linux内存管理之伙伴系统（建立）

内核使用伙伴系统来解决内存分配引起的外部碎片问题。

一、数据结构描述



结构zone中的free_area数组描述伙伴系统该数组为free_area结构



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struct zone {

……

struct free_area free_area[MAX_ORDER];

……

}；

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struct free_area {/*链表类型为5类，对于分类为新加入的*/

struct list_head free_list[MIGRATE_TYPES];

unsigned long nr_free;

};

下图为伙伴系统在管理区中的表示。





二、伙伴系统的初始化



在初始化物理管理区的时候初始化伙伴系统的，具体实现在下面的函数中：



Start_kernel()->setup_arch()->paging_init()->zone_sizes_init()->free_area_init_nodes()->free_area_init_node()->free_area_init_core()->init_currently_empty_zone()->zone_init_free_lists()



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/*初始化对应zone中所有order和所有类型的链表*/

static void __meminit zone_init_free_lists(struct zone *zone)

{

int order, t;

for_each_migratetype_order(order, t) {

INIT_LIST_HEAD(&zone->free_area[order].free_list[t]);

zone->free_area[order].nr_free = 0;

}

}

三、伙伴系统中数据初始化



将bootmem分配器中的数据回收到伙伴系统中



start_kernel()->mm_init()->mem_init()



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void __init mem_init(void)

{

int codesize, reservedpages, datasize, initsize;

int tmp;

/*和具体硬件相关*/

pci_iommu_alloc();



#ifdef CONFIG_FLATMEM

BUG_ON(!mem_map);

#endif

/* this will put all low memory onto the freelists */

/*释放bootmem中的内存到伙伴系统中，包括bootmem占有的位图

返回总共释放的页面数**/

totalram_pages += free_all_bootmem();



reservedpages = 0;

for (tmp = 0; tmp < max_low_pfn; tmp++)

/*

* Only count reserved RAM pages:

*/

if (page_is_ram(tmp) && PageReserved(pfn_to_page(tmp)))

reservedpages++;

/*初始化高端内存区，将高端内存区放入伙伴系统中*/

set_highmem_pages_init();

/*内核代码段、数据段、初始化端长度*/

codesize = (unsigned long) &_etext - (unsigned long) &_text;

datasize = (unsigned long) &_edata - (unsigned long) &_etext;

initsize = (unsigned long) &__init_end - (unsigned long) &__init_begin;

/*打印输出各种内存初始化后的信息*/

printk(KERN_INFO "Memory: %luk/%luk available (%dk kernel code, "

"%dk reserved, %dk data, %dk init, %ldk highmem)\n",

nr_free_pages() << (PAGE_SHIFT-10),

num_physpages << (PAGE_SHIFT-10),

codesize >> 10,

reservedpages << (PAGE_SHIFT-10),

datasize >> 10,

initsize >> 10,

(unsigned long) (totalhigh_pages << (PAGE_SHIFT-10))

);



printk(KERN_INFO "virtual kernel memory layout:\n"

" fixmap : 0x%08lx - 0x%08lx (%4ld kB)\n"

#ifdef CONFIG_HIGHMEM

" pkmap : 0x%08lx - 0x%08lx (%4ld kB)\n"

#endif

" vmalloc : 0x%08lx - 0x%08lx (%4ld MB)\n"

" lowmem : 0x%08lx - 0x%08lx (%4ld MB)\n"

" .init : 0x%08lx - 0x%08lx (%4ld kB)\n"

" .data : 0x%08lx - 0x%08lx (%4ld kB)\n"

" .text : 0x%08lx - 0x%08lx (%4ld kB)\n",

FIXADDR_START, FIXADDR_TOP,

(FIXADDR_TOP - FIXADDR_START) >> 10,



#ifdef CONFIG_HIGHMEM

PKMAP_BASE, PKMAP_BASE+LAST_PKMAP*PAGE_SIZE,

(LAST_PKMAP*PAGE_SIZE) >> 10,

#endif



VMALLOC_START, VMALLOC_END,

(VMALLOC_END - VMALLOC_START) >> 20,



(unsigned long)__va(0), (unsigned long)high_memory,

((unsigned long)high_memory - (unsigned long)__va(0)) >> 20,



(unsigned long)&__init_begin, (unsigned long)&__init_end,

((unsigned long)&__init_end -

(unsigned long)&__init_begin) >> 10,



(unsigned long)&_etext, (unsigned long)&_edata,

((unsigned long)&_edata - (unsigned long)&_etext) >> 10,



(unsigned long)&_text, (unsigned long)&_etext,

((unsigned long)&_etext - (unsigned long)&_text) >> 10);



/*

* Check boundaries twice: Some fundamental inconsistencies can

* be detected at build time already.

