Kernel-4.18.0-80.el8_dm-io

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.