apr内存池中疑惑

ovipgdft

1. APR_DECLARE(void) apr_allocator_max_free_set(apr_allocator_t *allocator,
   
2.                                              apr_size_t in_size)
   
3. {
   
4.     apr_uint32_t max_free_index;
   
5.     apr_uint32_t size = (APR_UINT32_TRUNC_CAST)in_size;
   
6. 
   
7. #if APR_HAS_THREADS
   
8.     apr_thread_mutex_t *mutex;
   
9. 
   
10.     mutex = apr_allocator_mutex_get(allocator);
    
11.     if (mutex != NULL)
    
12.         apr_thread_mutex_lock(mutex);
    
13. #endif /* APR_HAS_THREADS */
    
14. 
    
15.     max_free_index = APR_ALIGN(size, BOUNDARY_SIZE) >> BOUNDARY_INDEX;
    
16.     allocator->current_free_index += max_free_index;
    
17.     allocator->current_free_index -= allocator->max_free_index;
    
18.     allocator->max_free_index = max_free_index;
    
19.     if (allocator->current_free_index > max_free_index)
    
20.         allocator->current_free_index = max_free_index;
    
21. 
    
22. #if APR_HAS_THREADS
    
23.     if (mutex != NULL)
    
24.         apr_thread_mutex_unlock(mutex);
    
25. #endif
    
26. }

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看代码看到这个函数的时候不是很明白，请高人指点下
这个函数是设置max_free_index的，这个时候为什么要对current_free_index进行加减操作呢？
current_free_index是指当前内存池中实际存在最大的内存块大小