Linux Slab分配器(5)-释放对象

Linux Slab分配器(五)--释放对象

缓存回收对象基于以下原则

1.本地高速缓存的空间还可以容纳空闲对象，则直接将对象放回本地高速缓存

2.本地高速缓存的空间已满，则按batchcount的值将对象从本地高速缓存转移到slab中，转移是基于先进先出的原则的，也就是转移entry数组最前面的batchcount个空闲对象，因为这些对象在数组中存在的时间相对较长，不太可能仍然驻留在CPU高速缓存中

释放对象通过函数kmem_cache_free()来完成，下图给出了主要的工作流程

Linux Slab分配器(5)-释放对象

我们以__cache_free函数作为入口进行分析

static inline void __cache_free(struct kmem_cache *cachep, void *objp){struct array_cache *ac = cpu_cache_get(cachep);check_irq_off();kmemleak_free_recursive(objp, cachep-&gt;flags);objp = cache_free_debugcheck(cachep, objp, __builtin_return_address(0));kmemcheck_slab_free(cachep, objp, obj_size(cachep));/* * Skip calling cache_free_alien() when the platform is not numa. * This will avoid cache misses that happen while accessing slabp (which * is per page memory  reference) to get nodeid. Instead use a global * variable to skip the call, which is mostly likely to be present in * the cache. */if (nr_online_nodes &gt; 1 && cache_free_alien(cachep, objp))return;/*如果本地高速缓存中的空闲对象小于空闲对象上限，则直接用entry中的元素记录对象的地址*/if (likely(ac-&gt;avail &lt; ac-&gt;limit)) {STATS_INC_FREEHIT(cachep);ac-&gt;entry[ac-&gt;avail++] = objp;return;} else {/*否则将本地高速缓存中的空闲对象批量转移到slab中*/STATS_INC_FREEMISS(cachep);cache_flusharray(cachep, ac);ac-&gt;entry[ac-&gt;avail++] = objp;}}

static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac){int batchcount;struct kmem_list3 *l3;int node = numa_node_id();batchcount = ac-&gt;batchcount;#if DEBUGBUG_ON(!batchcount || batchcount &gt; ac-&gt;avail);#endifcheck_irq_off();l3 = cachep-&gt;nodelists[node];spin_lock(&l3-&gt;list_lock);if (l3-&gt;shared) {/*如果开启了共享本地高速缓存*//*获取共享的array_cache*/struct array_cache *shared_array = l3-&gt;shared;/*计算共享本地高速缓存还可容纳的空闲对象数*/int max = shared_array-&gt;limit - shared_array-&gt;avail;if (max) {if (batchcount &gt; max)batchcount = max;/*将batchcount个对象移到共享本地高速缓存中*/memcpy(&(shared_array-&gt;entry[shared_array-&gt;avail]),       ac-&gt;entry, sizeof(void *) * batchcount);shared_array-&gt;avail += batchcount;goto free_done;}}/*将本地高速缓存的前batchcount个对象放回slab*/free_block(cachep, ac-&gt;entry, batchcount, node);free_done:#if STATS{int i = 0;struct list_head *p;p = l3-&gt;slabs_free.next;while (p != &(l3-&gt;slabs_free)) {struct slab *slabp;slabp = list_entry(p, struct slab, list);BUG_ON(slabp-&gt;inuse);i++;p = p-&gt;next;}STATS_SET_FREEABLE(cachep, i);}#endifspin_unlock(&l3-&gt;list_lock);ac-&gt;avail -= batchcount;/*刷新本地高速缓存的avail值*//*将从batchcount开始的元素搬到前面去*/memmove(ac-&gt;entry, &(ac-&gt;entry[batchcount]), sizeof(void *)*ac-&gt;avail);}/* * Release an obj back to its cache. If the obj has a constructed state, it must * be in this state _before_ it is released.  Called with disabled ints. */static inline void __cache_free(struct kmem_cache *cachep, void *objp){struct array_cache *ac = cpu_cache_get(cachep);check_irq_off();kmemleak_free_recursive(objp, cachep-&gt;flags);objp = cache_free_debugcheck(cachep, objp, __builtin_return_address(0));kmemcheck_slab_free(cachep, objp, obj_size(cachep));/* * Skip calling cache_free_alien() when the platform is not numa. * This will avoid cache misses that happen while accessing slabp (which * is per page memory  reference) to get nodeid. Instead use a global * variable to skip the call, which is mostly likely to be present in * the cache. */if (nr_online_nodes &gt; 1 && cache_free_alien(cachep, objp))return;/*如果本地高速缓存中的空闲对象小于空闲对象上限，则直接用entry中的元素记录对象的地址*/if (likely(ac-&gt;avail &lt; ac-&gt;limit)) {STATS_INC_FREEHIT(cachep);ac-&gt;entry[ac-&gt;avail++] = objp;return;} else {/*否则将本地高速缓存中的空闲对象批量转移到slab中*/STATS_INC_FREEMISS(cachep);cache_flusharray(cachep, ac);ac-&gt;entry[ac-&gt;avail++] = objp;}}

