From 58faef9328f7d66b377cb34a20331c4310cdbd5e Mon Sep 17 00:00:00 2001 From: Mike Rapoport Date: Sat, 30 Jun 2018 17:55:00 +0300 Subject: [PATCH] docs/mm: bootmem: add overview documentation Signed-off-by: Mike Rapoport Signed-off-by: Jonathan Corbet --- mm/bootmem.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 47 insertions(+) diff --git a/mm/bootmem.c b/mm/bootmem.c index 76fc17eb78ae..97db0e8e362b 100644 --- a/mm/bootmem.c +++ b/mm/bootmem.c @@ -21,6 +21,53 @@ #include "internal.h" +/** + * DOC: bootmem overview + * + * Bootmem is a boot-time physical memory allocator and configurator. + * + * It is used early in the boot process before the page allocator is + * set up. + * + * Bootmem is based on the most basic of allocators, a First Fit + * allocator which uses a bitmap to represent memory. If a bit is 1, + * the page is allocated and 0 if unallocated. To satisfy allocations + * of sizes smaller than a page, the allocator records the Page Frame + * Number (PFN) of the last allocation and the offset the allocation + * ended at. Subsequent small allocations are merged together and + * stored on the same page. + * + * The information used by the bootmem allocator is represented by + * :c:type:`struct bootmem_data`. An array to hold up to %MAX_NUMNODES + * such structures is statically allocated and then it is discarded + * when the system initialization completes. Each entry in this array + * corresponds to a node with memory. For UMA systems only entry 0 is + * used. + * + * The bootmem allocator is initialized during early architecture + * specific setup. Each architecture is required to supply a + * :c:func:`setup_arch` function which, among other tasks, is + * responsible for acquiring the necessary parameters to initialise + * the boot memory allocator. These parameters define limits of usable + * physical memory: + * + * * @min_low_pfn - the lowest PFN that is available in the system + * * @max_low_pfn - the highest PFN that may be addressed by low + * memory (%ZONE_NORMAL) + * * @max_pfn - the last PFN available to the system. + * + * After those limits are determined, the :c:func:`init_bootmem` or + * :c:func:`init_bootmem_node` function should be called to initialize + * the bootmem allocator. The UMA case should use the `init_bootmem` + * function. It will initialize ``contig_page_data`` structure that + * represents the only memory node in the system. In the NUMA case the + * `init_bootmem_node` function should be called to initialize the + * bootmem allocator for each node. + * + * Once the allocator is set up, it is possible to use either single + * node or NUMA variant of the allocation APIs. + */ + #ifndef CONFIG_NEED_MULTIPLE_NODES struct pglist_data __refdata contig_page_data = { .bdata = &bootmem_node_data[0]