Linux内存管理之slab机制（创建cache）
Linux内核中创建cache节点由函数kmem_cache_create()实现。
该函数的执行流程：
1，从全局cache_cache中获得cache结构，因为全局cache_cache初始化对象的大小就是kmem_cache结构的大小，所以返回的指针正好可以转换为cache结构；调用 kmem_cache_zalloc(&cache_cache, gfp);
2，获得slab中碎片大小，由函数calculate_slab_order()实现；
3，计算并初始化cache的各种属性，如果是外置式，需要用kmem_find_general_cachep(slab_size, 0u)指定cachep->slabp_cache，用于存放slab对象和kmem_bufctl_t[]数组；
4，设置每个CPU上得本地cache，setup_cpu_cache();
5，cache创建完毕，将其加入到全局slab cache链表中；
一、主实现
/**
* kmem_cache_create - Create a cache.
* @name: A string which is used in /proc/slabinfo to identify this cache.
* @size: The size of objects to be created in this cache.
* @align: The required alignment for the objects.
* @flags: SLAB flags
* @ctor: A constructor for the objects.
*
* Returns a ptr to the cache on success, NULL on failure.
* Cannot be called within a int, but can be interrupted.
* The @ctor is run when new pages are allocated by the cache.
*
* @name must be valid until the cache is destroyed. This implies that
* the module calling this has to destroy the cache before getting unloaded.
* Note that kmem_cache_name() is not guaranteed to return the same pointer,
* therefore applications must manage it themselves.
*
* The flags are
*
* %SLAB_POISON - Poison the slab with a known test pattern (a5a5a5a5)
* to catch references to uninitialised memory.
*
* %SLAB_RED_ZONE - Insert `Red' zones around the allocated memory to check
* for buffer overruns.
*
* %SLAB_HWCACHE_ALIGN - Align the objects in this cache to a hardware
* cacheline. This can be beneficial if you're counting cycles as closely
* as davem.
*/
/*创建slab系统顶层的cache节点。创建完成后，cache
里并没有任何slab以及对象，只有当分配对象
，并且cache中没有空闲对象时，才会创建新的slab。*/
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;
gfp_t gfp;
/*
* Sanity checks... these are all serious usage bugs.
*//* 安全性检查 */
if (!name || in_interrupt() || (size < BYTES_PER_WORD) ||
size > KMALLOC_MAX_SIZE) {
printk(KERN_ERR "%s: Early error in slab %s\n", __func__,
name);
BUG();
}
/*
* We use cache_chain_mutex to ensure a consistent view of
* cpu_online_mask as well. Please see cpuup_callback
*/
/* slab分配器是否已经初始化好，如果是内核启动阶段
，则只有一个cpu执行slab分配器的初始化动作，无需加锁，否则需要加锁 */
if (slab_is_available()) {
get_online_cpus();
mutex_lock(&cache_chain_mutex);
}
/* 遍历cache链，做些校验工作 */
list_for_each_entry(pc, &cache_chain, next) {
char tmp;
int res;
/*
* This happens when the module gets unloaded and doesn't
* destroy its slab cache and no-one else reuses the vmalloc
* area of the module. Print a warning.
*/
/* 检查cache链表中的cache是否都有名字 */
res = probe_kernel_address(pc->name, tmp);
if (res) {/*没有名字，报错*/
printk(KERN_ERR
"SLAB: cache with size %d has lost its name\n",
pc->buffer_size);
continue;
}
/* 检查cache链表中是否已经存在相同名字的cache */
if (!strcmp(pc->name, name)) {
printk(KERN_ERR
"kmem_cache_create: duplicate cache %s\n", name);
dump_stack();
goto oops;
}
}
#if DEBUG
WARN_ON(strchr(name, ' ')); /* It confuses parsers */
#if FORCED_DEBUG
/*
* Enable redzoning and last user accounting, except for caches with
* large objects, if the increased size would increase the object size
* above the next power of two: caches with object sizes just above a
* power of two have a significant amount of internal fragmentation.
*/
if (size < 4096 || fls(size - 1) == fls(size-1 + REDZONE_ALIGN +
2 * sizeof(unsigned long long)))
flags |= SLAB_RED_ZONE | SLAB_STORE_USER;
if (!(flags & SLAB_DESTROY_BY_RCU))
flags |= SLAB_POISON;
#endif
if (flags & SLAB_DESTROY_BY_RCU)
BUG_ON(flags & SLAB_POISON);
#endif
/*
* Always checks flags, a caller might be expecting debug support which
* isn't available.
