dm: add WRITE SAME support

WRITE SAME bios have a payload that contain a single page.  When
cloning WRITE SAME bios DM has no need to modify the bi_io_vec
attributes (and doing so would be detrimental).  DM need only alter the
start and end of the WRITE SAME bio accordingly.

Rather than duplicate __clone_and_map_discard, factor out a common
function that is also used by __clone_and_map_write_same.

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

drivers/md/dm.c

@@ -1174,7 +1174,28 @@ static void __clone_and_map_simple(struct clone_info *ci, struct dm_target *ti)
 	ci->sector_count = 0;
 }
 
-static int __clone_and_map_discard(struct clone_info *ci)
+typedef unsigned (*get_num_requests_fn)(struct dm_target *ti);
+
+static unsigned get_num_discard_requests(struct dm_target *ti)
+{
+	return ti->num_discard_requests;
+}
+
+static unsigned get_num_write_same_requests(struct dm_target *ti)
+{
+	return ti->num_write_same_requests;
+}
+
+typedef bool (*is_split_required_fn)(struct dm_target *ti);
+
+static bool is_split_required_for_discard(struct dm_target *ti)
+{
+	return ti->split_discard_requests;
+}
+
+static int __clone_and_map_changing_extent_only(struct clone_info *ci,
+						get_num_requests_fn get_num_requests,
+						is_split_required_fn is_split_required)
 {
 	struct dm_target *ti;
 	sector_t len;
@@ -1185,15 +1206,15 @@ static int __clone_and_map_discard(struct clone_info *ci)
 			return -EIO;
 
 		/*
-		 * Even though the device advertised discard support,
-		 * that does not mean every target supports it, and
+		 * Even though the device advertised support for this type of
+		 * request, that does not mean every target supports it, and
 		 * reconfiguration might also have changed that since the
 		 * check was performed.
 		 */
-		if (!ti->num_discard_requests)
+		if (!get_num_requests || !get_num_requests(ti))
 			return -EOPNOTSUPP;
 
-		if (!ti->split_discard_requests)
+		if (is_split_required && !is_split_required(ti))
 			len = min(ci->sector_count, max_io_len_target_boundary(ci->sector, ti));
 		else
 			len = min(ci->sector_count, max_io_len(ci->sector, ti));
@@ -1206,6 +1227,17 @@ static int __clone_and_map_discard(struct clone_info *ci)
 	return 0;
 }
 
+static int __clone_and_map_discard(struct clone_info *ci)
+{
+	return __clone_and_map_changing_extent_only(ci, get_num_discard_requests,
+						    is_split_required_for_discard);
+}
+
+static int __clone_and_map_write_same(struct clone_info *ci)
+{
+	return __clone_and_map_changing_extent_only(ci, get_num_write_same_requests, NULL);
+}
+
 static int __clone_and_map(struct clone_info *ci)
 {
 	struct bio *bio = ci->bio;
@@ -1215,6 +1247,8 @@ static int __clone_and_map(struct clone_info *ci)
 
 	if (unlikely(bio->bi_rw & REQ_DISCARD))
 		return __clone_and_map_discard(ci);
+	else if (unlikely(bio->bi_rw & REQ_WRITE_SAME))
+		return __clone_and_map_write_same(ci);
 
 	ti = dm_table_find_target(ci->map, ci->sector);
 	if (!dm_target_is_valid(ti))