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[dm-devel] [RFC PATCH 1/4] dm-mpath: add a path selector interface

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:

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>
 drivers/md/dm-mpath.c         |    3 +++
 drivers/md/dm-path-selector.h |    1 +
 2 files changed, 4 insertions(+)

Index: 2.6.27-rc6/drivers/md/dm-mpath.c
--- 2.6.27-rc6.orig/drivers/md/dm-mpath.c
+++ 2.6.27-rc6/drivers/md/dm-mpath.c
@@ -343,6 +343,9 @@ 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: 2.6.27-rc6/drivers/md/dm-path-selector.h
--- 2.6.27-rc6.orig/drivers/md/dm-path-selector.h
+++ 2.6.27-rc6/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);

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