linux内核源码分析之slab

Posted 2022-05-17 为了维护世界和平_

目录

创建对象obj

释放对象

销毁缓存

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创建对象obj

static inline void *____cache_alloc(struct kmem_cache *cachep, gfp_t flags)

	void *objp;
	struct array_cache *ac;

	check_irq_off();

	//从per-CPU缓存中获取数据
	ac = cpu_cache_get(cachep);
	if (likely(ac->avail)) 
		ac->touched = 1;
		objp = ac->entry[--ac->avail];

		STATS_INC_ALLOCHIT(cachep);
		goto out;
	

	//重新分配
	STATS_INC_ALLOCMISS(cachep);
	objp = cache_alloc_refill(cachep, flags);

	ac = cpu_cache_get(cachep);

out:
	if (objp)
		kmemleak_erase(&ac->entry[ac->avail]);
	return objp;

如果在per-CPU缓存中有对象获得，当前活动CPU相关的array_cache实例

在per-CPU缓存中没有对象时，重新填充操作由cache_alloc_refill实现
 

static struct page *get_first_slab(struct kmem_cache_node *n, bool pfmemalloc)

	struct page *page;

	assert_spin_locked(&n->list_lock);
	//先从kmem_cache_node结构体中slabs_partial 链表上查看有没有page
	page = list_first_entry_or_null(&n->slabs_partial, struct page,
					slab_list);
	if (!page) 
		//如果没有
		n->free_touched = 1;
		//从slabs_free 链表上查看有没有page
		page = list_first_entry_or_null(&n->slabs_free, struct page,
						slab_list);
		if (page)
			n->free_slabs--;//空闲数-1
	

	if (sk_memalloc_socks())
		page = get_valid_first_slab(n, page, pfmemalloc);

	//返回page
	return page;



static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)

	int batchcount;
	struct kmem_cache_node *n;
	struct array_cache *ac, *shared;
	int node;
	void *list = NULL;
	struct page *page;

	check_irq_off();
	//获取内存节点
	node = numa_mem_id();

	ac = cpu_cache_get(cachep);
	batchcount = ac->batchcount;
	if (!ac->touched && batchcount > BATCHREFILL_LIMIT) 
		batchcount = BATCHREFILL_LIMIT;
	
	//获取cachep所属的kmem_cache_node
	n = get_node(cachep, node);

	BUG_ON(ac->avail > 0 || !n);
	shared = READ_ONCE(n->shared);
	if (!n->free_objects && (!shared || !shared->avail))
		goto direct_grow;

	spin_lock(&n->list_lock);
	shared = READ_ONCE(n->shared);

	if (shared && transfer_objects(ac, shared, batchcount)) 
		shared->touched = 1;
		goto alloc_done;
	

	while (batchcount > 0) 
		//获取kmem_cache_node结构体中的kmem_cache，返回page
		page = get_first_slab(n, false);
		if (!page)
			goto must_grow;

		check_spinlock_acquired(cachep);

		batchcount = alloc_block(cachep, ac, page, batchcount);
		fixup_slab_list(cachep, n, page, &list);
	

must_grow:
	n->free_objects -= ac->avail;
alloc_done:
	spin_unlock(&n->list_lock);
	fixup_objfreelist_debug(cachep, &list);
direct_grow:
	if (unlikely(!ac->avail)) 
		if (sk_memalloc_socks()) 
			void *obj = cache_alloc_pfmemalloc(cachep, n, flags);
			if (obj)
				return obj;
		
		//分配新的kmem_cache 并初始化
		page = cache_grow_begin(cachep, gfp_exact_node(flags), node);
		ac = cpu_cache_get(cachep);
		if (!ac->avail && page)
			alloc_block(cachep, ac, page, batchcount);
		//让page挂载到kmem_cache_node结构体的slabs_list上
		cache_grow_end(cachep, page);
		if (!ac->avail)
			return NULL;
	
	ac->touched = 1;
	//重新分配
	return ac->entry[--ac->avail];

• 获取 cachep 所属的 kmem_cache_node
• 调用 get_first_slab，获取 kmem_cache_node 结构还有没有包含空闲对象的 kmem_cache
• kmem_cache_node 结构没有包含空闲对象的 kmem_cache 了，调用cache_grow_begin 函数
• cache_grow_end 函数，把分配的 page 挂载到 kmem_cache_node 结构的slabs_list 链表上

static struct page *cache_grow_begin(struct kmem_cache *cachep,
				gfp_t flags, int nodeid)

	void *freelist;
	size_t offset;
	gfp_t local_flags;
	int page_node;
	struct kmem_cache_node *n;
	struct page *page;

	if (unlikely(flags & GFP_SLAB_BUG_MASK)) 
		gfp_t invalid_mask = flags & GFP_SLAB_BUG_MASK;
		flags &= ~GFP_SLAB_BUG_MASK;
		pr_warn("Unexpected gfp: %#x (%pGg). Fixing up to gfp: %#x (%pGg). Fix your code!\\n",
				invalid_mask, &invalid_mask, flags, &flags);
		dump_stack();
	
	WARN_ON_ONCE(cachep->ctor && (flags & __GFP_ZERO));
	local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);

	check_irq_off();
	if (gfpflags_allow_blocking(local_flags))
		local_irq_enable();

