把握linux内核设计思想（十二）：内存管理之slab分配器

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上一节最后说到对于小内存区的请求，假设採用伙伴系统来进行分配，则会在页内产生非常多空暇空间无法使用。因此产生slab分配器来处理对小内存区（几十或几百字节）的请求。Linux中引入Slab的主要目的是为了降低对伙伴算法的调用次数。

内核常常重复使用某一内存区。比如。仅仅要内核创建一个新的进程，就要为该进程相关的数据结构（task_struct、打开文件对象等）分配内存区。当进程结束时。收回这些内存区。由于进程的创建和撤销很频繁。linux把那些频繁使用的页面保存在快速缓存中并又一次使用。

slab分配器基于对象进行管理，同样类型的对象归为一类(如进程描写叙述符就是一类)，每当要申请这样一个对象。slab分配器就分配一个空暇对象出去，而当要释放时，将其又一次保存在slab分配器中，而不是直接返回给伙伴系统。

对于频繁请求的对象。创建适当大小的专用对象来处理。对于不频繁的对象。用一系列几何分布大小的对象来处理（详见通用对象）。

Slab分配模式把对象分组放进缓冲区，为缓冲区的组织和管理与硬件快速缓存的命中率密切相关，因此。Slab缓冲区并不是由各个对象直接构成。而是由一连串的“大块（Slab）”构成，而每一个大块中则包括了若干个同种类型的对象。这些对象或已被分配。或空暇。实际上。缓冲区就是主存中的一片区域，把这片区域划分为多个块。每块就是一个Slab，每一个Slab由一个或多个页面组成，每一个Slab中存放的就是对象。

slab相关数据结构：

缓冲区数据结构使用kmem_cache结构来表示。

struct kmem_cache {
/* 1) per-cpu data, touched during every alloc/free */
struct array_cache *array[NR_CPUS];
/* 2) Cache tunables. Protected by cache_chain_mutex */
unsigned int batchcount;
unsigned int limit;
unsigned int shared;

unsigned int buffer_size;
u32 reciprocal_buffer_size;
/* 3) touched by every alloc & free from the backend */

unsigned int flags;		/* constant flags */
unsigned int num;		/* # of objs per slab */

/* 4) cache_grow/shrink */
/* order of pgs per slab (2^n) */
unsigned int gfporder;

/* force GFP flags, e.g. GFP_DMA */
gfp_t gfpflags;

size_t colour;			/* cache colouring range */
unsigned int colour_off;	/* colour offset */
struct kmem_cache *slabp_cache;
unsigned int slab_size;
unsigned int dflags;		/* dynamic flags */

/* constructor func */
void (*ctor)(void *obj);

/* 5) cache creation/removal */
const char *name;
struct list_head next;

/* 6) statistics */
#ifdef CONFIG_DEBUG_SLAB
unsigned long num_active;
unsigned long num_allocations;
unsigned long high_mark;
unsigned long grown;
unsigned long reaped;
unsigned long errors;
unsigned long max_freeable;
unsigned long node_allocs;
unsigned long node_frees;
unsigned long node_overflow;
atomic_t allochit;
atomic_t allocmiss;
atomic_t freehit;
atomic_t freemiss;

/*
* If debugging is enabled, then the allocator can add additional
* fields and/or padding to every object. buffer_size contains the total
* object size including these internal fields, the following two
* variables contain the offset to the user object and its size.
*/
int obj_offset;
int obj_size;
#endif /* CONFIG_DEBUG_SLAB */

/*
* We put nodelists[] at the end of kmem_cache, because we want to size
* this array to nr_node_ids slots instead of MAX_NUMNODES
* (see kmem_cache_init())
* We still use [MAX_NUMNODES] and not [1] or [0] because cache_cache
* is statically defined, so we reserve the max number of nodes.
*/
struct kmem_list3 *nodelists[MAX_NUMNODES];
/*
* Do not add fields after nodelists[]
*/
};

当中struct kmem_list3结构体链接slab，共享快速缓存。其定义例如以下:

/*
* The slab lists for all objects.
*/
struct kmem_list3 {
struct list_head slabs_partial;	/* partial list first, better asm code */
struct list_head slabs_full;
struct list_head slabs_free;
unsigned long free_objects;
unsigned int free_limit;
unsigned int colour_next;	/* Per-node cache coloring */
spinlock_t list_lock;
struct array_cache *shared;	/* shared per node */
struct array_cache **alien;	/* on other nodes */
unsigned long next_reap;	/* updated without locking */
int free_touched;		/* updated without locking */
};

该结构包括三个链表：slabs_partial、slabs_full、slabs_free，这些链表包括缓冲区全部slab。slab描写叙述符struct
slab用于描写叙述每一个slab：

