iOS —— malloc分析(1)

一、malloc_zone_t 分析

这个家伙是一个非常重要的家伙，我们先来看看 malloc_zone_t 的结构

typedef struct _malloc_zone_t {
void    *reserved1;    /* RESERVED FOR CFAllocator DO NOT USE */
void    *reserved2;    /* RESERVED FOR CFAllocator DO NOT USE */

    size_t     (* MALLOC_ZONE_FN_PTR(size))(struct _malloc_zone_t *zone, const void *ptr); /* returns the size of a block or 0 if not in this zone; must be fast, especially for negative answers */
void     *(* MALLOC_ZONE_FN_PTR(malloc))(struct _malloc_zone_t *zone, size_t size);
void     *(* MALLOC_ZONE_FN_PTR(calloc))(struct _malloc_zone_t *zone, size_t num_items, size_t size); /* same as malloc, but block returned is set to zero */
void     *(* MALLOC_ZONE_FN_PTR(valloc))(struct _malloc_zone_t *zone, size_t size); /* same as malloc, but block returned is set to zero and is guaranteed to be page aligned */
void     (* MALLOC_ZONE_FN_PTR(free))(struct _malloc_zone_t *zone, void *ptr);
void     *(* MALLOC_ZONE_FN_PTR(realloc))(struct _malloc_zone_t *zone, void *ptr, size_t size);
void     (* MALLOC_ZONE_FN_PTR(destroy))(struct _malloc_zone_t *zone);
const char    *zone_name;

unsigned    (* MALLOC_ZONE_FN_PTR(batch_malloc))(struct _malloc_zone_t *zone, size_t size, void **results, unsigned num_requested);

struct malloc_introspection_t    * MALLOC_INTROSPECT_TBL_PTR(introspect);
unsigned    version;

void *(* MALLOC_ZONE_FN_PTR(memalign))(struct _malloc_zone_t *zone, size_t alignment, size_t size);

void (* MALLOC_ZONE_FN_PTR(free_definite_size))(struct _malloc_zone_t *zone, void *ptr, size_t size);


    size_t     (* MALLOC_ZONE_FN_PTR(pressure_relief))(struct _malloc_zone_t *zone, size_t goal);


    boolean_t (* MALLOC_ZONE_FN_PTR(claimed_address))(struct _malloc_zone_t *zone, void *ptr);


} malloc_zone_t;

void * calloc(size_t num_items, size_t size)

{
void *retval;

    retval = malloc_zone_calloc(default_zone, num_items, size);
if (retval == NULL) {

        errno = ENOMEM;

    }
return retval;

}

void * malloc_zone_calloc(malloc_zone_t *zone, size_t num_items, size_t size)

{
MALLOC_TRACE(TRACE_calloc | DBG_FUNC_START, (uintptr_t)zone, num_items, size, 0);

void *ptr;
if (malloc_check_start && (malloc_check_counter++ >= malloc_check_start)) {
internal_check();

    }


    ptr = zone->calloc(zone, num_items, size);

if (malloc_logger) {
malloc_logger(MALLOC_LOG_TYPE_ALLOCATE | MALLOC_LOG_TYPE_HAS_ZONE | MALLOC_LOG_TYPE_CLEARED, (uintptr_t)zone,

                (uintptr_t)(num_items * size), 0, (uintptr_t)ptr, 0);

    }

MALLOC_TRACE(TRACE_calloc | DBG_FUNC_END, (uintptr_t)zone, num_items, size, (uintptr_t)ptr);
return ptr;

}

static virtual_default_zone_t virtual_default_zone
__attribute__((section("__DATA,__v_zone")))
__attribute__((aligned(PAGE_MAX_SIZE))) = {
NULL,
NULL,

    default_zone_size,

    default_zone_malloc,

    default_zone_calloc,

    default_zone_valloc,

    default_zone_free,

    default_zone_realloc,

    default_zone_destroy,

    DEFAULT_MALLOC_ZONE_STRING,

    default_zone_batch_malloc,

    default_zone_batch_free,
&default_zone_introspect,
10,

    default_zone_memalign,

    default_zone_free_definite_size,

    default_zone_pressure_relief,

    default_zone_malloc_claimed_address,

};

