From: Mikulas Patocka <[email-masked]>

When processing barriers, skip the second flush if processing the bio
failed with -EOPNOTSUPP.  This can happen with discard+barrier requests.
If the device doesn't support discard, there would be two useless
SYNCHRONIZE CACHE commands.  The first dm_flush cannot be so easily
optimized out, so we leave it there.

Previously, -EOPNOTSUPP could be received in dec_pending only with empty
barriers and we ignored that error, assuming the device not supporting
cache flushes has cache always consistent.  With the addition of discard
barriers, this -EOPNOTSUPP can also be generated by discards and we
must record it in md->barrier_error for process_barrier.

Signed-off-by: Mikulas Patocka <[email-masked]>
Signed-off-by: Alasdair G Kergon <[email-masked]>

---
 drivers/md/dm.c |   15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

Index: linux-2.6.35/drivers/md/dm.c
===================================================================
--- linux-2.6.35.orig/drivers/md/dm.c
+++ linux-2.6.35/drivers/md/dm.c
@@ -639,8 +639,14 @@ static void dec_pending(struct dm_io *io
 			 * There can be just one barrier request so we use
 			 * a per-device variable for error reporting.
 			 * Note that you can't touch the bio after end_io_acct
+			 *
+			 * We ignore -EOPNOTSUPP for empty flush reported by
+			 * underlying devices. We assume that if the device
+			 * doesn't support empty barriers, it doesn't need
+			 * cache flushing commands.
 			 */
-			if (!md->barrier_error && io_error != -EOPNOTSUPP)
+			if (!md->barrier_error &&
+			    !(bio_empty_barrier(bio) && io_error == -EOPNOTSUPP))
 				md->barrier_error = io_error;
 			end_io_acct(io);
 			free_io(md, io);
@@ -2284,7 +2290,12 @@ static void process_barrier(struct mappe
 
 	if (!bio_empty_barrier(bio)) {
 		__split_and_process_bio(md, bio);
-		dm_flush(md);
+		/*
+		 * If the request isn't supported, don't waste time with
+		 * the second flush.
+		 */
+		if (md->barrier_error != -EOPNOTSUPP)
+			dm_flush(md);
 	}
 
 	if (md->barrier_error != DM_ENDIO_REQUEUE)