text: #include <linux/module.h>
  #include <linux/scatterlist.h>
  #include <linux/mempool.h>
  #include <linux/slab.h>
  
  #define SG_MEMPOOL_NR		ARRAY_SIZE(sg_pools)
  #define SG_MEMPOOL_SIZE		2
  
  struct sg_pool {
  	size_t		size;
  	char		*name;
  	struct kmem_cache	*slab;
  	mempool_t	*pool;
  };
  
  #define SP(x) { .size = x, "sgpool-" __stringify(x) }
  #if (SG_CHUNK_SIZE < 32)
  #error SG_CHUNK_SIZE is too small (must be 32 or greater)
  #endif
  static struct sg_pool sg_pools[] = {
  	SP(8),
  	SP(16),
  #if (SG_CHUNK_SIZE > 32)
  	SP(32),
  #if (SG_CHUNK_SIZE > 64)
  	SP(64),
  #if (SG_CHUNK_SIZE > 128)
  	SP(128),
  #if (SG_CHUNK_SIZE > 256)
  #error SG_CHUNK_SIZE is too large (256 MAX)
  #endif
  #endif
  #endif
  #endif
  	SP(SG_CHUNK_SIZE)
  };
  #undef SP
  
  static inline unsigned int sg_pool_index(unsigned short nents)
  {
  	unsigned int index;
  
  	BUG_ON(nents > SG_CHUNK_SIZE);
  
  	if (nents <= 8)
  		index = 0;
  	else
  		index = get_count_order(nents) - 3;
  
  	return index;
  }
  
  static void sg_pool_free(struct scatterlist *sgl, unsigned int nents)
  {
  	struct sg_pool *sgp;
  
  	sgp = sg_pools + sg_pool_index(nents);
  	mempool_free(sgl, sgp->pool);
  }
  
  static struct scatterlist *sg_pool_alloc(unsigned int nents, gfp_t gfp_mask)
  {
  	struct sg_pool *sgp;
  
  	sgp = sg_pools + sg_pool_index(nents);
  	return mempool_alloc(sgp->pool, gfp_mask);
  }
  
  /**
   * sg_free_table_chained - Free a previously mapped sg table
   * @table:	The sg table header to use
   * @first_chunk: was first_chunk not NULL in sg_alloc_table_chained?
   *
   *  Description:
   *    Free an sg table previously allocated and setup with
   *    sg_alloc_table_chained().
   *
   **/
  void sg_free_table_chained(struct sg_table *table, bool first_chunk)
  {
  	if (first_chunk && table->orig_nents <= SG_CHUNK_SIZE)
  		return;
  	__sg_free_table(table, SG_CHUNK_SIZE, first_chunk, sg_pool_free);
  }
  EXPORT_SYMBOL_GPL(sg_free_table_chained);
  
  /**
   * sg_alloc_table_chained - Allocate and chain SGLs in an sg table
   * @table:	The sg table header to use
   * @nents:	Number of entries in sg list
   * @first_chunk: first SGL
   *
   *  Description:
   *    Allocate and chain SGLs in an sg table. If @nents@ is larger than
   *    SG_CHUNK_SIZE a chained sg table will be setup.
   *
   **/
  int sg_alloc_table_chained(struct sg_table *table, int nents,
  		struct scatterlist *first_chunk)
  {
  	int ret;
  
  	BUG_ON(!nents);
  
  	if (first_chunk) {
  		if (nents <= SG_CHUNK_SIZE) {
  			table->nents = table->orig_nents = nents;
  			sg_init_table(table->sgl, nents);
  			return 0;
  		}
  	}
  
  	ret = __sg_alloc_table(table, nents, SG_CHUNK_SIZE,
  			       first_chunk, GFP_ATOMIC, sg_pool_alloc);
  	if (unlikely(ret))
  		sg_free_table_chained(table, (bool)first_chunk);
  	return ret;
  }
  EXPORT_SYMBOL_GPL(sg_alloc_table_chained);
  
  static __init int sg_pool_init(void)
  {
  	int i;
  
  	for (i = 0; i < SG_MEMPOOL_NR; i++) {
  		struct sg_pool *sgp = sg_pools + i;
  		int size = sgp->size * sizeof(struct scatterlist);
  
  		sgp->slab = kmem_cache_create(sgp->name, size, 0,
  				SLAB_HWCACHE_ALIGN, NULL);
  		if (!sgp->slab) {
  			printk(KERN_ERR "SG_POOL: can't init sg slab %s\n",
  					sgp->name);
  			goto cleanup_sdb;
  		}
  
  		sgp->pool = mempool_create_slab_pool(SG_MEMPOOL_SIZE,
  						     sgp->slab);
  		if (!sgp->pool) {
  			printk(KERN_ERR "SG_POOL: can't init sg mempool %s\n",
  					sgp->name);
  			goto cleanup_sdb;
  		}
  	}
  
  	return 0;
  
  cleanup_sdb:
  	for (i = 0; i < SG_MEMPOOL_NR; i++) {
  		struct sg_pool *sgp = sg_pools + i;
  
  		mempool_destroy(sgp->pool);
  		kmem_cache_destroy(sgp->slab);
  	}
  
  	return -ENOMEM;
  }
  
  static __exit void sg_pool_exit(void)
  {
  	int i;
  
  	for (i = 0; i < SG_MEMPOOL_NR; i++) {
  		struct sg_pool *sgp = sg_pools + i;
  		mempool_destroy(sgp->pool);
  		kmem_cache_destroy(sgp->slab);
  	}
  }
  
  module_init(sg_pool_init);
  module_exit(sg_pool_exit);
  