Linux内存管理之slab机制（初始化）

发布时间：2014-09-05 16:36:53作者：知识屋

一、内核启动早期初始化
start_kernel()->mm_init()->kmem_cache_init()

执行流程：

1，初始化静态initkmem_list3三链；

2，初始化cache_cache的nodelists字段为1中的三链；

3，根据内存情况初始化每个slab占用的页面数变量slab_break_gfp_order；

4，将cache_cache加入cache_chain链表中，初始化cache_cache；

5，创建kmalloc所用的general cache：

1）cache的名称和大小存放在两个数据结构对应的数组中，对应大小的cache可以从size数组中找到；

2）先创建INDEX_AC和INDEX_L3下标的cache；

3）循环创建size数组中各个大小的cache；

6，替换静态本地cache全局变量：

1) 替换cache_cache中的arry_cache,本来指向静态变量initarray_cache.cache；

2) 替换malloc_sizes[INDEX_AC].cs_cachep的local cache，原本指向静态变量initarray_generic.cache；

7，替换静态三链

1）替换cache_cache三链，原本指向静态变量initkmem_list3；

2）替换malloc_sizes[INDEX_AC].cs_cachep三链，原本指向静态变量initkmem_list3；

8，更新初始化进度

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/*
* Initialisation. Called after the page allocator have been initialised and
* before smp_init().
*/
void __init kmem_cache_init(void)
{
    size_t left_over;
    struct cache_sizes *sizes;
    struct cache_names *names;
    int i;
    int order;
    int node;
    /* 在slab初始化好之前，无法通过kmalloc分配初始化过程中必要的一些对象
    ，只能使用静态的全局变量
    ，待slab初始化后期，再使用kmalloc动态分配的对象替换全局变量*/

    /* 如前所述，先借用全局变量initkmem_list3表示的slab三链
    ，每个内存节点对应一组slab三链。initkmem_list3是个slab三链数组，对于每个内存节点，包含三组
    ：struct kmem_cache的slab三链、struct arraycache_init的slab 三链、struct kmem_list3的slab三链
    。这里循环初始化所有内存节点的所有slab三链*/
    if (num_possible_nodes() == 1)
        use_alien_caches = 0;
    /*初始化所有node的所有slab中的三个链表*/
    for (i = 0; i < NUM_INIT_LISTS; i++) {
        kmem_list3_init(&initkmem_list3[i]);
        /* 全局变量cache_cache指向的slab cache包含所有struct kmem_cache对象，不包含cache_cache本身
        。这里初始化所有内存节点的struct kmem_cache的slab三链为空。*/
        if (i < MAX_NUMNODES)
            cache_cache.nodelists[i] = NULL;
    }
    /* 设置struct kmem_cache的slab三链指向initkmem_list3中的一组slab三链，
    CACHE_CACHE为cache在内核cache链表中的索引，
    struct kmem_cache对应的cache是内核中创建的第一个cache
    ，故CACHE_CACHE为0 */
    set_up_list3s(&cache_cache, CACHE_CACHE);

    /*
     * Fragmentation resistance on low memory - only use bigger
     * page orders on machines with more than 32MB of memory.
     */
     /* 全局变量slab_break_gfp_order为每个slab最多占用几个页面
     ，用来抑制碎片，比如大小为3360的对象
     ，如果其slab只占一个页面，碎片为736
     ，slab占用两个页面，则碎片大小也翻倍
     。只有当对象很大
     ，以至于slab中连一个对象都放不下时
     ，才可以超过这个值
     。有两个可能的取值
     ：当可用内存大于32MB时
     ，BREAK_GFP_ORDER_HI为1
     ，即每个slab最多占用2个页面
     ，只有当对象大小大于8192时
     ，才可以突破slab_break_gfp_order的限制
     。小于等于32MB时BREAK_GFP_ORDER_LO为0。*/
    if (totalram_pages > (32 << 20) >> PAGE_SHIFT)
        slab_break_gfp_order = BREAK_GFP_ORDER_HI;

