dm: introduce num_flush_requests

Introduce num_flush_requests for a target to set to say how many flush
instructions (empty barriers) it wants to receive.  These are sent by
__clone_and_map_empty_barrier with map_info->flush_request going from 0
to (num_flush_requests - 1).

Old targets without flush support won't receive any flush requests.

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

drivers/md/dm.c

@@ -750,13 +750,50 @@ static struct bio *clone_bio(struct bio *bio, sector_t sector,
 	return clone;
 }
 
+static void __flush_target(struct clone_info *ci, struct dm_target *ti,
+			  unsigned flush_nr)
+{
+	struct dm_target_io *tio = alloc_tio(ci->md);
+	struct bio *clone;
+
+	tio->io = ci->io;
+	tio->ti = ti;
+
+	memset(&tio->info, 0, sizeof(tio->info));
+	tio->info.flush_request = flush_nr;
+
+	clone = bio_alloc_bioset(GFP_NOIO, 0, ci->md->bs);
+	__bio_clone(clone, ci->bio);
+	clone->bi_destructor = dm_bio_destructor;
+
+	__map_bio(ti, clone, tio);
+}
+
+static int __clone_and_map_empty_barrier(struct clone_info *ci)
+{
+	unsigned target_nr = 0, flush_nr;
+	struct dm_target *ti;
+
+	while ((ti = dm_table_get_target(ci->map, target_nr++)))
+		for (flush_nr = 0; flush_nr < ti->num_flush_requests;
+		     flush_nr++)
+			__flush_target(ci, ti, flush_nr);
+
+	ci->sector_count = 0;
+
+	return 0;
+}
+
 static int __clone_and_map(struct clone_info *ci)
 {
 	struct bio *clone, *bio = ci->bio;
 	struct dm_target *ti;
 	sector_t len = 0, max;
 	struct dm_target_io *tio;
 
+	if (unlikely(bio_empty_barrier(bio)))
+		return __clone_and_map_empty_barrier(ci);
+
 	ti = dm_table_find_target(ci->map, ci->sector);
 	if (!dm_target_is_valid(ti))
 		return -EIO;
@@ -877,6 +914,8 @@ static void __split_and_process_bio(struct mapped_device *md, struct bio *bio)
 	ci.io->md = md;
 	ci.sector = bio->bi_sector;
 	ci.sector_count = bio_sectors(bio);
+	if (unlikely(bio_empty_barrier(bio)))
+		ci.sector_count = 1;
 	ci.idx = bio->bi_idx;
 
 	start_io_acct(ci.io);

include/linux/device-mapper.h

@@ -21,6 +21,7 @@ typedef enum { STATUSTYPE_INFO, STATUSTYPE_TABLE } status_type_t;
 union map_info {
 	void *ptr;
 	unsigned long long ll;
+	unsigned flush_request;
 };
 
 /*
@@ -167,6 +168,16 @@ struct dm_target {
 	/* Always a power of 2 */
 	sector_t split_io;
 
+	/*
+	 * A number of zero-length barrier requests that will be submitted
+	 * to the target for the purpose of flushing cache.
+	 *
+	 * The request number will be placed in union map_info->flush_request.
+	 * It is a responsibility of the target driver to remap these requests
+	 * to the real underlying devices.
+	 */
+	unsigned num_flush_requests;
+
 	/*
 	 * These are automatically filled in by
 	 * dm_table_get_device.