dm-io
  =====
  
  Dm-io provides synchronous and asynchronous I/O services. There are three
  types of I/O services available, and each type has a sync and an async
  version.
  
  The user must set up an io_region structure to describe the desired location
  of the I/O. Each io_region indicates a block-device along with the starting
  sector and size of the region.
  
     struct io_region {
        struct block_device *bdev;
        sector_t sector;
        sector_t count;
     };
  
  Dm-io can read from one io_region or write to one or more io_regions. Writes
  to multiple regions are specified by an array of io_region structures.
  
  The first I/O service type takes a list of memory pages as the data buffer for
  the I/O, along with an offset into the first page.
  
     struct page_list {
        struct page_list *next;
        struct page *page;
     };
  
     int dm_io_sync(unsigned int num_regions, struct io_region *where, int rw,
                    struct page_list *pl, unsigned int offset,
                    unsigned long *error_bits);
     int dm_io_async(unsigned int num_regions, struct io_region *where, int rw,
                     struct page_list *pl, unsigned int offset,
                     io_notify_fn fn, void *context);
  
  The second I/O service type takes an array of bio vectors as the data buffer
  for the I/O. This service can be handy if the caller has a pre-assembled bio,
  but wants to direct different portions of the bio to different devices.
  
     int dm_io_sync_bvec(unsigned int num_regions, struct io_region *where,
                         int rw, struct bio_vec *bvec,
                         unsigned long *error_bits);
     int dm_io_async_bvec(unsigned int num_regions, struct io_region *where,
                          int rw, struct bio_vec *bvec,
                          io_notify_fn fn, void *context);
  
  The third I/O service type takes a pointer to a vmalloc'd memory buffer as the
  data buffer for the I/O. This service can be handy if the caller needs to do
  I/O to a large region but doesn't want to allocate a large number of individual
  memory pages.
  
     int dm_io_sync_vm(unsigned int num_regions, struct io_region *where, int rw,
                       void *data, unsigned long *error_bits);
     int dm_io_async_vm(unsigned int num_regions, struct io_region *where, int rw,
                        void *data, io_notify_fn fn, void *context);
  
  Callers of the asynchronous I/O services must include the name of a completion
  callback routine and a pointer to some context data for the I/O.
  
     typedef void (*io_notify_fn)(unsigned long error, void *context);
  
  The "error" parameter in this callback, as well as the "*error" parameter in
  all of the synchronous versions, is a bitset (instead of a simple error value).
  In the case of an write-I/O to multiple regions, this bitset allows dm-io to
  indicate success or failure on each individual region.
  
  Before using any of the dm-io services, the user should call dm_io_get()
  and specify the number of pages they expect to perform I/O on concurrently.
  Dm-io will attempt to resize its mempool to make sure enough pages are
  always available in order to avoid unnecessary waiting while performing I/O.
  
  When the user is finished using the dm-io services, they should call
  dm_io_put() and specify the same number of pages that were given on the
  dm_io_get() call.