*/
BUG_ON(flags & ~CREATE_MASK);
/*
* Check that size is in terms of words. This is needed to avoid
* unaligned accesses for some archs when redzoning is used, and makes
* sure any on-slab bufctl's are also correctly aligned.
*/
if (size & (BYTES_PER_WORD - 1)) {
size += (BYTES_PER_WORD - 1);
size &= ~(BYTES_PER_WORD - 1);
}
/* calculate the final buffer alignment: */
/* 1) arch recommendation: can be overridden for debug */
if (flags & SLAB_HWCACHE_ALIGN) {
/*
* Default alignment: as specified by the arch code. Except if
* an object is really small, then squeeze multiple objects into
* one cacheline.
*/
ralign = cache_line_size();
while (size <= ralign / 2)
ralign /= 2;
} else {
ralign = BYTES_PER_WORD;
}
/*
* Redzoning and user store require word alignment or possibly larger.
* Note this will be overridden by architecture or caller mandated
* alignment if either is greater than BYTES_PER_WORD.
*/
if (flags & SLAB_STORE_USER)
ralign = BYTES_PER_WORD;
if (flags & SLAB_RED_ZONE) {
ralign = REDZONE_ALIGN;
/* If redzoning, ensure that the second redzone is suitably
* aligned, by adjusting the object size accordingly. */
size += REDZONE_ALIGN - 1;
size &= ~(REDZONE_ALIGN - 1);
}
/* 2) arch mandated alignment */
if (ralign < ARCH_SLAB_MINALIGN) {
ralign = ARCH_SLAB_MINALIGN;
}
/* 3) caller mandated alignment */
if (ralign < align) {
ralign = align;
}
/* disable debug if necessary */
if (ralign > __alignof__(unsigned long long))
flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
/*
* 4) Store it.
*/
align = ralign;
/* slab分配器是否已经可用 */
if (slab_is_available())
gfp = GFP_KERNEL;
else
/* slab初始化好之前，不允许阻塞，且只能在低端内存区分配 */
gfp = GFP_NOWAIT;
/* Get cache's description obj. */
/* 获得struct kmem_cache对象 ,为什么能从cache中获得的对象是
kmem_cache结构呢，因为这里的全局变量cache_cache的对象大小
就是kmem_cache结构大小*/
cachep = kmem_cache_zalloc(&cache_cache, gfp);
if (!cachep)
goto oops;
#if DEBUG
cachep->obj_size = size;
/*
* Both debugging options require word-alignment which is calculated
* into align above.
*/
if (flags & SLAB_RED_ZONE) {
/* add space for red zone words */
cachep->obj_offset += sizeof(unsigned long long);
size += 2 * sizeof(unsigned long long);
}
if (flags & SLAB_STORE_USER) {
/* user store requires one word storage behind the end of
* the real object. But if the second red zone needs to be
* aligned to 64 bits, we must allow that much space.
*/
if (flags & SLAB_RED_ZONE)
size += REDZONE_ALIGN;
else
size += BYTES_PER_WORD;
}
#if FORCED_DEBUG && defined(CONFIG_DEBUG_PAGEALLOC)
if (size >= malloc_sizes[INDEX_L3 + 1].cs_size
&& cachep->obj_size > cache_line_size() && size < PAGE_SIZE) {
cachep->obj_offset += PAGE_SIZE - size;
size = PAGE_SIZE;
}
#endif
#endif
/*
* Determine if the slab management is 'on' or 'off' slab.
* (bootstrapping cannot cope with offslab caches so don't do
* it too early on.)
*/
/* 确定slab管理对象的存储方式：内置还是外置
。通常，当对象大于等于512时，使用外置方式
。初始化阶段采用内置式。
slab_early_init 参见kmem_cache_init函数 */
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).