static void free_block(struct kmem_cache *cachep, void **objpp, int nr_objects,       int node){int i;struct kmem_list3 *l3;for (i = 0; i &lt; nr_objects; i++) {void *objp = objpp[i];struct slab *slabp;/*通过对象的虚拟地址得到slab描述符*/slabp = virt_to_slab(objp);/*获取kmem_list3*/l3 = cachep-&gt;nodelists[node];/*先将slab从所在链表中删除*/list_del(&slabp-&gt;list);check_spinlock_acquired_node(cachep, node);check_slabp(cachep, slabp);/*将一个对象放回slab上*/slab_put_obj(cachep, slabp, objp, node);STATS_DEC_ACTIVE(cachep);/*kmem_list3中的空闲对象数加1*/l3-&gt;free_objects++;check_slabp(cachep, slabp);/* fixup slab chains *//*slab的对象全部空闲*/if (slabp-&gt;inuse == 0) {/*如果空闲对象数大于了空闲对象上限*/if (l3-&gt;free_objects &gt; l3-&gt;free_limit) {/*总空闲对象数减去一个slab的对象数*/l3-&gt;free_objects -= cachep-&gt;num;/* No need to drop any previously held * lock here, even if we have a off-slab slab * descriptor it is guaranteed to come from * a different cache, refer to comments before * alloc_slabmgmt. */ /*销毁该slab*/slab_destroy(cachep, slabp);} else {/*将该slab添加到free链表*/list_add(&slabp-&gt;list, &l3-&gt;slabs_free);}} else {/*否则添加到partial链表*//* Unconditionally move a slab to the end of the * partial list on free - maximum time for the * other objects to be freed, too. */list_add_tail(&slabp-&gt;list, &l3-&gt;slabs_partial);}}}

static void slab_put_obj(struct kmem_cache *cachep, struct slab *slabp,void *objp, int nodeid){/*得到对象的偏移*/unsigned int objnr = obj_to_index(cachep, slabp, objp);#if DEBUG/* Verify that the slab belongs to the intended node */WARN_ON(slabp-&gt;nodeid != nodeid);if (slab_bufctl(slabp)[objnr] + 1 &lt;= SLAB_LIMIT + 1) {printk(KERN_ERR "slab: double free detected in cache ""'%s', objp %p\n", cachep-&gt;name, objp);BUG();}#endif    /*bufctl数组的相应元素更新为free*/slab_bufctl(slabp)[objnr] = slabp-&gt;free;/*free更新为objnr*/slabp-&gt;free = objnr;/*非空闲数减1*/slabp-&gt;inuse--;}

1楼zi77642475前天 21:18: 神马情况？妙发都可以抢不到沙发？

Linux Slab分配器(5)-释放对象

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