内存管理 ---slab机制 分配对象

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从一个缓存中分配对象总是遵循下面的原则：

1.本地高速缓存中是否有空闲对象，如果有的话则从其中获取对象，
2.如果本地高速缓存中没有对象，则从kmem_list3中的slab链表中寻找空闲对象并填充到本地高速缓存再分配；
3.如果所有的slab中都没有空闲对象了，那么就要创建新的slab,再分配 。


 

来自：http://blog.csdn.net/vanbreaker/article/details/7671211   

 Linux内核从slab中分配内存空间由kmalloc()或kmem_cache_alloc()函数实现。

kmalloc()->__kmalloc()->__do_kmalloc()；

/**
 * __do_kmalloc - allocate memory
 * @size: how many bytes of memory are required.
 * @flags: the type of memory to allocate (see kmalloc).
 * @caller: function caller for debug tracking of the caller
 */
static __always_inline void *__do_kmalloc(size_t size, gfp_t flags,
					  void *caller)
{
	struct kmem_cache *cachep;
	void *ret;

	/* If you want to save a few bytes .text space: replace
	 * __ with kmem_.
	 * Then kmalloc uses the uninlined functions instead of the inline
	 * functions.
	 */   /*查找指定大小的通用cache，关于sizes数组，在前面 
    的初始化中就已经分析过了*/  
	cachep = __find_general_cachep(size, flags);
	if (unlikely(ZERO_OR_NULL_PTR(cachep)))
		return cachep;
	ret = __cache_alloc(cachep, flags, caller); /*实际的分配工作*/ 

	trace_kmalloc((unsigned long) caller, ret,
		      size, cachep->size, flags);

	return ret;
}

static inline void *____cache_alloc(struct kmem_cache *cachep, gfp_t flags)
{
	void *objp;
	struct array_cache *ac;
	bool force_refill = false;

	check_irq_off();

	ac = cpu_cache_get(cachep); /* 获得本CPU的local cache */  
	if (likely(ac->avail)) {/* 如果local cache中有可用的空闲对象 */  
		ac->touched = 1;
<span style="white-space:pre">		 /* 从local cache的entry数组中提取最后面的空闲对象 */  </span>
		objp = ac_get_obj(cachep, ac, flags, false); 

		/*
		 * Allow for the possibility all avail objects are not allowed
		 * by the current flags
		 */
		if (objp) {
			STATS_INC_ALLOCHIT(cachep);
			goto out;
		}
		force_refill = true;  //标志位 是否需要refill
	}

	STATS_INC_ALLOCMISS(cachep); /* 从slab三链中提取空闲对象填充到local cache中 */  
	objp = cache_alloc_refill(cachep, flags, force_refill);
	/*
	 * the 'ac' may be updated by cache_alloc_refill(),
	 * and kmemleak_erase() requires its correct value.
	 */ /* cache_alloc_refill的cache_grow打开了中断，local cache指针可能发生了变化，需要重新获得
	ac = cpu_cache_get(cachep);

out:
	/*
	 * To avoid a false negative, if an object that is in one of the
	 * per-CPU caches is leaked, we need to make sure kmemleak doesn't
	 * treat the array pointers as a reference to the object.
	 */
	if (objp)
		kmemleak_erase(&ac->entry[ac->avail]);/* 分配出去的对象，其entry指针指向空 */  
	return objp;
}

static inline void *ac_get_obj(struct kmem_cache *cachep,
			struct array_cache *ac, gfp_t flags, bool force_refill)
{
	void *objp;

	if (unlikely(sk_memalloc_socks()))
		objp = __ac_get_obj(cachep, ac, flags, force_refill);
	else  
<span style="white-space:pre">	 /*先将avail的值减1，这样avail对应的空闲对象是最热的，即最近释放出来的， 
          更有可能驻留在CPU高速缓存中*/  </span>
	objp = ac->entry[--ac->avail];

	return objp;
}

tatic void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags,
							bool force_refill)
{
	int batchcount;
	struct kmem_list3 *l3;
	struct array_cache *ac;
	int node;

	check_irq_off();
	node = numa_mem_id(); /* 获得本内存节点，UMA只有一个节点 */  
	if (unlikely(force_refill))
		goto force_grow;
retry:
	ac = cpu_cache_get(cachep);
	batchcount = ac->batchcount; /*获取批量转移的数目*/  
	if (!ac->touched && batchcount > BATCHREFILL_LIMIT) {
		/*
		 * If there was little recent activity on this cache, then
		 * perform only a partial refill.  Otherwise we could generate
		 * refill bouncing.
		 */   /* 最近未使用过此local cache，没有必要添加过多的对象 
         ，添加的数目为默认的限定值 */  
		batchcount = BATCHREFILL_LIMIT;
	}
	l3 = cachep->nodelists[node];/*获取kmem_list3*/  

	BUG_ON(ac->avail > 0 || !l3);
	spin_lock(&l3->list_lock);

