dm: add switch target

dm-switch is a new target that maps IO to underlying block devices
efficiently when there is a large number of fixed-sized address regions
but there is no simple pattern to allow for a compact mapping
representation such as dm-stripe.

Though we have developed this target for a specific storage device, Dell
EqualLogic, we have made an effort to keep it as general purpose as
possible in the hope that others may benefit.

Originally developed by Jim Ramsay. Simplified by Mikulas Patocka.

Signed-off-by: Jim Ramsay <jim_ramsay@dell.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

4 files changed, 679 insertions, 0 deletions

diff --git a/Documentation/device-mapper/switch.txt b/Documentation/device-mapper/switch.txt
new file mode 100644
index 000000000000..2fa749387be8
--- /dev/null
+++ b/Documentation/device-mapper/switch.txt
@@ -0,0 +1,126 @@
+dm-switch
+=========
+
+The device-mapper switch target creates a device that supports an
+arbitrary mapping of fixed-size regions of I/O across a fixed set of
+paths.  The path used for any specific region can be switched
+dynamically by sending the target a message.
+
+It maps I/O to underlying block devices efficiently when there is a large
+number of fixed-sized address regions but there is no simple pattern
+that would allow for a compact representation of the mapping such as
+dm-stripe.
+
+Background
+----------
+
+Dell EqualLogic and some other iSCSI storage arrays use a distributed
+frameless architecture.  In this architecture, the storage group
+consists of a number of distinct storage arrays ("members") each having
+independent controllers, disk storage and network adapters.  When a LUN
+is created it is spread across multiple members.  The details of the
+spreading are hidden from initiators connected to this storage system.
+The storage group exposes a single target discovery portal, no matter
+how many members are being used.  When iSCSI sessions are created, each
+session is connected to an eth port on a single member.  Data to a LUN
+can be sent on any iSCSI session, and if the blocks being accessed are
+stored on another member the I/O will be forwarded as required.  This
+forwarding is invisible to the initiator.  The storage layout is also
+dynamic, and the blocks stored on disk may be moved from member to
+member as needed to balance the load.
+
+This architecture simplifies the management and configuration of both
+the storage group and initiators.  In a multipathing configuration, it
+is possible to set up multiple iSCSI sessions to use multiple network
+interfaces on both the host and target to take advantage of the
+increased network bandwidth.  An initiator could use a simple round
+robin algorithm to send I/O across all paths and let the storage array
+members forward it as necessary, but there is a performance advantage to
+sending data directly to the correct member.
+
+A device-mapper table already lets you map different regions of a
+device onto different targets.  However in this architecture the LUN is
+spread with an address region size on the order of 10s of MBs, which
+means the resulting table could have more than a million entries and
+consume far too much memory.
+
+Using this device-mapper switch target we can now build a two-layer
+device hierarchy:
+
+    Upper Tier – Determine which array member the I/O should be sent to.
+    Lower Tier – Load balance amongst paths to a particular member.
+
+The lower tier consists of a single dm multipath device for each member.
+Each of these multipath devices contains the set of paths directly to
+the array member in one priority group, and leverages existing path
+selectors to load balance amongst these paths.  We also build a
+non-preferred priority group containing paths to other array members for
+failover reasons.
+
+The upper tier consists of a single dm-switch device.  This device uses
+a bitmap to look up the location of the I/O and choose the appropriate
+lower tier device to route the I/O.  By using a bitmap we are able to
+use 4 bits for each address range in a 16 member group (which is very
+large for us).  This is a much denser representation than the dm table
+b-tree can achieve.
+
+Construction Parameters
+=======================
+
+    <num_paths> <region_size> <num_optional_args> [<optional_args>...]
+    [<dev_path> <offset>]+
+
+<num_paths>
+    The number of paths across which to distribute the I/O.
+
+<region_size>
+    The number of 512-byte sectors in a region. Each region can be redirected
+    to any of the available paths.
+
+<num_optional_args>
+    The number of optional arguments. Currently, no optional arguments
+    are supported and so this must be zero.
+
+<dev_path>
+    The block device that represents a specific path to the device.
