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