kcopyd
  ======
  
  Kcopyd provides the ability to copy a range of sectors from one block-device
  to one or more other block-devices, with an asynchronous completion
  notification. It is used by dm-snapshot and dm-mirror.
  
  Users of kcopyd must first create a client and indicate how many memory pages
  to set aside for their copy jobs. This is done with a call to
  kcopyd_client_create().
  
     int kcopyd_client_create(unsigned int num_pages,
                              struct kcopyd_client **result);
  
  To start a copy job, the user must set up io_region structures to describe
  the source and destinations of the copy. Each io_region indicates a
  block-device along with the starting sector and size of the region. The source
  of the copy is given as one io_region structure, and the destinations of the
  copy are given as an array of io_region structures.
  
     struct io_region {
        struct block_device *bdev;
        sector_t sector;
        sector_t count;
     };
  
  To start the copy, the user calls kcopyd_copy(), passing in the client
  pointer, pointers to the source and destination io_regions, the name of a
  completion callback routine, and a pointer to some context data for the copy.
  
     int kcopyd_copy(struct kcopyd_client *kc, struct io_region *from,
                     unsigned int num_dests, struct io_region *dests,
                     unsigned int flags, kcopyd_notify_fn fn, void *context);
  
     typedef void (*kcopyd_notify_fn)(int read_err, unsigned int write_err,
  				    void *context);
  
  When the copy completes, kcopyd will call the user's completion routine,
  passing back the user's context pointer. It will also indicate if a read or
  write error occurred during the copy.
  
  When a user is done with all their copy jobs, they should call
  kcopyd_client_destroy() to delete the kcopyd client, which will release the
  associated memory pages.
  
     void kcopyd_client_destroy(struct kcopyd_client *kc);