kmem_cache是slab的核心结构体，主要描述slab的各种信息和链接空闲slab，还保存高速缓存的指针数组。所以要想使用slab分配得先创建kmem_cache结构体。
代码语言：javascript复制struct kmem_cache *
kmem_cache_create (const char *name, size_t size, size_t align,
unsigned long flags, void (*ctor)(void *))
{
size_t left_over, slab_size, ralign;
struct kmem_cache *cachep = NULL, *pc;
if (!name || (size < BYTES_PER_WORD) ||
size > KMALLOC_MAX_SIZE) {
printf("slab create errorn");
}
//size和字对齐，32位4字节对齐，64位8字节对齐
if (size & (BYTES_PER_WORD - 1)) {
size = (BYTES_PER_WORD - 1);
size &= ~(BYTES_PER_WORD - 1);
}
//如果标记了cache对齐,cache行大小在现代的处理器中大部分是64B
if (flags & SLAB_HWCACHE_ALIGN) {
ralign = cache_line_size(); //获取cache行大小
while (size <= ralign / 2) //获取size最小对齐
ralign /= 2;
} else {
ralign = BYTES_PER_WORD;
}
/* 特定体系结构最小的slab对象大小*/
if (ralign < ARCH_SLAB_MINALIGN) {
ralign = ARCH_SLAB_MINALIGN;
}
if (ralign < align) {
ralign = align; //取更大的对齐
}
/*
* 计算完成对齐
*/
align = ralign;
//获取kmem_cache结构体指针，从cache_cache结构体中分配
cachep = kmem_cache_zalloc(&cache_cache, GFP_KERNEL);
if (!cachep)
goto oops;
//如果申请的对象size大于等于512B，则slab管理结构不在本slab页面上,在早期初始化slab阶段所分配的cache都是onslab的
//早期初始化完成后可以offslab
if ((size >= (PAGE_SIZE >> 3)) && !slab_early_init)
/*
* Size is large, assume best to place the slab management obj
* off-slab (should allow better packing of objs).
*/
//如果分配的对象很大，则最好将slab管理结构放在slab外面，也就是动态的从其它cache分配一块
flags |= CFLGS_OFF_SLAB;
//对齐申请的size
size = ALIGN(size, align);
//计算剩余字节和一个slab的页面的阶
left_over = calculate_slab_order(cachep, size, align, flags);
//如果对象个数是0则说明分配出错，内存不够了
if (!cachep->num) {
printf(
"kmem_cache_create: couldn't create cache %s.n", name);
cachep = NULL;
goto oops;
}
//计算对齐后的slab管理结构的大小
slab_size = ALIGN(cachep->num * sizeof(kmem_bufctl_t)
sizeof(struct slab), align);
//如果slab管理结构在本cache外面，并且剩余的字节数大于等于管理结构体的
//大小则把标记位置为onslab，并且剩余字节数减去slab管理结构的大小
//这样做应该是为了节省空间，减少内部碎片，但是可能会造成比较多的cache miss
if (flags & CFLGS_OFF_SLAB && left_over >= slab_size) {
flags &= ~CFLGS_OFF_SLAB;
left_over -= slab_size;
}
//如果slab管理结构不在本cache上，则slab管理结构的大小不需要对齐，只需要计算真实的slab管理结构大小
if (flags & CFLGS_OFF_SLAB) {
/* really off slab. No need for manual alignment */
slab_size =
cachep->num * sizeof(kmem_bufctl_t) sizeof(struct slab);
}
//着色偏移等于cache line的大小
cachep->colour_off = cache_line_size();
/* Offset must be a multiple of the alignment. */
//如果指定的对齐大于cache line大小，则着色偏移等于指定对齐
if (cachep->colour_off < align)
cachep->colour_off = align;
//着色颜色个数等于剩余字节数除以着色偏移
cachep->colour = left_over / cachep->colour_off;
//slab管理结构的大小
cachep->slab_size = slab_size;
//标记位
cachep->flags = flags;
cachep->gfpflags = 0;
//cache中的object大小
cachep->buffer_size = size;
//cache对象大小的倒数
cachep->reciprocal_buffer_size = reciprocal_value(size);
//如果slab管理结构不在本cache，则需要给slab管理结构指定一个大小适合的kmem_cache
//给slab管理结构分配内存
if (flags & CFLGS_OFF_SLAB) {
cachep->slabp_cache = kmem_find_general_cachep(slab_size, 0u);
/*
* This is a possibility for one of the malloc_sizes caches.