    /* Bootstrap is tricky, because several objects are allocated
     * from caches that do not exist yet:
     * 1) initialize the cache_cache cache: it contains the struct
     *    kmem_cache structures of all caches, except cache_cache itself:
     *    cache_cache is statically allocated.
     *    Initially an __init data area is used for the head array and the
     *    kmem_list3 structures, it's replaced with a kmalloc allocated
     *    array at the end of the bootstrap.
     * 2) Create the first kmalloc cache.
     *    The struct kmem_cache for the new cache is allocated normally.
     *    An __init data area is used for the head array.
     * 3) Create the remaining kmalloc caches, with minimally sized
     *    head arrays.
     * 4) Replace the __init data head arrays for cache_cache and the first
     *    kmalloc cache with kmalloc allocated arrays.
     * 5) Replace the __init data for kmem_list3 for cache_cache and
     *    the other cache's with kmalloc allocated memory.
     * 6) Resize the head arrays of the kmalloc caches to their final sizes.
     */

    node = numa_node_id();

    /* 1) create the cache_cache */
    /* 第一步，创建struct kmem_cache所在的cache，由全局变量cache_cache指向
    ，这里只是初始化数据结构
    ，并未真正创建这些对象，要待分配时才创建。*/
    /* 全局变量cache_chain是内核slab cache链表的表头*/
    INIT_LIST_HEAD(&cache_chain);

    /* 将cache_cache加入到slab cache链表*/
    list_add(&cache_cache.next, &cache_chain);

    /* 设置cache着色基本单位为cache line的大小：32字节*/
    cache_cache.colour_off = cache_line_size();
    /* 初始化cache_cache的local cache，同样这里也不能使用kmalloc
    ，需要使用静态分配的全局变量initarray_cache */
    cache_cache.array[smp_processor_id()] = &initarray_cache.cache;
    /* 初始化slab链表,用全局变量*/
    cache_cache.nodelists[node] = &initkmem_list3[CACHE_CACHE + node];

    /*
     * struct kmem_cache size depends on nr_node_ids, which
     * can be less than MAX_NUMNODES.
     */
     /* buffer_size保存slab中对象的大小，这里是计算struct kmem_cache的大小
     ，nodelists是最后一个成员
     ，nr_node_ids保存内存节点个数，UMA为1
     ，所以nodelists偏移加上1个struct kmem_list3 的大小即为struct kmem_cache的大小*/
    cache_cache.buffer_size = offsetof(struct kmem_cache, nodelists) +
                 nr_node_ids * sizeof(struct kmem_list3 *);
#if DEBUG
    cache_cache.obj_size = cache_cache.buffer_size;
#endif
    /* 将对象大小与cache line大小对齐*/
    cache_cache.buffer_size = ALIGN(cache_cache.buffer_size,
                    cache_line_size());
    /* 计算对象大小的倒数，用于计算对象在slab中的索引*/
    cache_cache.reciprocal_buffer_size =
        reciprocal_value(cache_cache.buffer_size);

    for (order = 0; order < MAX_ORDER; order++) {
        /* 计算cache_cache中的对象数目*/
        cache_estimate(order, cache_cache.buffer_size,
            cache_line_size(), 0, &left_over, &cache_cache.num);
        /* num不为0意味着创建struct kmem_cache对象成功，退出*/
        if (cache_cache.num)
            break;
    }
    BUG_ON(!cache_cache.num);
     /* gfporder表示本slab包含2^gfporder个页面*/
    cache_cache.gfporder = order;
      /* 着色区的大小，以colour_off为单位*/
    cache_cache.colour = left_over / cache_cache.colour_off;
    /* slab管理对象的大小*/
    cache_cache.slab_size = ALIGN(cache_cache.num * sizeof(kmem_bufctl_t) +
                      sizeof(struct slab), cache_line_size());

    /* 2+3) create the kmalloc caches */
    /* 第二步，创建kmalloc所用的general cache
    ，kmalloc所用的对象按大小分级
    ，malloc_sizes保存大小，cache_names保存cache名*/
    sizes = malloc_sizes;
    names = cache_names;

