函数mem_area_alloc

/********************************************************************//**
Allocates memory from a pool. NOTE: This low-level function should only be
used in mem0mem.*!
@return    own: allocated memory buffer */
UNIV_INTERN
void*
mem_area_alloc(
/*===========*/
    ulint*        psize,    /*!< in: requested size in bytes; for optimum
                space usage, the size should be a power of 2
                minus MEM_AREA_EXTRA_SIZE;
                out: allocated size in bytes (greater than
                or equal to the requested size) */
    mem_pool_t*    pool)    /*!< in: memory pool */
{
    mem_area_t*    area;
    ulint        size;
    ulint        n;
    ibool        ret;

    /* If we are using os allocator just make a simple call
    to malloc */
    if (UNIV_LIKELY(srv_use_sys_malloc)) {
        return(malloc(*psize));
    }

    size = *psize;
    n = ut_2_log(ut_max(size + MEM_AREA_EXTRA_SIZE, MEM_AREA_MIN_SIZE));

    mutex_enter(&(pool->mutex));
    mem_n_threads_inside++;

    ut_a(mem_n_threads_inside == 1);

    area = UT_LIST_GET_FIRST(pool->free_list[n]);

    if (area == NULL) {
        /**
         *详见

　　　　　*pool->free_list[i]找不到内存块的话,尝试从pool->free_list[i+1]中查找
        *如果找不到,一直向上查找
        *
        *如果找到了,从list列表中摘除,并插入到向下的数组元素所在列表中
        *
        *例如：要分配100字节内存,通过计算在pool->free_list[7]中查找 2^7=128 而 2^6=64不符合要求
        *如果pool->free_list[7]的头结点为空,
        *那么从pool->free_list[7+1]中查找并赋值为area,如果找到,将area从pool->free_list[8]中的链表中删除
        *将此area取类型强制转为byte*,然后取(byte*)area+ut_2_exp(7) 即取出area一半的地址
        *再设置size为ut_2_exp(7)
        *并设置为pool->free_list[7]的头结点

        */

        ret = mem_pool_fill_free_list(n, pool);

        if (ret == FALSE) {
            /* Out of memory in memory pool: we try to allocate
            from the operating system with the regular malloc: */

            mem_n_threads_inside--;
            mutex_exit(&(pool->mutex));

            return(ut_malloc(size));
        }
        //此area为合适的内存块
        area = UT_LIST_GET_FIRST(pool->free_list[n]);
    }
    /**
     *判断此area是否是空闲的,false为已使用
     *area->size_and_free & MEM_AREA_FREE
     */

    if (!mem_area_get_free(area)) {
        fprintf(stderr,
            "InnoDB: Error: Removing element from mem pool"
            " free list %lu though the\n"
            "InnoDB: element is not marked free!\n",
            (ulong) n);

        mem_analyze_corruption(area);

        /* Try to analyze a strange assertion failure reported at
        mysql@lists.mysql.com where the free bit IS 1 in the
        hex dump above */

        if (mem_area_get_free(area)) {
            fprintf(stderr,
                "InnoDB: Probably a race condition"
                " because now the area is marked free!\n");
        }

        ut_error;
    }

    if (UT_LIST_GET_LEN(pool->free_list[n]) == 0) {
        fprintf(stderr,
            "InnoDB: Error: Removing element from mem pool"
            " free list %lu\n"
            "InnoDB: though the list length is 0!\n",
            (ulong) n);
        mem_analyze_corruption(area);

        ut_error;
    }

    ut_ad(mem_area_get_size(area) == ut_2_exp(n));
    
    /**
     *设置为已使用状态
     *并从pool->free_list[n]中删除头结点
     */
    mem_area_set_free(area, FALSE);

    UT_LIST_REMOVE(free_list, pool->free_list[n], area);

    pool->reserved += mem_area_get_size(area);

    mem_n_threads_inside--;
    mutex_exit(&(pool->mutex));

    ut_ad(mem_pool_validate(pool));

    *psize = ut_2_exp(n) - MEM_AREA_EXTRA_SIZE;
    UNIV_MEM_ALLOC(MEM_AREA_EXTRA_SIZE + (byte*)area, *psize);

    return((void*)(MEM_AREA_EXTRA_SIZE + ((byte*)area)));
}