From: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Subject: dm-mpath: add a path selector interface
References: FATE#303862,FATE#302108

This patch adds a new hook for dm path selector: start_io.
Target drivers should call this hook before submitting I/O to
the selected path.
Path selectors can use it to start accounting of the I/O.
(e.g. counting the number of in-flight I/Os.)

The code is based on the patch posted by Stefan Bader:
https://www.redhat.com/archives/dm-devel/2005-October/msg00050.html


Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>

Index: linux-2.6.27/drivers/md/dm-mpath.c
===================================================================
--- linux-2.6.27.orig/drivers/md/dm-mpath.c
+++ linux-2.6.27/drivers/md/dm-mpath.c
@@ -359,6 +359,9 @@ static int map_io(struct multipath *m, s
 
 	mpio->pgpath = pgpath;
 
+	if (r == DM_MAPIO_REMAPPED && pgpath->pg->ps.type->start_io)
+		pgpath->pg->ps.type->start_io(&pgpath->pg->ps, &pgpath->path);
+
 	spin_unlock_irqrestore(&m->lock, flags);
 
 	return r;
Index: linux-2.6.27/drivers/md/dm-path-selector.h
===================================================================
--- linux-2.6.27.orig/drivers/md/dm-path-selector.h
+++ linux-2.6.27/drivers/md/dm-path-selector.h
@@ -75,6 +75,7 @@ struct path_selector_type {
 	int (*status) (struct path_selector *ps, struct dm_path *path,
 		       status_type_t type, char *result, unsigned int maxlen);
 
+	int (*start_io) (struct path_selector *ps, struct dm_path *path);
 	int (*end_io) (struct path_selector *ps, struct dm_path *path);
 };