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分类: LINUX

2011-10-11 00:08:17

五:kmem_cache_create()分析
我们以一个例子来跟踪分析一下slab的机制:
下面是一个测试模块的代码:
#include
#include
#include
 
MODULE_LICENSE("GPL");
MODULE_AUTHOR("ericxiao <>");
MODULE_DESCRIPTION("slab test module");
 
static kmem_cache_t *test_cachep = NULL;
struct slab_test
{
       int val;
};
 
void fun_ctor(struct slab_test *object , kmem_cache_t *cachep , unsigned long flags )
{
                printk("in ctor fuction ...\n");
                object->val = 1;
}
 
void fun_dtor(struct slab_test *object , kmem_cache_t *cachep , unsigned long flags)
{
     printk("in dtor fuction ...\n");
     object -> val = 0;
}
 
static int __init init(void)
{
       struct slab_test *object = NULL;
       printk("slab test moudle init ... \n");
       test_cachep = kmem_cache_create("test_cachep",sizeof(struct slab_test),0,SLAB_HWCACHE_ALIGN, \
                    fun_ctor, fun_dtor);
       if(!test_cachep)
                        return;           
       object = kmem_cache_alloc( test_cachep, GFP_KERNEL );
       if(object)
       {
                 printk("alloc one val = %d\n",object->val);
                 kmem_cache_free( test_cachep, object );
                 object = NULL;
       }else
            return;
       object = kmem_cache_alloc( test_cachep, GFP_KERNEL );
       if(object)
       {
                 printk("alloc two val = %d\n",object->val);
                 kmem_cache_free( test_cachep, object );
                 object = NULL;
       }else
            return;                
      
}
 
static void fini(void)
{
       printk("test moudle exit ...\n");
       if(test_cachep)
                      kmem_cache_destroy( test_cachep );
}
 
module_init(init);
module_exit(fini);
我们把模块加载之后,用dmesg的命令可以看到如下的输出信息:
slab test moudle init ...
in ctor fuction ...
in ctor fuction ...
……
alloc one val = 1
alloc two val = 1
将模块卸载之后可以看到:
test moudle exit ...
in dtor fuction ...
……
从上我们可以看到,当从cache中分配一个对象时,会初始化很多object(dmesg输出信息中,出现多次in ctor fuction ...),当一个对象释放时,并没有马上调用其析构函数。
我们来看看具体的代码
kmem_cache_create()是创建一个专用cache.同样的,所有专用缓冲区头部也由一个slab分配器维护,它的名字叫:cache_cache。其中每个大个对象的大小均为sizeof(cache).它是静态初始化的:
static kmem_cache_t cache_cache = {
     .lists        = LIST3_INIT(cache_cache.lists),
     .batchcount   = 1,
     .limit        = BOOT_CPUCACHE_ENTRIES,
     .objsize = sizeof(kmem_cache_t),
     .flags        = SLAB_NO_REAP,
     .spinlock = SPIN_LOCK_UNLOCKED,
     .name         = "kmem_cache",
#if DEBUG
     .reallen = sizeof(kmem_cache_t),
#endif
};
Kmem_cache_creat的代码在slab.c中,如下所示:
//参数含义:
//name:cache名字。Align:对齐量.flags:分配标志,ctor:初始化函数 ,dtor析构函数
kmem_cache_t *kmem_cache_create (const char *name, size_t size, size_t align,
     unsigned long flags, void (*ctor)(void*, kmem_cache_t *, unsigned long),
     void (*dtor)(void*, kmem_cache_t *, unsigned long))
{
     size_t left_over, slab_size;
     kmem_cache_t *cachep = NULL;
//参数检测名字不能为空,有析构函数,必须要用初始化函数,不能在中断中,对像不能太大也不能太小(不//能超过2^5个页)
     if ((!name) ||in_interrupt() ||(size < BYTES_PER_WORD) ||
         (size > (1<
         (dtor && !ctor)) {
              printk(KERN_ERR "%s: Early error in slab %s\n",
                       __FUNCTION__, name);
              BUG();
         }
 
     if (flags & SLAB_DESTROY_BY_RCU)
         BUG_ON(dtor);
//flag参数的有效性检查
     if (flags & ~CREATE_MASK)
         BUG();
 