    /*
     * Initialize the caches that provide memory for the array cache and the
     * kmem_list3 structures first. Without this, further allocations will
     * bug.
     */
    /* 首先创建struct array_cache和struct kmem_list3所用的general cache
    ，它们是后续初始化动作的基础*/
    /* INDEX_AC是计算local cache所用的struct arraycache_init对象在kmalloc size中的索引
    ，即属于哪一级别大小的general cache
    ，创建此大小级别的cache为local cache所用*/
    sizes[INDEX_AC].cs_cachep = kmem_cache_create(names[INDEX_AC].name,
                    sizes[INDEX_AC].cs_size,
                    ARCH_KMALLOC_MINALIGN,
                    ARCH_KMALLOC_FLAGS|SLAB_PANIC,
                    NULL);
    /* 如果struct kmem_list3和struct arraycache_init对应的kmalloc size索引不同
    ，即大小属于不同的级别
    ，则创建struct kmem_list3所用的cache，否则共用一个cache */
    if (INDEX_AC != INDEX_L3) {
        sizes[INDEX_L3].cs_cachep =
            kmem_cache_create(names[INDEX_L3].name,
                sizes[INDEX_L3].cs_size,
                ARCH_KMALLOC_MINALIGN,
                ARCH_KMALLOC_FLAGS|SLAB_PANIC,
                NULL);
    }
    /* 创建完上述两个general cache后，slab early init阶段结束，在此之前
    ，不允许创建外置式slab */
    slab_early_init = 0;

    /* 循环创建kmalloc各级别的general cache */
    while (sizes->cs_size != ULONG_MAX) {
        /*
         * For performance, all the general caches are L1 aligned.
         * This should be particularly beneficial on SMP boxes, as it
         * eliminates "false sharing".
         * Note for systems short on memory removing the alignment will
         * allow tighter packing of the smaller caches.
         */
         /* 某级别的kmalloc cache还未创建，创建之，struct kmem_list3和
         struct arraycache_init对应的cache已经创建过了*/
        if (!sizes->cs_cachep) {
            sizes->cs_cachep = kmem_cache_create(names->name,
                    sizes->cs_size,
                    ARCH_KMALLOC_MINALIGN,
                    ARCH_KMALLOC_FLAGS|SLAB_PANIC,
                    NULL);
        }
#ifdef CONFIG_ZONE_DMA
        sizes->cs_dmacachep = kmem_cache_create(
                    names->name_dma,
                    sizes->cs_size,
                    ARCH_KMALLOC_MINALIGN,
                    ARCH_KMALLOC_FLAGS|SLAB_CACHE_DMA|
                        SLAB_PANIC,
                    NULL);
#endif
        sizes++;
        names++;
    }
    /* 至此，kmalloc general cache已经创建完毕，可以拿来使用了*/
    /* 4) Replace the bootstrap head arrays */
    /* 第四步，用kmalloc对象替换静态分配的全局变量
    。到目前为止一共使用了两个全局local cache
    ，一个是cache_cache的local cache指向initarray_cache.cache
    ，另一个是malloc_sizes[INDEX_AC].cs_cachep的local cache指向initarray_generic.cache
    ，参见setup_cpu_cache函数。这里替换它们。*/
    {
        struct array_cache *ptr;
        /* 申请cache_cache所用local cache的空间*/
        ptr = kmalloc(sizeof(struct arraycache_init), GFP_NOWAIT);

        BUG_ON(cpu_cache_get(&cache_cache) != &initarray_cache.cache);
        /* 复制原cache_cache的local cache，即initarray_cache，到新的位置*/
        memcpy(ptr, cpu_cache_get(&cache_cache),
               sizeof(struct arraycache_init));
        /*
         * Do not assume that spinlocks can be initialized via memcpy:
         */
        spin_lock_init(&ptr->lock);
        /* cache_cache的local cache指向新的位置*/
        cache_cache.array[smp_processor_id()] = ptr;
        /* 申请malloc_sizes[INDEX_AC].cs_cachep所用local cache的空间*/
        ptr = kmalloc(sizeof(struct arraycache_init), GFP_NOWAIT);

        BUG_ON(cpu_cache_get(malloc_sizes[INDEX_AC].cs_cachep)
               != &initarray_generic.cache);
        /* 复制原local cache到新分配的位置，注意此时local cache的大小是固定的*/
        memcpy(ptr, cpu_cache_get(malloc_sizes[INDEX_AC].cs_cachep),
               sizeof(struct arraycache_init));
        /*
         * Do not assume that spinlocks can be initialized via memcpy:
         */
        spin_lock_init(&ptr->lock);

