linux内存管理之伙伴系统（建立）

发布时间：2014-09-05 16:36:53作者：知识屋

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的里面具体的东西，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

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