     //align参数的调整。如无特别要求,align设为零,flag设为SLAB_HWCACHE_ALIGN。按照处理器缓//存对齐
     if (align) {
         flags &= ~(SLAB_RED_ZONE|SLAB_STORE_USER);
     } else {
         if (flags & SLAB_HWCACHE_ALIGN) {
              //cache_line_size取得处理平始的cache line.前面已经分析过
align = cache_line_size();
//如果对象太小,为了提高利用了,取cache line半数对齐
              while (size <= align/2)
                   align /= 2;
         } else {
              align = BYTES_PER_WORD;
         }
     }
 
//从cache_cache中分得一个缓存描述符 kmem_cache_alloc函数在后面讲述
     cachep = (kmem_cache_t *) kmem_cache_alloc(&cache_cache, SLAB_KERNEL);
     if (!cachep)
         goto opps;
//初始化
     memset(cachep, 0, sizeof(kmem_cache_t));
 
     //把大小按照BYTES_PER_WORD 对齐。BYTES_PER_WORD也即处理器的地址单元,在i32 为32
     if (size & (BYTES_PER_WORD-1)) {
         size += (BYTES_PER_WORD-1);
         size &= ~(BYTES_PER_WORD-1);
     }
    
//如果size 大于1/8 个页面。就把slab放到缓存区的外面
     if (size >= (PAGE_SIZE>>3))
         flags |= CFLGS_OFF_SLAB;
     //使size按照align对齐
     size = ALIGN(size, align);
 
     if ((flags & SLAB_RECLAIM_ACCOUNT) && size <= PAGE_SIZE) {
         cachep->gfporder = 0;
         cache_estimate(cachep->gfporder, size, align, flags,
                       &left_over, &cachep->num);
     } else {
         //在这里,为cache中每个slab的大小以及slab中的对象个数取得一个平衡点
         do {
              unsigned int break_flag = 0;
cal_wastage:
              //cache_estimate:指定slab的大小后,返回slab中的对像个数
              //以及剩余空间数
              cache_estimate(cachep->gfporder, size, align, flags,
                            &left_over, &cachep->num);
              if (break_flag)
                   break;
              if (cachep->gfporder >= MAX_GFP_ORDER)
                   break;
              if (!cachep->num)
                   goto next;
              if (flags & CFLGS_OFF_SLAB &&
                       cachep->num > offslab_limit) {
                   /* This num of objs will cause problems. */
                   cachep->gfporder--;
                   break_flag++;
                   goto cal_wastage;
              }
 
              /*
               * Large num of objs is good, but v. large slabs are
               * currently bad for the gfp()s.
               */
              if (cachep->gfporder >= slab_break_gfp_order)
                   break;
 
              if ((left_over*8) <= (PAGE_SIZE<gfporder))
                   break;   /* Acceptable internal fragmentation. */
next:
              cachep->gfporder++;
         } while (1);
     }
    
     if (!cachep->num) {
         //出现意外,打印出常现的oops错误
         printk("kmem_cache_create: couldn't create cache %s.\n", name);
         kmem_cache_free(&cache_cache, cachep);
         cachep = NULL;
         goto opps;
     }
使slab大小按照align对齐
     slab_size = ALIGN(cachep->num*sizeof(kmem_bufctl_t)
                   + sizeof(struct slab), align);
 
    
     if (flags & CFLGS_OFF_SLAB && left_over >= slab_size) {
     //如果剩余空间足间大,就把slab描述符放到缓存区里面
         flags &= ~CFLGS_OFF_SLAB;
         left_over -= slab_size;
     }
 
     if (flags & CFLGS_OFF_SLAB) {
         //如果slab描述符依然只能放到缓存区外面。则取slab_size大小的实际值
         //也就是说不需要与alin 对齐了
         slab_size = cachep->num*sizeof(kmem_bufctl_t)+sizeof(struct slab);
     }
 
