device-mapper: Add dm-bow

dm-bow is a device mapper driver that allows to create a checkpoint on
the volume so that it is possible to restore the state if necessary.
This is needed to protect the ext4 partition during a system upgrade in
case of a power failure.

Change-Id: Ic62e83dc5b04cfa250d1b7eaa3721546782e88eb
Originally-by: Paul Lawrence <paullawrence@google.com>
Origin: https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/drivers/md/dm-bow.c
Signed-off-by: Mateusz Moscicki <m.moscicki2@samsung.com>

4 files changed, 1420 insertions, 0 deletions

diff --git a/Documentation/device-mapper/dm-bow.txt b/Documentation/device-mapper/dm-bow.txt
new file mode 100644
index 000000000000..f84eb5c90921
--- /dev/null
+++ b/Documentation/device-mapper/dm-bow.txt
@@ -0,0 +1,100 @@
+dm_bow (backup on write)
+========================
+
+dm_bow is a device mapper driver that uses the free space on a device to back up
+data that is overwritten. The changes can then be committed by a simple state
+change, or rolled back by removing the dm_bow device and running a command line
+utility over the underlying device.
+
+dm_bow has three states, set by writing ‘1’ or ‘2’ to /sys/block/dm-?/bow/state.
+It is only possible to go from state 0 (initial state) to state 1, and then from
+state 1 to state 2.
+
+State 0: dm_bow collects all trims to the device and assumes that these mark
+free space on the overlying file system that can be safely used. Typically the
+mount code would create the dm_bow device, mount the file system, call the
+FITRIM ioctl on the file system then switch to state 1. These trims are not
+propagated to the underlying device.
+
+State 1: All writes to the device cause the underlying data to be backed up to
+the free (trimmed) area as needed in such a way as they can be restored.
+However, the writes, with one exception, then happen exactly as they would
+without dm_bow, so the device is always in a good final state. The exception is
+that sector 0 is used to keep a log of the latest changes, both to indicate that
+we are in this state and to allow rollback. See below for all details. If there
+isn't enough free space, writes are failed with -ENOSPC.
+
+State 2: The transition to state 2 triggers replacing the special sector 0 with
+the normal sector 0, and the freeing of all state information. dm_bow then
+becomes a pass-through driver, allowing the device to continue to be used with
+minimal performance impact.
+
+Usage
+=====
+dm-bow takes one command line parameter, the name of the underlying device.
+
+dm-bow will typically be used in the following way. dm-bow will be loaded with a
+suitable underlying device and the resultant device will be mounted. A file
+system trim will be issued via the FITRIM ioctl, then the device will be
+switched to state 1. The file system will now be used as normal. At some point,
+the changes can either be committed by switching to state 2, or rolled back by
+unmounting the file system, removing the dm-bow device and running the command
+line utility. Note that rebooting the device will be equivalent to unmounting
+and removing, but the command line utility must still be run
+
+Details of operation in state 1
+===============================
+
+dm_bow maintains a type for all sectors. A sector can be any of:
+
+SECTOR0
+SECTOR0_CURRENT
+UNCHANGED
+FREE
+CHANGED
+BACKUP
+
+SECTOR0 is the first sector on the device, and is used to hold the log of
+changes. This is the one exception.
+
+SECTOR0_CURRENT is a sector picked from the FREE sectors, and is where reads and
+writes from the true sector zero are redirected to. Note that like any backup
+sector, if the sector is written to directly, it must be moved again.
+
+UNCHANGED means that the sector has not been changed since we entered state 1.
+Thus if it is written to or trimmed, the contents must first be backed up.
+
+FREE means that the sector was trimmed in state 0 and has not yet been written
+to or used for backup. On being written to, a FREE sector is changed to CHANGED.
+
+CHANGED means that the sector has been modified, and can be further modified
+without further backup.
+
+BACKUP means that this is a free sector being used as a backup. On being written
+to, the contents must first be backed up again.
+
+All backup operations are logged to the first sector. The log sector has the
+format:
+--------------------------------------------------------
+| Magic | Count | Sequence | Log entry | Log entry | …
+--------------------------------------------------------
+
+Magic is a magic number. Count is the number of log entries. Sequence is 0
+initially. A log entry is
+
+-----------------------------------
+| Source | Dest | Size | Checksum |
+-----------------------------------
+
+When SECTOR0 is full, the log sector is backed up and another empty log sector
+created with sequence number one higher. The first entry in any log entry with
+sequence > 0 therefore must be the log of the backing up of the previous log
+sector. Note that sequence is not strictly needed, but is a useful sanity check
+and potentially limits the time spent trying to restore a corrupted snapshot.