*/
flags |= CFLGS_OFF_SLAB;
size = ALIGN(size, align);
/* 获得slab中碎片的大小 */
left_over = calculate_slab_order(cachep, size, align, flags);
/* cachep->num为该cache中每个slab的对象数，为0，表示为该对象创建cache失败 */
if (!cachep->num) {
printk(KERN_ERR
"kmem_cache_create: couldn't create cache %s.\n", name);
kmem_cache_free(&cache_cache, cachep);
cachep = NULL;
goto oops;
}
/* 计算slab管理对象的大小，包括struct slab对象和kmem_bufctl_t数组 */
slab_size = ALIGN(cachep->num * sizeof(kmem_bufctl_t)
+ sizeof(struct slab), align);
/*
* If the slab has been placed off-slab, and we have enough space then
* move it on-slab. This is at the expense of any extra colouring.
*/
/* 如果这是一个外置式slab，并且碎片大小大于slab管理对象的大小
，则可将slab管理对象移到slab中，改造成一个内置式slab */
if (flags & CFLGS_OFF_SLAB && left_over >= slab_size) {
/* 除去off-slab标志位 */
flags &= ~CFLGS_OFF_SLAB;
/* 更新碎片大小 */
left_over -= slab_size;
}
if (flags & CFLGS_OFF_SLAB) {
/* really off slab. No need for manual alignment */
/* align是针对slab对象的，如果slab管理对象是外置存储
，自然不会像内置那样影响到后面slab对象的存储位置
，也就不需要对齐了 */
slab_size =
cachep->num * sizeof(kmem_bufctl_t) + sizeof(struct slab);
#ifdef CONFIG_PAGE_POISONING
/* If we're going to use the generic kernel_map_pages()
* poisoning, then it's going to smash the contents of
* the redzone and userword anyhow, so switch them off.
*/
if (size % PAGE_SIZE == 0 && flags & SLAB_POISON)
flags &= ~(SLAB_RED_ZONE | SLAB_STORE_USER);
#endif
}
/* cache的着色块的单位大小 */
cachep->colour_off = cache_line_size();
/* Offset must be a multiple of the alignment. */
/* 着色块大小必须是对象要求对齐方式的倍数 */
if (cachep->colour_off < align)
cachep->colour_off = align;
/* 计算碎片区需要多少个着色快 */
cachep->colour = left_over / cachep->colour_off;
/* slab管理对象的大小 */
cachep->slab_size = slab_size;
cachep->flags = flags;
cachep->gfpflags = 0;
if (CONFIG_ZONE_DMA_FLAG && (flags & SLAB_CACHE_DMA))
cachep->gfpflags |= GFP_DMA;
/* slab对象的大小 */
cachep->buffer_size = size;
/* 计算对象在slab中索引时用，参见obj_to_index函数 */
cachep->reciprocal_buffer_size = reciprocal_value(size);
if (flags & CFLGS_OFF_SLAB) {
/* 分配一个slab管理区域对象，保存在slabp_cache中，
这个函数传入的大小为slab_size,也就是分配slab_size大小的cache
,在slab创建的时候如果是外置式，那么需要从分配的这里面
分配出slab对象，剩下的空间放kmem_bufctl_t[]数组，
如果是内置式的slab，此指针为空 */
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));
}
cachep->ctor = ctor;
cachep->name = name;
/* 设置每个cpu上的local cache */
if (setup_cpu_cache(cachep, gfp)) {
__kmem_cache_destroy(cachep);
cachep = NULL;
goto oops;
}
/* cache setup completed, link it into the list */
/* cache创建完毕，将其加入到全局slab cache链表中 */
list_add(&cachep->next, &cache_chain);
oops:
if (!cachep && (flags & SLAB_PANIC))
panic("kmem_cache_create(): failed to create slab `%s'\n",
name);
if (slab_is_available()) {
mutex_unlock(&cache_chain_mutex);
put_online_cpus();
}
return cachep;
}
其中，cache_cache
/* internal cache of cache description objs */
static struct kmem_cache cache_cache = {
.batchcount = 1,
.limit = BOOT_CPUCACHE_ENTRIES,
.shared = 1,
.buffer_size = sizeof(struct kmem_cache),/*大小为cache结构，难怪名称为cache_cache*/
.name = "kmem_cache",
};
二、计算slab碎片大小
/**
* calculate_slab_order - calculate size (page order) of slabs
* @cachep: pointer to the cache that is being created
* @size: size of objects to be created in this cache.
* @align: required alignment for the objects.
* @flags: slab allocation flags
*
* Also calculates the number of objects per slab.