//着色偏移量,至少为一个cache_size.若align值是自己指定的,且超出了一个cache size.这样//值就会取设定的align
     cachep->colour_off = cache_line_size();
     if (cachep->colour_off < align)
         cachep->colour_off = align;
     //颜色的总数,为剩余的空间数/着色偏移量
,     //从这里我们可以看到,如果偏移量太少,着色机制是没有任何意义的
     //这是值得提醒的是colour_next没有被特别赋值,即为默认值0
     cachep->colour = left_over/cachep->colour_off;
     //各种成员的初始化
     cachep->slab_size = slab_size;
     cachep->flags = flags;
     cachep->gfpflags = 0;
     if (flags & SLAB_CACHE_DMA)
         cachep->gfpflags |= GFP_DMA;
     spin_lock_init(&cachep->spinlock);
     cachep->objsize = size;
     /* NUMA */
     INIT_LIST_HEAD(&cachep->lists.slabs_full);
     INIT_LIST_HEAD(&cachep->lists.slabs_partial);
     INIT_LIST_HEAD(&cachep->lists.slabs_free);
    
     //如果slab描述符是放在缓存区外面的。那就为slab描述符指定一个分配缓存
     if (flags & CFLGS_OFF_SLAB)
         cachep->slabp_cache = kmem_find_general_cachep(slab_size,0);
     cachep->ctor = ctor;
     cachep->dtor = dtor;
     cachep->name = name;
 
     /* Don't let CPUs to come and go */
     lock_cpu_hotplug();
 
     //g_cpucache_up:判断普通缓存是否就绪的标志
     //NONE是初始值 PARTIAL:是一个中间的状态,即普通缓存正在初始化
     //FULL:普通缓存已经初始化完成
     if (g_cpucache_up == FULL) {
         enable_cpucache(cachep);
     } else {
         if (g_cpucache_up == NONE) {
              /* Note: the first kmem_cache_create must create
               * the cache that's used by kmalloc(24), otherwise
               * the creation of further caches will BUG().
               */
              cachep->array[smp_processor_id()] =
                       &initarray_generic.cache;
              g_cpucache_up = PARTIAL;
         } else {
              cachep->array[smp_processor_id()] =
                   kmalloc(sizeof(struct arraycache_init),
                       GFP_KERNEL);
         }
         BUG_ON(!ac_data(cachep));
         ac_data(cachep)->avail = 0;
         ac_data(cachep)->limit = BOOT_CPUCACHE_ENTRIES;
         ac_data(cachep)->batchcount = 1;
         ac_data(cachep)->touched = 0;
         cachep->batchcount = 1;
         cachep->limit = BOOT_CPUCACHE_ENTRIES;
         cachep->free_limit = (1+num_online_cpus())*cachep->batchcount
                       + cachep->num;
     }
 
     cachep->lists.next_reap = jiffies + REAPTIMEOUT_LIST3 +
                   ((unsigned long)cachep)%REAPTIMEOUT_LIST3;
 
     //查看是否有相同名字的cache
     down(&cache_chain_sem);
     {
         struct list_head *p;
         mm_segment_t old_fs;
 
         old_fs = get_fs();
         set_fs(KERNEL_DS);
         list_for_each(p, &cache_chain) {
              kmem_cache_t *pc = list_entry(p, kmem_cache_t, next);
              char tmp;
 
              /*
               * This happens when the module gets unloaded and
               * doesn't destroy its slab cache and noone else reuses
               * the vmalloc area of the module. Print a warning.
               */
#ifdef CONFIG_X86_UACCESS_INDIRECT
              if (__direct_get_user(tmp,pc->name)) {
#else
              if (__get_user(tmp,pc->name)) {
#endif
                   printk("SLAB: cache with size %d has lost its "
                            "name\n", pc->objsize);
                   continue;
              }   
              if (!strcmp(pc->name,name)) {
                   printk("kmem_cache_create: duplicate "
                            "cache %s\n",name);
                   up(&cache_chain_sem);
                   unlock_cpu_hotplug();
                   BUG();
              }   
         }
         set_fs(old_fs);
     }
 