*/

#define __FIXADDR_TOP (-PAGE_SIZE)

#ifdef CONFIG_HIGHMEM

BUILD_BUG_ON(PKMAP_BASE + LAST_PKMAP*PAGE_SIZE > FIXADDR_START);

BUILD_BUG_ON(VMALLOC_END > PKMAP_BASE);

#endif

#define high_memory (-128UL << 20)

BUILD_BUG_ON(VMALLOC_START >= VMALLOC_END);

#undef high_memory

#undef __FIXADDR_TOP



#ifdef CONFIG_HIGHMEM

BUG_ON(PKMAP_BASE + LAST_PKMAP*PAGE_SIZE > FIXADDR_START);

BUG_ON(VMALLOC_END > PKMAP_BASE);

#endif

BUG_ON(VMALLOC_START >= VMALLOC_END);

BUG_ON((unsigned long)high_memory > VMALLOC_START);



if (boot_cpu_data.wp_works_ok < 0)

test_wp_bit();



save_pg_dir();

/*调用zap_low_mappings函数清low_memory的映射，内核线程只访问内核空间是不能访问用户空间的

，其实low_memory的映射被设置的部分也就是当初为

8MB建立的恒等映射填充了临时内核页全局目录的第0项，第1项

这里将用户空间的页目录项<3G的PGD清0；*/

zap_low_mappings(true);

}

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/**

* free_all_bootmem - release free pages to the buddy allocator

*

* Returns the number of pages actually released.

*/

unsigned long __init free_all_bootmem(void)

{

return free_all_bootmem_core(NODE_DATA(0)->bdata);

}

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static unsigned long __init free_all_bootmem_core(bootmem_data_t *bdata)

{

int aligned;

struct page *page;

unsigned long start, end, pages, count = 0;



if (!bdata->node_bootmem_map)

return 0;

/*节点内存开始和结束处*/

start = bdata->node_min_pfn;

end = bdata->node_low_pfn;



/*

* If the start is aligned to the machines wordsize, we might

* be able to free pages in bulks of that order.

*/

aligned = !(start & (BITS_PER_LONG - 1));



bdebug("nid=%td start=%lx end=%lx aligned=%d\n",

bdata - bootmem_node_data, start, end, aligned);

/*用于释放整个bootmem所涉及的内存*/

while (start < end) {

unsigned long *map, idx, vec;



map = bdata->node_bootmem_map;

idx = start - bdata->node_min_pfn;/*相对于开始处的偏移*/

vec = ~map[idx / BITS_PER_LONG];/*vec值为页面分配情况*/

/*如果开始地址以32位对其、连续的32个页面都没有被分配(空闲),并且

释放起点以上的32个页面都是合法的(不超过end值)，则释放连续的32个

页面，即1<<5个页面*/

if (aligned && vec == ~0UL && start + BITS_PER_LONG < end) {

int order = ilog2(BITS_PER_LONG);/*32位下为5*/

/*释放到伙伴系统中*/

__free_pages_bootmem(pfn_to_page(start), order);

count += BITS_PER_LONG;/*释放的总页面数更新*/

} else {

unsigned long off = 0;

/*vec!=0表示这个区间存在页面空闲，off为这个区间的下标，从0开始*/

while (vec && off < BITS_PER_LONG) {

if (vec & 1) {/*如果页面空闲*/

/*偏移转化为具体的页面*/

page = pfn_to_page(start + off);

/*一个页面一个页面的释放*/

__free_pages_bootmem(page, 0);/*释放单个页面*/

count++;/*更新释放页面总数*/

}

vec >>= 1;/*vec向右移动一位，表示访问下一个页面*/

off++;/*偏移加一*/

}

}

start += BITS_PER_LONG;/*偏移向后移动*/

}

/*虚拟地址转化为page

用于释放bdata中的位图所占有的内存*/

page = virt_to_page(bdata->node_bootmem_map);

pages = bdata->node_low_pfn - bdata->node_min_pfn;



/*计算bootmem分配器中所使用的页面数,即位图使用的页面数*/

pages = bootmem_bootmap_pages(pages);

count += pages;/*释放的总页面数加*/

while (pages--)/*每次释放一个页面，释放

总共的pages个页面*/

__free_pages_bootmem(page++, 0);



bdebug("nid=%td released=%lx\n", bdata - bootmem_node_data, count);



return count;/*返回释放的总页面数*/

}

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/*

* permit the bootmem allocator to evade page validation on high-order frees

*/

void __meminit __free_pages_bootmem(struct page *page, unsigned int order)

{

if (order == 0) {

__ClearPageReserved(page);

set_page_count(page, 0);/*设置页面的引用位为0*/

set_page_refcounted(page);/*设置页面的引用位为1*/

__free_page(page);/*释放单个页面到伙伴系统中*/

} else {

int loop;



