本文将mark下Linux kernel内存分配函数的相关notes。

内核函数

kmalloc系列

kmalloc() 申请的内存位于物理内存映射区域，而且在物理上也是连续的，它们与真实的物理地址只有一个固定的偏移，因而存在较简单的转换关系。

  • kmalloc
  • kzalloc
  • kcalloc
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/**
 * kzalloc - allocate memory. The memory is set to zero.
 * @size: how many bytes of memory are required.
 * @flags: the type of memory to allocate (see kmalloc).
 */
static inline void *kzalloc(size_t size, gfp_t flags)
{
	return kmalloc(size, flags | __GFP_ZERO);
}

/**
 * kcalloc - allocate memory for an array. The memory is set to zero.
 * @n: number of elements.
 * @size: element size.
 * @flags: the type of memory to allocate (see kmalloc).
 */
static inline void *kcalloc(size_t n, size_t size, gfp_t flags)
{
	return kmalloc_array(n, size, flags | __GFP_ZERO);
}

vmalloc系列

物理地址不连续：通过映射非连续的物理页帧实现。
虚拟地址连续：虚拟地址空间是连续的。
逐页映射: 用 alloc_page() 从伙伴系统获取多个不连续的物理页帧。 修改内核页表，将这些物理页映射到连续的虚拟地址空间（VMALLOC_START ~ VMALLOC_END）。

  • vmalloc
  • vzalloc
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/**
 *	vmalloc  -  allocate virtually contiguous memory
 *	@size:		allocation size
 *	Allocate enough pages to cover @size from the page level
 *	allocator and map them into contiguous kernel virtual space.
 *
 *	For tight control over page level allocator and protection flags
 *	use __vmalloc() instead.
 */
void *vmalloc(unsigned long size)
{
	return __vmalloc_node_flags(size, NUMA_NO_NODE,
				    GFP_KERNEL);
}

/**
 *	vzalloc - allocate virtually contiguous memory with zero fill
 *	@size:	allocation size
 *	Allocate enough pages to cover @size from the page level
 *	allocator and map them into contiguous kernel virtual space.
 *	The memory allocated is set to zero.
 *
 *	For tight control over page level allocator and protection flags
 *	use __vmalloc() instead.
 */
void *vzalloc(unsigned long size)
{
	return __vmalloc_node_flags(size, NUMA_NO_NODE,
				GFP_KERNEL | __GFP_ZERO);
}

kvzalloc系列

  • kvzalloc
  • kvcalloc
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static inline void *kvzalloc(size_t size, gfp_t flags)
{
	return kvmalloc(size, flags | __GFP_ZERO);
}

static inline void *kvmalloc(size_t size, gfp_t flags)
{
	return kvmalloc_node(size, flags, NUMA_NO_NODE);
}

/**
 * kvmalloc_node - attempt to allocate physically contiguous memory, but upon
 * failure, fall back to non-contiguous (vmalloc) allocation.
 * @size: size of the request.
 * @flags: gfp mask for the allocation - must be compatible (superset) with GFP_KERNEL.
 * @node: numa node to allocate from
 *
 * Uses kmalloc to get the memory but if the allocation fails then falls back
 * to the vmalloc allocator. Use kvfree for freeing the memory.
 *
 * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported.
 * __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
 * preferable to the vmalloc fallback, due to visible performance drawbacks.
 *
 * Please note that any use of gfp flags outside of GFP_KERNEL is careful to not
 * fall back to vmalloc.
 */
void *kvmalloc_node(size_t size, gfp_t flags, int node)
{
	gfp_t kmalloc_flags = flags;
	void *ret;

	/*
	 * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
	 * so the given set of flags has to be compatible.
	 */
	if ((flags & GFP_KERNEL) != GFP_KERNEL)
		return kmalloc_node(size, flags, node);

	/*
	 * We want to attempt a large physically contiguous block first because
	 * it is less likely to fragment multiple larger blocks and therefore
	 * contribute to a long term fragmentation less than vmalloc fallback.
	 * However make sure that larger requests are not too disruptive - no
	 * OOM killer and no allocation failure warnings as we have a fallback.
	 */
	if (size > PAGE_SIZE) {
		kmalloc_flags |= __GFP_NOWARN;

		if (!(kmalloc_flags & __GFP_RETRY_MAYFAIL))
			kmalloc_flags |= __GFP_NORETRY;
	}

	ret = kmalloc_node(size, kmalloc_flags, node);

	/*
	 * It doesn't really make sense to fallback to vmalloc for sub page
	 * requests
	 */
	if (ret || size <= PAGE_SIZE)
		return ret;

	return __vmalloc_node_flags_caller(size, node, flags,
			__builtin_return_address(0));
}

物理地址获取

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static phys_addr_t kvm_kaddr_to_phys(void *kaddr)
{
	if (!is_vmalloc_addr(kaddr)) {
		return __pa(kaddr);
	} else {
		return page_to_phys(vmalloc_to_page(kaddr)) +
		       offset_in_page(kaddr);
	}
}

Note that the virtual address may come from different kernel memory zones, including the vmalloc region or the direct memory region. Checking whether the a virtual address belongs to the vmalloc region by invoking the is_vmalloc_addr function. If so, the vmalloc_to_page function is used to get the corresponding physical page structure; otherwise, the virt_to_page function is used to obtain the right page.

参考资料:

  1. https://elixir.bootlin.com/linux/v4.19/
  2. what are differences between kmalloc() kcalloc() vmalloc() and kzalloc()?
  3. 带你看懂Linux内核空间内存申请函数kmalloc.、kzalloc、 vmalloc的区别（一篇就够了）
  4. 傻傻分不清楚 kmalloc、vmalloc和malloc之间有什么区别以及实现上的差异
  5. QZFS: QAT Accelerated Compression in File System for Application Agnostic and Cost Efficient Data Storage