kernel malloc

最近依旧在偷闲看 linux 的源码. 途中遇到了很关键的内存分配.

想看看内存是如何分配的. 所以有了这篇笔记. 一开始时, 仅阅读 slab 部分.

/**
 * kmalloc - allocate memory
 * @size: how many bytes of memory are required.
 * @flags: the type of memory to allocate.
 *
 * kmalloc is the normal method of allocating memory
 * for objects smaller than page size in the kernel.
 *
 * The allocated object address is aligned to at least ARCH_KMALLOC_MINALIGN
 * bytes. For @size of power of two bytes, the alignment is also guaranteed
 * to be at least to the size.
 *
 * The @flags argument may be one of the GFP flags defined at
 * include/linux/gfp.h and described at
 * :ref:`Documentation/core-api/mm-api.rst <mm-api-gfp-flags>`
 *
 * The recommended usage of the @flags is described at
 * :ref:`Documentation/core-api/memory-allocation.rst <memory_allocation>`
 *
 * Below is a brief outline of the most useful GFP flags
 *
 * %GFP_KERNEL
 *	Allocate normal kernel ram. May sleep.
 *
 * %GFP_NOWAIT
 *	Allocation will not sleep.
 *
 * %GFP_ATOMIC
 *	Allocation will not sleep.  May use emergency pools.
 *
 * %GFP_HIGHUSER
 *	Allocate memory from high memory on behalf of user.
 *
 * Also it is possible to set different flags by OR'ing
 * in one or more of the following additional @flags:
 *
 * %__GFP_HIGH
 *	This allocation has high priority and may use emergency pools.
 *
 * %__GFP_NOFAIL
 *	Indicate that this allocation is in no way allowed to fail
 *	(think twice before using).
 *
 * %__GFP_NORETRY
 *	If memory is not immediately available,
 *	then give up at once.
 *
 * %__GFP_NOWARN
 *	If allocation fails, don't issue any warnings.
 *
 * %__GFP_RETRY_MAYFAIL
 *	Try really hard to succeed the allocation but fail
 *	eventually.
 */
static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
	if (__builtin_constant_p(size)) {
#ifndef CONFIG_SLOB
		unsigned int index;
#endif
		if (size > KMALLOC_MAX_CACHE_SIZE)
			return kmalloc_large(size, flags);
#ifndef CONFIG_SLOB
		index = kmalloc_index(size);

		if (!index)
			return ZERO_SIZE_PTR;

		return kmem_cache_alloc_trace(
				kmalloc_caches[kmalloc_type(flags)][index],
				flags, size);
#endif
	}
	return __kmalloc(size, flags);
}

内核分配内存. 参数总是 size + flag.

而 flag 总是 GFP_KERNEL. 完整的参数列表在 include/linux/gfp.h 文件中.

其中:

#define GFP_KERNEL	(__GFP_RECLAIM | __GFP_IO | __GFP_FS)

%__GFP_RECLAIM is shorthand to allow/forbid both direct and kswapd reclaim.
%__GFP_IO can start physical IO.
%__GFP_FS can call down to the low-level FS. Clearing the flag avoids the
 * allocator recursing into the filesystem which might already be holding
 * locks.

reclaim 控制分配过程中. 是否允许回收(直接/kswapd).

__always_inline  // inline __attribute__((__always_inline__))

参见:

Common Function Attributes (Using the GNU Compiler Collection (GCC))

这个关键字很重要. 代码中

__builtin_constant_p(size)

若没有这个关键字. 上述代码很可能就是无效的代码. 整个分支都会无效.

如果 size 为常量 && 分配大小 大于 8k(我的代码版本默认页大小). 则调用 kmalloc_large . 否则调用 __kmalloc (重申: 只包含 slab 逻辑)

那么. 先来看看 kmalloc_large

/**
 * get_order - Determine the allocation order of a memory size
 * @size: The size for which to get the order
 *
 * Determine the allocation order of a particular sized block of memory.  This
 * is on a logarithmic scale, where:
 *
 *	0 -> 2^0 * PAGE_SIZE and below
 *	1 -> 2^1 * PAGE_SIZE to 2^0 * PAGE_SIZE + 1
 *	2 -> 2^2 * PAGE_SIZE to 2^1 * PAGE_SIZE + 1
 *	3 -> 2^3 * PAGE_SIZE to 2^2 * PAGE_SIZE + 1
 *	4 -> 2^4 * PAGE_SIZE to 2^3 * PAGE_SIZE + 1
 *	...
 *
 * The order returned is used to find the smallest allocation granule required
 * to hold an object of the specified size.
 *
 * The result is undefined if the size is 0.
 */
static inline __attribute_const__ int get_order(unsigned long size)
{
	if (__builtin_constant_p(size)) {
		if (!size)
			return BITS_PER_LONG - PAGE_SHIFT;

		if (size < (1UL << PAGE_SHIFT))
			return 0;

		return ilog2((size) - 1) - PAGE_SHIFT + 1;
	}

	size--;
	size >>= PAGE_SHIFT;
#if BITS_PER_LONG == 32
	return fls(size);
#else
	return fls64(size);
#endif
}

static __always_inline void *kmalloc_large(size_t size, gfp_t flags)
{
	unsigned int order = get_order(size);
	return kmalloc_order_trace(size, flags, order);
}