目录
一、概述
伙伴系统最小的分配单位是页，这对于较小的内存需求会造成浪费，所以在伙伴系统之上引入了slab。slab是相同大小内存块的集合，在具体设计上，考虑NUMA上node到cpu的层级关系，slub被设计成一个三级缓存的形式【1】：
- 第一个层次，每cpu一个，无锁
- 第二个层次，每node一个，本node，cpu之间的锁
- 第三个层次，每NUMA一个，node之间的锁
和缓存的思想一样，越是下层开销越大，每个级别的cache增加内存分配/释放的性能。
二、SLUB
我们说，slab可以看做是相同大小的小块内存的集合，它的规格由kmem_cache描述。
2.1 数据结构
[include/linux/slub_def.h]
struct kmem_cache {
struct kmem_cache_cpu __percpu *cpu_slab;
/* Used for retriving partial slabs etc */
slab_flags_t flags;
unsigned long min_partial;
unsigned int size; /* The size of an object including meta data */
unsigned int object_size;/* The size of an object without meta data */
unsigned int offset; /* Free pointer offset. */
struct kmem_cache_order_objects oo;
/* Allocation and freeing of slabs */
struct kmem_cache_order_objects max;
struct kmem_cache_order_objects min;
gfp_t allocflags; /* gfp flags to use on each alloc */
int refcount; /* Refcount for slab cache destroy */
void (*ctor)(void *);
unsigned int inuse; /* Offset to metadata */
unsigned int align; /* Alignment */
unsigned int reserved; /* Reserved bytes at the end of slabs */
unsigned int red_left_pad; /* Left redzone padding size */
const char *name; /* Name (only for display!) */
struct list_head list; /* List of slab caches */
unsigned int useroffset; /* Usercopy region offset */
unsigned int usersize; /* Usercopy region size */
struct kmem_cache_node *node[MAX_NUMNODES];
};其中描述slub规格的有：
- object_size 小块内存的实际大小
- size 每个小块内存实际需要的大小
- offset 到空闲指针的偏移
- oo 记录slab占用页面大小(order)和slab obj的数量
- inuse 到metadata的偏移
- align 对齐
- reserved slab 一般不能正好容纳obj，表示剩余的空间
一个slab的简单示意如下：
- 一个slab 占用2^order个页面，容纳nr_obj个obj
- slab使用空闲链表组织,空闲指针如上图void*所示(当然也可以使用占用obj的模式)
下面来看一下slab三层缓存的描述：
struct kmem_cache_cpu {
void **freelist; /* Pointer to next available object */
unsigned long tid; /* Globally unique transaction id */
struct page *page; /* The slab from which we are allocating */
};
struct kmem_cache_node {
spinlock_t list_lock;
unsigned long nr_partial;
struct list_head partial;
};上述结构的关系如下图所示：
- 各级缓存的门限值分别由cpu_partial, min_partial控制
- kmem_cache_node 是一组：node[MAX_NUMNODES]，图中没有画出
2.2 初始化
mm_init->kmem_cache_init
2.2.1 静态建立过程
由于kmem_cache这个管理的slub的结构也要纳入slub的, 其在kmem_cache_init中使用静态构建的bootstrap方式
[mm/slub.c]
void __init kmem_cache_init(void)
{
static __initdata struct kmem_cache boot_kmem_cache, boot_kmem_cache_node;
kmem_cache_node = &boot_kmem_cache_node;
kmem_cache = &boot_kmem_cache;
create_boot_cache(kmem_cache_node, "kmem_cache_node",
sizeof(struct kmem_cache_node), SLAB_HWCACHE_ALIGN, 0, 0);
register_hotmemory_notifier(&slab_memory_callback_nb);
/* Able to allocate the per node structures */
slab_state = PARTIAL;
create_boot_cache(kmem_cache, "kmem_cache",
offsetof(struct kmem_cache, node) +
nr_node_ids * sizeof(struct kmem_cache_node *),
SLAB_HWCACHE_ALIGN, 0, 0);
kmem_cache = bootstrap(&boot_kmem_cache);
kmem_cache_node = bootstrap(&boot_kmem_cache_node);
