dm: add target num_write_bios fn

Add a num_write_bios function to struct target.

If an instance of a target sets this, it will be queried before the
target's mapping function is called on a write bio, and the response
controls the number of copies of the write bio that the target will
receive.

This provides a convenient way for a target to send the same data to
more than one device.  The new cache target uses this in writethrough
mode, to send the data both to the cache and the backing device.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 15 insertions, 7 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index e417cf0a69e..7e469260fe5 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1152,15 +1152,23 @@ static void __clone_and_map_data_bio(struct clone_info *ci, struct dm_target *ti
 {
 	struct bio *bio = ci->bio;
 	struct dm_target_io *tio;
+	unsigned target_bio_nr;
+	unsigned num_target_bios = 1;
 
-	tio = alloc_tio(ci, ti, nr_iovecs, 0);
-
-	if (split_bvec)
-		clone_split_bio(tio, bio, sector, idx, offset, len);
-	else
-		clone_bio(tio, bio, sector, idx, bv_count, len);
+	/*
+	 * Does the target want to receive duplicate copies of the bio?
+	 */
+	if (bio_data_dir(bio) == WRITE && ti->num_write_bios)
+		num_target_bios = ti->num_write_bios(ti, bio);
 
-	__map_bio(tio);
+	for (target_bio_nr = 0; target_bio_nr < num_target_bios; target_bio_nr++) {
+		tio = alloc_tio(ci, ti, nr_iovecs, target_bio_nr);
+		if (split_bvec)
+			clone_split_bio(tio, bio, sector, idx, offset, len);
+		else
+			clone_bio(tio, bio, sector, idx, bv_count, len);
+		__map_bio(tio);
+	}
 }
 
 typedef unsigned (*get_num_bios_fn)(struct dm_target *ti);