+
+<offset>
+    The offset of the start of data on the specific <dev_path> (in units
+    of 512-byte sectors). This number is added to the sector number when
+    forwarding the request to the specific path. Typically it is zero.
+
+Messages
+========
+
+set_region_mappings <index>:<path_nr> [<index>]:<path_nr> [<index>]:<path_nr>...
+
+Modify the region table by specifying which regions are redirected to
+which paths.
+
+<index>
+    The region number (region size was specified in constructor parameters).
+    If index is omitted, the next region (previous index + 1) is used.
+    Expressed in hexadecimal (WITHOUT any prefix like 0x).
+
+<path_nr>
+    The path number in the range 0 ... (<num_paths> - 1).
+    Expressed in hexadecimal (WITHOUT any prefix like 0x).
+
+Status
+======
+
+No status line is reported.
+
+Example
+=======
+
+Assume that you have volumes vg1/switch0 vg1/switch1 vg1/switch2 with
+the same size.
+
+Create a switch device with 64kB region size:
+    dmsetup create switch --table "0 `blockdev --getsize /dev/vg1/switch0`
+        switch 3 128 0 /dev/vg1/switch0 0 /dev/vg1/switch1 0 /dev/vg1/switch2 0"
+
+Set mappings for the first 7 entries to point to devices switch0, switch1,
+switch2, switch0, switch1, switch2, switch1:
+    dmsetup message switch 0 set_region_mappings 0:0 :1 :2 :0 :1 :2 :1
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 3bfc8f1da9fe..30b426ed744b 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -412,4 +412,18 @@ config DM_VERITY
 
           If unsure, say N.
 
+config DM_SWITCH
+        tristate "Switch target support (EXPERIMENTAL)"
+        depends on BLK_DEV_DM
+        ---help---
+          This device-mapper target creates a device that supports an arbitrary
+          mapping of fixed-size regions of I/O across a fixed set of paths.
+          The path used for any specific region can be switched dynamically
+          by sending the target a message.
+
+          To compile this code as a module, choose M here: the module will
+          be called dm-switch.
+
+          If unsure, say N.
+
 endif # MD
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 1439fd4ad9b1..5ef78efc27f2 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -40,6 +40,7 @@ obj-$(CONFIG_DM_FLAKEY)		+= dm-flakey.o
 obj-$(CONFIG_DM_MULTIPATH)      += dm-multipath.o dm-round-robin.o
 obj-$(CONFIG_DM_MULTIPATH_QL)   += dm-queue-length.o
 obj-$(CONFIG_DM_MULTIPATH_ST)   += dm-service-time.o
+obj-$(CONFIG_DM_SWITCH)         += dm-switch.o
 obj-$(CONFIG_DM_SNAPSHOT)       += dm-snapshot.o
 obj-$(CONFIG_DM_PERSISTENT_DATA)        += persistent-data/
 obj-$(CONFIG_DM_MIRROR)         += dm-mirror.o dm-log.o dm-region-hash.o
diff --git a/drivers/md/dm-switch.c b/drivers/md/dm-switch.c
new file mode 100644
index 000000000000..ff9ac4be4721
--- /dev/null
+++ b/drivers/md/dm-switch.c
@@ -0,0 +1,538 @@
+/*
+ * Copyright (C) 2010-2012 by Dell Inc.  All rights reserved.
+ * Copyright (C) 2011-2013 Red Hat, Inc.
+ *
+ * This file is released under the GPL.
+ *
+ * dm-switch is a device-mapper target that maps IO to underlying block
+ * devices efficiently when there are a large number of fixed-sized
+ * address regions but there is no simple pattern to allow for a compact
+ * mapping representation such as dm-stripe.
+ */
+
+#include <linux/device-mapper.h>
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/vmalloc.h>
+
+#define DM_MSG_PREFIX "switch"
+
+/*
+ * One region_table_slot_t holds <region_entries_per_slot> region table
+ * entries each of which is <region_table_entry_bits> in size.
+ */
+typedef unsigned long region_table_slot_t;
+
+/*
+ * A device with the offset to its start sector.
+ */
+struct switch_path {
+        struct dm_dev *dmdev;
+        sector_t start;
+};
+
+/*
+ * Context block for a dm switch device.