        malloc_sizes[INDEX_AC].cs_cachep->array[smp_processor_id()] =
            ptr;
    }
    /* 5) Replace the bootstrap kmem_list3's */
    /* 第五步，与第四步类似，用kmalloc的空间替换静态分配的slab三链*/
    {
        int nid;
      /* UMA只有一个节点*/
        for_each_online_node(nid) {
            /* 复制struct kmem_cache的slab三链*/
            init_list(&cache_cache, &initkmem_list3[CACHE_CACHE + nid], nid);
            /* 复制struct arraycache_init的slab三链*/
            init_list(malloc_sizes[INDEX_AC].cs_cachep,
                  &initkmem_list3[SIZE_AC + nid], nid);
            /* 复制struct kmem_list3的slab三链*/
            if (INDEX_AC != INDEX_L3) {
                init_list(malloc_sizes[INDEX_L3].cs_cachep,
                      &initkmem_list3[SIZE_L3 + nid], nid);
            }
        }
    }
    /* 更新slab系统初始化进度*/
    g_cpucache_up = EARLY;
}
辅助操作

1，slab三链初始化

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static void kmem_list3_init(struct kmem_list3 *parent)
{
    INIT_LIST_HEAD(&parent->slabs_full);
    INIT_LIST_HEAD(&parent->slabs_partial);
    INIT_LIST_HEAD(&parent->slabs_free);
    parent->shared = NULL;
    parent->alien = NULL;
    parent->colour_next = 0;
    spin_lock_init(&parent->list_lock);
    parent->free_objects = 0;
    parent->free_touched = 0;
}
2，slab三链静态数据初始化

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/*设置cache的slab三链指向静态分配的全局变量*/
static void __init set_up_list3s(struct kmem_cache *cachep, int index)
{
    int node;
    /* UMA只有一个节点*/
    for_each_online_node(node) {
        /* 全局变量initkmem_list3是初始化阶段使用的slab三链*/
        cachep->nodelists[node] = &initkmem_list3[index + node];
        /* 设置回收时间*/
        cachep->nodelists[node]->next_reap = jiffies +
            REAPTIMEOUT_LIST3 +
            ((unsigned long)cachep) % REAPTIMEOUT_LIST3;
    }
}
3，计算每个slab中对象的数目

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/*
* Calculate the number of objects and left-over bytes for a given buffer size.
*/
/*计算每个slab中对象的数目。*/
/*
1)        gfporder：slab由2gfporder个页面组成。
2)        buffer_size：对象的大小。
3)        align：对象的对齐方式。
4)        flags：内置式slab还是外置式slab。
5)       left_over：slab中浪费空间的大小。
6)        num：slab中的对象数目。
*/
static void cache_estimate(unsigned long gfporder, size_t buffer_size,
               size_t align, int flags, size_t *left_over,
               unsigned int *num)
{
    int nr_objs;
    size_t mgmt_size;
    /* slab大小为1<<order个页面*/
    size_t slab_size = PAGE_SIZE << gfporder;

    /*
     * The slab management structure can be either off the slab or
     * on it. For the latter case, the memory allocated for a
     * slab is used for:
     *
     * - The struct slab
     * - One kmem_bufctl_t for each object
     * - Padding to respect alignment of @align
     * - @buffer_size bytes for each object
     *
     * If the slab management structure is off the slab, then the
     * alignment will already be calculated into the size. Because
     * the slabs are all pages aligned, the objects will be at the
     * correct alignment when allocated.
     */
    if (flags & CFLGS_OFF_SLAB) {
        /* 外置式slab */
      mgmt_size = 0;
        /* slab页面不含slab管理对象，全部用来存储slab对象*/
        nr_objs = slab_size / buffer_size;
        /* 对象数不能超过上限*/
        if (nr_objs > SLAB_LIMIT)
            nr_objs = SLAB_LIMIT;
    } else {
        /*
         * Ignore padding for the initial guess. The padding
         * is at most @align-1 bytes, and @buffer_size is at
         * least @align. In the worst case, this result will
         * be one greater than the number of objects that fit
         * into the memory allocation when taking the padding
         * into account.
         *//* 内置式slab，slab管理对象与slab对象在一起
         ，此时slab页面中包含：一个struct slab对象，一个kmem_bufctl_t数组，slab对象。
         kmem_bufctl_t数组大小与slab对象数目相同*/
        nr_objs = (slab_size - sizeof(struct slab)) /
              (buffer_size + sizeof(kmem_bufctl_t));

