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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.