2 files changed, 30 insertions, 7 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index e417cf0a69ef..7e469260fe5e 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1152,15 +1152,23 @@ static void __clone_and_map_data_bio(struct clone_info *ci, struct dm_target *ti
 {
         struct bio *bio = ci->bio;
         struct dm_target_io *tio;
+        unsigned target_bio_nr;
+        unsigned num_target_bios = 1;
 
-        tio = alloc_tio(ci, ti, nr_iovecs, 0);
-
-        if (split_bvec)
-                clone_split_bio(tio, bio, sector, idx, offset, len);
-        else
-                clone_bio(tio, bio, sector, idx, bv_count, len);
+        /*
+         * Does the target want to receive duplicate copies of the bio?
+         */
+        if (bio_data_dir(bio) == WRITE && ti->num_write_bios)
+                num_target_bios = ti->num_write_bios(ti, bio);
 
-        __map_bio(tio);
+        for (target_bio_nr = 0; target_bio_nr < num_target_bios; target_bio_nr++) {
+                tio = alloc_tio(ci, ti, nr_iovecs, target_bio_nr);
+                if (split_bvec)
+                        clone_split_bio(tio, bio, sector, idx, offset, len);
+                else
+                        clone_bio(tio, bio, sector, idx, bv_count, len);
+                __map_bio(tio);
+        }
 }
 
 typedef unsigned (*get_num_bios_fn)(struct dm_target *ti);
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index d5f984b07466..1e483fa7afb4 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -175,6 +175,14 @@ struct target_type {
 #define DM_TARGET_IMMUTABLE             0x00000004
 #define dm_target_is_immutable(type)    ((type)->features & DM_TARGET_IMMUTABLE)
 
+/*
+ * Some targets need to be sent the same WRITE bio severals times so
+ * that they can send copies of it to different devices.  This function
+ * examines any supplied bio and returns the number of copies of it the
+ * target requires.
+ */
+typedef unsigned (*dm_num_write_bios_fn) (struct dm_target *ti, struct bio *bio);
+
 struct dm_target {
         struct dm_table *table;
         struct target_type *type;
@@ -214,6 +222,13 @@ struct dm_target {
          */
         unsigned per_bio_data_size;
 
+        /*
+         * If defined, this function is called to find out how many
+         * duplicate bios should be sent to the target when writing
+         * data.
+         */
+        dm_num_write_bios_fn num_write_bios;
+
         /* target specific data */
         void *private;