     //将cache挂至cache_chain链
     list_add(&cachep->next, &cache_chain);
     up(&cache_chain_sem);
     unlock_cpu_hotplug();
opps:
     if (!cachep && (flags & SLAB_PANIC))
         panic("kmem_cache_create(): failed to create slab `%s'\n",
              name);
     return cachep;
}
首先我们遇到的问题是第一个鸡与鸡蛋的问题:新建cache描述符是从cache_cache中分配cache描述符,那cache_cache是从何而来呢?cache_cache是静态定义的一个数据结构,只要静态初始化它的成员就可以了。另一个鸡与鸡蛋的问题就是cache中array数组的初始化问题。例如:
cachep->array[smp_processor_id()] =
                   kmalloc(sizeof(struct arraycache_init),
                       GFP_KERNEL);
也就是说从普通缓存中分得空间,那普通缓存区中的arry如何取得空间呢?这也是一个静态定义的数组:initarray_generic.cache。我们以后再详细分析内存各子系统的初始化过程。详情请关注本站更新。
另外,我们也接触到了着色部份的代码。如下所示:
cachep->colour_off = cache_line_size();
         if (cachep->colour_off < align)
         cachep->colour_off = align;
     cachep->colour = left_over/cachep->colour_off;
着色的原理在前面已经分析过了。Colour_off:每一个slab中偏移值。以colour:颜色的总数,即最大的偏移位置,它的大小为剩余大小/偏移值,colour_next初始化为零。
举例说明:
Colour_off = 32  colour = 2; colour_next = 0
第一个slab偏移colour_next* Colour_off = 0*32 = 0 然后colour_next加1。即为1
第二个slab偏移colour_next* Colour_off = 1*32 = 32然后colour_next加1。即为2
第三个slab偏移colour_next* Colour_off = 2*32 = 64然后colour_next加1。即为3,由于colour为2。所以,colour_next = 0;
第四个slab偏移colour_next* Colour_off = 0*32 = 0
……
另外:要注意的是slab大小计算的时候:
slab_size = ALIGN(cachep->num*sizeof(kmem_bufctl_t) + sizeof(struct slab), align);
虽然在struct slab里没有定义kmem_bufctl_t.但在为slab申请空间的时候申请了num个kmem_bufctl_t的多余空间,也就是说kmem_bufctl_t数组紧放在slab描述符之后
此外,array被初始化了arraycache_init大小。
struct arraycache_init {
     struct array_cache cache;
     void * entries[BOOT_CPUCACHE_ENTRIES];
};
为什么要这样做?我们在后面再给出分析
 
 
六:kmem_cache_alloc的实现分析:
我们在上面可以看到,创建一个cache描述符的时候,并没有这之分配slab数据。现在我们来看一下怎么从cache中申请对象
void * kmem_cache_alloc (kmem_cache_t *cachep, int flags)
{
     return __cache_alloc(cachep, flags);
}
实际上会调用__cache_alloc
如下:
static inline void * __cache_alloc (kmem_cache_t *cachep, int flags)
{
     unsigned long save_flags;
     void* objp;
     struct array_cache *ac;
     //如果定义了__GFP_WAIT。可能会引起睡眠
     cache_alloc_debugcheck_before(cachep, flags);
 
     local_irq_save(save_flags);
     //取得当前处理器所在的array_cache(简称为AC,我们下面也这样称呼它)
     ac = ac_data(cachep);
     //ac->avail:AC中第后一个可用的对象索引
    