/* Now we can use the kmem_cache to allocate kmalloc slabs */
setup_kmalloc_cache_index_table();
create_kmalloc_caches(0);
/* Setup random freelists for each cache */
init_freelist_randomization();
cpuhp_setup_state_nocalls(CPUHP_SLUB_DEAD, "slub:dead", NULL,
slub_cpu_dead);
pr_info("SLUB: HWalign=%d, Order=%u-%u, MinObjects=%u, CPUs=%u, Nodes=%d\n",
cache_line_size(),
slub_min_order, slub_max_order, slub_min_objects,
nr_cpu_ids, nr_node_ids);
}create_boot_cache创建struct kmem_cache_node的kmem_cache,其核心是__kmem_cache_create，在这个过程中slub需要计算几个量，这里展示如下：
unsigned long calculate_alignment(unsigned long flags,
unsigned long align, unsigned long size)
{
if (flags & SLAB_HWCACHE_ALIGN) {
unsigned long ralign = cache_line_size();
while (size <= ralign / 2)
ralign /= 2;
align = max(align, ralign);
}
if (align < ARCH_SLAB_MINALIGN)
align = ARCH_SLAB_MINALIGN;
return ALIGN(align, sizeof(void *));
}- calculate_alignment计算slub的对齐属性
- 如果指定了SLAB_HWCACHE_ALIGN，尽可能将obj放入一个cache中，但最终对齐不能小于指定align
- align至少关于word对齐(不能跨越word边界)
static inline int slab_order(int size, int min_objects,
int max_order, int fract_leftover, int reserved)
{
int order;
int rem;
int min_order = slub_min_order;
if (order_objects(min_order, size, reserved) > MAX_OBJS_PER_PAGE)
return get_order(size * MAX_OBJS_PER_PAGE) - 1;
for (order = max(min_order, get_order(min_objects * size + reserved));
order <= max_order; order++) {
unsigned long slab_size = PAGE_SIZE << order;
rem = (slab_size - reserved) % size;
if (rem <= slab_size / fract_leftover)
break;
}
return order;
}- slab_order 计算需要为指定size的obj分配page的数量(以order表示，范围在[slub_min_order, slub_max_order]
- 需要保证不浪费空间，不超过总空间的 1/ fract_leftover(rem/slab_size <= 1 / fract_leftover)
static inline int calculate_order(int size, int reserved)
{
int order;
int min_objects;
int fraction;
int max_objects;
min_objects = slub_min_objects;
if (!min_objects)
min_objects = 4 * (fls(nr_cpu_ids) + 1);
max_objects = order_objects(slub_max_order, size, reserved);
min_objects = min(min_objects, max_objects);
while (min_objects > 1) {
fraction = 16;
while (fraction >= 4) {
order = slab_order(size, min_objects,
slub_max_order, fraction, reserved);
if (order <= slub_max_order)
return order;
fraction /= 2;
}
min_objects--;
}
order = slab_order(size, 1, slub_max_order, 1, reserved);
if (order <= slub_max_order)
return order;
order = slab_order(size, 1, MAX_ORDER, 1, reserved);
if (order < MAX_ORDER)
return order;
return -ENOSYS;
}
- 在min_objects条件下，合理的order数
- 如果object太大，尝试放开(min_objects=1或者slub_max_order=MAX_ORDER)
static int calculate_sizes(struct kmem_cache *s, int forced_order)
{
unsigned long flags = s->flags;
size_t size = s->object_size;
int order;
size = ALIGN(size, sizeof(void *));
s->inuse = size;
size = ALIGN(size, s->align);
s->size = size;
if (forced_order >= 0)
order = forced_order;
else
order = calculate_order(size, s->reserved);
if (order < 0)
return 0;
s->allocflags = 0;
if (order)