        /*
         * This calculated number will be either the right
         * amount, or one greater than what we want.
         *//* 计算cache line对齐后的大小，如果超出了slab总的大小，则对象数减一*/
        if (slab_mgmt_size(nr_objs, align) + nr_objs*buffer_size
               > slab_size)
            nr_objs--;

        if (nr_objs > SLAB_LIMIT)
            nr_objs = SLAB_LIMIT;
        /* 计算cache line对齐后slab管理对象的大小*/
        mgmt_size = slab_mgmt_size(nr_objs, align);
    }
    *num = nr_objs;/* 保存slab对象数目*/
    /* 计算浪费空间的大小*/
    *left_over = slab_size - nr_objs*buffer_size - mgmt_size;
}
辅助数据结构与变量

Linux内核中将所有的通用cache以不同的大小存放在数组中，以方便查找。其中malloc_sizes[]数组为cache_sizes类型的数组，存放各个cache的大小；cache_names[]数组为cache_names结构类型数组，存放各个cache大小的名称；malloc_sizes[]数组和cache_names[]数组下标对应，也就是说cache_names[i]名称的cache对应的大小为malloc_sizes[i]。

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/* Size description struct for general caches. */
struct cache_sizes {
    size_t          cs_size;
    struct kmem_cache   *cs_cachep;
#ifdef CONFIG_ZONE_DMA
    struct kmem_cache   *cs_dmacachep;
#endif
};
/*
* These are the default caches for kmalloc. Custom caches can have other sizes.
*/
struct cache_sizes malloc_sizes[] = {
#define CACHE(x) { .cs_size = (x) },
#include <linux/kmalloc_sizes.h>
    CACHE(ULONG_MAX)
#undef CACHE
};
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/* Must match cache_sizes above. Out of line to keep cache footprint low. */
struct cache_names {
    char *name;
    char *name_dma;
};

static struct cache_names __initdata cache_names[] = {
#define CACHE(x) { .name = "size-" #x, .name_dma = "size-" #x "(DMA)" },
#include <linux/kmalloc_sizes.h>
    {NULL,}
#undef CACHE
};
二、内核启动末期初始化

1，根据对象大小计算local cache中对象数目上限；

2，借助数据结构ccupdate_struct操作cpu本地cache。为每个在线cpu分配cpu本地cache；

3，用新分配的cpu本地cache替换原有的cache；

4，更新slab三链以及cpu本地共享cache。

第二阶段代码分析

Start_kernel()->kmem_cache_init_late()

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/*Slab系统初始化分两个部分，先初始化一些基本的，待系统初始化工作进行的差不多时，再配置一些特殊功能。*/
void __init kmem_cache_init_late(void)
{
    struct kmem_cache *cachep;
    /* 初始化阶段local cache的大小是固定的，要根据对象大小重新计算*/
    /* 6) resize the head arrays to their final sizes */
    mutex_lock(&cache_chain_mutex);
    list_for_each_entry(cachep, &cache_chain, next)
        if (enable_cpucache(cachep, GFP_NOWAIT))
            BUG();
    mutex_unlock(&cache_chain_mutex);

    /* Done! */
    /* 大功告成，general cache终于全部建立起来了*/
    g_cpucache_up = FULL;

    /* Annotate slab for lockdep -- annotate the malloc caches */
    init_lock_keys();

    /*
     * Register a cpu startup notifier callback that initializes
     * cpu_cache_get for all new cpus
     */
     /* 注册cpu up回调函数，cpu up时配置local cache */
    register_cpu_notifier(&cpucache_notifier);

    /*
     * The reap timers are started later, with a module init call: That part
     * of the kernel is not yet operational.
     */
}
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/* Called with cache_chain_mutex held always */
/*local cache 初始化*/
static int enable_cpucache(struct kmem_cache *cachep, gfp_t gfp)
{
    int err;
    int limit, shared;

    /*
     * The head array serves three purposes:
     * - create a LIFO ordering, i.e. return objects that are cache-warm
     * - reduce the number of spinlock operations.
     * - reduce the number of linked list operations on the slab and
     *   bufctl chains: array operations are cheaper.
     * The numbers are guessed, we should auto-tune as described by
     * Bonwick.
     */ /* 根据对象大小计算local cache中对象数目上限*/
    if (cachep->buffer_size > 131072)
        limit = 1;
    else if (cachep->buffer_size > PAGE_SIZE)
        limit = 8;
    else if (cachep->buffer_size > 1024)
        limit = 24;
    else if (cachep->buffer_size > 256)
        limit = 54;
    else
        limit = 120;