//如果AC中还有可用的对象
if (likely(ac->avail)) {
         STATS_INC_ALLOCHIT(cachep);
         //每次分配都会把ac->touched置为1
         ac->touched = 1;
         objp = ac_entry(ac)[--ac->avail];
     } else {
         //如果AC中没有可用对象,那只能从l3中“搬出”对象到AC中
         STATS_INC_ALLOCMISS(cachep);
         objp = cache_alloc_refill(cachep, flags);
     }
     local_irq_restore(save_flags);
     objp = cache_alloc_debugcheck_after(cachep, flags, objp, __builtin_return_address(0));
     return objp;
}
首先,会从AC中分配对象,如果AC中无可用对象,那就从l3链表中分配对象了,首先它会从share链表中取对象,然后再从末满,空链表中取对象,如果都没有空闲对象的话,只能从伙伴系统中分配内存了.接着看下面的代码:
static void* cache_alloc_refill(kmem_cache_t* cachep, int flags)
{
     int batchcount;
     struct kmem_list3 *l3;
     struct array_cache *ac;
 
     check_irq_off();
     ac = ac_data(cachep);
retry:
     //batchcount:一次向AC填充的对象值
     batchcount = ac->batchcount;
     if (!ac->touched && batchcount > BATCHREFILL_LIMIT) {
         batchcount = BATCHREFILL_LIMIT;
     }
     //取得cache所对象的l3
     l3 = list3_data(cachep);
     //如果Ac中依然有可用对象,则退出
     BUG_ON(ac->avail > 0);
     spin_lock(&cachep->spinlock);
     //首先会从shared中取对象
     if (l3->shared) {
         struct array_cache *shared_array = l3->shared;
         if (shared_array->avail) {
              如果share的剩余量不足batchcount。则把它全部都移至AC中
              if (batchcount > shared_array->avail)
                   batchcount = shared_array->avail;
              shared_array->avail -= batchcount;
              ac->avail = batchcount;
              //把share链中的object拷贝到AC中
              memcpy(ac_entry(ac), &ac_entry(shared_array)[shared_array->avail],
                       sizeof(void*)*batchcount);
              shared_array->touched = 1;
              //AC中已经有数据了,那么,可以直接从AC中分配了
              goto alloc_done;
         }
     }

//运行到这里的话,那说明share链中没有对象了
     while (batchcount > 0) {
         //先从末满的链表中获取,若末满链为空的话,从全空链表中获取
         struct list_head *entry;
         struct slab *slabp;
         /* Get slab alloc is to come from. */
         entry = l3->slabs_partial.next;
         //判断slabs_partial是否为空
         if (entry == &l3->slabs_partial) {
              l3->free_touched = 1;
              //判断slabs_free链是否为空
              entry = l3->slabs_free.next;
              if (entry == &l3->slabs_free)
                   //若全为空的话,就从伙伴系统中分配页面了
                   goto must_grow;
         }
         //从链表中取得slab描述符  
         slabp = list_entry(entry, struct slab, list);
         check_slabp(cachep, slabp);
          check_spinlock_acquired(cachep);
         //对象取尽,或者已经满尽分配要求
         while (slabp->inuse < cachep->num && batchcount--) {
              //从相应的slab中分配对象
              kmem_bufctl_t next;
              STATS_INC_ALLOCED(cachep);
              STATS_INC_ACTIVE(cachep);
              STATS_SET_HIGH(cachep);
 
              //得到空闲对象指针
              ac_entry(ac)[ac->avail++] = slabp->s_mem + slabp->free*cachep->objsize;
              //更新计数
              slabp->inuse++;
              next = slab_bufctl(slabp)[slabp->free];
#if DEBUG
              slab_bufctl(slabp)[slabp->free] = BUFCTL_FREE;
#endif
                   //使free指向下一人空闲对像的索引
                  slabp->free = next;
         }
         check_slabp(cachep, slabp);
 
         /* move slabp to correct slabp list: */
         //slab从链中脱落
         list_del(&slabp->list);
         if (slabp->free == BUFCTL_END)
              //如果slab中没有空闲对象了,则把它加入slabs_full链
              list_add(&slabp->list, &l3->slabs_full);
         else
              //如果slab中没有空闲对象了,则把它加入slabs_partial链
              list_add(&slabp->list, &l3->slabs_partial);
     }
 
must_grow:
     //更新free_objects计数.(如果三链都为空的情况下:ac->avail为进入函数的初始值,即为0)
     l3->free_objects -= ac->avail;
alloc_done:
     spin_unlock(&cachep->spinlock);
 
     if (unlikely(!ac->avail)) {
         int x;
         x = cache_grow(cachep, flags);
        