	//获取页面
	page = kmem_getpages(cachep, local_flags, nodeid);
	if (!page)
		goto failed;
	//获取页面所在的内存节点号
	page_node = page_to_nid(page);
	//根据内存节点获取对应kmem_cache_node结构
	n = get_node(cachep, page_node);

	/* Get colour for the slab, and cal the next value. */
	n->colour_next++;
	if (n->colour_next >= cachep->colour)
		n->colour_next = 0;

	offset = n->colour_next;
	if (offset >= cachep->colour)
		offset = 0;

	offset *= cachep->colour_off;

	kasan_poison_slab(page);

	/* Get slab management. */
	//分配管理空闲对象的数据结构
	freelist = alloc_slabmgmt(cachep, page, offset,
			local_flags & ~GFP_CONSTRAINT_MASK, page_node);
	if (OFF_SLAB(cachep) && !freelist)
		goto opps1;

	//让页面中相关的字段指向kmem_cache和空闲对象
	slab_map_pages(cachep, page, freelist);
	//初始化空闲对象管理数据
	cache_init_objs(cachep, page);

	if (gfpflags_allow_blocking(local_flags))
		local_irq_disable();

	return page;

opps1:
	kmem_freepages(cachep, page);
failed:
	if (gfpflags_allow_blocking(local_flags))
		local_irq_disable();
	return NULL;


static void cache_grow_end(struct kmem_cache *cachep, struct page *page)

	struct kmem_cache_node *n;
	void *list = NULL;

	check_irq_off();

	if (!page)
		return;
	//初始化page结构的slab_list链表
	INIT_LIST_HEAD(&page->slab_list);
	//根据内存节点获取对应kmem_cache_node结构
	n = get_node(cachep, page_to_nid(page));

	spin_lock(&n->list_lock);
	//slab数增加
	n->total_slabs++;
	if (!page->active) 
		//把这个page结构加入到kmem_cache_node结构的空闲链表中
		list_add_tail(&page->slab_list, &n->slabs_free);
		n->free_slabs++;
	 else
		fixup_slab_list(cachep, n, page, &list);

	STATS_INC_GROWN(cachep);
	n->free_objects += cachep->num - page->active;
	spin_unlock(&n->list_lock);

	fixup_objfreelist_debug(cachep, &list);

释放对象

使用kmem_cache_free返回给slab分配器。

1）如果per-CPU缓存中的对象数目低于允许的限制，则在其中存储一个指向缓存中对象的指针

2）必须将一些对象（准确的数目由array_cache->batchcount给出）从缓存移回slab，从编号最低的数组元素开始。
 

void ___cache_free(struct kmem_cache *cachep, void *objp,
		unsigned long caller)

	struct array_cache *ac = cpu_cache_get(cachep);

    //小于极限值
	if (ac->avail < ac->limit) 
		STATS_INC_FREEHIT(cachep);
	 else 
		STATS_INC_FREEMISS(cachep);
		cache_flusharray(cachep, ac);
	
	if (sk_memalloc_socks()) 
		struct page *page = virt_to_head_page(objp);

		if (unlikely(PageSlabPfmemalloc(page))) 
			cache_free_pfmemalloc(cachep, page, objp);
			return;
		
	
	ac->entry[ac->avail++] = objp;

//归还给slab
static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)


    ...
    //释放
	free_block(cachep, ac->entry, batchcount, node, &list);
free_done:
	spin_unlock(&n->list_lock);
	slabs_destroy(cachep, &list);
	ac->avail -= batchcount;
	memmove(ac->entry, &(ac->entry[batchcount]), sizeof(void *)*ac->avail);

 free_block将对象从缓存移动到原来的slab，并将剩余的对象向数组起始处移动。

static void free_block(struct kmem_cache *cachep, void **objpp,
			int nr_objects, int node, struct list_head *list)

	int i;
	struct kmem_cache_node *n = get_node(cachep, node);
	struct page *page;
	n->free_objects += nr_objects;

	for (i = 0; i < nr_objects; i++) 
		void *objp;
		struct page *page;

		objp = objpp[i];
		//找到对象所在的页
		page = virt_to_head_page(objp);
		//slab从缓存列表中删除
		list_del(&page->slab_list);
		check_spinlock_acquired_node(cachep, node);
		slab_put_obj(cachep, page, objp);
		STATS_DEC_ACTIVE(cachep);

		/* fixup slab chains */
		if (page->active == 0) 
			list_add(&page->slab_list, &n->slabs_free);
			n->free_slabs++;
		 else 
			//添加到partial缓存列表中
			list_add_tail(&page->slab_list, &n->slabs_partial);
		
	
	//移动剩余的对象
	while (n->free_objects > n->free_limit && !list_empty(&n->slabs_free)) 
		n->free_objects -= cachep->num;
		page = list_last_entry(&n->slabs_free, struct page, slab_list);
		list_move(&page->slab_list, list);
		n->free_slabs--;
		n->total_slabs--;
	

销毁缓存

        kmem_cache_destroy函数。该函数主要在删除模块时调用，此时需要将分配的内存都释放。

 参考

《深入Linux内核架构》

https://course.0voice.com/v1/course/intro?courseId=2&agentId=0

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