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简介：bootmem分配器是系统启动初期的内存分配方式，在耳熟能详的伙伴系统，slab系统建立前内存都是利用bootmem分配器来分配的，伙伴系统框架建立起来后，bootmem会过度到伙伴系统，bootmem大致思想就是收集内存中的可用内存，然后建立bit位图，然后需要的内存从这些空闲内存中分配，分配了就标记占用，当然这种分配方式很低效，但是由于只占用启动阶段很少一部分，所以也大可接受了，好了，废话不多说，一起看代码吧，
相关阅读：http://www.linuxidc.com/Linux/2012-02/53138.htm
一、初始化部分
init/main.c中
start_kernel函数
{
…
setup_arch(&command_line);
…
}
代码位于arch/arm/kernel/setup.c中，
void __init setup_arch(char **cmdline_p)
{
…
paging_init(mdesc);
…
}
*******************************************************
* 继续跟进paging_init函数在arch/arm/mm/mmu.c中
*
*******************************************************
* paging_init() sets up the page tables, initialises the zone memory
* maps, and sets up the zero page, bad page and bad page tables.
*/
void __init paging_init(struct machine_desc *mdesc)
{
void *zero_page;
memblock_set_current_limit(lowmem_limit);
build_mem_type_table();
prepare_page_table();
map_lowmem();
devicemaps_init(mdesc);
kmap_init();
top_pmd = pmd_off_k(0xffff0000);
/* allocate the zero page. */
zero_page = early_alloc(PAGE_SIZE);
bootmem_init();
empty_zero_page = virt_to_page(zero_page);
__flush_dcache_page(NULL, empty_zero_page);
}
* 其中的bootmem_init()就是我们要找的，对应arm的定义在
* arch/arm/mm/init.c
***************************************************************
void __init bootmem_init(void)
{
unsigned long min, max_low, max_high;
max_low = max_high = 0;
/*
*找到内存区域大小，max_low低端内存上界限
*max_high 总内存上界
*/
find_limits(&min, &max_low, &max_high);
//arm bootmem初始化
arm_bootmem_init(min, max_low);
/*
* Sparsemem tries to allocate bootmem in memory_present(),
* so must be done after the fixed reservations
*/
arm_memory_present();
/*
* sparse_init() needs the bootmem allocator up and running.
*/
sparse_init();
/*
* Now free the memory – free_area_init_node needs
* the sparse mem_map arrays initialized by sparse_init()
* for memmap_init_zone(), otherwise all PFNs are invalid.
*/
arm_bootmem_free(min, max_low, max_high);
high_memory = __va(((phys_addr_t)max_low << PAGE_SHIFT) – 1) + 1;
/*
* This doesn’t seem to be used by the Linux memory manager any
* more, but is used by ll_rw_block. If we can get rid of it, we
* also get rid of some of the stuff above as well.
*
* Note: max_low_pfn and max_pfn reflect the number of _pages_ in
* the system, not the maximum PFN.
*/
max_low_pfn = max_low – PHYS_PFN_OFFSET;
max_pfn = max_high – PHYS_PFN_OFFSET;
}
*******************************************************
* 这里的函数需要一个一个的过一下，先看这个
* find_limits(&min, &max_low, &max_high)函数
* 同文件夹下
*******************************************************
static void __init find_limits(unsigned long *min, unsigned long *max_low,
unsigned long *max_high)
{
struct meminfo *mi = &meminfo; //内存bank的表述结构
int i;
*min = -1UL;
*max_low = *max_high = 0;
for_each_bank (i, mi) {
struct membank *bank = &mi->bank[i];
unsigned long start, end;
start = bank_pfn_start(bank);
end = bank_pfn_end(bank);
if (*min > start)
*min = start; //保证min最后是最小值
if (*max_high < end) //保证最后的max_high是最大值
*max_high = end;
if (bank->highmem) //如果bank是高端内存那么不再标记max_low
continue;
if (*max_low < end) //如果是普通内存，那么max_low继续跟着增大
*max_low = end;
}
}这样会遍历所有的bank，start和end分别表示每个bank的开始和结束地址，max_low标记成低端最大值，例如：896M，平时耳熟能详的x86上面，这个值不是个固定值，是根据总内存大小来的，896M比较常见.