*
* This could be made much more intelligent. For now, try to avoid using
* high order pages for slabs. When the gfp() functions are more friendly
* towards high-order requests, this should be changed.
*/
/*计算slab由几个页面组成，同时计算每个slab中有多少对象*/
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;
for (gfporder = 0; gfporder <= KMALLOC_MAX_ORDER; gfporder++) {
unsigned int num;
size_t remainder;
/* 计算slab中对象数 */
cache_estimate(gfporder, size, align, flags, &remainder, &num);
/* 对象数为0，表示此order下，一个对象都放不下，检查下一order */
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().
*/
/* 创建一个外置式slab时，要相应分配该slab的管理对象
，包含struct slab对象和kmem_bufctl_t数组，分配管理对象的流程就是分配普通对象的流程
，再来看一下分配对象的流程：
kmem_cache_alloc->__cache_alloc-> __do_cache_alloc-> ____cache_alloc-> cache_alloc_refill-> cache_grow-> alloc_slabmgmt-> kmem_cache_alloc_node-> kmem_cache_alloc
可以看出这里可能存在一个循环，循环的关键在于alloc_slabmgmt函数
，当slab管理对象是off-slab方式时，就形成了循环
。那么什么时候slab管理对象会采用外置式slab呢？显然当其管理的slab中对象很多
，从而kmem_bufctl_t数组很大，致使整个管理对象也很大，此时才会形成循环
。故需要对kmem_bufctl_t的数目做限制，下面的算法是很粗略的，既然对象大小为size时
，是外置式slab，那么我们假设管理对象的大小也是size，计算出kmem_bufctl_t数组的大小
，即此大小的kmem_bufctl_t数组一定会造成管理对象是外置式slab。之所以说粗略
，是指数组大小小于这个限制时，也不能确保管理对象一定是内置式slab。但这也不会引发错误
，因为还有一个slab_break_gfp_order阀门来控制每个slab所占页面数，通常其值为1，即每个slab最多两个页面
，外置式slab存放的都是大于512的大对象，所以
slab中不会有太多的大对象，kmem_bufctl_t数组也不会很大，粗略判断一下就足够了。
*/
offslab_limit = size - sizeof(struct slab);
offslab_limit /= sizeof(kmem_bufctl_t);
/* 对象数目大于限制，跳出循环，不再尝试更大的order
，避免slab中对象数目过多
，此时计算的对象数也是有效的，循环一次没什么 */
if (num > offslab_limit)
break;
}
/* Found something acceptable - save it away */
/* 每个slab中的对象数 */
cachep->num = num;
/* slab的order，即由几个页面组成 */
cachep->gfporder = gfporder;
/* slab中剩余空间（碎片）的大小 */
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.
*/
/* SLAB_RECLAIM_ACCOUNT表示此slab所占页面为可回收的
，当内核检测是否有足够的页面满足用户态的需求时
，此类页面将被计算在内，通过调用
kmem_freepages()函数可以释放分配给slab的页框。由于是可回收的
，所以不需要做后面的碎片检测了 */
if (flags & SLAB_RECLAIM_ACCOUNT)
break;
/*
* Large number of objects is good, but very large slabs are
* currently bad for the gfp()s.
*/
/* slab_break_gfp_order为slab所占页面的阀门，超过这个阀门时
，无论碎片大小，都不再检测更高的order了 */
if (gfporder >= slab_break_gfp_order)
break;
/*
* Acceptable internal fragmentation?
*/
/* slab所占页面的大小是碎片大小的8倍以上
，页面利用率较高，可以接受这样的order */
if (left_over * 8 <= (PAGE_SIZE << gfporder))
break;
}
/* 返回碎片大小 */
return left_over;
}
三、查找指定大小cache
/*在general cache中分配一个struct kmem_cache对象。直接调用__find_general_cachep。*/
static struct kmem_cache *kmem_find_general_cachep(size_t size, gfp_t gfpflags)
{
return __find_general_cachep(size, gfpflags);
}
static inline struct kmem_cache *__find_general_cachep(size_t size,
gfp_t gfpflags)
{
struct cache_sizes *csizep = malloc_sizes;
#if DEBUG
/* This happens if someone tries to call
* kmem_cache_create(), or __kmalloc(), before
* the generic caches are initialized.
