Linux內存模型之bootmem分配器<一>

簡介：bootmem分配器是系統啟動初期的內存分配方式，在耳熟能詳的伙伴系統，slab系統建立前內存都是利用bootmem分配器來分配的，伙伴系統框架建立起來後，bootmem會過度到伙伴系統，bootmem大致思想就是收集內存中的可用內存，然後建立bit位圖，然後需要的內存從這些空閒內存中分配，分配了就標記占用，當然這種分配方式很低效，但是由於只占用啟動階段很少一部分，所以也大可接受了，好了，廢話不多說，一起看代碼吧，

相關閱讀：http://www.linuxidc.com/Linux/2012-02/53138.htm

一、初始化部分
init/main.c中
start_kernel函數
{
...
setup_arch(&command_line);
...
}
代碼位於arch/arm/kernel/setup.c中，
void __init setup_arch(char **cmdline_p)
{
...
paging_init(mdesc);
...
}

*******************************************************
* 繼續跟進paging_init函數在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()就是我們要找的，對應arm的定義在
* arch/arm/mm/init.c
***************************************************************
void __init bootmem_init(void)
{
unsigned long min, max_low, max_high;

max_low = max_high = 0;

/*
*找到內存區域大小，max_low低端內存上界限
*max_high 總內存上界
*/
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;
}

*******************************************************
* 這裡的函數需要一個一個的過一下，先看這個
* find_limits(&min, &max_low, &max_high)函數
* 同文件夾下
*******************************************************
static void __init find_limits(unsigned long *min, unsigned long *max_low,
unsigned long *max_high)
{
struct meminfo *mi = &meminfo; //內存bank的表述結構
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; //保證min最後是最小值
if (*max_high < end) //保證最後的max_high是最大值
*max_high = end;
if (bank->highmem) //如果bank是高端內存那麼不再標記max_low
continue;
if (*max_low < end) //如果是普通內存，那麼max_low繼續跟著增大
*max_low = end;
}
}