Linux内核中创建slab主要由函数cache_grow()实现,从slab的创建中我们可以完整地看到slab与对象、页面的组织方式。
- /*
- * Grow (by 1) the number of slabs within a cache. This is called by
- * kmem_cache_alloc() when there are no active objs left in a cache.
- */
- /*使用一个或多个页面创建一个空slab。
- objp:页面虚拟地址,为空表示还未申请内存页,不为空
- ,说明已申请内存页,可直接用来创建slab*/
- static int cache_grow(struct kmem_cache *cachep,
- gfp_t flags, int nodeid, void *objp)
- {
- struct slab *slabp;
- size_t offset;
- gfp_t local_flags;
- struct kmem_list3 *l3;
- /*
- * Be lazy and only check for valid flags here, keeping it out of the
- * critical path in kmem_cache_alloc().
- */
- BUG_ON(flags & GFP_SLAB_BUG_MASK);
- local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
- /* Take the l3 list lock to change the colour_next on this node */
- check_irq_off();
- /* 获得本内存节点的slab三链 */
- l3 = cachep->nodelists[nodeid];
- spin_lock(&l3->list_lock);
- /* Get colour for the slab, and cal the next value. */
- /* 获得本slab的着色区偏移 */
- offset = l3->colour_next;
- /* 更新着色区偏移,使不同slab的着色偏移不同 */
- l3->colour_next++;
- /* 不能超过着色区的总大小,如果超过了,重置为0。这就是前面分析过的着色循环问题
- 。事实上,如果slab中浪费的空间很少,那么很快就会循环一次。*/
- if (l3->colour_next >= cachep->colour)
- l3->colour_next = 0;
- spin_unlock(&l3->list_lock);
- /* 将着色单位区间的个数转换为着色区大小 */
- offset *= cachep->colour_off;
- if (local_flags & __GFP_WAIT)
- local_irq_enable();
- /*
- * The test for missing atomic flag is performed here, rather than
- * the more obvious place, simply to reduce the critical path length
- * in kmem_cache_alloc(). If a caller is seriously mis-behaving they
- * will eventually be caught here (where it matters).
- */
- kmem_flagcheck(cachep, flags);
- /*
- * Get mem for the objs. Attempt to allocate a physical page from
- * ‘nodeid’.
- */
- if (!objp)/* 还未分配页面,从本内存节点分配1<<cachep->gfporder个页面
- ,objp为slab首页面的虚拟地址 */
- objp = kmem_getpages(cachep, local_flags, nodeid);
- if (!objp)
- goto failed;
- /* Get slab management. */
- /* 分配slab管理对象 */
- slabp = alloc_slabmgmt(cachep, objp, offset,
- local_flags & ~GFP_CONSTRAINT_MASK, nodeid);
- if (!slabp)
- goto opps1;
- /* 设置page到cache、slab的映射 */
- slab_map_pages(cachep, slabp, objp);
- /* 初始化slab中的对象 */
- cache_init_objs(cachep, slabp);
- if (local_flags & __GFP_WAIT)
- local_irq_disable();
- check_irq_off();
- spin_lock(&l3->list_lock);
- /* Make slab active. */
- list_add_tail(&slabp->list, &(l3->slabs_free));
- /* 更新本cache增长计数 */
- STATS_INC_GROWN(cachep);
- /* 更新slab链表中空闲对象计数 */
- l3->free_objects += cachep->num;
- spin_unlock(&l3->list_lock);
- return 1;
- opps1:
- kmem_freepages(cachep, objp);
- failed:
- if (local_flags & __GFP_WAIT)
- local_irq_disable();
- return 0;
- }
执行流程:
1,从cache结构中获得并计算着色区偏移量;
2,从伙伴系统中获得1<<cachep->gfporder个页面用于slab;
3,初始化slab中相关变量,如果是外置式slab需要从新申请slab管理区的空间,由函数alloc_slabmgmt()实现。
- /*分配slab管理对象*/
- static struct slab *alloc_slabmgmt(struct kmem_cache *cachep, void *objp,
- int colour_off, gfp_t local_flags,
- int nodeid)
- {
- struct slab *slabp;
- if (OFF_SLAB(cachep)) {
- /* Slab management obj is off-slab. */
- /* 外置式slab。从general slab cache中分配一个管理对象,
- slabp_cache指向保存有struct slab对象的general slab cache。
- slab初始化阶段general slab cache可能还未创建,slabp_cache指针为空
- ,故初始化阶段创建的slab均为内置式slab。*/
- slabp = kmem_cache_alloc_node(cachep->slabp_cache,
- local_flags, nodeid);
- /*
- * If the first object in the slab is leaked (it’s allocated
- * but no one has a reference to it), we want to make sure
- * kmemleak does not treat the ->s_mem pointer as a reference
- * to the object. Otherwise we will not report the leak.