+
+On entering state 1, dm_bow has a list of free sectors. All other sectors are
+unchanged. Sector0_current is selected from the free sectors and the contents of
+sector 0 are copied there. The sector 0 is backed up, which triggers the first
+log entry to be written.
+
+
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 8b8c123cae66..adb3e1f86062 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -459,6 +459,26 @@ config DM_FLAKEY
        ---help---
          A target that intermittently fails I/O for debugging purposes.
 
+config DM_BOW
+	tristate "BOW support"
+	depends on BLK_DEV_DM
+	select CRYPTO
+	select CRYPTO_HASH
+	select DM_BUFIO
+	help
+	  This device-mapper target creates a read-only device that
+	  transparently validates the data on one underlying device against
+	  a pre-generated tree of cryptographic checksums stored on a second
+	  device.
+
+	  You'll need to activate the digests you're going to use in the
+	  cryptoapi configuration.
+
+	  To compile this code as a module, choose M here: the module will
+	  be called dm-verity.
+
+	  If unsure, say N.
+
 config DM_VERITY
 	tristate "Verity target support"
 	depends on BLK_DEV_DM
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 822f4e8753bc..f93731e760ee 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -68,6 +68,7 @@ obj-$(CONFIG_DM_LOG_WRITES)	+= dm-log-writes.o
 obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o
 obj-$(CONFIG_DM_ZONED)		+= dm-zoned.o
 obj-$(CONFIG_DM_WRITECACHE)	+= dm-writecache.o
+obj-$(CONFIG_DM_BOW)		+= dm-bow.o
 
 ifeq ($(CONFIG_DM_UEVENT),y)
 dm-mod-objs			+= dm-uevent.o
diff --git a/drivers/md/dm-bow.c b/drivers/md/dm-bow.c
new file mode 100644
index 000000000000..5cd953068f7c
--- /dev/null
+++ b/drivers/md/dm-bow.c
@@ -0,0 +1,1299 @@
+/*
+ * Copyright (C) 2018 Google Limited.
+ *
+ * This file is released under the GPL.
+ */
+
+#include "dm.h"
+#include "dm-core.h"
+
+#include <linux/crc32.h>
+#include <linux/dm-bufio.h>
+#include <linux/module.h>
+
+#define DM_MSG_PREFIX "bow"
+
+struct log_entry {
+	u64 source;
+	u64 dest;
+	u32 size;
+	u32 checksum;
+} __packed;
+
+struct log_sector {
+	u32 magic;
+	u16 header_version;
+	u16 header_size;
+	u32 block_size;
+	u32 count;
+	u32 sequence;
+	sector_t sector0;
+	struct log_entry entries[];
+} __packed;
+
+/*
+ * MAGIC is BOW in ascii
+ */
+#define MAGIC 0x00574f42
+#define HEADER_VERSION 0x0100
+
+/*
+ * A sorted set of ranges representing the state of the data on the device.
+ * Use an rb_tree for fast lookup of a given sector
+ * Consecutive ranges are always of different type - operations on this
+ * set must merge matching consecutive ranges.