s->allocflags |= __GFP_COMP;
if (s->flags & SLAB_CACHE_DMA)
s->allocflags |= GFP_DMA;
if (s->flags & SLAB_RECLAIM_ACCOUNT)
s->allocflags |= __GFP_RECLAIMABLE;
s->oo = oo_make(order, size, s->reserved);
s->min = oo_make(get_order(size), size, s->reserved);
if (oo_objects(s->oo) > oo_objects(s->max))
s->max = s->oo;
return !!oo_objects(s->oo);
}- calculate_sizes计算object实际占用的空间，及需要分配page的数量(2^order)
- 主要区分s->object_size，s->inuse，s->size(obj大小，跨word对齐，s->align对齐)
- s->oo记录slub 最终的order和其包含的object数量
static inline int order_objects(int order, unsigned long size, int reserved)
{
return ((PAGE_SIZE << order) - reserved) / size;
}
static inline struct kmem_cache_order_objects oo_make(int order,
unsigned long size, int reserved)
{
struct kmem_cache_order_objects x = {
(order << OO_SHIFT) + order_objects(order, size, reserved)
};
return x;
}slub的各指标都确定了，现在可以分配空间了，最终就是在底层调用前面计算的order大小的配置，这里主要关注page的一系列操作。先看一下和slub有关的page的结构：
struct page {
unsigned long flags;
union {
};
union {
void *freelist;
};
union {
struct {
unsigned inuse:16;
unsigned objects:15;
unsigned frozen:1;
};
};
union {
struct {
struct page *next;
int pages;
int pobjects;
};
}
union {
struct kmem_cache *slab_cache;
};
};- freelist 空闲链表头
- inuse 当前已使用object数量
- objects slub中object总数
- frozen
- slab_cache
kmem_cache未建立起来之前，使用early_kmem_cache_node_alloc分配kmem_cache_node, 重点看一下过程中分配一个slab的过程。
static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
{
page = alloc_slab_page(s, alloc_gfp, node, oo);
page->objects = oo_objects(oo);
order = compound_order(page);
page->slab_cache = s;
__SetPageSlab(page);
if (page_is_pfmemalloc(page))
SetPageSlabPfmemalloc(page);
start = page_address(page);
shuffle = shuffle_freelist(s, page);
if (!shuffle) {
for_each_object_idx(p, idx, s, start, page->objects) {
setup_object(s, page, p);
if (likely(idx < page->objects))
set_freepointer(s, p, p + s->size);
else
set_freepointer(s, p, NULL);
}
page->freelist = fixup_red_left(s, start);
}
page->inuse = page->objects;
page->frozen = 1;
out:
if (gfpflags_allow_blocking(flags))
local_irq_disable();
if (!page)
return NULL;
mod_lruvec_page_state(page,
(s->flags & SLAB_RECLAIM_ACCOUNT) ?
NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
1 << oo_order(oo));
inc_slabs_node(s, page_to_nid(page), page->objects);
return page;
}- alloc_slab_page分配对于order的page
- 设置page的slab属性：__SetPageSlab(page);
- 初始化freelist链表
一个slab分配后如下图所示：
下面来看上述slub结构是如何管理的：
static void early_kmem_cache_node_alloc(int node)
{
struct page *page;
struct kmem_cache_node *n;
page = new_slab(kmem_cache_node, GFP_NOWAIT, node);
n = page->freelist;
BUG_ON(!n);
page->freelist = get_freepointer(kmem_cache_node, n);
page->inuse = 1;
page->frozen = 0;
kmem_cache_node->node[node] = n;
init_kmem_cache_node(n);
inc_slabs_node(kmem_cache_node, node, page->objects);
/*
* No locks need to be taken here as it has just been
* initialized and there is no concurrent access.