+ */
+struct switch_ctx {
+        struct dm_target *ti;
+
+        unsigned nr_paths;              /* Number of paths in path_list. */
+
+        unsigned region_size;           /* Region size in 512-byte sectors */
+        unsigned long nr_regions;       /* Number of regions making up the device */
+        signed char region_size_bits;   /* log2 of region_size or -1 */
+
+        unsigned char region_table_entry_bits;  /* Number of bits in one region table entry */
+        unsigned char region_entries_per_slot;  /* Number of entries in one region table slot */
+        signed char region_entries_per_slot_bits;       /* log2 of region_entries_per_slot or -1 */
+
+        region_table_slot_t *region_table;      /* Region table */
+
+        /*
+         * Array of dm devices to switch between.
+         */
+        struct switch_path path_list[0];
+};
+
+static struct switch_ctx *alloc_switch_ctx(struct dm_target *ti, unsigned nr_paths,
+                                           unsigned region_size)
+{
+        struct switch_ctx *sctx;
+
+        sctx = kzalloc(sizeof(struct switch_ctx) + nr_paths * sizeof(struct switch_path),
+                       GFP_KERNEL);
+        if (!sctx)
+                return NULL;
+
+        sctx->ti = ti;
+        sctx->region_size = region_size;
+
+        ti->private = sctx;
+
+        return sctx;
+}
+
+static int alloc_region_table(struct dm_target *ti, unsigned nr_paths)
+{
+        struct switch_ctx *sctx = ti->private;
+        sector_t nr_regions = ti->len;
+        sector_t nr_slots;
+
+        if (!(sctx->region_size & (sctx->region_size - 1)))
+                sctx->region_size_bits = __ffs(sctx->region_size);
+        else
+                sctx->region_size_bits = -1;
+
+        sctx->region_table_entry_bits = 1;
+        while (sctx->region_table_entry_bits < sizeof(region_table_slot_t) * 8 &&
+               (region_table_slot_t)1 << sctx->region_table_entry_bits < nr_paths)
+                sctx->region_table_entry_bits++;
+
+        sctx->region_entries_per_slot = (sizeof(region_table_slot_t) * 8) / sctx->region_table_entry_bits;
+        if (!(sctx->region_entries_per_slot & (sctx->region_entries_per_slot - 1)))
+                sctx->region_entries_per_slot_bits = __ffs(sctx->region_entries_per_slot);
+        else
+                sctx->region_entries_per_slot_bits = -1;
+
+        if (sector_div(nr_regions, sctx->region_size))
+                nr_regions++;
+
+        sctx->nr_regions = nr_regions;
+        if (sctx->nr_regions != nr_regions || sctx->nr_regions >= ULONG_MAX) {
+                ti->error = "Region table too large";
+                return -EINVAL;
+        }
+
+        nr_slots = nr_regions;
+        if (sector_div(nr_slots, sctx->region_entries_per_slot))
+                nr_slots++;
+
+        if (nr_slots > ULONG_MAX / sizeof(region_table_slot_t)) {
+                ti->error = "Region table too large";
+                return -EINVAL;
+        }
+
+        sctx->region_table = vmalloc(nr_slots * sizeof(region_table_slot_t));
+        if (!sctx->region_table) {
+                ti->error = "Cannot allocate region table";
+                return -ENOMEM;
+        }
+
+        return 0;
+}
+
+static void switch_get_position(struct switch_ctx *sctx, unsigned long region_nr,
+                                unsigned long *region_index, unsigned *bit)
+{
+        if (sctx->region_entries_per_slot_bits >= 0) {
+                *region_index = region_nr >> sctx->region_entries_per_slot_bits;
+                *bit = region_nr & (sctx->region_entries_per_slot - 1);
+        } else {
+                *region_index = region_nr / sctx->region_entries_per_slot;
+                *bit = region_nr % sctx->region_entries_per_slot;
+        }
+
+        *bit *= sctx->region_table_entry_bits;
+}
+
+/*
+ * Find which path to use at given offset.