+ *
+ * Top range is always of type TOP
+ */
+struct bow_range {
+	struct rb_node		node;
+	sector_t		sector;
+	enum {
+		INVALID,	/* Type not set */
+		SECTOR0,	/* First sector - holds log record */
+		SECTOR0_CURRENT,/* Live contents of sector0 */
+		UNCHANGED,	/* Original contents */
+		TRIMMED,	/* Range has been trimmed */
+		CHANGED,	/* Range has been changed */
+		BACKUP,		/* Range is being used as a backup */
+		TOP,		/* Final range - sector is size of device */
+	} type;
+	struct list_head	trimmed_list; /* list of TRIMMED ranges */
+};
+
+static const char * const readable_type[] = {
+	"Invalid",
+	"Sector0",
+	"Sector0_current",
+	"Unchanged",
+	"Free",
+	"Changed",
+	"Backup",
+	"Top",
+};
+
+enum state {
+	TRIM,
+	CHECKPOINT,
+	COMMITTED,
+};
+
+struct bow_context {
+	struct dm_dev *dev;
+	u32 block_size;
+	u32 block_shift;
+	struct workqueue_struct *workqueue;
+	struct dm_bufio_client *bufio;
+	struct mutex ranges_lock; /* Hold to access this struct and/or ranges */
+	struct rb_root ranges;
+	struct dm_kobject_holder kobj_holder;	/* for sysfs attributes */
+	atomic_t state; /* One of the enum state values above */
+	u64 trims_total;
+	struct log_sector *log_sector;
+	struct list_head trimmed_list;
+	bool forward_trims;
+};
+
+sector_t range_top(struct bow_range *br)
+{
+	return container_of(rb_next(&br->node), struct bow_range, node)
+		->sector;
+}
+
+u64 range_size(struct bow_range *br)
+{
+	return (range_top(br) - br->sector) * SECTOR_SIZE;
+}
+
+static sector_t bvec_top(struct bvec_iter *bi_iter)
+{
+	return bi_iter->bi_sector + bi_iter->bi_size / SECTOR_SIZE;
+}
+
+/*
+ * Find the first range that overlaps with bi_iter
+ * bi_iter is set to the size of the overlapping sub-range
+ */
+static struct bow_range *find_first_overlapping_range(struct rb_root *ranges,
+						      struct bvec_iter *bi_iter)
+{
+	struct rb_node *node = ranges->rb_node;
+	struct bow_range *br;
+
+	while (node) {
+		br = container_of(node, struct bow_range, node);
+
+		if (br->sector <= bi_iter->bi_sector
+		    && bi_iter->bi_sector < range_top(br))
+			break;
+
+		if (bi_iter->bi_sector < br->sector)
+			node = node->rb_left;
+		else
+			node = node->rb_right;
+	}
+
+	WARN_ON(!node);
+	if (!node)
+		return NULL;
+
+	if (range_top(br) - bi_iter->bi_sector
+	    < bi_iter->bi_size >> SECTOR_SHIFT)
+		bi_iter->bi_size = (range_top(br) - bi_iter->bi_sector)
+			<< SECTOR_SHIFT;
+
+	return br;
+}
+
+void add_before(struct rb_root *ranges, struct bow_range *new_br,
+		struct bow_range *existing)
+{
+	struct rb_node *parent = &(existing->node);
+	struct rb_node **link = &(parent->rb_left);
+
+	while (*link) {
+		parent = *link;
+		link = &((*link)->rb_right);
+	}
+
+	rb_link_node(&new_br->node, parent, link);
+	rb_insert_color(&new_br->node, ranges);
+}
+
+/*
+ * Given a range br returned by find_first_overlapping_range, split br into a
+ * leading range, a range matching the bi_iter and a trailing range.
+ * Leading and trailing may end up size 0 and will then be deleted. The
+ * new range matching the bi_iter is then returned and should have its type
+ * and type specific fields populated.
+ * If bi_iter runs off the end of the range, bi_iter is truncated accordingly
+ */
+static int split_range(struct bow_context *bc, struct bow_range **br,
+		       struct bvec_iter *bi_iter)
+{
+	struct bow_range *new_br;
+
+	if (bi_iter->bi_sector < (*br)->sector) {
+		WARN_ON(true);
+		return BLK_STS_IOERR;
+	}
+
+	if (bi_iter->bi_sector > (*br)->sector) {
+		struct bow_range *leading_br =
+			kzalloc(sizeof(*leading_br), GFP_KERNEL);
+
+		if (!leading_br)
+			return BLK_STS_RESOURCE;
+
+		*leading_br = **br;
+		if (leading_br->type == TRIMMED)
+			list_add(&leading_br->trimmed_list, &bc->trimmed_list);
+
+		add_before(&bc->ranges, leading_br, *br);
+		(*br)->sector = bi_iter->bi_sector;
+	}
+
+	if (bvec_top(bi_iter) >= range_top(*br)) {
+		bi_iter->bi_size = (range_top(*br) - (*br)->sector)
+					* SECTOR_SIZE;
+		return BLK_STS_OK;
+	}
+
+	/* new_br will be the beginning, existing br will be the tail */