* But since we go off slab only for object size greater than
* PAGE_SIZE/8, and malloc_sizes gets created in ascending order,
* this should not happen at all.
* But leave a BUG_ON for some lucky dude.
*/
//BUG_ON(ZERO_OR_NULL_PTR(cachep->slabp_cache));
}
//构造函数指针，分配完回调
cachep->ctor = ctor;
//kmem_cache的名字
cachep->name = name;
//设置cache的array缓存
if (setup_cpu_cache(cachep)) {
__kmem_cache_destroy(cachep);
cachep = NULL;
goto oops;
}
/* cache setup completed, link it into the list */
//把分配的cache加入到kmem_cache链表
list_add(&cachep->next, &cache_chain);
oops:
//如果cachep指针为空，则说明没分配成功报错
if (!cachep && (flags & SLAB_PANIC))
printf("kmem_cache_create(): failed to create slab `%s'n",
name);
//mutex_unlock(&cache_chain_mutex);
// put_online_cpus();
//返回cachep
return cachep;
}接下来主要分析calculate_slab_order函数，此函数主要计算一个slab占用的页面个数，以最小对象个数为单位。
代码语言：javascript复制static size_t calculate_slab_order(struct kmem_cache *cachep,
size_t size, size_t align, unsigned long flags)
{
unsigned long offslab_limit;
size_t left_over = 0;
int gfporder;
//计算一个slab占用页面的最小阶
for (gfporder = 0; gfporder <= KMALLOC_MAX_ORDER; gfporder ) {
unsigned int num;
size_t remainder;
cache_estimate(gfporder, size, align, flags, &remainder, &num);
if (!num)
continue;
if (flags & CFLGS_OFF_SLAB) {
/*
* Max number of objs-per-slab for caches which
* use off-slab slabs. Needed to avoid a possible
* looping condition in cache_grow().
*/
/*
*如果slab管理结构不在本slab页面上则需要计算slab大小的限制，主要针对
*大内存对象
*/
offslab_limit = size - sizeof(struct slab);
offslab_limit /= sizeof(kmem_bufctl_t);
if (num > offslab_limit)
break;
}
//一个slab对象个数
cachep->num = num;
//一个slab占用的页面的个数的阶
cachep->gfporder = gfporder;
//内部碎片大小
left_over = remainder;
/*
* A VFS-reclaimable slab tends to have most allocations
* as GFP_NOFS and we really don't want to have to be allocating
* higher-order pages when we are unable to shrink dcache.
*/
//if (flags & SLAB_RECLAIM_ACCOUNT)
// break;
/*
* Large number of objects is good, but very large slabs are
* currently bad for the gfp()s.
*/
//一个slab按照最小的页面数计算，比如不超过4KB的对象，每次分配slab只需要
//一页即可
if (gfporder >= slab_break_gfp_order)
break;
/*
* Acceptable internal fragmentation?
*/
//内部碎片乘以8，要小于等于所分配的页大小
//比如此时只分配了一页，那么leftover的大小不能大于512B
if (left_over * 8 <= (PAGE_SIZE << gfporder))
break;
}
return left_over;
}具体的计算函数cache_estimate
代码语言：javascript复制//计算一个slab中的object的数目和slab剩余的字节数
static void cache_estimate(unsigned long gfporder, size_t buffer_size,
size_t align, int flags, size_t *left_over,
unsigned int *num)
{
int nr_objs;
size_t mgmt_size;
size_t slab_size = PAGE_SIZE << gfporder;
//如果slab管理结构不在本slab页面上
if (flags & CFLGS_OFF_SLAB) {
mgmt_size = 0;
nr_objs = slab_size / buffer_size; //一个slab上对象的个数
if (nr_objs > SLAB_LIMIT)
nr_objs = SLAB_LIMIT; //如果超过个数限制就等于最大的
} else {//如果在本页面上
//计算对象个数
nr_objs = (slab_size - sizeof(struct slab)) /
(buffer_size sizeof(kmem_bufctl_t));
/*
* This calculated number will be either the right
* amount, or one greater than what we want.
*/
//计算的结构可能超过slab_size大小，需要减去一个对象
if (slab_mgmt_size(nr_objs, align) nr_objs*buffer_size
> slab_size)
nr_objs--;
if (nr_objs > SLAB_LIMIT)
nr_objs = SLAB_LIMIT;
//slab管理结构的最终大小
mgmt_size = slab_mgmt_size(nr_objs, align);
}
*num = nr_objs;
//内部碎片大小
*left_over = slab_size - nr_objs*buffer_size - mgmt_size;
}
