linux vmalloc分析

vmalloc是内核中使用到的内存分配函数，一般用来分配大块内存，
这个函数得到的是连续的虚拟地址，物理上地址不连续。
具体看内核实现：

void *vmalloc(unsigned long size)
{
return __vmalloc_node_flags(size, NUMA_NO_NODE,
   GFP_KERNEL | __GFP_HIGHMEM);
}

最终调用：

void *__vmalloc_node_range(unsigned long size, unsigned long align,
			unsigned long start, unsigned long end, gfp_t gfp_mask,
			pgprot_t prot, unsigned long vm_flags, int node,
			const void *caller)
{
	struct vm_struct *area;
	void *addr;
	unsigned long real_size = size;
#ifdef CONFIG_FIX_MOVABLE_ZONE
	unsigned long total_pages = total_unmovable_pages;
#else
	unsigned long total_pages = totalram_pages; //total free pages
#endif

	size = PAGE_ALIGN(size);
	if (!size || (size >> PAGE_SHIFT) > total_pages)
		goto fail;

	area = __get_vm_area_node(size, align, VM_ALLOC | VM_UNLIST | vm_flags,
			start, end, node, gfp_mask, caller);
	if (!area)
		goto fail;

	addr = __vmalloc_area_node(area, gfp_mask, prot, node, caller);
	if (!addr)
		return NULL;

	/*
	 * In this function, newly allocated vm_struct has VM_UNLIST flag.
	 * It means that vm_struct is not fully initialized.
	 * Now, it is fully initialized, so remove this flag here.
	 */
	clear_vm_unlist(area);

	/*
	 * A ref_count = 2 is needed because vm_struct allocated in
	 * __get_vm_area_node() contains a reference to the virtual address of
	 * the vmalloc'ed block.
	 */
	kmemleak_alloc(addr, real_size, 2, gfp_mask);

	return addr;

fail:
	warn_alloc_failed(gfp_mask, 0,
			  "vmalloc: allocation failure: %lu bytes\n",
			  real_size);
	return NULL;
}

static struct vm_struct *__get_vm_area_node(unsigned long size,
		unsigned long align, unsigned long flags, unsigned long start,
		unsigned long end, int node, gfp_t gfp_mask, const void *caller)
{
	struct vmap_area *va;
	struct vm_struct *area;
	//该函数不能在中断中调用
	BUG_ON(in_interrupt());
	if (flags & VM_IOREMAP) {
		int bit = fls(size);

		if (bit > IOREMAP_MAX_ORDER)
			bit = IOREMAP_MAX_ORDER;
		else if (bit < PAGE_SHIFT)
			bit = PAGE_SHIFT;

		align = 1ul << bit;
	}

	size = PAGE_ALIGN(size);
	if (unlikely(!size))
		return NULL;
	//申请vm_struct
	area = kzalloc_node(sizeof(*area), gfp_mask & GFP_RECLAIM_MASK, node);
	if (unlikely(!area))
		return NULL;

	/*
	 * We always allocate a guard page.
	 */
	size += PAGE_SIZE;
	
	//类似vma，从vmap_area rb tree中查找适合新申请vmap_area的插入点
	va = alloc_vmap_area(size, align, start, end, node, gfp_mask);
	if (IS_ERR(va)) {
		kfree(area);
		return NULL;
	}

	/*
	 * When this function is called from __vmalloc_node_range,
	 * we add VM_UNLIST flag to avoid accessing uninitialized
	 * members of vm_struct such as pages and nr_pages fields.
	 * They will be set later.
	 */
	//用vmap_area来初始化vm_struct成员
	if (flags & VM_UNLIST)
		setup_vmalloc_vm(area, va, flags, caller);
	else
		insert_vmalloc_vm(area, va, flags, caller);

	return area;
}

static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
				 pgprot_t prot, int node, const void *caller)
{
	const int order = 0;
	struct page **pages;
	unsigned int nr_pages, array_size, i;
	gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
	
	//申请的page数量
	nr_pages = (area->size - PAGE_SIZE) >> PAGE_SHIFT;
	//page 数组大小
	array_size = (nr_pages * sizeof(struct page *));

	area->nr_pages = nr_pages;
	/* Please note that the recursion is strictly bounded. */
	//array_size大于PAGE_SIZE的话，就需要递归调用__vmalloc_node申请内存，
	//否则直接调用kmalloc_node申请内存
	if (array_size > PAGE_SIZE) {
		pages = __vmalloc_node(array_size, 1, nested_gfp|__GFP_HIGHMEM,
				PAGE_KERNEL, node, caller);
		area->flags |= VM_VPAGES;
	} else {
		pages = kmalloc_node(array_size, nested_gfp, node);
	}
	area->pages = pages;
	area->caller = caller;
	if (!area->pages) {
		remove_vm_area(area->addr);
		kfree(area);
		return NULL;
	}

	//循环依次给每个page size申请物理内存，可以看到物理内存地址不是连续的
	for (i = 0; i < area->nr_pages; i++) {
		struct page *page;
		gfp_t tmp_mask = gfp_mask | __GFP_NOWARN;

		if (node < 0)
			page = alloc_page(tmp_mask);
		else
			page = alloc_pages_node(node, tmp_mask, order);

		if (unlikely(!page)) {
			/* Successfully allocated i pages, free them in __vunmap() */
			area->nr_pages = i;
			goto fail;
		}
		area->pages[i] = page;
	}

	//将虚拟地址跟每个物理页面建立映射
	if (map_vm_area(area, prot, &pages))
		goto fail;
	return area->addr;

fail:
	warn_alloc_failed(gfp_mask, order,
			  "vmalloc: allocation failure, allocated %ld of %ld bytes\n",
			  (area->nr_pages*PAGE_SIZE), area->size);
	vfree(area->addr);
	return NULL;
}