+ */
+static unsigned switch_get_path_nr(struct switch_ctx *sctx, sector_t offset)
+{
+        unsigned long region_index;
+        unsigned bit, path_nr;
+        sector_t p;
+
+        p = offset;
+        if (sctx->region_size_bits >= 0)
+                p >>= sctx->region_size_bits;
+        else
+                sector_div(p, sctx->region_size);
+
+        switch_get_position(sctx, p, &region_index, &bit);
+        path_nr = (ACCESS_ONCE(sctx->region_table[region_index]) >> bit) &
+               ((1 << sctx->region_table_entry_bits) - 1);
+
+        /* This can only happen if the processor uses non-atomic stores. */
+        if (unlikely(path_nr >= sctx->nr_paths))
+                path_nr = 0;
+
+        return path_nr;
+}
+
+static void switch_region_table_write(struct switch_ctx *sctx, unsigned long region_nr,
+                                      unsigned value)
+{
+        unsigned long region_index;
+        unsigned bit;
+        region_table_slot_t pte;
+
+        switch_get_position(sctx, region_nr, &region_index, &bit);
+
+        pte = sctx->region_table[region_index];
+        pte &= ~((((region_table_slot_t)1 << sctx->region_table_entry_bits) - 1) << bit);
+        pte |= (region_table_slot_t)value << bit;
+        sctx->region_table[region_index] = pte;
+}
+
+/*
+ * Fill the region table with an initial round robin pattern.
+ */
+static void initialise_region_table(struct switch_ctx *sctx)
+{
+        unsigned path_nr = 0;
+        unsigned long region_nr;
+
+        for (region_nr = 0; region_nr < sctx->nr_regions; region_nr++) {
+                switch_region_table_write(sctx, region_nr, path_nr);
+                if (++path_nr >= sctx->nr_paths)
+                        path_nr = 0;
+        }
+}
+
+static int parse_path(struct dm_arg_set *as, struct dm_target *ti)
+{
+        struct switch_ctx *sctx = ti->private;
+        unsigned long long start;
+        int r;
+
+        r = dm_get_device(ti, dm_shift_arg(as), dm_table_get_mode(ti->table),
+                          &sctx->path_list[sctx->nr_paths].dmdev);
+        if (r) {
+                ti->error = "Device lookup failed";
+                return r;
+        }
+
+        if (kstrtoull(dm_shift_arg(as), 10, &start) || start != (sector_t)start) {
+                ti->error = "Invalid device starting offset";
+                dm_put_device(ti, sctx->path_list[sctx->nr_paths].dmdev);
+                return -EINVAL;
+        }
+
+        sctx->path_list[sctx->nr_paths].start = start;
+
+        sctx->nr_paths++;
+
+        return 0;
+}
+
+/*
+ * Destructor: Don't free the dm_target, just the ti->private data (if any).
+ */
+static void switch_dtr(struct dm_target *ti)
+{
+        struct switch_ctx *sctx = ti->private;
+
+        while (sctx->nr_paths--)
+                dm_put_device(ti, sctx->path_list[sctx->nr_paths].dmdev);
+
+        vfree(sctx->region_table);
+        kfree(sctx);
+}
+
+/*
+ * Constructor arguments:
+ *   <num_paths> <region_size> <num_optional_args> [<optional_args>...]
+ *   [<dev_path> <offset>]+
+ *
+ * Optional args are to allow for future extension: currently this
+ * parameter must be 0.