• 从上面的结构可以看出 defaultzone->calloc 实际的函数实现为 default_zone_calloc。

static void *
default_zone_calloc(malloc_zone_t *zone, size_t num_items, size_t size)

{

    zone = runtime_default_zone();

return zone->calloc(zone, num_items, size);

}

• 引导创建真正的 zone
• 使用真正的 zone 进行 calloc

2.4 zone分析

在创建正在的 zone时，其实系统是有对应的一套创建策略的。在跟踪 runtime_default_zone 方法后，最终会进入如下调用

static void
_malloc_initialize(void *context __unused)

{

    ...... - 省略多余代码
//创建helper_zone,

    malloc_zone_t *helper_zone = create_scalable_zone(0, malloc_debug_flags);
//创建 nano zone
if (_malloc_engaged_nano == NANO_V2) {

    zone = nanov2_create_zone(helper_zone, malloc_debug_flags);

    } else if (_malloc_engaged_nano == NANO_V1) {

    zone = nano_create_zone(helper_zone, malloc_debug_flags);

    }
//如果上面的if else if 成立，这进入 nonazone
if (zone) {
malloc_zone_register_while_locked(zone);
malloc_zone_register_while_locked(helper_zone);

// Must call malloc_set_zone_name() *after* helper and nano are hooked together.
malloc_set_zone_name(zone, DEFAULT_MALLOC_ZONE_STRING);
malloc_set_zone_name(helper_zone, MALLOC_HELPER_ZONE_STRING);

    } else {
//使用helper_zone分配内存

    zone = helper_zone;
malloc_zone_register_while_locked(zone);
malloc_set_zone_name(zone, DEFAULT_MALLOC_ZONE_STRING);

    }
//缓存default_zone

    initial_default_zone = zone;

    .....    

}

typedef struct nano_meta_s {

 OSQueueHead            slot_LIFO MALLOC_NANO_CACHE_ALIGN;
unsigned int        slot_madvised_log_page_count;
volatile uintptr_t        slot_current_base_addr;
volatile uintptr_t        slot_limit_addr;
volatile size_t        slot_objects_mapped;
volatile size_t        slot_objects_skipped;

    bitarray_t            slot_madvised_pages;
// position on cache line distinct from that of slot_LIFO
volatile uintptr_t        slot_bump_addr MALLOC_NANO_CACHE_ALIGN;
volatile boolean_t        slot_exhausted;
unsigned int        slot_bytes;
unsigned int        slot_objects;

} *nano_meta_admin_t;

// vm_allocate()'d, so page-aligned to begin with.
typedef struct nanozone_s {
// first page will be given read-only protection

    malloc_zone_t        basic_zone;

    uint8_t            pad[PAGE_MAX_SIZE - sizeof(malloc_zone_t)];

// remainder of structure is R/W (contains no function pointers)
// page-aligned
// max: NANO_MAG_SIZE cores x NANO_SLOT_SIZE slots for nano blocks {16 .. 256}
//以Mag、Slot为维度，维护申请的band内存部分 slot 的范围为 1~16
struct nano_meta_s        meta_data[NANO_MAG_SIZE][NANO_SLOT_SIZE];//

    _malloc_lock_s            band_resupply_lock[NANO_MAG_SIZE];

    uintptr_t           band_max_mapped_baseaddr[NANO_MAG_SIZE];

    size_t            core_mapped_size[NANO_MAG_SIZE];
unsigned            debug_flags;

    uintptr_t            cookie;

    malloc_zone_t        *helper_zone;

} nanozone_t;

• nanozone_t 同样是 malloc_zone_t 类型。在nano_create_zone 函数内部会完成对 calloc等函数的重新赋值。

2.6  nano_create_zone 分析

malloc_zone_t *
nano_create_zone(malloc_zone_t *helper_zone, unsigned debug_flags)

{

    nanozone_t *nanozone;
int i, j;
//构造nano zone
/* Note: It is important that nano_create_zone resets _malloc_engaged_nano

     * if it is unable to enable the nanozone (and chooses not to abort). As

     * several functions rely on _malloc_engaged_nano to determine if they

     * should manipulate the nanozone, and these should not run if we failed

     * to create the zone.