	/* See if we can refill from the shared array */
<span style="white-space:pre">	 /* shared local cache用于多核系统中，为所有cpu共享 
    ，如果有共享本地高速缓存 
    ，那么首先从shared local cache中批量搬运空闲对象到local cache中 
    。通过shared local cache使填充工作变得简单。*/  </span>
	if (l3->shared && transfer_objects(ac, l3->shared, batchcount)) {
		l3->shared->touched = 1;
		goto alloc_done;
	}
<span style="white-space:pre">		  /* 如果没有shared local cache，或是其中没有空闲的对象 
    ，从slab链表中分配 */  </span>
	while (batchcount > 0) {
		struct list_head *entry;
		struct slab *slabp;
		/* Get slab alloc is to come from. */
<span style="white-space:pre">		/*扫描slab链表，先从partial链表开始，如果整个partial链表都无法找到batchcount个空闲对象， 
       </span>    <span style="white-space:pre"> 再扫描free链表*/ </span>
		entry = l3->slabs_partial.next;
		if (entry == &l3->slabs_partial) { /*entry回到表头说明partial链表已经扫描完毕，开始扫描free链表*/ 
			l3->free_touched = 1; /* 表示刚刚访问了slab空链表 */  
			entry = l3->slabs_free.next;
			if (entry == &l3->slabs_free)/* 空slab链表也为空，必须增加slab了 */  
				goto must_grow;
		}

		slabp = list_entry(entry, struct slab, list);
		check_slabp(cachep, slabp);
		check_spinlock_acquired(cachep);

		/*
		 * The slab was either on partial or free list so
		 * there must be at least one object available for
		 * allocation.
		 */
		BUG_ON(slabp->inuse >= cachep->num);
<span style="white-space:pre">		  /*如果slabp中还存在空闲对象并且还需要继续填充对象到本地高速缓存*/  </span>
		while (slabp->inuse < cachep->num && batchcount--) {
			STATS_INC_ALLOCED(cachep);
			STATS_INC_ACTIVE(cachep);
			STATS_SET_HIGH(cachep);
<span style="white-space:pre">			 /*填充的本质就是用ac后面的void*数组元素指向一个空闲对象</span>
<span style="white-space:pre">				 ac->entry[ac->avail++] = slab_get_obj(cachep, slabp, node); </span>
<span style="white-space:pre">				 */  </span>
			ac_put_obj(cachep, ac, slab_get_obj(cachep, slabp,node));
		}
		check_slabp(cachep, slabp);

		/* move slabp to correct slabp list: */
		list_del(&slabp->list);
		if (slabp->free == BUFCTL_END) /*free等于BUFCTL_END表示空闲对象已耗尽，将slab插入full链表*/  
			list_add(&slabp->list, &l3->slabs_full);
		else
			list_add(&slabp->list, &l3->slabs_partial);
	}

must_grow:
<span style="white-space:pre">	 /* 前面从slab链表中添加avail个空闲对象到local cache中 
    ，更新slab链表的空闲对象数 */ </span>
	l3->free_objects -= ac->avail;
alloc_done:
	spin_unlock(&l3->list_lock);

	if (unlikely(!ac->avail)) { /* local cache中仍没有可用的空闲对象，说明slab 
                             三链中也没有空闲对象，需要创建新的空slab了 */ 
		int x;
force_grow:
		x = cache_grow(cachep, flags | GFP_THISNODE, node, NULL); /* 创建一个空slab */  

		/* cache_grow can reenable interrupts, then ac could change. */
		ac = cpu_cache_get(cachep);
		node = numa_mem_id();

		/* no objects in sight? abort */
		if (!x && (ac->avail == 0 || force_refill))
			return NULL;

		if (!ac->avail)		/* objects refilled by interrupt? */
			goto retry;
	}
	ac->touched = 1;

	return ac_get_obj(cachep, ac, flags, force_refill); /* 返回local cache中最后一个空闲对象的虚拟地址 objp = ac->entry[--ac->avail];*/  
}

/*
 * Transfer objects in one arraycache to another.
 * Locking must be handled by the caller.
 *
 * Return the number of entries transferred.
 */
static int transfer_objects(struct array_cache *to,
		struct array_cache *from, unsigned int max)
{
	/* Figure out how many entries to transfer */
	int nr = min(min(from->avail, max), to->limit - to->avail);

	if (!nr)
		return 0;
	/*拷贝*/
	memcpy(to->entry + to->avail, from->entry + from->avail -nr,
			sizeof(void *) *nr);
	/*两边数据更新*/
	from->avail -= nr;
	to->avail += nr;
	to->touched = 1;
	return nr;
}

/*从slab中提取一个空闲对象*/
static void *slab_get_obj(struct kmem_cache *cachep, struct slab *slabp,
				int nodeid)
{
	/* 获得一个空闲的对象，free是本slab中第一个空闲对象的索引 */
	void *objp = index_to_obj(cachep, slabp, slabp->free);
	kmem_bufctl_t next;
 	/* 更新在用对象计数 */
	slabp->inuse++;
 	/* 获得下一个空闲对象的索引 */
	next = slab_bufctl(slabp)[slabp->free];
#if DEBUG
	slab_bufctl(slabp)[slabp->free] = BUFCTL_FREE;
	WARN_ON(slabp->nodeid != nodeid);
#endif
	/* free指向下一个空闲的对象 */
	slabp->free = next;

	return objp;
}

static inline void *index_to_obj(struct kmem_cache *cache, struct slab *slab,
				 unsigned int idx)
{	/* s_mem是slab中第一个对象的起始地址，buffer_size是每个对象的大小
	，这里根据对象索引计算对象的地址 */
	return slab->s_mem + cache->buffer_size * idx;
}

static inline kmem_bufctl_t *slab_bufctl(struct slab *slabp)
{
	return (kmem_bufctl_t *) (slabp + 1);
}

static inline struct array_cache *cpu_cache_get(struct kmem_cache *cachep)
{
	return cachep->array[smp_processor_id()];
}

内存管理 ---slab机制 分配对象

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原文地址：http://blog.csdn.net/u012681083/article/details/51345432