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Linux Tutorial - The Operating System - The Kernel - Memory Management - Page Allocation and Deallocation
  Linux Page Tables ---- Memory Mapping  


Page Allocation and Deallocation

There are many demands on the physical pages in the system. For example, when an image is loaded into memory the operating system needs to allocate pages. These will be freed when the image has finished executing and is unloaded. Another use for physical pages is to hold kernel specific data structures such as the page tables themselves. The mechanisms and data structures used for page allocation and deallocation are perhaps the most critical in maintaining the efficiency of the virtual memory subsystem.

All of the physical pages in the system are described by the mem_map data structure which is a list of mem_map_t** structures which is initialized at boot time. Each mem_map_t describes a single physical page in the system. Important fields (so far as memory management is concerned) are:

count
This is a count of the number of users of this page. The count is greater than one when the page is shared between many processes,
age
This field describes the age of the page and is used to decide if the page is a good candidate for discarding or swapping,
map_nr
This is the physical page frame number that this mem_map_t describes.

The free_area vector is used by the page allocation code to find and free pages. The whole buffer management scheme is supported by this mechanism and, as far as the code is concerned, the size of the page and physical paging mechanisms used by the processor are irrelevant.

Each element of free_area contains information about blocks of pages. The first element in the array describes single pages, the next blocks of 2 pages, the next blocks of 4 pages and so on upwards in powers of two. The list element is used as a queue head and has pointers to the page data structures in the mem_map array. Free blocks of pages are queued here. map is a pointer to a bitmap which keeps track of allocated groups of pages of this size. Bit N of the bitmap is set if the Nth block of pages is free.

The figure below, shows the free_area structure. Element 0 has one free page (page frame number 0) and element 2 has 2 free blocks of 4 pages, the first starting at page frame number 4 and the second at page frame number 56.

Page Allocation

Linux uses the Buddy algorithm 2 to effectively allocate and deallocate blocks of pages. The page allocation code

attempts to allocate a block of one or more physical pages. Pages are allocated in blocks which are powers of 2 in size. That means that it can allocate a block 1 page, 2 pages, 4 pages and so on. So long as there are enough free pages in the system to grant this request (nr_free_pages > min_free_pages) the allocation code will search the free_area for a block of pages of the size requested. Each element of the free_area has a map of the allocated and free blocks of pages for that sized block. For example, element 2 of the array has a memory map that describes free and allocated blocks each of 4 pages long.

The allocation algorithm first searches for blocks of pages of the size requested. It follows the chain of free pages that is queued on the list element of the free_area data structure. If no blocks of pages of the requested size are free, blocks of the next size (which is twice that of the size requested) are looked for. This process continues until all of the free_area has been searched or until a block of pages has been found. If the block of pages found is larger than that requested it must be broken down until there is a block of the right size. Because the blocks are each a power of 2 pages big then this breaking down process is easy as you simply break the blocks in half. The free blocks are queued on the appropriate queue and the allocated block of pages is returned to the caller.


Figure: The free_area data structure

For example, in the Figure above if a block of 2 pages was requested, the first block of 4 pages (starting at page frame number 4) would be broken into two 2 page blocks. The first, starting at page frame number 4 would be returned to the caller as the allocated pages and the second block, starting at page frame number 6 would be queued as a free block of 2 pages onto element 1 of the free_area array.

Page Deallocation

Allocating blocks of pages tends to fragment memory with larger blocks of free pages being broken down into smaller ones. The page deallocation code

recombines pages into larger blocks of free pages whenever it can. In fact the page block size is important as it allows for easy combination of blocks into larger blocks.

Whenever a block of pages is freed, the adjacent or buddy block of the same size is checked to see if it is free. If it is, then it is combined with the newly freed block of pages to form a new free block of pages for the next size block of pages. Each time two blocks of pages are recombined into a bigger block of free pages the page deallocation code attempts to recombine that block into a yet larger one. In this way the blocks of free pages are as large as memory usage will allow.

For example, in the Figure above, if page frame number 1 were to be freed, then that would be combined with the already free page frame number 0 and queued onto element 1 of the free_area as a free block of size 2 pages.

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