cache_estimate()函数分析

 

/*
 * Calculate the number of objects and left-over bytes for a given buffer size.
 */
/*内核提供了计算一个Slab缓存的大小的函数，Slab缓存的内存布局分成以下两种： *1. Slab缓存管理区在Slab内，即同对象存放在一起。 *2. Slab缓存管理区存放在单独的一片区域，即同对象分别存放。 *通过宏CFLGS_OFF_SLAB来区分。
 *参数介绍： * gfporder: Slab缓存大小为2^gfporder个页面 * buffer_size： 每个对象的大小 * align：对齐 * flags：CFLGS_OFF_SLAB表示Slab管理区在缓存外，否则表示在缓存内。 * left_over：输出参数，在一个Slab中被浪费的大小。 * num： 输出参数，一个Slab中，对象的数量。
 */
static void cache_estimate(unsigned long gfporder, size_t buffer_size,
      size_t align, int flags, size_t *left_over,
      unsigned int *num)
{
 int nr_objs;
 size_t mgmt_size;
 size_t slab_size = PAGE_SIZE << gfporder;
 /*
  * The slab management structure can be either off the slab or
  * on it. For the latter case, the memory allocated for a
  * slab is used for:
  *
  * - The struct slab
  * - One kmem_bufctl_t for each object
  * - Padding to respect alignment of @align
  * - @buffer_size bytes for each object
  *
  * If the slab management structure is off the slab, then the
  * alignment will already be calculated into the size. Because
  * the slabs are all pages aligned, the objects will be at the
  * correct alignment when allocated.
  */
 /*如果slab描述符在外部，那么对象数为slab所占内存页长度除以对象大小*/
 if (flags & CFLGS_OFF_SLAB) {
  mgmt_size = 0;
  nr_objs = slab_size / buffer_size;
 /*如果在一个Slab中对象数量，超过一定数量，将对象数重置为最大值*/
 if (nr_objs > SLAB_LIMIT)
   nr_objs = SLAB_LIMIT;
 } else {
  /*
   * Ignore padding for the initial guess. The padding
   * is at most @align-1 bytes, and @buffer_size is at
   * least @align. In the worst case, this result will
   * be one greater than the number of objects that fit
   * into the memory allocation when taking the padding
   * into account.
   */
  /*如果slab描述符在内部，那么对象数为slab所占内存页减去
    slab描述符的大小，然后除以对象大小加上每个对象的偏移量*/
  nr_objs = (slab_size - sizeof(struct slab)) /
     (buffer_size + sizeof(kmem_bufctl_t));
  /*
   * This calculated number will be either the right
   * amount, or one greater than what we want.
   */
  /*调用slab_mgmt_size()调整slab中第一个对象的地址对其align，
    然后如果调整后的长度超过slab所占内存页的大小，减少slab中
    对象的个数*/
  if (slab_mgmt_size(nr_objs, align) + nr_objs*buffer_size
         > slab_size)
   nr_objs--;
  if (nr_objs > SLAB_LIMIT)
   nr_objs = SLAB_LIMIT;
  /*再次调整slab中第一个对象的地址对其align*/
  mgmt_size = slab_mgmt_size(nr_objs, align);
 }
 *num = nr_objs;
 *left_over = slab_size - nr_objs*buffer_size - mgmt_size;
}