Linux-3.14.12内存管理笔记【伙伴管理算法（2）】

前面已经分析了linux内存管理算法（伙伴管理算法）的准备工作。

具体的算法初始化则回到start_kernel()函数接着往下走，下一个函数是mm_init()：

【file：/init/main.c】
/*
 * Set up kernel memory allocators
 */
static void __init mm_init(void)
{
    /*
     * page_cgroup requires contiguous pages,
     * bigger than MAX_ORDER unless SPARSEMEM.
     */
    page_cgroup_init_flatmem();
    mem_init();
    kmem_cache_init();
    percpu_init_late();
    pgtable_init();
    vmalloc_init();
}

乍看仅仅是几个函数的调用，实际上这里的事情远远没这么简单。其中page_cgroup_init_flatmem()与cgroup相关，而mem_init()则是管理伙伴管理算法的初始化，此外kmem_cache_init()是用于内核slub内存分配体系的初始化，而vmalloc_init()则是用于vmalloc的初始化。

当前主要分析伙伴管理算法，则仅对mem_init()做专门的分析，其余的暂且后面再分析。

伙伴管理算法的初始化函数入口是mem_init()，其实现：

【file：/arch/x86/mm/init_32.c】
void __init mem_init(void)
{
    pci_iommu_alloc();

#ifdef CONFIG_FLATMEM
    BUG_ON(!mem_map);
#endif
    /*
     * With CONFIG_DEBUG_PAGEALLOC initialization of highmem pages has to
     * be done before free_all_bootmem(). Memblock use free low memory for
     * temporary data (see find_range_array()) and for this purpose can use
     * pages that was already passed to the buddy allocator, hence marked as
     * not accessible in the page tables when compiled with
     * CONFIG_DEBUG_PAGEALLOC. Otherwise order of initialization is not
     * important here.
     */
    set_highmem_pages_init();

    /* this will put all low memory onto the freelists */
    free_all_bootmem();

    after_bootmem = 1;

    mem_init_print_info(NULL);
    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_RANDOMIZE_BASE
    BUILD_BUG_ON(CONFIG_RANDOMIZE_BASE_MAX_OFFSET > KERNEL_IMAGE_SIZE);
#endif

#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();
}

其中pci_iommu_alloc()不是伙伴算法重点相关的函数，不过还是稍微记录一下：

【file:/arch/x86/kernel/pci-dma.c】
void __init pci_iommu_alloc(void)
{
    struct iommu_table_entry *p;

    sort_iommu_table(__iommu_table, __iommu_table_end);
    check_iommu_entries(__iommu_table, __iommu_table_end);

    for (p = __iommu_table; p < __iommu_table_end; p++) {
        if (p && p->detect && p->detect() > 0) {
            p->flags |= IOMMU_DETECTED;
            if (p->early_init)
                p->early_init();
            if (p->flags & IOMMU_FINISH_IF_DETECTED)
                break;
        }
    }
}

该函数主要是将iommu table先行排序检查，然后调用各个表项注册的函数进行初始化。

而接着的set_highmem_pages_init()则是伙伴算法的开始：

【file:/arch/x86/mm/highmem_32.c】
void __init set_highmem_pages_init(void)
{
    struct zone *zone;
    int nid;

    /*
     * Explicitly reset zone->managed_pages because set_highmem_pages_init()
     * is invoked before free_all_bootmem()
     */
    reset_all_zones_managed_pages();
    for_each_zone(zone) {
        unsigned long zone_start_pfn, zone_end_pfn;

        if (!is_highmem(zone))
            continue;

        zone_start_pfn = zone->zone_start_pfn;
        zone_end_pfn = zone_start_pfn + zone->spanned_pages;

        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);

        add_highpages_with_active_regions(nid, zone_start_pfn,
                 zone_end_pfn);
    }
}