/*
* struct slab
*
* Manages the objs in a slab. Placed either at the beginning of mem allocated
* for a slab, or allocated from an general cache.
* Slabs are chained into three list: fully used, partial, fully free slabs.
*/
struct slab {
struct list_head list;
unsigned long colouroff;
void *s_mem;		/* including colour offset */
unsigned int inuse;	/* num of objs active in slab */
kmem_bufctl_t free;
unsigned short nodeid;
};

一个新的缓冲区使用例如以下函数创建：

struct kmem_cache *kmem_cache_create (const char *name, size_t size, size_t align, unsigned long flags, void (*ctor)(void *));

函数创建成功会返回一个指向所创建缓冲区的指针；撤销一个缓冲区调用例如以下函数：

<span style="font-family:Microsoft YaHei;">void kmem_cache_destroy(struct kmem_cache *cachep)；</span>

上面两个函数都不能在中断上下文中使用。由于它可能睡眠。

在创建来缓冲区之后，能够通过下列函数获取对象：

/**
* kmem_cache_alloc - Allocate an object
* @cachep: The cache to allocate from.
* @flags: See kmalloc().
*
* Allocate an object from this cache.  The flags are only relevant
* if the cache has no available objects.
*/
void *kmem_cache_alloc(struct kmem_cache *cachep, gfp_t flags)
{
void *ret = __cache_alloc(cachep, flags, __builtin_return_address(0));

trace_kmem_cache_alloc(_RET_IP_, ret,
obj_size(cachep), cachep->buffer_size, flags);

return ret;
}

该函数从给点缓冲区cachep中返回一个指向对象的指针。

假设缓冲区的全部slab中都没有空暇对象，那么slab层必须通过kmem_getpages()获取新的页。參数flags传递给_get_free_pages()。

<span style="font-family:Microsoft YaHei;">static void *kmem_getpages(struct kmem_cache *cachep, gfp_t flags, int nodeid)；</span>

释放对象使用例如以下函数：

/**
* kmem_cache_free - Deallocate an object
* @cachep: The cache the allocation was from.
* @objp: The previously allocated object.
*
* Free an object which was previously allocated from this
* cache.
*/
void kmem_cache_free(struct kmem_cache *cachep, void *objp)
{
unsigned long flags;

local_irq_save(flags);
debug_check_no_locks_freed(objp, obj_size(cachep));
if (!(cachep->flags & SLAB_DEBUG_OBJECTS))
debug_check_no_obj_freed(objp, obj_size(cachep));
__cache_free(cachep, objp);
local_irq_restore(flags);

trace_kmem_cache_free(_RET_IP_, objp);
}

假设你要频繁的创建非常多同样类型的对象，就要当考虑使用slab快速缓存区。

实际上上一节所讲kmalloc()函数也是使用slab分配器分配的。

static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
struct kmem_cache *cachep;
void *ret;

if (__builtin_constant_p(size)) {
int i = 0;

if (!size)
return ZERO_SIZE_PTR;

#define CACHE(x) \
if (size <= x) \
goto found; \
else \
i++;
#include <linux/kmalloc_sizes.h>
#undef CACHE
return NULL;
found:
#ifdef CONFIG_ZONE_DMA
if (flags & GFP_DMA)
cachep = malloc_sizes[i].cs_dmacachep;
else
#endif
cachep = malloc_sizes[i].cs_cachep;

ret = kmem_cache_alloc_notrace(cachep, flags);

trace_kmalloc(_THIS_IP_, ret,
size, slab_buffer_size(cachep), flags);

return ret;
}
return __kmalloc(size, flags);
}

kfree函数实现例如以下：

/**
* kfree - free previously allocated memory
* @objp: pointer returned by kmalloc.
*
* If @objp is NULL, no operation is performed.
*
* Don't free memory not originally allocated by kmalloc()
* or you will run into trouble.
*/
void kfree(const void *objp)
{
struct kmem_cache *c;
unsigned long flags;

trace_kfree(_RET_IP_, objp);

if (unlikely(ZERO_OR_NULL_PTR(objp)))
return;
local_irq_save(flags);
kfree_debugcheck(objp);
c = virt_to_cache(objp);
debug_check_no_locks_freed(objp, obj_size(c));
debug_check_no_obj_freed(objp, obj_size(c));
__cache_free(c, (void *)objp);
local_irq_restore(flags);
}

最后。结合上一节。看看分配函数的选择：

假设须要连续的物理页，就能够使用某个低级页分配器或kmalloc()。

假设想从高端内存进行分配，使用alloc_pages()。

假设不须要物理上连续的页，而不过虚拟地址上连续的页，那么就是用vmalloc。

假设要创建和销毁非常多大的数据结构，那么考虑建立slab快速缓存。