         // cache_grow can reenable interrupts, then ac could change.
         ac = ac_data(cachep);
         //如果grow失败,返回NULL
         if (!x && ac->avail == 0)   // no objects in sight? abort
              return NULL;
         //如果grow成功,则重复上述操作,即从三链表中取空闲对象^_^
         if (!ac->avail)        // objects refilled by interrupt?
              goto retry;
     }
     ac->touched = 1;
     return ac_entry(ac)[--ac->avail];
}
这段代码涉及到slab_bufctl(),等我们看完分配,释放的全过程后。再来详细分析它涉及到的各项操作,cache_grow()用来做slab分配器与slab的交互。它的代码如下示:
static int cache_grow (kmem_cache_t * cachep, int flags)
{
     struct slab   *slabp;
     void     *objp;
     size_t        offset;
     int      local_flags;
     unsigned long ctor_flags;
     if (flags & ~(SLAB_DMA|SLAB_LEVEL_MASK|SLAB_NO_GROW))
         BUG();
     if (flags & SLAB_NO_GROW)
         return 0;
 
     ctor_flags = SLAB_CTOR_CONSTRUCTOR;
     local_flags = (flags & SLAB_LEVEL_MASK);
     if (!(local_flags & __GFP_WAIT))
         /*
          * Not allowed to sleep.  Need to tell a constructor about
          * this - it might need to know...
          */
         ctor_flags |= SLAB_CTOR_ATOMIC;
 
     /* About to mess with non-constant members - lock. */
     check_irq_off();
     spin_lock(&cachep->spinlock);
     //取得下一个偏移索引(着色机制在前面已经详细分析了)
     offset = cachep->colour_next;
     cachep->colour_next++;
     //如果大于允许的最大颜色,那就把计数归位,即为0
     if (cachep->colour_next >= cachep->colour)
         cachep->colour_next = 0;
     //计算偏移量
     offset *= cachep->colour_off;
 
     spin_unlock(&cachep->spinlock);
 
     if (local_flags & __GFP_WAIT)
         local_irq_enable();
     kmem_flagcheck(cachep, flags);
     //向伙伴系统申请内存
     if (!(objp = kmem_getpages(cachep, flags, -1)))
         goto failed;
     //分配slab描述符,这里有两种情况,一种是slab在缓存外部,另一种是内部
     if (!(slabp = alloc_slabmgmt(cachep, objp, offset, local_flags)))
         goto opps1;
 
     set_slab_attr(cachep, slabp, objp);
     //初始化slab的对像
     cache_init_objs(cachep, slabp, ctor_flags);
 
     if (local_flags & __GFP_WAIT)
         local_irq_disable();
     check_irq_off();
     spin_lock(&cachep->spinlock);
 
     //将新构建的slab加至slabs_free链
     list_add_tail(&slabp->list, &(list3_data(cachep)->slabs_free));
     STATS_INC_GROWN(cachep);
     //更新计数
     list3_data(cachep)->free_objects += cachep->num;
     spin_unlock(&cachep->spinlock);
     return 1;
opps1:
     //发生了错误,把内存归还伙伴系统
     kmem_freepages(cachep, objp);
failed:
     if (local_flags & __GFP_WAIT)
         local_irq_disable();
     return 0;
}
我们看到了cache_grow的概貌,接着分析它里面调用的子函数。
kmem_getpages()用于slab分配器向伙伴系统分配内存,代码如下:
 
//nodeid:分配内面的cpu结点。如果从当前CPU分存,nodeid置为-1
static void *kmem_getpages(kmem_cache_t *cachep, int flags, int nodeid)
{
     struct page *page;
     void *addr;
     int i;
 
     flags |= cachep->gfpflags;
//__get_free_pages与alloc_pages_node在《linux内存管理之伙伴系统分析》一文中已有详
//细分析,请参考
     if (likely(nodeid == -1)) {
         //从当前cpu结点分配内存
         addr = (void*)__get_free_pages(flags, cachep->gfporder);
         if (!addr)
              return NULL;
         //将地址转换为页描述符
         page = virt_to_page(addr);
     } else {
         //从指定结点分配内存
         page = alloc_pages_node(nodeid, flags, cachep->gfporder);
         if (!page)
              return NULL;
         addr = page_address(page);
     }
 