*/
__add_partial(n, page, DEACTIVATE_TO_HEAD);
}- 注意这里是分配struct kmem_cache_node结构，所以kmem_cache_node->node[node] = n; 这句话直接将kmem_cache中的node指向第一个kmem_cache_node
接下来是分配每cpu结构：
static inline int alloc_kmem_cache_cpus(struct kmem_cache *s)
{
s->cpu_slab = __alloc_percpu(sizeof(struct kmem_cache_cpu),
2 * sizeof(void *));
if (!s->cpu_slab)
return 0;
init_kmem_cache_cpus(s);
return 1;
}- 可以看到就是简单的分配并初始化kmem_cache_cpu
至此，静态的"kmem_cache_node"结构就建立了，kmem_cache的状态变为slab_state = PARTIAL，那么接下来的"kmem_cache"的静态分配kmem_cache_node就可以从前面建立的slub中取了！
kmem_cache和kmem_cache_node的kmem_cache就创建完成了，可以调用标准的接口了，所以此时要将kmem_cache和kmem_cache_node纳入标准的kmem_cache管理中，由静态变为动态，先列函数流程：
static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
{
int node;
struct kmem_cache *s = kmem_cache_zalloc(kmem_cache, GFP_NOWAIT);
struct kmem_cache_node *n;
memcpy(s, static_cache, kmem_cache->object_size);
__flush_cpu_slab(s, smp_processor_id());
for_each_kmem_cache_node(s, node, n) {
struct page *p;
list_for_each_entry(p, &n->partial, lru)
p->slab_cache = s;
}
slab_init_memcg_params(s);
list_add(&s->list, &slab_caches);
memcg_link_cache(s);
return s;
}- 首先分配一个kmem_cache的kmem_cache, 通过memcpy将静态转变为动态的，其中分配kmem_cache_zalloc->kmem_cache_alloc->slab_alloc->slab_alloc_node，我们上面已经分析过了
接下来一路通过__flush_cpu_slab将kmem_cache_cpu flush掉，主要函数是：
static void deactivate_slab(struct kmem_cache *s, struct page *page,
void *freelist)
{
enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
struct kmem_cache_node *n = get_node(s, page_to_nid(page));
int lock = 0;
enum slab_modes l = M_NONE, m = M_NONE;
void *nextfree;
int tail = DEACTIVATE_TO_HEAD;
struct page new;
struct page old;
if (page->freelist) {
stat(s, DEACTIVATE_REMOTE_FREES);
tail = DEACTIVATE_TO_TAIL;
}
/*
* Stage one: Free all available per cpu objects back
* to the page freelist while it is still frozen. Leave the
* last one.
*
* There is no need to take the list->lock because the page
* is still frozen.
*/
while (freelist && (nextfree = get_freepointer(s, freelist))) {
void *prior;
unsigned long counters;
do {
prior = page->freelist;
counters = page->counters;
set_freepointer(s, freelist, prior);
new.counters = counters;
new.inuse--;
VM_BUG_ON(!new.frozen);
} while (!__cmpxchg_double_slab(s, page,
prior, counters,
freelist, new.counters,
"drain percpu freelist"));
freelist = nextfree;
}
...
}- 先来看第一阶段，注释写的很明白，将kmem_cache_cpu对应的freelist空闲的obj放入page对应的freelist，最终要剩余一个free的obj，此时frozen=1
下面来看第二阶段:
/*
* Stage two: Ensure that the page is unfrozen while the
* list presence reflects the actual number of objects
* during unfreeze.
*
* We setup the list membership and then perform a cmpxchg
* with the count. If there is a mismatch then the page
* is not unfrozen but the page is on the wrong list.
*
* Then we restart the process which may have to remove
* the page from the list that we just put it on again
* because the number of objects in the slab may have
* changed.
*/
redo:
old.freelist = page->freelist;
old.counters = page->counters;
VM_BUG_ON(!old.frozen);
/* Determine target state of the slab */
new.counters = old.counters;
if (freelist) {
new.inuse--;
set_freepointer(s, freelist, old.freelist);
new.freelist = freelist;
} else
new.freelist = old.freelist;
new.frozen = 0;
if (!new.inuse && n->nr_partial >= s->min_partial)
m = M_FREE;
else if (new.freelist) {
m = M_PARTIAL;
if (!lock) {
lock = 1;
/*
* Taking the spinlock removes the possiblity
* that acquire_slab() will see a slab page that
* is frozen
*/
spin_lock(&n->list_lock);
}
} else {
m = M_FULL;
if (kmem_cache_debug(s) && !lock) {
lock = 1;
/*
* This also ensures that the scanning of full
* slabs from diagnostic functions will not see
* any frozen slabs.
