Linux内存管理之slab分配器分析(四)

七：kmem_cache_free（）的实现

kmem_cache_free用于把从slab中分配的对象释放掉，同分配一样，它首先会把它放到AC中，如果AC满了，则把对象释放到share链中，如果share也满了，也就把它释放至slab。来看具体的代码：

void kmem_cache_free (kmem_cache_t *cachep, void *objp)

{

     unsigned long flags;

 

     local_irq_save(flags);

     __cache_free(cachep, objp);

     local_irq_restore(flags);

}

函数调用__cache_free(),代码如下：

static inline void __cache_free (kmem_cache_t *cachep, void* objp)

{

     struct array_cache *ac = ac_data(cachep);

 

     check_irq_off();

     //DEBUG用，忽略

     objp = cache_free_debugcheck(cachep, objp, __builtin_return_address(0));

     //如果AC中的对像没有超过限制，那就把它释放到AC中。

     if (likely(ac->avail < ac->limit)) {

         STATS_INC_FREEHIT(cachep);

         ac_entry(ac)[ac->avail++] = objp;

         return;

     } else {

         //如果AC中对象数目到了限值，则cache_flusharray（）后，再把对像加入AC中

         STATS_INC_FREEMISS(cachep);

         cache_flusharray(cachep, ac);

         ac_entry(ac)[ac->avail++] = objp;

     }

}

如果当前AC中对象数已经到达限值，就会调用cache_flusharray()将里面的对象“刷新”出去。代码如下：

static void cache_flusharray (kmem_cache_t* cachep, struct array_cache *ac)

{

     int batchcount;

     //要从AC中转移的对象数

     batchcount = ac->batchcount;

#if DEBUG

     BUG_ON(!batchcount || batchcount > ac->avail);

#endif

     check_irq_off();

     spin_lock(&cachep->spinlock);

     if (cachep->lists.shared) {

         struct array_cache *shared_array = cachep->lists.shared;

         int max = shared_array->limit-shared_array->avail;

         if (max) {

              //如果share中还有可闲

              if (batchcount > max)

                   batchcount = max;

              //将AC中的对象复制到share中

              memcpy(&ac_entry(shared_array)[shared_array->avail],

                       &ac_entry(ac)[0],

                       sizeof(void*)*batchcount);

              shared_array->avail += batchcount;

              goto free_done;

         }

     }

     //运行到这里。说明share也满了，只能把对象归还slab了

     free_block(cachep, &ac_entry(ac)[0], batchcount);

free_done:

//STATS只有在打开DEBUG开关的时候才会为1，起DEBUG作用，忽略之

#if STATS

     {

         int i = 0;

         struct list_head *p;

 

         p = list3_data(cachep)->slabs_free.next;

         while (p != &(list3_data(cachep)->slabs_free)) {

              struct slab *slabp;

 

              slabp = list_entry(p, struct slab, list);

              BUG_ON(slabp->inuse);

 

              i++;

              p = p->next;

         }

         STATS_SET_FREEABLE(cachep, i);

     }

#endif

     spin_unlock(&cachep->spinlock);

     //更新avail计数

     ac->avail -= batchcount;

     //从前面已经知道，实际上AC中的0~batchcount项都已经转移到share中了

     //所以，把batchcount后面的项移到前面

     memmove(&ac_entry(ac)[0], &ac_entry(ac)[batchcount],

              sizeof(void*)*ac->avail);

}

free_block()用于将对象归还给slab,代码如下：

static void free_block(kmem_cache_t *cachep, void **objpp, int nr_objects)

{

     int i;

 

     check_spinlock_acquired(cachep);

//更新free_objects计数

     cachep->lists.free_objects += nr_objects;

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

         void *objp = objpp[i];

         struct slab *slabp;

         unsigned int objnr;

         //取得对象所在的slab

         slabp = GET_PAGE_SLAB(virt_to_page(objp));

         list_del(&slabp->list);

         //对象在slab中的序号

         objnr = (objp - slabp->s_mem) / cachep->objsize;

         check_slabp(cachep, slabp);

#if DEBUG

         if (slab_bufctl(slabp)[objnr] != BUFCTL_FREE) {

              printk(KERN_ERR "slab: double free detected in cache '%s', objp %p.\n",

                            cachep->name, objp);

              BUG();

         }

#endif

         //更新bufctl

         slab_bufctl(slabp)[objnr] = slabp->free;

         slabp->free = objnr;

         STATS_DEC_ACTIVE(cachep);

         slabp->inuse--;

         check_slabp(cachep, slabp);

         //调整slab所在的链表

         if (slabp->inuse == 0) {

         //如果slabp中没有被使用的对像

              if (cachep->lists.free_objects > cachep->free_limit) {

                   //如果cahce中空闲对象数超过限值，则把slab释放掉

                   cachep->lists.free_objects -= cachep->num;

                   slab_destroy(cachep, slabp);

              } else {

                   //把slab移到“全空”链表

                   list_add(&slabp->list,

                   &list3_data_ptr(cachep, objp)->slabs_free);

              }

         } else {

              //把slabp加入到slabs_partial末尾

              list_add_tail(&slabp->list,

                   &list3_data_ptr(cachep, objp)->slabs_partial);

         }

     }

}

//slab_destroy用于将slabp所点的空间全部都释放掉

static void slab_destroy (kmem_cache_t *cachep, struct slab *slabp)

