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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.
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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 up previous contents index
Next: 4.6 Kernel Page Tables Up: 4. Page Table Management Previous: 4.4 Translating and Setting   Contents   Index
Mel 2004-02-15

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