dm-snap: use blkdev_issue_flush instead of open coding it

Use blkdev_issue_flush, which uses an on-stack bio instead of an
opencoded version with a bio embedded into struct dm_snapshot.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220124091107.642561-8-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/dm-snap.c

@@ -141,11 +141,6 @@ struct dm_snapshot {
 	 * for them to be committed.
 	 */
 	struct bio_list bios_queued_during_merge;
-
-	/*
-	 * Flush data after merge.
-	 */
-	struct bio flush_bio;
 };
 
 /*
@@ -1127,17 +1122,6 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
 
 static void error_bios(struct bio *bio);
 
-static int flush_data(struct dm_snapshot *s)
-{
-	struct bio *flush_bio = &s->flush_bio;
-
-	bio_reset(flush_bio);
-	bio_set_dev(flush_bio, s->origin->bdev);
-	flush_bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
-
-	return submit_bio_wait(flush_bio);
-}
-
 static void merge_callback(int read_err, unsigned long write_err, void *context)
 {
 	struct dm_snapshot *s = context;
@@ -1151,7 +1135,7 @@ static void merge_callback(int read_err, unsigned long write_err, void *context)
 		goto shut;
 	}
 
-	if (flush_data(s) < 0) {
+	if (blkdev_issue_flush(s->origin->bdev) < 0) {
 		DMERR("Flush after merge failed: shutting down merge");
 		goto shut;
 	}
@@ -1340,7 +1324,6 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	s->first_merging_chunk = 0;
 	s->num_merging_chunks = 0;
 	bio_list_init(&s->bios_queued_during_merge);
-	bio_init(&s->flush_bio, NULL, 0);
 
 	/* Allocate hash table for COW data */
 	if (init_hash_tables(s)) {
@@ -1528,8 +1511,6 @@ static void snapshot_dtr(struct dm_target *ti)
 
 	dm_exception_store_destroy(s->store);
 
-	bio_uninit(&s->flush_bio);
-
 	dm_put_device(ti, s->cow);
 
 	dm_put_device(ti, s->origin);