- *//* 对第一个对象做检查 */
- kmemleak_scan_area(slabp, offsetof(struct slab, list),
- sizeof(struct list_head), local_flags);
- if (!slabp)
- return NULL;
- } else {/* 内置式slab。objp为slab首页面的虚拟地址,加上着色偏移
- ,得到slab管理对象的虚拟地址 */
- slabp = objp + colour_off;
- /* 计算slab中第一个对象的页内偏移,slab_size保存slab管理对象的大小
- ,包含struct slab对象和kmem_bufctl_t数组 */
- colour_off += cachep->slab_size;
- } /* 在用(已分配)对象数为0 */
- slabp->inuse = 0;
- /* 第一个对象的页内偏移,可见对于内置式slab,colouroff成员不仅包括着色区
- ,还包括管理对象占用的空间
- ,外置式slab,colouroff成员只包括着色区。*/
- slabp->colouroff = colour_off;
- /* 第一个对象的虚拟地址 */
- slabp->s_mem = objp + colour_off;
- /* 内存节点ID */
- slabp->nodeid = nodeid;
- /* 第一个空闲对象索引为0,即kmem_bufctl_t数组的第一个元素 */
- slabp->free = 0;
- return slabp;
- }
通过初始化,我们画出下面图像。
4,设置slab中页面(1<<cachep->gfporder个)到slab、cache的映射。这样,可以通过page的lru链表找到page所属的slab和cache。slab_map_pages()实现
- /*设置page到cache、slab的指针,这样就能知道页面所在的cache、slab
- addr:slab首页面虚拟地址*/
- static void slab_map_pages(struct kmem_cache *cache, struct slab *slab,
- void *addr)
- {
- int nr_pages;
- struct page *page;
- /* 获得slab首页面*/
- page = virt_to_page(addr);
- nr_pages = 1;
- /* 如果不是大页面(关于大页面请参阅相关文档)
- ,计算页面的个数 */
- if (likely(!PageCompound(page)))
- nr_pages <<= cache->gfporder;
- do {
- /* struct page结构中的lru根据页面的用途有不同的含义
- ,当页面空闲或用于高速缓存时,
- lru成员用于构造双向链表将page串联起来,而当page用于slab时,
- next指向page所在的cache,prev指向page所在的slab */
- page_set_cache(page, cache);
- page_set_slab(page, slab);
- page++;
- } while (–nr_pages);
- }
代码实现结果如下图
5,初始化slab中kmem_bufctl_t[]数组,其中kmem_bufctl_t[]数组为一个静态链表,指定了slab对象(obj)的访问顺序。即kmem_bufctl_t[]中存放的是下一个访问的obj。在后面分析中slab_get_obj()函数从slab中提取一个空闲对象,他通过index_to_obj()函数找到空闲对象在kmem_bufctl_t[]数组中的下标,然后通过slab_bufctl(slabp)[slabp->free]获得下一个空闲对象的索引并用它更新静态链表。
- /*初始化slab中的对象,主要是通过kmem_bufctl_t数组将对象串联起来*/
- static void cache_init_objs(struct kmem_cache *cachep,
- struct slab *slabp)
- {
- int i;
- /* 逐一初始化slab中的对象 */
- for (i = 0; i < cachep->num; i++) {
- /* 获得slab中第i个对象 */
- void *objp = index_to_obj(cachep, slabp, i);
- #if DEBUG
- /* 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_offset(cachep));
- 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->buffer_size % PAGE_SIZE) == 0 &&
- OFF_SLAB(cachep) && cachep->flags & SLAB_POISON)
- kernel_map_pages(virt_to_page(objp),
- cachep->buffer_size / PAGE_SIZE, 0);
- #else
- /* 调用此对象的构造函数 */
- if (cachep->ctor)
- cachep->ctor(objp);
- #endif /* 初始时所有对象都是空闲的,只需按照数组顺序串起来即可 */
- /*相当于静态索引指针*/
- slab_bufctl(slabp)[i] = i + 1;
- }
- /* 最后一个指向BUFCTL_END */
- slab_bufctl(slabp)[i – 1] = BUFCTL_END;
- }