    /*
     * CPU bound tasks (e.g. network routing) can exhibit cpu bound
     * allocation behaviour: Most allocs on one cpu, most free operations
     * on another cpu. For these cases, an efficient object passing between
     * cpus is necessary. This is provided by a shared array. The array
     * replaces Bonwick's magazine layer.
     * On uniprocessor, it's functionally equivalent (but less efficient)
     * to a larger limit. Thus disabled by default.
     */
  shared = 0;
    /* 多核系统，设置shared local cache中对象数目*/
    if (cachep->buffer_size <= PAGE_SIZE && num_possible_cpus() > 1)
        shared = 8;

#if DEBUG
    /*
     * With debugging enabled, large batchcount lead to excessively long
     * periods with disabled local interrupts. Limit the batchcount
     */
    if (limit > 32)
        limit = 32;
#endif
    /* 配置local cache */
    err = do_tune_cpucache(cachep, limit, (limit + 1) / 2, shared, gfp);
    if (err)
        printk(KERN_ERR "enable_cpucache failed for %s, error %d./n",
               cachep->name, -err);
    return err;
}
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/* Always called with the cache_chain_mutex held */
/*配置local cache、shared local cache和slab三链*/
static int do_tune_cpucache(struct kmem_cache *cachep, int limit,
                int batchcount, int shared, gfp_t gfp)
{
    struct ccupdate_struct *new;
    int i;

    new = kzalloc(sizeof(*new), gfp);
    if (!new)
        return -ENOMEM;
    /* 为每个cpu分配新的struct array_cache对象*/
    for_each_online_cpu(i) {
        new->new[i] = alloc_arraycache(cpu_to_node(i), limit,
                        batchcount, gfp);
        if (!new->new[i]) {
            for (i--; i >= 0; i--)
                kfree(new->new[i]);
            kfree(new);
            return -ENOMEM;
        }
    }
    new->cachep = cachep;
    /* 用新的struct array_cache对象替换旧的struct array_cache对象
    ，在支持cpu热插拔的系统上，离线cpu可能没有释放local cache
    ，使用的仍是旧local cache，参见__kmem_cache_destroy函数
    。虽然cpu up时要重新配置local cache，也无济于事。考虑下面的情景
    ：共有Cpu A和Cpu B，Cpu B down后，destroy Cache X，由于此时Cpu B是down状态
    ，所以Cache X中Cpu B的local cache未释放，过一段时间Cpu B又up了
    ，更新cache_chain 链中所有cache的local cache，但此时Cache X对象已经释放回
    cache_cache中了，其Cpu B local cache并未被更新。又过了一段时间
    ，系统需要创建新的cache，将Cache X对象分配出去，其Cpu B仍然是旧的
    local cache，需要进行更新。
    */
    on_each_cpu(do_ccupdate_local, (void *)new, 1);

    check_irq_on();
    cachep->batchcount = batchcount;
    cachep->limit = limit;
    cachep->shared = shared;
    /* 释放旧的local cache */
    for_each_online_cpu(i) {
        struct array_cache *ccold = new->new[i];
        if (!ccold)
            continue;
        spin_lock_irq(&cachep->nodelists[cpu_to_node(i)]->list_lock);
        /* 释放旧local cache中的对象*/
        free_block(cachep, ccold->entry, ccold->avail, cpu_to_node(i));
        spin_unlock_irq(&cachep->nodelists[cpu_to_node(i)]->list_lock);
        /* 释放旧的struct array_cache对象*/
        kfree(ccold);
    }
    kfree(new);
    /* 初始化shared local cache 和slab三链*/
    return alloc_kmemlist(cachep, gfp);
}
更新本地cache

www.zhishiwu.com
/*更新每个cpu的struct array_cache对象*/
static void do_ccupdate_local(void *info)
{
    struct ccupdate_struct *new = info;
    struct array_cache *old;

    check_irq_off();
    old = cpu_cache_get(new->cachep);
     /* 指向新的struct array_cache对象*/
    new->cachep->array[smp_processor_id()] = new->new[smp_processor_id()];
      /* 保存旧的struct array_cache对象*/
    new->new[smp_processor_id()] = old;
}
www.zhishiwu.com
/*初始化shared local cache和slab三链，初始化完成后，slab三链中没有任何slab*/
static int alloc_kmemlist(struct kmem_cache *cachep, gfp_t gfp)
{
    int node;
    struct kmem_list3 *l3;
    struct array_cache *new_shared;
    struct array_cache **new_alien = NULL;