     //计算页面个数。即为2^ cachep->gfporder
     i = (1 << cachep->gfporder);
     if (cachep->flags & SLAB_RECLAIM_ACCOUNT)
         atomic_add(i, &slab_reclaim_pages);
     //更新cpu nr_slab状态计数
     add_page_state(nr_slab, i);
     while (i--) {
         //将页面标识为PG_slab,表示该页面已被slab使用
         SetPageSlab(page);
         page++;
     }
     return addr;
}
alloc_slabmgmt()是一个slab描述符分配器接口,代码如下:
static struct slab* alloc_slabmgmt (kmem_cache_t *cachep,
              void *objp, int colour_off, int local_flags)
{
     struct slab *slabp;
     //如果slab描述符是外置的
     if (OFF_SLAB(cachep)) {
         //从对应的cache中分配slab描述符
         slabp = kmem_cache_alloc(cachep->slabp_cache, local_flags);
         if (!slabp)
              return NULL;
     } else {
         //从偏移量后开始安置slab
         slabp = objp+colour_off;
         //更新偏移量,即加上slab的大小,这位置也是有效数据的起始偏移位置
         colour_off += cachep->slab_size;
     }
     slabp->inuse = 0;
     slabp->colouroff = colour_off;
     slabp->s_mem = objp+colour_off;
 
     return slabp;
}
cache_init_objs()初始化分配得到的每一个对象,代码如下:
static void cache_init_objs (kmem_cache_t * cachep,
              struct slab * slabp, unsigned long ctor_flags)
{
     int i;
 
     for (i = 0; i < cachep->num; i++) {
         //取slab中的每一个对像
         void* objp = slabp->s_mem+cachep->objsize*i;
#if DEBUG
         //忽略掉debug信息
         /* need to poison the objs? */
         if (cachep->flags & SLAB_POISON)
              poison_obj(cachep, objp, POISON_FREE);
         if (cachep->flags & SLAB_STORE_USER)
              *dbg_userword(cachep, objp) = NULL;
 
         if (cachep->flags & SLAB_RED_ZONE) {
              *dbg_redzone1(cachep, objp) = RED_INACTIVE;
              *dbg_redzone2(cachep, objp) = RED_INACTIVE;
         }
         /*
          * Constructors are not allowed to allocate memory from
          * the same cache which they are a constructor for.
          * Otherwise, deadlock. They must also be threaded.
          */
         if (cachep->ctor && !(cachep->flags & SLAB_POISON))
              cachep->ctor(objp+obj_dbghead(cachep), cachep, ctor_flags);
 
         if (cachep->flags & SLAB_RED_ZONE) {
              if (*dbg_redzone2(cachep, objp) != RED_INACTIVE)
                   slab_error(cachep, "constructor overwrote the"
                                 " end of an object");
              if (*dbg_redzone1(cachep, objp) != RED_INACTIVE)
                   slab_error(cachep, "constructor overwrote the"
                                 " start of an object");
         }
         if ((cachep->objsize % PAGE_SIZE) == 0 && OFF_SLAB(cachep) && cachep->flags & SLAB_POISON)
                   kernel_map_pages(virt_to_page(objp), cachep->objsize/PAGE_SIZE, 0);
#else
         //如果有初始化函数,则调用之
         if (cachep->ctor)
              cachep->ctor(objp, cachep, ctor_flags);
#endif
         //更新bufctl数组
         slab_bufctl(slabp)[i] = i+1;
     }
     //置末尾描述符
     slab_bufctl(slabp)[i-1] = BUFCTL_END;
     slabp->free = 0;
}
同样,slab_bufctl的分析,等讲完释放对像的时候再继续
到此,我们已经看完到分配对象的全过程,接着来看怎么释放一个对象
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