*/
spin_lock(&n->list_lock);
}
}
if (l != m) {
if (l == M_PARTIAL)
remove_partial(n, page);
else if (l == M_FULL)
remove_full(s, n, page);
if (m == M_PARTIAL) {
add_partial(n, page, tail);
stat(s, tail);
} else if (m == M_FULL) {
stat(s, DEACTIVATE_FULL);
add_full(s, n, page);
}
}
l = m;
if (!__cmpxchg_double_slab(s, page,
old.freelist, old.counters,
new.freelist, new.counters,
"unfreezing slab"))
goto redo;
if (lock)
spin_unlock(&n->list_lock);
if (m == M_FREE) {
stat(s, DEACTIVATE_EMPTY);
discard_slab(s, page);
stat(s, FREE_SLAB);
}2.3 API
- struct kmem_cache * kmem_cache_create(const char *name, size_t size, size_t align, unsigned long flags, void (*ctor)(void *))
- void *kmem_cache_alloc(struct kmem_cache *s, gfp_t gfpflags)
- void kmem_cache_free(struct kmem_cache *s, void *x)
- void *kmalloc(size_t size, gfp_t flags)
- void kfree(const void *x)
2.3.1 kmem_cache_create
先来看kmem_cache_create，这个函数调用create_cache->__kmem_cache_create,该流程和上面的流程是一致的，不同的地方体现在init_kmem_cache_nodes会调用kmem_cache_alloc_node：
static int init_kmem_cache_nodes(struct kmem_cache *s)
{
int node;
for_each_node_state(node, N_NORMAL_MEMORY) {
struct kmem_cache_node *n;
if (slab_state == DOWN) {
early_kmem_cache_node_alloc(node);
continue;
}
n = kmem_cache_alloc_node(kmem_cache_node,
GFP_KERNEL, node);
...
}其流程如下：
上述过程和kmem_cache_alloc是一致的，放到下面分析.
2.3.2 kmem_cache_alloc
kmem_cache_alloc申请过程中各级缓存的形态如下图：
先说明一下分配的时候的流程：
- L1 HIT, 即命中本地kmem_cache_cpu(c->freelist != NULL),直接从freelist头取一个，所谓fastpath
- L1 MISS, L2 HIT(c->partial != NULL),在L2中取一个slab挂到L1(c->page)上
- L1 MISS, L2 MISS, L3 HIT(n->partial != NULL),在L3中取一个slab挂到L1(c->page)上，并取数量为cpu_partial / 2为上限个obj挂到L2(c->partial)上
- L1 MISS, L2 MISS, L3 MISS,在伙伴系统中分配一个slab挂到L1(c->page)上
- 伙伴系统也分配失败，OOM
下面结合上面的步骤分析一下kmem_cache_alloc流程：kmem_cache_alloc->slab_alloc->slab_alloc_node
static __always_inline void *slab_alloc_node(struct kmem_cache *s,
gfp_t gfpflags, int node, unsigned long addr)
{
void *object;
struct kmem_cache_cpu *c;
struct page *page;
unsigned long tid;
s = slab_pre_alloc_hook(s, gfpflags);
if (!s)
return NULL;
redo:
do {
tid = this_cpu_read(s->cpu_slab->tid);
c = raw_cpu_ptr(s->cpu_slab);
} while (IS_ENABLED(CONFIG_PREEMPT) &&
unlikely(tid != READ_ONCE(c->tid)));
barrier();
object = c->freelist;
page = c->page;
if (unlikely(!object || !node_match(page, node))) {
object = __slab_alloc(s, gfpflags, node, addr, c);
stat(s, ALLOC_SLOWPATH);
} else {
void *next_object = get_freepointer_safe(s, object);
if (unlikely(!this_cpu_cmpxchg_double(
s->cpu_slab->freelist, s->cpu_slab->tid,
object, tid,
next_object, next_tid(tid)))) {
note_cmpxchg_failure("slab_alloc", s, tid);
goto redo;
}
prefetch_freepointer(s, next_object);
stat(s, ALLOC_FASTPATH);
}
return object;
}
先看本地cache不为空的情况( c->freelist)，即所谓fastpath操作，其实就是一个原子操作，移动freelist指针。
下面看一下slowpath的情况：
static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
unsigned long addr, struct kmem_cache_cpu *c)
{
void *freelist;
struct page *page;
page = c->page;
if (!page)
goto new_slab;
...
load_freelist:
/*
* freelist is pointing to the list of objects to be used.