{

     //将slabp->s_mem前移colouroff位，即为slab的起始地址

     void *addr = slabp->s_mem - slabp->colouroff;

//忽略掉DEBUG

#if DEBUG

     int i;

     for (i = 0; i < cachep->num; i++) {

         void *objp = slabp->s_mem + cachep->objsize * i;

 

         if (cachep->flags & SLAB_POISON) {

#ifdef CONFIG_DEBUG_PAGEALLOC

              if ((cachep->objsize%PAGE_SIZE)==0 && OFF_SLAB(cachep))

                   kernel_map_pages(virt_to_page(objp), cachep->objsize/PAGE_SIZE,1);

              else

                   check_poison_obj(cachep, objp);

#else

              check_poison_obj(cachep, objp);

#endif

         }

         if (cachep->flags & SLAB_RED_ZONE) {

              if (*dbg_redzone1(cachep, objp) != RED_INACTIVE)

                   slab_error(cachep, "start of a freed object "

                                 "was overwritten");

              if (*dbg_redzone2(cachep, objp) != RED_INACTIVE)

                   slab_error(cachep, "end of a freed object "

                                 "was overwritten");

         }

         if (cachep->dtor && !(cachep->flags & SLAB_POISON))

              (cachep->dtor)(objp+obj_dbghead(cachep), cachep, 0);

     }

#else

     //为其中的每一对象调用析构函数

     if (cachep->dtor) {

         int i;

         for (i = 0; i < cachep->num; i++) {

              void* objp = slabp->s_mem+cachep->objsize*i;

              (cachep->dtor)(objp, cachep, 0);

         }

     }

#endif

 

     if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU)) {

         struct slab_rcu *slab_rcu;

 

         slab_rcu = (struct slab_rcu *) slabp;

         slab_rcu->cachep = cachep;

         slab_rcu->addr = addr;

         call_rcu(&slab_rcu->head, kmem_rcu_free);

     } else {

         //将内存归还伙伴系统

         kmem_freepages(cachep, addr);

         if (OFF_SLAB(cachep))

              //如果slab是外置的，则将slab则相应的cache中释放掉

              kmem_cache_free(cachep->slabp_cache, slabp);

     }

}

八：kmem_cache_destroy（）实现:

Kmem_cache_destroy先会将AC和share中的对象释放到slab中，再把每一个slab都释放掉，如果当前cache中没有被分配对象的话，就会释放掉cache描述符，AC和share。代码如下：

int kmem_cache_destroy (kmem_cache_t * cachep)

{

     int i;

     //参数为空，或者在中断处理中。

     if (!cachep || in_interrupt())

         BUG();

 

     /* Don't let CPUs to come and go */

     lock_cpu_hotplug();

 

     /* Find the cache in the chain of caches. */

     down(&cache_chain_sem);

     //将cache脱链

     list_del(&cachep->next);

     up(&cache_chain_sem);

     //_cache_shrink(cachep)：将cachep中的所有slabp全都释放掉

     if (__cache_shrink(cachep)) {

         //如果当前cache中还有被分配对象，返回

         slab_error(cachep, "Can't free all objects");

         down(&cache_chain_sem);

         //重新加入链表

         list_add(&cachep->next,&cache_chain);

         up(&cache_chain_sem);

         unlock_cpu_hotplug();

         return 1;

     }

 

     if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU))

         synchronize_kernel();

 

     //释放掉AC

     for (i = 0; i < NR_CPUS; i++)

         kfree(cachep->array[i]);

 

     //释放掉share

     kfree(cachep->lists.shared);

     cachep->lists.shared = NULL;

     //将cache描述符也释放掉

     kmem_cache_free(&cache_cache, cachep);

 

     unlock_cpu_hotplug();

 

     return 0;

}

接着看_cache_shrink()：

static int __cache_shrink(kmem_cache_t *cachep)

{

     struct slab *slabp;

     int ret;

     //将AC share中的对象全都释放给slab.我们在前面的代码中看到，释放对象时，先将对象释放到

//AC中，如果AC满了，再释放到share中，若是share满了，才会释放到slab.当cache要销毁的时候，//这些缓冲中的对象用不着了，先释放到slab中，再把整个slab释放掉

     drain_cpu_caches(cachep);

     check_irq_on();

     spin_lock_irq(&cachep->spinlock);

     //将slabs_free中的slab全被释放掉

     for(;;) {

         struct list_head *p;

 

         p = cachep->lists.slabs_free.prev;

         if (p == &cachep->lists.slabs_free)

              break;

 

          slabp = list_entry(cachep->lists.slabs_free.prev, struct slab, list);

#if DEBUG

         if (slabp->inuse)

              BUG();

#endif

         list_del(&slabp->list);

 

         cachep->lists.free_objects -= cachep->num;

         spin_unlock_irq(&cachep->spinlock);

         //这函数我们在前面已经分析过了的

         slab_destroy(cachep, slabp);

         spin_lock_irq(&cachep->spinlock);

     }

     //如果cachep->lists.slabs_ful和slabs_partial还有对象，说明cache中还被分配的对象，

     ret = !list_empty(&cachep->lists.slabs_full) ||

         !list_empty(&cachep->lists.slabs_partial);

     spin_unlock_irq(&cachep->spinlock);

     return ret;

}