text: #ifndef DM_STATS_H
  #define DM_STATS_H
  
  #include <linux/types.h>
  #include <linux/mutex.h>
  #include <linux/list.h>
  
  int dm_statistics_init(void);
  void dm_statistics_exit(void);
  
  struct dm_stats {
  	struct mutex mutex;
  	struct list_head list;	/* list of struct dm_stat */
  	struct dm_stats_last_position __percpu *last;
  	sector_t last_sector;
  	unsigned last_rw;
  };
  
  struct dm_stats_aux {
  	bool merged;
  	unsigned long long duration_ns;
  };
  
  void dm_stats_init(struct dm_stats *st);
  void dm_stats_cleanup(struct dm_stats *st);
  
  struct mapped_device;
  
  int dm_stats_message(struct mapped_device *md, unsigned argc, char **argv,
  		     char *result, unsigned maxlen);
  
  void dm_stats_account_io(struct dm_stats *stats, unsigned long bi_rw,
  			 sector_t bi_sector, unsigned bi_sectors, bool end,
  			 unsigned long duration_jiffies,
  			 struct dm_stats_aux *aux);
  
  static inline bool dm_stats_used(struct dm_stats *st)
  {
  	return !list_empty(&st->list);
  }
  
  #endif
  