* page is pointing to the page from which the objects are obtained.
* That page must be frozen for per cpu allocations to work.
*/
VM_BUG_ON(!c->page->frozen);
c->freelist = get_freepointer(s, freelist);
c->tid = next_tid(c->tid);
return freelist;
new_slab:
if (slub_percpu_partial(c)) {
page = c->page = slub_percpu_partial(c);
slub_set_percpu_partial(c, page);
stat(s, CPU_PARTIAL_ALLOC);
goto redo;
}
freelist = new_slab_objects(s, gfpflags, node, &c);
if (unlikely(!freelist)) {
slab_out_of_memory(s, gfpflags, node);
return NULL;
}
page = c->page;
if (likely(!kmem_cache_debug(s) && pfmemalloc_match(page, gfpflags)))
goto load_freelist;
}- c->page为空，说明本地缓存为空，执行new_slab_objects,改函数首先尝试从kmem_cache_node对应的partial链表上去寻找合适的slub，对应get_partial函数，参照上图，这里分析一下acquire_slab
如果cpu partial为空，那么考虑kmem_cache_node，对应new_slab_objects->get_partial
static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
struct kmem_cache_cpu *c)
{
void *object;
int searchnode = node;
if (node == NUMA_NO_NODE)
searchnode = numa_mem_id();
else if (!node_present_pages(node))
searchnode = node_to_mem_node(node);
object = get_partial_node(s, get_node(s, searchnode), c, flags);
if (object || node != NUMA_NO_NODE)
return object;
return get_any_partial(s, flags, c);
}- get_partial先从kmem_cache_node的searchnode的partial链表，如果不满足，按照distance去其他node对应的partial链表中获取。
先来看get_partial_node
static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
struct kmem_cache_cpu *c, gfp_t flags)
{
struct page *page, *page2;
void *object = NULL;
int available = 0;
int objects;
if (!n || !n->nr_partial)
return NULL;
spin_lock(&n->list_lock);
list_for_each_entry_safe(page, page2, &n->partial, lru) {
void *t;
if (!pfmemalloc_match(page, flags))
continue;
t = acquire_slab(s, n, page, object == NULL, &objects);
if (!t)
break;
available += objects;
if (!object) {
c->page = page;
stat(s, ALLOC_FROM_PARTIAL);
object = t;
} else {
put_cpu_partial(s, page, 0);
stat(s, CPU_PARTIAL_NODE);
}
if (!kmem_cache_has_cpu_partial(s)
|| available > s->cpu_partial / 2)
break;
}
spin_unlock(&n->list_lock);
return object;
}get_partial_node描述了kmem_cache_node层次命中后的操作，可以看到主要是：
- 变量partial，acquire_slab获取partial链表上slab空闲的数量，对于第一个获取到的slab，挂到c->page上，即L1 上
- 然后将后续slab挂到c->partial，即L2上，直到满足其数量大于cpu_partial的一半
static inline void *acquire_slab(struct kmem_cache *s,
struct kmem_cache_node *n, struct page *page,
int mode, int *objects)
{
void *freelist;
unsigned long counters;
struct page new;
freelist = page->freelist;
counters = page->counters;
new.counters = counters;
*objects = new.objects - new.inuse;
if (mode) {
new.inuse = page->objects;
new.freelist = NULL;
} else {
new.freelist = freelist;
}
VM_BUG_ON(new.frozen);
new.frozen = 1;
if (!__cmpxchg_double_slab(s, page,
freelist, counters,
new.freelist, new.counters,
"acquire_slab"))
return NULL;
remove_partial(n, page);
return freelist;
