dm mpath: support discard

Enable discard support in the DM multipath target.

This discard support depends on a few discard-specific fixes to the
block layer's request stacking driver methods.

Discard requests are optional so don't allow a failed discard to trigger
path failures. If there is a real problem with a given path the
barriers associated with the discard (either before or after the
discard) will cause path failure. That said, unconditionally passing
discard failures up the stack is not ideal. This must be fixed once DM
has more information about the nature of the underlying storage failure.

Signed-off-by: Mike Snitzer <[e-mail]>
Signed-off-by: Alasdair G Kergon <[e-mail]>
Cc: Kiyoshi Ueda <[e-mail]>

drivers/md/dm-mpath.c

···
 	}
 
 	ti->num_flush_requests = 1;
+	ti->num_discard_requests = 1;
 
 	return 0;
 
···
 		return 0;	/* I/O complete */
 
 	if (error == -EOPNOTSUPP)
+		return error;
+
+	if (clone->cmd_flags & REQ_DISCARD)
+		/*
+		 * Pass all discard request failures up.
+		 * FIXME: only fail_path if the discard failed due to a
+		 * transport problem.  This requires precise understanding
+		 * of the underlying failure (e.g. the SCSI sense).
+		 */
 		return error;
 
 	if (mpio->pgpath)