*/
BUG_ON(malloc_sizes[INDEX_AC].cs_cachep == NULL);
#endif
if (!size)
return ZERO_SIZE_PTR;
/* 找到合适的malloc size */
while (size > csizep->cs_size)
csizep++;
/*
* Really subtle: The last entry with cs->cs_size==ULONG_MAX
* has cs_{dma,}cachep==NULL. Thus no special case
* for large kmalloc calls required.
*/
#ifdef CONFIG_ZONE_DMA
if (unlikely(gfpflags & GFP_DMA))
return csizep->cs_dmacachep;
#endif
/* 返回该大小级别的cache */
return csizep->cs_cachep;
}
四、设置CPU本地cache
/*配置local cache和slab三链。*/
static int __init_refok setup_cpu_cache(struct kmem_cache *cachep, gfp_t gfp)
{
/* general cache初始化完毕，配置每个cpu的local cache */
if (g_cpucache_up == FULL)
return enable_cpucache(cachep, gfp);
/* 此时处于系统初始化阶段，g_cpucache_up记录general cache初始化的进度
，比如PARTIAL_AC表示struct array_cache所在的cache已经创建，
PARTIAL_L3表示struct kmem_list3所在的cache已经创建
，注意创建这两个cache的先后顺序
。在初始化阶段只需配置主cpu的local cache和slab三链 */
if (g_cpucache_up == NONE) {
/*
* Note: the first kmem_cache_create must create the cache
* that's used by kmalloc(24), otherwise the creation of
* further caches will BUG().
*/
/* 初始化阶段创建struct array_cache所在cache时进入这个流程
，此时struct array_cache所在的general cache还未创建
，只能使用静态分配的全局变量initarray_generic表示的local cache */
cachep->array[smp_processor_id()] = &initarray_generic.cache;
/*
* If the cache that's used by kmalloc(sizeof(kmem_list3)) is
* the first cache, then we need to set up all its list3s,
* otherwise the creation of further caches will BUG().
*/
/* 创建struct kmem_list3所在的cache是在struct array_cache所在cache之后
，所以此时struct kmem_list3所在的
cache也一定没有创建，也需要使用全局变量 */
set_up_list3s(cachep, SIZE_AC);
/* 执行到这struct array_cache所在的cache创建完毕
，如果struct kmem_list3和struct array_cache位于同一个general cache中
，不会再重复创建了
，g_cpucache_up表示的进度更进一步 */
if (INDEX_AC == INDEX_L3)
g_cpucache_up = PARTIAL_L3;
else
g_cpucache_up = PARTIAL_AC;
} else {
/* g_cpucache_up至少为PARTIAL_AC时进入这个流程，struct array_cache所在的
general cache已经建立起来，可以通过kmalloc分配了 */
cachep->array[smp_processor_id()] =
kmalloc(sizeof(struct arraycache_init), gfp);
if (g_cpucache_up == PARTIAL_AC) {
/* struct kmem_list3所在cache仍未创建完毕，还需使用全局的slab三链 */
set_up_list3s(cachep, SIZE_L3);
/* 后面将会分析kmem_cache_init函数，只有创建struct kmem_list3所在
cache时才会进入此流程，上面的代码执行完，struct kmem_list3所在
cache也就创建完毕可以使用了，更新g_cpucache_up */
g_cpucache_up = PARTIAL_L3;
} else {
int node;
for_each_online_node(node) {
cachep->nodelists[node] =/* 通过kmalloc分配struct kmem_list3对象 */
kmalloc_node(sizeof(struct kmem_list3),
gfp, node);
BUG_ON(!cachep->nodelists[node]);
/* 初始化slab三链 */
kmem_list3_init(cachep->nodelists[node]);
}
}
}
/* 设置回收时间 */
cachep->nodelists[numa_node_id()]->next_reap =
jiffies + REAPTIMEOUT_LIST3 +
((unsigned long)cachep) % REAPTIMEOUT_LIST3;
cpu_cache_get(cachep)->avail = 0;
cpu_cache_get(cachep)->limit = BOOT_CPUCACHE_ENTRIES;
cpu_cache_get(cachep)->batchcount = 1;
cpu_cache_get(cachep)->touched = 0;
cachep->batchcount = 1;
cachep->limit = BOOT_CPUCACHE_ENTRIES;
return 0;
}