其中for_each_free_mem_range(i, nid, &start, &end, NULL)用于遍历查找memblock算法中空闲的空间区域，然后通过clamp_t()对空间区域进行去除内存空洞调整。里面的for ( ; pfn < e_pfn; pfn++)则用于将空间区域的各页面通过free_highmem_page()进行释放处理，其中if (pfn_valid(pfn))用于判断页面的有效性，而pfn_to_page(pfn)则是将页框号转换为页面管理结构。

进一步分析free_highmem_page()实现：

【file:/mm/page_alloc.c】
void free_highmem_page(struct page *page)
{
    __free_reserved_page(page);
    totalram_pages++;
    page_zone(page)->managed_pages++;
    totalhigh_pages++;
}

其中totalram_pages用于记录内存的总页面数，page_zone(page)->managed_pages则是记录管理区的管理页面数，totalhigh_pages则是记录高端内存的页面总数；

具体看一下__free_reserved_page()：

【file:/include/linux/mm.h】
/* Free the reserved page into the buddy system, so it gets managed. */
static inline void __free_reserved_page(struct page *page)
{
    ClearPageReserved(page);
    init_page_count(page);
    __free_page(page);
}

其中ClearPageReserved定义在/include/linux/page-flags.h中：

#define CLEARPAGEFLAG(uname, lname)                 \

static inline void ClearPage##uname(struct page *page)          \

                                    { clear_bit(PG_##lname, &page->flags); }

用于清除页面的flag中的reserved标志位，表示页面属于动态内存。

接着的init_page_count()这是设置页面的_count引用计数，设置为1，用于为__free_page()释放页面到内存管理算法中做准备。最后的__free_page()，该函数既是初始化伙伴管理算法，同时也是伙伴管理算法释放页面的操作函数。暂且搁置分析__free_page()的实现，后面再详细深入。

接着回到mem_init ()里面下一个调用free_all_bootmem()：

【file:/mm/nobootmem.c】
unsigned long __init free_all_bootmem(void)
{
    unsigned long pages;

    reset_all_zones_managed_pages();

    /*
     * We need to use NUMA_NO_NODE instead of NODE_DATA(0)->node_id
     * because in some case like Node0 doesn't have RAM installed
     * low ram will be on Node1
     */
    pages = free_low_memory_core_early();
    totalram_pages += pages;

    return pages;
}

其中reset_all_zones_managed_pages()是用于重置管理区zone结构中的managed_pages成员数据，着重分析一下free_low_memory_core_early()实现：

该函数通过for_each_free_mem_range()遍历memblock算法中的空闲内存空间，并调用__free_memory_core()来释放；而后面的get_allocated_memblock_reserved_regions_info()和get_allocated_memblock_memory_regions_info()用于获取通过申请而得的memblock管理算法空间，然后释放，其中如果其算法管理空间是系统定义的memblock_reserved_init_regions和memblock_memory_init_regions则仍保留不予以释放。

最后着重分析一下__free_memory_core()的实现：

【file:/mm/nobootmem.c】
static void __init __free_pages_memory(unsigned long start, unsigned long end)
{
    int order;

    while (start < end) {
        order = min(MAX_ORDER - 1UL, __ffs(start));

        while (start + (1UL << order) > end)
            order--;

        __free_pages_bootmem(pfn_to_page(start), order);

        start += (1UL << order);
    }
}

其里面的__free_pages_bootmem()则：

【file:/mm/nobootmem.c】
void __init __free_pages_bootmem(struct page *page, unsigned int order)
{
    unsigned int nr_pages = 1 << order;
    struct page *p = page;
    unsigned int loop;

    prefetchw(p);
    for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
        prefetchw(p + 1);
        __ClearPageReserved(p);
        set_page_count(p, 0);
    }
    __ClearPageReserved(p);
    set_page_count(p, 0);

    page_zone(page)->managed_pages += nr_pages;
    set_page_refcounted(page);
    __free_pages(page, order);
}

由此可以看到，其最终调用的还是__free_pages()将页面予以释放。该函数在后面集中进行分析。

至此，伙伴管理算法初始化完毕。