**************************************************************
* 回过头来继续看arm_bootmem_init(min, max_low)
* 将刚才得到的值传入，可见bootmem是操作的低端内存
**************************************************************
static void __init arm_bootmem_init(unsigned long start_pfn,
unsigned long end_pfn)
{
struct memblock_region *reg;
unsigned int boot_pages;
phys_addr_t bitmap;
pg_data_t *pgdat;
/*
* Allocate the bootmem bitmap page. This must be in a region
* of memory which has already been mapped.
*
* 分配bitmap所需的内存，bitmap是一个内存单元分配与否的标志，
* 用一个bit来表示，当然它自身也要占用一定内存，参数是低端起始
* 内存到低端上界内存，这里假设拿x86的896M来说，那么就是
* 0—896M，哦，忘了说一点，这里应当是以4k(一页)为单位，
* 所以应当是0—896M/4k
*/
boot_pages = bootmem_bootmap_pages(end_pfn – start_pfn);
//分配bitmap
bitmap = memblock_alloc_base(boot_pages << PAGE_SHIFT, L1_CACHE_BYTES,
__pfn_to_phys(end_pfn));
/*
* Initialise the bootmem allocator, handing the
* memory banks over to bootmem.
* 初始化分配器核心，包括bitmap内容清空
*/
node_set_online(0);
pgdat = NODE_DATA(0);
init_bootmem_node(pgdat, __phys_to_pfn(bitmap), start_pfn, end_pfn);
/* Free the lowmem regions from memblock into bootmem.
* 把memblock收集的那些内存标志到bootmem中，也就是把这些
* 内存在bootmem中标记成可用在bitmap中
*
*/
for_each_memblock(memory, reg) {
unsigned long start = memblock_region_memory_base_pfn(reg);
unsigned long end = memblock_region_memory_end_pfn(reg);
if (end >= end_pfn)
end = end_pfn;
if (start >= end)
break;
free_bootmem(__pfn_to_phys(start), (end – start) << PAGE_SHIFT);
}
/* Reserve the lowmem memblock reserved regions in bootmem.
* 再过滤一遍，把memblock已用的内存在bitmap中标记成已使用
*
*/
for_each_memblock(reserved, reg) {
unsigned long start = memblock_region_reserved_base_pfn(reg);
unsigned long end = memblock_region_reserved_end_pfn(reg);
if (end >= end_pfn)
end = end_pfn;
if (start >= end)
break;
reserve_bootmem(__pfn_to_phys(start),
(end – start) << PAGE_SHIFT, BOOTMEM_DEFAULT);
}
}
*******************************************************
* bootmem_bootmap_pages是个体系结构无关函数在
* mm/bootmem.c中定义
*******************************************************
/**
* bootmem_bootmap_pages – calculate bitmap size in pages
* @pages: number of pages the bitmap has to represent
*/
unsigned long __init bootmem_bootmap_pages(unsigned long pages)
{
unsigned long bytes = bootmap_bytes(pages);
return PAGE_ALIGN(bytes) >> PAGE_SHIFT;
}
static unsigned long __init bootmap_bytes(unsigned long pages)
{
//页数/8得到需要用多少个字节表示这些页
unsigned long bytes = (pages + 7) / 8;
return ALIGN(bytes, sizeof(long));
}返回的即是bitmap的size，下面函数将分配bitmap，
*******************************************************
* memblock_alloc_base这个东东以前没见过，顺着看看
* 在/mm/memblock.c中，这里的参数max_addr为896M
*******************************************************
phys_addr_t __init __memblock_alloc_base(phys_addr_t size, phys_addr_t align, phys_addr_t max_addr)
{
phys_addr_t found;
/* We align the size to limit fragmentation. Without this, a lot of
