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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.
Next: 4.6 Kernel Page Tables Up: 4. Page Table Management Previous: 4.4 Translating and Setting Contents Index
4.5 Allocating and Freeing Page Tables
The last set of functions deal with the allocation and freeing of page tables. Page tables, as stated, are physical pages containing an array of entries and the allocation and freeing of physical pages is a relatively expensive operation, both in terms of time and the fact that interrupts are disabled during page allocation. The allocation and deletion of page tables, at any of the three levels, is a very frequent operation so it is important the operation is as quick as possible.
Hence the pages used for the page tables are cached in a number of different lists called quicklists. Each architecture implements these caches differently but the principles used are the same. For example, not all architectures cache PGDs because the allocation and freeing of them only happens during process creation and exit. As both of these are very expensive operations, the allocation of another page is negligible.
PGDs, PMDs and PTEs have two sets of functions each for the allocation and
freeing of page tables. The allocation functions are pgd_alloc(),
pmd_alloc() and pte_alloc() respectively and the
free functions are, predictably enough, called pgd_free(),
pmd_free() and pte_free().
Broadly speaking, the three implement caching with the use of three
caches called pgd_quicklist, pmd_quicklist
and pte_quicklist. Architectures implement these three
lists in different ways but one method is through the use of a LIFO type
structure. Ordinarily, a page table entry contains pointers to other pages
containing page tables or data. While cached, the first element of the list
is used to point to the next free page table. During allocation, one page
is popped off the list and during free, one is placed as the new head of
the list. A count is kept of how many pages are used in the cache.
The quick allocation function from the pgd_quicklist
is not externally defined outside of the architecture although
get_pgd_fast() is a common choice for the function name. The
cached allocation function for PMDs and PTEs are publicly defined as
pmd_alloc_one_fast() and pte_alloc_one_fast().
If a page is not available from the cache, a page will be allocated using the
physical page allocator (see Chapter 7). The
functions for the three levels of page tables are get_pgd_slow(),
pmd_alloc_one() and pte_alloc_one().
Obviously a large number of pages may exist on these caches and so there
is a mechanism in place for pruning them. Each time the caches grow or
shrink, a counter is incremented or decremented and it has a high and low
watermark. check_pgt_cache() is called in two places to check
these watermarks. When the high watermark is reached, entries from the cache
will be freed until the cache size returns to the low watermark. The function
is called after clear_page_tables() when a large number of page
tables are potentially reached and is also called by the system idle task.
Next: 4.6 Kernel Page Tables Up: 4. Page Table Management Previous: 4.4 Translating and Setting Contents Index Mel 2004-02-15