}- acquire_slab 主要的操作就是将slub从kmem_cache_node的partial链摘除，这里注意，第一次获取的slab是挂在L1上的，所以要：
new.inuse = page->objects;
new.freelist = NULL;- remove_partial 对应摘链动作
如果L3 miss了，会执行new_slab从伙伴系统中分配：
page = new_slab(s, flags, node);
if (page) {
c = raw_cpu_ptr(s->cpu_slab);
if (c->page)
flush_slab(s, c);
/*
* No other reference to the page yet so we can
* muck around with it freely without cmpxchg
*/
freelist = page->freelist;
page->freelist = NULL;
stat(s, ALLOC_SLAB);
c->page = page;
*pc = c;
} else
freelist = NULL;将申请到的slab，挂在L1上，相应的字段处理一下就好了，具体参照文章开始的框图。如果伙伴系统也失败了，只能OOM了。
综上，如果L1 miss，L2，L3，乃至从伙伴系统找到一个新的slab都要保证下面的状态：
page->freelist = NULL;
c->page = page;这个很容易理解，一个slab不完全是free的，因此freelist指向也是不确定的。最终在下面
load_freelist:
VM_BUG_ON(!c->page->frozen);
c->freelist = get_freepointer(s, freelist); //这里跳过第一个元素，慢速路径返回原freelist就能直接使用了
2.3.3 kmem_cache_free
下面分析kmem_cache_free，其函数调用流程大体如下：
释放的流程，可以参照下图，来自【2】
- 如果slab所在L1， L2， L3 中直接释放
- 如果slab是full，加入L2(cpu partial)链表中，这种情况下要检查L2 obj总数是否超过cpu_partial,如果超过，将L2移动到L3。
- 如果释放obj后slab变成empty且在L3中，如果slab数量nr_partial > min_partial,考虑将slab回收到伙伴系统中。
分段来看__slab_free：
static void __slab_free(struct kmem_cache *s, struct page *page,
void *head, void *tail, int cnt, unsigned long addr)
{
void *prior;
int was_frozen;
struct page new;
unsigned long counters;
struct kmem_cache_node *n = NULL;
unsigned long uninitialized_var(flags);
stat(s, FREE_SLOWPATH);
do {
if (unlikely(n)) {
spin_unlock_irqrestore(&n->list_lock, flags);
n = NULL;
}
prior = page->freelist;
counters = page->counters;
set_freepointer(s, tail, prior);
new.counters = counters;
was_frozen = new.frozen;
new.inuse -= cnt;
if ((!new.inuse || !prior) && !was_frozen) {
if (kmem_cache_has_cpu_partial(s) && !prior) {
new.frozen = 1;
} else { /* Needs to be taken off a list */
n = get_node(s, page_to_nid(page));
spin_lock_irqsave(&n->list_lock, flags);
}
}
} while (!cmpxchg_double_slab(s, page,
prior, counters,
head, new.counters,
"__slab_free"));如果page不在kmem_cache_cpu中，有以下几种情况：
- 在cpu partial链表中，此时frozen=1
- slab是满的(page->freelist == NULL)，不在任何链表中，此时frozen=0
- 在node partial中，此时frozen=0
上面代码片段就是将obj释放到对应的slab中，需要考虑下面的情况：
- 如果slab是满的(!prior)，因为没有在任何链表中，需要将其加入cpu partial中，因此将frozen=1
- 如果slab在node partial链表中，释放n个obj之后为slab变空，需要选择一个node
如果是上述情况1，即在full slab释放n个obj，那么将该slab放入cpu partial中，使用put_cpu_partial，即要考虑是否超过空闲门限cpu_partial：
if (likely(!n)) {
/*
* If we just froze the page then put it onto the
* per cpu partial list.
*/
if (new.frozen && !was_frozen) {
put_cpu_partial(s, page, 1);
stat(s, CPU_PARTIAL_FREE);
}
/*
* The list lock was not taken therefore no list
* activity can be necessary.
*/
if (was_frozen)
stat(s, FREE_FROZEN);
return;
}如果是情况2，需要判断空slab以及是否超过L3门限min_partial而将其释放给buddy allcator，根据slab情况将其从node partial移除，接着使用discard_slab释放：
if (unlikely(!new.inuse && n->nr_partial >= s->min_partial))
goto slab_empty;
slab_empty:
if (prior) {
/*
* Slab on the partial list.