+ */
+static int switch_ctr(struct dm_target *ti, unsigned argc, char **argv)
+{
+        static struct dm_arg _args[] = {
+                {1, (KMALLOC_MAX_SIZE - sizeof(struct switch_ctx)) / sizeof(struct switch_path), "Invalid number of paths"},
+                {1, UINT_MAX, "Invalid region size"},
+                {0, 0, "Invalid number of optional args"},
+        };
+
+        struct switch_ctx *sctx;
+        struct dm_arg_set as;
+        unsigned nr_paths, region_size, nr_optional_args;
+        int r;
+
+        as.argc = argc;
+        as.argv = argv;
+
+        r = dm_read_arg(_args, &as, &nr_paths, &ti->error);
+        if (r)
+                return -EINVAL;
+
+        r = dm_read_arg(_args + 1, &as, &region_size, &ti->error);
+        if (r)
+                return r;
+
+        r = dm_read_arg_group(_args + 2, &as, &nr_optional_args, &ti->error);
+        if (r)
+                return r;
+        /* parse optional arguments here, if we add any */
+
+        if (as.argc != nr_paths * 2) {
+                ti->error = "Incorrect number of path arguments";
+                return -EINVAL;
+        }
+
+        sctx = alloc_switch_ctx(ti, nr_paths, region_size);
+        if (!sctx) {
+                ti->error = "Cannot allocate redirection context";
+                return -ENOMEM;
+        }
+
+        r = dm_set_target_max_io_len(ti, region_size);
+        if (r)
+                goto error;
+
+        while (as.argc) {
+                r = parse_path(&as, ti);
+                if (r)
+                        goto error;
+        }
+
+        r = alloc_region_table(ti, nr_paths);
+        if (r)
+                goto error;
+
+        initialise_region_table(sctx);
+
+        /* For UNMAP, sending the request down any path is sufficient */
+        ti->num_discard_bios = 1;
+
+        return 0;
+
+error:
+        switch_dtr(ti);
+
+        return r;
+}
+
+static int switch_map(struct dm_target *ti, struct bio *bio)
+{
+        struct switch_ctx *sctx = ti->private;
+        sector_t offset = dm_target_offset(ti, bio->bi_sector);
+        unsigned path_nr = switch_get_path_nr(sctx, offset);
+
+        bio->bi_bdev = sctx->path_list[path_nr].dmdev->bdev;
+        bio->bi_sector = sctx->path_list[path_nr].start + offset;
+
+        return DM_MAPIO_REMAPPED;
+}
+
+/*
+ * We need to parse hex numbers in the message as quickly as possible.
+ *
+ * This table-based hex parser improves performance.
+ * It improves a time to load 1000000 entries compared to the condition-based
+ * parser.
+ *              table-based parser      condition-based parser
+ * PA-RISC      0.29s                   0.31s
+ * Opteron      0.0495s                 0.0498s
+ */
+static const unsigned char hex_table[256] = {
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 255, 255, 255, 255, 255, 255,
+255, 10, 11, 12, 13, 14, 15, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 10, 11, 12, 13, 14, 15, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255
+};
+
+static __always_inline unsigned long parse_hex(const char **string)
+{
+        unsigned char d;
+        unsigned long r = 0;
+
+        while ((d = hex_table[(unsigned char)**string]) < 16) {
+                r = (r << 4) | d;
+                (*string)++;
+        }
+
+        return r;
+}
+
+static int process_set_region_mappings(struct switch_ctx *sctx,
+                             unsigned argc, char **argv)
+{
+        unsigned i;
+        unsigned long region_index = 0;
+
+        for (i = 1; i < argc; i++) {
+                unsigned long path_nr;
+                const char *string = argv[i];
+
+                if (*string == ':')
+                        region_index++;
+                else {
+                        region_index = parse_hex(&string);
+                        if (unlikely(*string != ':')) {
+                                DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+                                return -EINVAL;
+                        }
+                }
+
+                string++;
+                if (unlikely(!*string)) {
+                        DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+                        return -EINVAL;
+                }
+
+                path_nr = parse_hex(&string);
+                if (unlikely(*string)) {
+                        DMWARN("invalid set_region_mappings argument: '%s'", argv[i]);
+                        return -EINVAL;
+                }
+                if (unlikely(region_index >= sctx->nr_regions)) {
+                        DMWARN("invalid set_region_mappings region number: %lu >= %lu", region_index, sctx->nr_regions);
+                        return -EINVAL;
+                }
+                if (unlikely(path_nr >= sctx->nr_paths)) {
+                        DMWARN("invalid set_region_mappings device: %lu >= %u", path_nr, sctx->nr_paths);
+                        return -EINVAL;
+                }
+
+                switch_region_table_write(sctx, region_index, path_nr);
+        }
+
+        return 0;
+}
+
+/*
+ * Messages are processed one-at-a-time.
+ *
+ * Only set_region_mappings is supported.