/*这个不是很明白，可能和特定的体系相关???*/

prefetchw(page);

for (loop = 0; loop < BITS_PER_LONG; loop++) {

struct page *p = &page[loop];



if (loop + 1 < BITS_PER_LONG)

prefetchw(p + 1);

__ClearPageReserved(p);

set_page_count(p, 0);

}



set_page_refcounted(page);/*设置页面的引用计数为1*/

/*这里具体释放到那个类型里面，

要看page的里***体的东西，也就是

可以用相关函数来获取他所属的类型*/

__free_pages(page, order);/*释放order个页面*/

}

}

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void __init set_highmem_pages_init(void)

{

struct zone *zone;

int nid;



for_each_zone(zone) {

unsigned long zone_start_pfn, zone_end_pfn;



if (!is_highmem(zone))/*验证是否属于高端内存区域中*/

/*如果不属于，将不执行下面的操作*/

continue;



zone_start_pfn = zone->zone_start_pfn;

zone_end_pfn = zone_start_pfn + zone->spanned_pages;

/*返回zone中的node的id*/

nid = zone_to_nid(zone);

printk(KERN_INFO "Initializing %s for node %d (%08lx:%08lx)\n",

zone->name, nid, zone_start_pfn, zone_end_pfn);

/*将区间中的内存放到伙伴系统中*/

add_highpages_with_active_regions(nid, zone_start_pfn,

zone_end_pfn);

}

totalram_pages += totalhigh_pages;

｝

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void __init add_highpages_with_active_regions(int nid, unsigned long start_pfn,

unsigned long end_pfn)

{

struct add_highpages_data data;



data.start_pfn = start_pfn;

data.end_pfn = end_pfn;

/*对节点中的每个区域进行页面的回收到伙伴系统中*/

work_with_active_regions(nid, add_highpages_work_fn, &data);

}

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/*用指定函数来操作活动区，在高端内存初始化时用了*/

void __init work_with_active_regions(int nid, work_fn_t work_fn, void *data)

{

int i;

int ret;



for_each_active_range_index_in_nid(i, nid) {

ret = work_fn(early_node_map[i].start_pfn,

early_node_map[i].end_pfn, data);

if (ret)

break;

}

}

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static int __init add_highpages_work_fn(unsigned long start_pfn,

unsigned long end_pfn, void *datax)

{

int node_pfn;

struct page *page;

unsigned long final_start_pfn, final_end_pfn;

struct add_highpages_data *data;



data = (struct add_highpages_data *)datax;

/*活动内存区间与指定考虑区间交集*/

final_start_pfn = max(start_pfn, data->start_pfn);

final_end_pfn = min(end_pfn, data->end_pfn);

if (final_start_pfn >= final_end_pfn)

return 0;



for (node_pfn = final_start_pfn; node_pfn < final_end_pfn;

node_pfn++) {

if (!pfn_valid(node_pfn))/*验证页面是否有效*/

continue;

page = pfn_to_page(node_pfn);/*将下标转换为具体的页面*/

/*初始化页面的count值，将页面释放到伙伴系统中*/

add_one_highpage_init(page, node_pfn);

}



return 0;



}

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static void __init add_one_highpage_init(struct page *page, int pfn)

{

/*ClearPageReserved清除了该页面flag中的reserved标志，表示该页面属于动态内存*/

ClearPageReserved(page);

init_page_count(page);/*设置page的count值为1*/

__free_page(page); /*释放页面到伙伴系统*/

totalhigh_pages++;/*更新高端页面总数*/

}

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void zap_low_mappings(bool early)

{

int i;



/*

* Zap initial low-memory mappings.

*

* Note that "pgd_clear()" doesn't do it for

* us, because pgd_clear() is a no-op on i386.

*/

/*这个函数很简单，就是把前面我们在arch/x86/kernel/head_32.S中设置的页全局目录的前若干项清零

。这若干项到底是多少

不错，0xc0000000>>22 & 1023= 768，这些也全局目录项代表虚拟地址前3G的页面，也就是所谓的用户区

，我们在这里把它全清零了。*/

for (i = 0; i < KERNEL_PGD_BOUNDARY; i++) {

#ifdef CONFIG_X86_PAE

set_pgd(swapper_pg_dir+i, __pgd(1 + __pa(empty_zero_page)));

#else

set_pgd(swapper_pg_dir+i, __pgd(0));

#endif

}



if (early)

__flush_tlb();

else

flush_tlb_all();

}

到此，伙伴系统已经建立并且里面存放了应有的内存数据。要从伙伴系统中分配内存，必须要有分配和释放机制。后面总结具体的分配和释放工作