* small allocs quickly eat up the whole reserve array on sparc
*/
size = memblock_align_up(size, align);
found = memblock_find_base(size, align, 0, max_addr);
if (found != MEMBLOCK_ERROR &&
!memblock_add_region(&memblock.reserved, found, size))
return found;
return 0;
}
*******************************************************
* memblock_find_base在可操作内存范围内，找到第一个bitmap
* size大小的空闲内存
*******************************************************
static phys_addr_t __init_memblock memblock_find_base(phys_addr_t size,
phys_addr_t align, phys_addr_t start, phys_addr_t end)
{
long i;
BUG_ON(0 == size);
/* Pump up max_addr */
if (end == MEMBLOCK_ALLOC_ACCESSIBLE)
end = memblock.current_limit;
/* We do a top-down search, this tends to limit memory
* fragmentation by keeping early boot allocs near the
* top of memory
*/
for (i = memblock.memory.cnt – 1; i >= 0; i–) {
phys_addr_t memblockbase = memblock.memory.regions[i].base;
phys_addr_t memblocksize = memblock.memory.regions[i].size;
phys_addr_t bottom, top, found;
if (memblocksize < size)
continue;
if ((memblockbase + memblocksize) <= start)
break;
bottom = max(memblockbase, start);
top = min(memblockbase + memblocksize, end);
if (bottom >= top)
continue;
found = memblock_find_region(bottom, top, size, align);
if (found != MEMBLOCK_ERROR)
return found;
}
return MEMBLOCK_ERROR;
}
memblock应当是收集的内存信息的一个描述，回头再看一下，这里会找到这么个基地之然后将其
返回，以前这个地址应当是在紧挨着内核镜像bss后面，继续回到arm_bootmem_init
函数，下一个是
/**
* init_bootmem_node – register a node as boot memory
* @pgdat: node to register
* @freepfn: pfn where the bitmap for this node is to be placed
* @startpfn: first pfn on the node
* @endpfn: first pfn after the node
*
* Returns the number of bytes needed to hold the bitmap for this node.
*/
unsigned long __init init_bootmem_node(pg_data_t *pgdat, unsigned long freepfn,
unsigned long startpfn, unsigned long endpfn)
{
return init_bootmem_core(pgdat->bdata, freepfn, startpfn, endpfn);
}
/*
* Called once to set up the allocator itself.
* bootmem 核心初始化
*/
static unsigned long __init init_bootmem_core(bootmem_data_t *bdata,
unsigned long mapstart, unsigned long start, unsigned long end)
{
unsigned long mapsize;
mminit_validate_memmodel_limits(&start, &end);
bdata->node_bootmem_map = phys_to_virt(PFN_PHYS(mapstart)); //bitmap起始地址
bdata->node_min_pfn = start; //节点起始地址
bdata->node_low_pfn = end; //结束地址
link_bootmem(bdata);
/*
* Initially all pages are reserved – setup_arch() has to
* register free RAM areas explicitly.
*/
mapsize = bootmap_bytes(end – start);
//又算了一遍？？？？
memset(bdata->node_bootmem_map, 0xff, mapsize);
//node_bootmem_map记录bitmap起始地址，清空这里
bdebug(“nid=%td start=%lx map=%lx end=%lx mapsize=%lx\n”,
bdata – bootmem_node_data, start, mapstart, end, mapsize);
return mapsize;
}
靠，现在的分配器真成分配器了，不带收集的？？
总结：总括了bootmem分配器，介绍了其原理，后文准备跟一下memblock这个东东，以及那些分配函数具体实现^.^!.
相关阅读：http://www.linuxidc.com/Linux/2012-02/53140.htm