+ */
+static int switch_message(struct dm_target *ti, unsigned argc, char **argv)
+{
+        static DEFINE_MUTEX(message_mutex);
+
+        struct switch_ctx *sctx = ti->private;
+        int r = -EINVAL;
+
+        mutex_lock(&message_mutex);
+
+        if (!strcasecmp(argv[0], "set_region_mappings"))
+                r = process_set_region_mappings(sctx, argc, argv);
+        else
+                DMWARN("Unrecognised message received.");
+
+        mutex_unlock(&message_mutex);
+
+        return r;
+}
+
+static void switch_status(struct dm_target *ti, status_type_t type,
+                          unsigned status_flags, char *result, unsigned maxlen)
+{
+        struct switch_ctx *sctx = ti->private;
+        unsigned sz = 0;
+        int path_nr;
+
+        switch (type) {
+        case STATUSTYPE_INFO:
+                result[0] = '\0';
+                break;
+
+        case STATUSTYPE_TABLE:
+                DMEMIT("%u %u 0", sctx->nr_paths, sctx->region_size);
+                for (path_nr = 0; path_nr < sctx->nr_paths; path_nr++)
+                        DMEMIT(" %s %llu", sctx->path_list[path_nr].dmdev->name,
+                               (unsigned long long)sctx->path_list[path_nr].start);
+                break;
+        }
+}
+
+/*
+ * Switch ioctl:
+ *
+ * Passthrough all ioctls to the path for sector 0
+ */
+static int switch_ioctl(struct dm_target *ti, unsigned cmd,
+                        unsigned long arg)
+{
+        struct switch_ctx *sctx = ti->private;
+        struct block_device *bdev;
+        fmode_t mode;
+        unsigned path_nr;
+        int r = 0;
+
+        path_nr = switch_get_path_nr(sctx, 0);
+
+        bdev = sctx->path_list[path_nr].dmdev->bdev;
+        mode = sctx->path_list[path_nr].dmdev->mode;
+
+        /*
+         * Only pass ioctls through if the device sizes match exactly.
+         */
+        if (ti->len + sctx->path_list[path_nr].start != i_size_read(bdev->bd_inode) >> SECTOR_SHIFT)
+                r = scsi_verify_blk_ioctl(NULL, cmd);
+
+        return r ? : __blkdev_driver_ioctl(bdev, mode, cmd, arg);
+}
+
+static int switch_iterate_devices(struct dm_target *ti,
+                                  iterate_devices_callout_fn fn, void *data)
+{
+        struct switch_ctx *sctx = ti->private;
+        int path_nr;
+        int r;
+
+        for (path_nr = 0; path_nr < sctx->nr_paths; path_nr++) {
+                r = fn(ti, sctx->path_list[path_nr].dmdev,
+                         sctx->path_list[path_nr].start, ti->len, data);
+                if (r)
+                        return r;
+        }
+
+        return 0;
+}
+
+static struct target_type switch_target = {
+        .name = "switch",
+        .version = {1, 0, 0},
+        .module = THIS_MODULE,
+        .ctr = switch_ctr,
+        .dtr = switch_dtr,
+        .map = switch_map,
+        .message = switch_message,
+        .status = switch_status,
+        .ioctl = switch_ioctl,
+        .iterate_devices = switch_iterate_devices,
+};
+
+static int __init dm_switch_init(void)
+{
+        int r;
+
+        r = dm_register_target(&switch_target);
+        if (r < 0)
+                DMERR("dm_register_target() failed %d", r);
+
+        return r;
+}
+
+static void __exit dm_switch_exit(void)
+{
+        dm_unregister_target(&switch_target);
+}
+
+module_init(dm_switch_init);
+module_exit(dm_switch_exit);
+
+MODULE_DESCRIPTION(DM_NAME " dynamic path switching target");
+MODULE_AUTHOR("Kevin D. O'Kelley <Kevin_OKelley@dell.com>");
+MODULE_AUTHOR("Narendran Ganapathy <Narendran_Ganapathy@dell.com>");
+MODULE_AUTHOR("Jim Ramsay <Jim_Ramsay@dell.com>");
+MODULE_AUTHOR("Mikulas Patocka <mpatocka@redhat.com>");
+MODULE_LICENSE("GPL");