x86, this takes place at the end of the pagetable_init() function.