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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: 7. Physical Page Allocation Up: 6. Boot Memory Allocator Previous: 6.4 Freeing Memory   Contents   Index
Late in the bootstrapping process, the function
is called which knows it is safe to remove the boot allocator and all
its associated data structures. Each architecture is required to provide
mem_init() that is responsible for destroying the
boot memory allocator and its associated structures.
The purpose of the function is quite simple. It is responsible for calculating
the dimensions of low and high memory and printing out an informational
message to the user as well as performing final initialisations of the hardware
if necessary. On the x86, the principal function of concern for the VM is the
This function first tells the boot memory allocator to retire itself
free_all_bootmem() for UMA architectures or
free_all_bootmem_node() for NUMA. Both call the core function
free_all_bootmem_core() with different parameters. The core
function is simple in principle and performs the following tasks:
- For all unallocated pages known to the allocator for this node;
- Clear the
PG_reservedflag in its struct page;
- Set the count to 1;
__free_pages()so that the buddy allocator (discussed next chapter) can build its free lists.
- Clear the
- Free all pages used for the bitmap and give them to the buddy allocator.
At this stage, the buddy allocator now has control of all the pages
in low memory which leaves only the high memory pages. The remainder
free_pages_init() function is responsible for
free_all_bootmem() returns, it first counts
the number of reserved pages for accounting purposes and then calls
one_highpage_init() for every page between
This function simple clears the
PG_highmem flag, sets the count to 1 and calls
__free_pages() to release it to the buddy allocator in the same
At this point, the boot memory allocator is no longer required and the buddy
allocator is the main physical page allocator for the system. An interesting
feature to note is that not only is the data for the boot allocator removed
but also the code. All the init function declarations used for bootstrapping
the system are marked
__init such as the following;
321 unsigned long __init free_all_bootmem (void)
All of these functions are placed together in the
.init section by
the linker. On the x86, the function
free_initmem() walks through
all pages from
and frees up the pages to the buddy allocator. With this method, Linux can
free up a considerable amount of memory6.3that is used by bootstrapping code that is no longer required.
- ... memory6.3
- 27 pages were freed while booting the kernel running on the machine this document is composed on.
Next: 7. Physical Page Allocation Up: 6. Boot Memory Allocator Previous: 6.4 Freeing Memory   Contents   Index Mel 2004-02-15
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