From: Andi Kleen <ak@suse.de>
Subject: Implement barrier support for single device DM devices
Patch-mainline: 2.6.28
References: FATE#304489

This patch implements barrier support in DM for the common case of dm linear
just remapping a single underlying device. In this case we can safely
pass the barrier through because there can be no reordering between
devices.

Signed-off-by: Andi Kleen <ak@suse.de>

This patch is slightly different from the one which made it to the mainline.
But this is not replaced by that as it would break kABI. The mainline
commit is ab4c1424882be9cd70b89abf2b484add355712fa

Acked-by: Nikanth Karthikesan <knikanth@suse.de>

---
 drivers/md/dm-linear.c |    1 +
 drivers/md/dm-table.c  |   26 +++++++++++++++++++++++++-
 drivers/md/dm.c        |   14 ++++----------
 drivers/md/dm.h        |    2 ++
 4 files changed, 32 insertions(+), 11 deletions(-)

Index: linux-2.6.27/drivers/md/dm.c
===================================================================
--- linux-2.6.27.orig/drivers/md/dm.c
+++ linux-2.6.27/drivers/md/dm.c
@@ -1149,7 +1149,11 @@ static int __split_bio(struct mapped_dev
 	ci.map = dm_get_table(md);
 	if (unlikely(!ci.map))
 		return -EIO;
-
+	if (unlikely(bio_barrier(bio) && !dm_table_barrier_ok(ci.map))) {
+		bio_endio(bio, -EOPNOTSUPP);
+		dm_table_put(ci.map);
+		return 0;
+	}
 	ci.md = md;
 	ci.bio = bio;
 	ci.io = alloc_io(md);
@@ -1232,15 +1236,6 @@ static int _dm_request(struct request_qu
 	int rw = bio_data_dir(bio);
 	struct mapped_device *md = q->queuedata;
 
-	/*
-	 * There is no use in forwarding any barrier request since we can't
-	 * guarantee it is (or can be) handled by the targets correctly.
-	 */
-	if (unlikely(bio_barrier(bio))) {
-		bio_endio(bio, -EOPNOTSUPP);
-		return 0;
-	}
-
 	down_read(&md->io_lock);
 
 	disk_stat_inc(dm_disk(md), ios[rw]);
Index: linux-2.6.27/drivers/md/dm.h
===================================================================
--- linux-2.6.27.orig/drivers/md/dm.h
+++ linux-2.6.27/drivers/md/dm.h
@@ -66,6 +66,8 @@ void dm_table_unplug_all(struct dm_table
  * To check the return value from dm_table_find_target().
  */
 #define dm_target_is_valid(t) ((t)->table)
+int dm_table_barrier_ok(struct dm_table *t);
+void dm_table_support_barrier(struct dm_table *t);
 
 /*-----------------------------------------------------------------
  * A registry of target types.
Index: linux-2.6.27/drivers/md/dm-linear.c
===================================================================
--- linux-2.6.27.orig/drivers/md/dm-linear.c
+++ linux-2.6.27/drivers/md/dm-linear.c
@@ -52,6 +52,7 @@ static int linear_ctr(struct dm_target *
 		ti->error = "dm-linear: Device lookup failed";
 		goto bad;
 	}
+	dm_table_support_barrier(ti->table);
 
 	ti->private = lc;
 	return 0;
Index: linux-2.6.27/drivers/md/dm-table.c
===================================================================
--- linux-2.6.27.orig/drivers/md/dm-table.c
+++ linux-2.6.27/drivers/md/dm-table.c
@@ -38,6 +38,9 @@ struct dm_table {
 	sector_t *highs;
 	struct dm_target *targets;
 
+	unsigned single_device : 1;
+	unsigned barrier_supported : 1;
+
 	/*
 	 * Indicates the rw permissions for the new logical
 	 * device.  This should be a combination of FMODE_READ
@@ -532,12 +535,21 @@ EXPORT_SYMBOL_GPL(dm_set_device_limits);
 int dm_get_device(struct dm_target *ti, const char *path, sector_t start,
 		  sector_t len, int mode, struct dm_dev **result)
 {
-	int r = __table_get_device(ti->table, ti, path,
+	struct dm_table *t = ti->table;
+	int r = __table_get_device(t, ti, path,
 				   start, len, mode, result);
 
 	if (!r)
 		dm_set_device_limits(ti, (*result)->bdev);
 
+	if (!r) {
+		/* Only got single device? */
+		if (t->devices.next->next == &t->devices)
+			t->single_device = 1;
+		else
+			t->single_device = 0;
+	}
+
 	return r;
 }
 
@@ -1050,6 +1062,16 @@ struct mapped_device *dm_table_get_md(st
 	return t->md;
 }
 
+int dm_table_barrier_ok(struct dm_table *t)
+{
+	return t->single_device && t->barrier_supported;
+}
+
+void dm_table_support_barrier(struct dm_table *t)
+{
+	t->barrier_supported = 1;
+}
+
 EXPORT_SYMBOL(dm_vcalloc);
 EXPORT_SYMBOL(dm_get_device);
 EXPORT_SYMBOL(dm_put_device);
@@ -1060,3 +1082,5 @@ EXPORT_SYMBOL(dm_table_get_md);
 EXPORT_SYMBOL(dm_table_put);
 EXPORT_SYMBOL(dm_table_get);
 EXPORT_SYMBOL(dm_table_unplug_all);
+EXPORT_SYMBOL(dm_table_barrier_ok);
+EXPORT_SYMBOL(dm_table_support_barrier);