Subject: [RFC PATCH 1/4] dm-mpath: add a path selector interface
To: [email-masked], [email-masked]
Cc: [email-masked], [email-masked], [email-masked]

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 <[email-masked]>
Signed-off-by: Kiyoshi Ueda <[email-masked]>
Signed-off-by: Jun'ichi Nomura <[email-masked]>
---
 drivers/md/dm-mpath.c         |    3 +++
 drivers/md/dm-path-selector.h |    1 +
 2 files changed, 4 insertions(+)

Index: linux-2.6.28-rc3/drivers/md/dm-mpath.c
===================================================================
--- linux-2.6.28-rc3.orig/drivers/md/dm-mpath.c	2008-11-07 13:15:05.000000000 +0000
+++ linux-2.6.28-rc3/drivers/md/dm-mpath.c	2008-11-07 13:15:20.000000000 +0000
@@ -362,6 +362,9 @@ 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-2.6.28-rc3/drivers/md/dm-path-selector.h
===================================================================
--- linux-2.6.28-rc3.orig/drivers/md/dm-path-selector.h	2008-11-04 14:10:55.000000000 +0000
+++ linux-2.6.28-rc3/drivers/md/dm-path-selector.h	2008-11-07 13:15:20.000000000 +0000
@@ -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);
 };