     */
//     MALLOC_ASSERT(_malloc_engaged_nano == NANO_V1);

/* get memory for the zone. */

    nanozone = nano_common_allocate_based_pages(NANOZONE_PAGED_SIZE, 0, 0, VM_MEMORY_MALLOC, 0);
if (!nanozone) {

        _malloc_engaged_nano = NANO_NONE;
return NULL;

    }
//构造对zone 的一些函数进行重新赋值
/* set up the basic_zone portion of the nanozone structure */

    nanozone->basic_zone.version = 10;

    nanozone->basic_zone.size = (void *)nano_size;

    nanozone->basic_zone.malloc = (debug_flags & MALLOC_DO_SCRIBBLE) ? (void *)nano_malloc_scribble : (void *)nano_malloc;

    nanozone->basic_zone.calloc = (void *)nano_calloc;

    nanozone->basic_zone.valloc = (void *)nano_valloc;

    nanozone->basic_zone.free = (debug_flags & MALLOC_DO_SCRIBBLE) ? (void *)nano_free_scribble : (void *)nano_free;

    nanozone->basic_zone.realloc = (void *)nano_realloc;

    nanozone->basic_zone.destroy = (void *)nano_destroy;

    nanozone->basic_zone.batch_malloc = (void *)nano_batch_malloc;

    nanozone->basic_zone.batch_free = (void *)nano_batch_free;

    nanozone->basic_zone.introspect = (struct malloc_introspection_t *)&nano_introspect;

    nanozone->basic_zone.memalign = (void *)nano_memalign;

    nanozone->basic_zone.free_definite_size = (debug_flags & MALLOC_DO_SCRIBBLE) ? (void *)nano_free_definite_size_scribble
: (void *)nano_free_definite_size;


    nanozone->basic_zone.pressure_relief = (void *)nano_pressure_relief;

    nanozone->basic_zone.claimed_address = (void *)nano_claimed_address;


    nanozone->basic_zone.reserved1 = 0; /* Set to zero once and for all as required by CFAllocator. */

    nanozone->basic_zone.reserved2 = 0; /* Set to zero once and for all as required by CFAllocator. */

mprotect(nanozone, sizeof(nanozone->basic_zone), PROT_READ); /* Prevent overwriting the function pointers in basic_zone. */

/* Nano zone does not support MALLOC_ADD_GUARD_PAGES. */
if (debug_flags & MALLOC_ADD_GUARD_PAGES) {
malloc_report(ASL_LEVEL_INFO, "nano zone does not support guard pages\n");

        debug_flags &= ~MALLOC_ADD_GUARD_PAGES;

    }

/* set up the remainder of the nanozone structure */

    nanozone->debug_flags = debug_flags;

if (phys_ncpus > sizeof(nanozone->core_mapped_size) /
sizeof(nanozone->core_mapped_size[0])) {
MALLOC_REPORT_FATAL_ERROR(phys_ncpus,
"nanozone abandoned because NCPUS > max magazines.\n");

    }

/* Initialize slot queue heads and resupply locks. */

    OSQueueHead q0 = OS_ATOMIC_QUEUE_INIT;
for (i = 0; i < nano_common_max_magazines; ++i) {
_malloc_lock_init(&nanozone->band_resupply_lock[i]);

for (j = 0; j < NANO_SLOT_SIZE; ++j) {

            nanozone->meta_data[i][j].slot_LIFO = q0;

        }

    }

/* Initialize the security token. */

    nanozone->cookie = (uintptr_t)malloc_entropy[0] & 0x0000ffffffff0000ULL; // scramble central 32bits with this cookie


    nanozone->helper_zone = helper_zone;

return (malloc_zone_t *)nanozone;

}

static void *
nano_calloc(nanozone_t *nanozone, size_t num_items, size_t size)

{

    size_t total_bytes;

if (calloc_get_size(num_items, size, 0, &total_bytes)) {
return NULL;

    }
// 如果要开辟的空间小于 NANO_MAX_SIZE 则进行nanozone_t的malloc。
if (total_bytes <= NANO_MAX_SIZE) {
void *p = _nano_malloc_check_clear(nanozone, total_bytes, 1);
if (p) {
return p;

        } else {
/* FALLTHROUGH to helper zone */

        }

    }
//否则就进行helper_zone的流程

    malloc_zone_t *zone = (malloc_zone_t *)(nanozone->helper_zone);
return zone->calloc(zone, 1, total_bytes);