    for_each_online_node(node) {
         /* NUMA相关*/
                if (use_alien_caches) {
                        new_alien = alloc_alien_cache(node, cachep->limit, gfp);
                        if (!new_alien)
                                goto fail;
                }

        new_shared = NULL;
        if (cachep->shared) {
            /* 分配shared local cache */
            new_shared = alloc_arraycache(node,
                cachep->shared*cachep->batchcount,
                    0xbaadf00d, gfp);
            if (!new_shared) {
                free_alien_cache(new_alien);
                goto fail;
            }
        }
        /* 获得旧的slab三链*/
        l3 = cachep->nodelists[node];
        if (l3) {
            /* 就slab三链指针不为空，需要先释放旧的资源*/
            struct array_cache *shared = l3->shared;

            spin_lock_irq(&l3->list_lock);
            /* 释放旧的shared local cache中的对象*/
            if (shared)
                free_block(cachep, shared->entry,
                        shared->avail, node);
            /* 指向新的shared local cache */
            l3->shared = new_shared;
            if (!l3->alien) {
                l3->alien = new_alien;
                new_alien = NULL;
            }/* 计算cache中空闲对象的上限*/
            l3->free_limit = (1 + nr_cpus_node(node)) *
                    cachep->batchcount + cachep->num;
            spin_unlock_irq(&l3->list_lock);
            /* 释放旧shared local cache的struct array_cache对象*/
            kfree(shared);
            free_alien_cache(new_alien);
            continue;/*访问下一个节点*/
        }
         /* 如果没有旧的l3，分配新的slab三链*/
        l3 = kmalloc_node(sizeof(struct kmem_list3), gfp, node);
        if (!l3) {
            free_alien_cache(new_alien);
            kfree(new_shared);
            goto fail;
        }
         /* 初始化slab三链*/
        kmem_list3_init(l3);
        l3->next_reap = jiffies + REAPTIMEOUT_LIST3 +
                ((unsigned long)cachep) % REAPTIMEOUT_LIST3;
        l3->shared = new_shared;
        l3->alien = new_alien;
        l3->free_limit = (1 + nr_cpus_node(node)) *
                    cachep->batchcount + cachep->num;
        cachep->nodelists[node] = l3;
    }
    return 0;

fail:
    if (!cachep->next.next) {
        /* Cache is not active yet. Roll back what we did */
        node--;
        while (node >= 0) {
            if (cachep->nodelists[node]) {
                l3 = cachep->nodelists[node];

                kfree(l3->shared);
                free_alien_cache(l3->alien);
                kfree(l3);
                cachep->nodelists[node] = NULL;
            }
            node--;
        }
    }
    return -ENOMEM;
}
看一个辅助函数

/*分配struct array_cache对象。*/
static struct array_cache *alloc_arraycache(int node, int entries,
                        int batchcount, gfp_t gfp)
{
    /* struct array_cache后面紧接着的是entry数组，合在一起申请内存*/
    int memsize = sizeof(void *) * entries + sizeof(struct array_cache);
    struct array_cache *nc = NULL;
    /* 分配一个local cache对象，kmalloc从general cache中分配*/
    nc = kmalloc_node(memsize, gfp, node);
    /*
     * The array_cache structures contain pointers to free object.
     * However, when such objects are allocated or transfered to another
     * cache the pointers are not cleared and they could be counted as
     * valid references during a kmemleak scan. Therefore, kmemleak must
     * not scan such objects.
     */
    kmemleak_no_scan(nc);
     /* 初始化local cache */
    if (nc) {
        nc->avail = 0;
        nc->limit = entries;
        nc->batchcount = batchcount;
        nc->touched = 0;
        spin_lock_init(&nc->lock);
    }
    return nc;
}
源代码中涉及了slab的分配、释放等操作在后面分析中陆续总结。slab相关数据结构、工作机制以及整体框架在分析完了slab的创建、释放工作后再做总结，这样可能会对slab机制有更好的了解。当然，从代码中看运行机制会更有说服了，也是一种习惯

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