system contig heap与system heap

• 从代码中我们看到system contig heap与system heap同属一个文件中，ion_system_heap.c
• 相同点：它们都是根据用户传递的字节len，转换成order，从buddy中申请内存页
• 不同点：

1. contig heap申请的是连续内存页，最大能到order 10（受限于buddy）。而system heap就只能通过order 8/4/0三种拼凑出来，凑够用户需要的内存
2. contig heap没有自己的内存pool，释放时直接返还给buddy。system heap有自己的pool（分为order 8/4/0）。

system contig heap创建

//指定heap name，heap type和ops
static struct ion_heap *__ion_system_contig_heap_create(void)
{
	struct ion_heap *heap;

	heap = kzalloc(sizeof(*heap), GFP_KERNEL);
	if (!heap)
		return ERR_PTR(-ENOMEM);
	heap->ops = &kmalloc_ops;
	heap->type = ION_HEAP_TYPE_SYSTEM_CONTIG;
	heap->name = "ion_system_contig_heap";
	return heap;
}

static int ion_system_contig_heap_create(void)
{
	struct ion_heap *heap;

	heap = __ion_system_contig_heap_create();
	if (IS_ERR(heap))
		return PTR_ERR(heap);
	//添加到ion dev，并且在/d/ion/heaps目录下创建debug节点，节点名以heap name命名："ion_system_contig_heap"
	ion_device_add_heap(heap);
	return 0;
}
//开机过程中，驱动加载时创建，当然这只是参考代码，具体还要看不同平台的具体实现
device_initcall(ion_system_contig_heap_create);

system contig heap内存分配

static struct ion_heap_ops kmalloc_ops = {
	.allocate = ion_system_contig_heap_allocate,
	.free = ion_system_contig_heap_free,
	.map_kernel = ion_heap_map_kernel,
	.unmap_kernel = ion_heap_unmap_kernel,
	.map_user = ion_heap_map_user,
};

• ion_system_contig_heap_allocate()函数分配：

直接从buddy中分配连续的内存块

static int ion_system_contig_heap_allocate(struct ion_heap *heap,
					   struct ion_buffer *buffer,
					   unsigned long len,
					   unsigned long flags)
{
	int order = get_order(len);
	struct page *page;
	struct sg_table *table;
	unsigned long i;
	int ret;
    //直接从buddy中分配内存页
    //分配的内存页是可能比实际请求的大的，比如申请len是3个page大小，那么order就为2，实际申请了4个page
	page = alloc_pages(low_order_gfp_flags, order);
	if (!page)
		return -ENOMEM;
    //将申请到的连续内存页分割成一页页
	split_page(page, order);

    //由于在分配时可能多分配，因此需要将多余的page释放回去。比如申请3个page，实际分配了4个
	len = PAGE_ALIGN(len);
	for (i = len >> PAGE_SHIFT; i < (1 << order); i++)
		__free_page(page + i);
    //接着申请sg_table
	table = kmalloc(sizeof(*table), GFP_KERNEL);
	if (!table) {
		ret = -ENOMEM;
		goto free_pages;
	}
    //由于是连续的内存，因此只需要申请一个scatterlist
	ret = sg_alloc_table(table, 1, GFP_KERNEL);
	if (ret)
		goto free_table;
    //将连续内存首页地址存到sg_table中
	sg_set_page(table->sgl, page, len, 0);

	buffer->sg_table = table;

	return 0;

free_table:
	kfree(table);
free_pages:
	for (i = 0; i < len >> PAGE_SHIFT; i++)
		__free_page(page + i);

	return ret;
}

system contig heap内存释放

• ion_system_contig_heap_free()函数：

将内存全部返还给buddy

static void ion_system_contig_heap_free(struct ion_buffer *buffer)
{
	struct sg_table *table = buffer->sg_table;
	struct page *page = sg_page(table->sgl);
	unsigned long pages = PAGE_ALIGN(buffer->size) >> PAGE_SHIFT;
	unsigned long i;
    //释放时就直接将内存页归还给buddy
	for (i = 0; i < pages; i++)
		__free_page(page + i);
	sg_free_table(table);
	kfree(table);
}

• 完