}

static void *
_nano_malloc_check_clear(nanozone_t *nanozone, size_t size, boolean_t cleared_requested)

{
MALLOC_TRACE(TRACE_nano_malloc, (uintptr_t)nanozone, size, cleared_requested, 0);

void *ptr;

    size_t slot_key;
// 获取16字节对齐之后的大小,slot_key非常关键，为slot_bytes/16的值，也是数组的二维下下标

    size_t slot_bytes = segregated_size_to_fit(nanozone, size, &slot_key); // Note slot_key is set here
//根据_os_cpu_number经过运算获取 mag_index(meta_data的一维索引)

    mag_index_t mag_index = nano_mag_index(nanozone);
//确定当前cpu对应的mag和通过size参数计算出来的slot，去对应metadata的链表中取已经被释放过的内存区块缓存

    nano_meta_admin_t pMeta = &(nanozone->meta_data[mag_index][slot_key]);
//检测是否存在已经释放过，可以直接拿来用的内存,已经被释放的内存会缓存在 chained_block_s 链表
//每一次free。同样会根据 index 和slot 的值回去 pMeta，然后把slot_LIFO的指针指向释放的内存。

    ptr = OSAtomicDequeue(&(pMeta->slot_LIFO), offsetof(struct chained_block_s, next));
if (ptr) {


    ...省略无关代码

//如果缓存的内存存在，这进行指针地址检查等异常检测，最后返回
//第一次调用malloc时，不会执行这一块代码。

    } else {
//没有释放过的内存，所以调用函数 获取内存

        ptr = segregated_next_block(nanozone, pMeta, slot_bytes, mag_index);

    }

if (cleared_requested && ptr) {
memset(ptr, 0, slot_bytes); // TODO: Needs a memory barrier after memset to ensure zeroes land first?

    }
return ptr;

}

static MALLOC_INLINE void *
segregated_next_block(nanozone_t *nanozone, nano_meta_admin_t pMeta, size_t slot_bytes, unsigned int mag_index)

{
while (1) {
//当前这块pMeta可用内存的结束地址

        uintptr_t theLimit = pMeta->slot_limit_addr; // Capture the slot limit that bounds slot_bump_addr right now
//原子的为pMeta->slot_bump_addr添加slot_bytes的长度，偏移到下一个地址

        uintptr_t b = OSAtomicAdd64Barrier(slot_bytes, (volatile int64_t *)&(pMeta->slot_bump_addr));
//减去添加的偏移量，获取当前可以获取的地址

        b -= slot_bytes; // Atomic op returned addr of *next* free block. Subtract to get addr for *this* allocation.

if (b < theLimit) {   // Did we stay within the bound of the present slot allocation?
//如果地址还在范围之内，则返回地址
return (void *)b; // Yep, so the slot_bump_addr this thread incremented is good to go

        } else {
//已经用尽了
if (pMeta->slot_exhausted) { // exhausted all the bands availble for this slot?

                pMeta->slot_bump_addr = theLimit;
return 0;                 // We're toast

            } else {
// One thread will grow the heap, others will see its been grown and retry allocation
_malloc_lock_lock(&nanozone->band_resupply_lock[mag_index]);
// re-check state now that we've taken the lock
//多线程的缘故，重新检查是否用尽
if (pMeta->slot_exhausted) {
_malloc_lock_unlock(&nanozone->band_resupply_lock[mag_index]);
return 0; // Toast

                } else if (b < pMeta->slot_limit_addr) {
//如果小于最大限制地址，当重新申请一个新的band后，重新尝试while
_malloc_lock_unlock(&nanozone->band_resupply_lock[mag_index]);
continue; // ... the slot was successfully grown by first-taker (not us). Now try again.

                } else if (segregated_band_grow(nanozone, pMeta, slot_bytes, mag_index)) {
//申请新的band成功，重新尝试while
_malloc_lock_unlock(&nanozone->band_resupply_lock[mag_index]);
continue; // ... the slot has been successfully grown by us. Now try again.

                } else {

                    pMeta->slot_exhausted = TRUE;

                    pMeta->slot_bump_addr = theLimit;
_malloc_lock_unlock(&nanozone->band_resupply_lock[mag_index]);
return 0;

                }

            }

        }

    }

}