*/
remove_partial(n, page);
stat(s, FREE_REMOVE_PARTIAL);
} else {
/* Slab must be on the full list */
remove_full(s, n, page);
}
spin_unlock_irqrestore(&n->list_lock, flags);
stat(s, FREE_SLAB);
discard_slab(s, page);2.3.4 kmalloc/kfree
kmalloc基于kmem_cache，下面的函数建立各个长度的slab，存储在kmalloc_caches中。
void __init create_kmalloc_caches(unsigned long flags)
{
int i;
for (i = KMALLOC_SHIFT_LOW; i <= KMALLOC_SHIFT_HIGH; i++) {
if (!kmalloc_caches[i])
new_kmalloc_cache(i, flags);
/*
* Caches that are not of the two-to-the-power-of size.
* These have to be created immediately after the
* earlier power of two caches
*/
if (KMALLOC_MIN_SIZE <= 32 && !kmalloc_caches[1] && i == 6)
new_kmalloc_cache(1, flags);
if (KMALLOC_MIN_SIZE <= 64 && !kmalloc_caches[2] && i == 7)
new_kmalloc_cache(2, flags);
}
/* Kmalloc array is now usable */
slab_state = UP;
#ifdef CONFIG_ZONE_DMA
for (i = 0; i <= KMALLOC_SHIFT_HIGH; i++) {
struct kmem_cache *s = kmalloc_caches[i];
if (s) {
int size = kmalloc_size(i);
char *n = kasprintf(GFP_NOWAIT,
"dma-kmalloc-%d", size);
BUG_ON(!n);
kmalloc_dma_caches[i] = create_kmalloc_cache(n,
size, SLAB_CACHE_DMA | flags);
}
}
#endif
}在我的机器上KMALLOC_SHIFT_LOW是3， KMALLOC_SHIFT_HIGH是13，而kmalloc_info如下：
const struct kmalloc_info_struct kmalloc_info[] __initconst = {
{NULL, 0}, {"kmalloc-96", 96},
{"kmalloc-192", 192}, {"kmalloc-8", 8},
{"kmalloc-16", 16}, {"kmalloc-32", 32},
{"kmalloc-64", 64}, {"kmalloc-128", 128},
{"kmalloc-256", 256}, {"kmalloc-512", 512},
{"kmalloc-1024", 1024}, {"kmalloc-2048", 2048},
{"kmalloc-4096", 4096}, {"kmalloc-8192", 8192},
{"kmalloc-16384", 16384}, {"kmalloc-32768", 32768},
{"kmalloc-65536", 65536}, {"kmalloc-131072", 131072},
{"kmalloc-262144", 262144}, {"kmalloc-524288", 524288},
{"kmalloc-1048576", 1048576}, {"kmalloc-2097152", 2097152},
{"kmalloc-4194304", 4194304}, {"kmalloc-8388608", 8388608},
{"kmalloc-16777216", 16777216}, {"kmalloc-33554432", 33554432},
{"kmalloc-67108864", 67108864}
};可见上述创建了表中kmalloc-8~kmalloc-8192，dma-kmalloc-8~dma-kmalloc-8192的slab
顺便看一下kmalloc
static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
if (__builtin_constant_p(size)) {
if (size > KMALLOC_MAX_CACHE_SIZE)
return kmalloc_large(size, flags);
#ifndef CONFIG_SLOB
if (!(flags & GFP_DMA)) {
int index = kmalloc_index(size);
if (!index)
return ZERO_SIZE_PTR;
return kmem_cache_alloc_trace(kmalloc_caches[index],
flags, size);
}
#endif
}
return __kmalloc(size, flags);
}- size 大于8192(4K),使用伙伴系统分配kmalloc_large
- 没有指定GFP_DMA标志，使用kmalloc_index(size)寻找size对应kmalloc_caches中的下标
kfree涉及到的函数上面也都分析过了，略过。
三、参考
【1】多核心Linux内核路径优化的不二法门之-slab与伙伴系统
【2】图解slub
8370
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