+	new_br = kzalloc(sizeof(*new_br), GFP_KERNEL);
+	if (!new_br)
+		return BLK_STS_RESOURCE;
+
+	new_br->sector = (*br)->sector;
+	(*br)->sector = bvec_top(bi_iter);
+	add_before(&bc->ranges, new_br, *br);
+	*br = new_br;
+
+	return BLK_STS_OK;
+}
+
+/*
+ * Sets type of a range. May merge range into surrounding ranges
+ * Since br may be invalidated, always sets br to NULL to prevent
+ * usage after this is called
+ */
+static void set_type(struct bow_context *bc, struct bow_range **br, int type)
+{
+	struct bow_range *prev = container_of(rb_prev(&(*br)->node),
+						      struct bow_range, node);
+	struct bow_range *next = container_of(rb_next(&(*br)->node),
+						      struct bow_range, node);
+
+	if ((*br)->type == TRIMMED) {
+		bc->trims_total -= range_size(*br);
+		list_del(&(*br)->trimmed_list);
+	}
+
+	if (type == TRIMMED) {
+		bc->trims_total += range_size(*br);
+		list_add(&(*br)->trimmed_list, &bc->trimmed_list);
+	}
+
+	(*br)->type = type;
+
+	if (next->type == type) {
+		if (type == TRIMMED)
+			list_del(&next->trimmed_list);
+		rb_erase(&next->node, &bc->ranges);
+		kfree(next);
+	}
+
+	if (prev->type == type) {
+		if (type == TRIMMED)
+			list_del(&(*br)->trimmed_list);
+		rb_erase(&(*br)->node, &bc->ranges);
+		kfree(*br);
+	}
+
+	*br = NULL;
+}
+
+static struct bow_range *find_free_range(struct bow_context *bc)
+{
+	if (list_empty(&bc->trimmed_list)) {
+		DMERR("Unable to find free space to back up to");
+		return NULL;
+	}
+
+	return list_first_entry(&bc->trimmed_list, struct bow_range,
+				trimmed_list);
+}
+
+static sector_t sector_to_page(struct bow_context const *bc, sector_t sector)
+{
+	WARN_ON((sector & (((sector_t)1 << (bc->block_shift - SECTOR_SHIFT)) - 1))
+		!= 0);
+	return sector >> (bc->block_shift - SECTOR_SHIFT);
+}
+
+static int copy_data(struct bow_context const *bc,
+		     struct bow_range *source, struct bow_range *dest,
+		     u32 *checksum)
+{
+	int i;
+
+	if (range_size(source) != range_size(dest)) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+	if (checksum)
+		*checksum = sector_to_page(bc, source->sector);
+
+	for (i = 0; i < range_size(source) >> bc->block_shift; ++i) {
+		struct dm_buffer *read_buffer, *write_buffer;
+		u8 *read, *write;
+		sector_t page = sector_to_page(bc, source->sector) + i;
+
+		read = dm_bufio_read(bc->bufio, page, &read_buffer);
+		if (IS_ERR(read)) {
+			DMERR("Cannot read page %llu",
+			      (unsigned long long)page);
+			return PTR_ERR(read);
+		}
+
+		if (checksum)
+			*checksum = crc32(*checksum, read, bc->block_size);
+
+		write = dm_bufio_new(bc->bufio,
+				     sector_to_page(bc, dest->sector) + i,
+				     &write_buffer);
+		if (IS_ERR(write)) {
+			DMERR("Cannot write sector");
+			dm_bufio_release(read_buffer);
+			return PTR_ERR(write);
+		}
+
+		memcpy(write, read, bc->block_size);
+
+		dm_bufio_mark_buffer_dirty(write_buffer);
+		dm_bufio_release(write_buffer);
+		dm_bufio_release(read_buffer);
+	}
+
+	dm_bufio_write_dirty_buffers(bc->bufio);
+	return BLK_STS_OK;
+}
+
+/****** logging functions ******/
+
+static int add_log_entry(struct bow_context *bc, sector_t source, sector_t dest,
+			 unsigned int size, u32 checksum);
+
+static int backup_log_sector(struct bow_context *bc)
+{
+	struct bow_range *first_br, *free_br;
+	struct bvec_iter bi_iter;
+	u32 checksum = 0;
+	int ret;
+
+	first_br = container_of(rb_first(&bc->ranges), struct bow_range, node);
+
+	if (first_br->type != SECTOR0) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+	if (range_size(first_br) != bc->block_size) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+	free_br = find_free_range(bc);
+	/* No space left - return this error to userspace */
+	if (!free_br)
+		return BLK_STS_NOSPC;
+	bi_iter.bi_sector = free_br->sector;
+	bi_iter.bi_size = bc->block_size;
+	ret = split_range(bc, &free_br, &bi_iter);
+	if (ret)
+		return ret;
+	if (bi_iter.bi_size != bc->block_size) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+	ret = copy_data(bc, first_br, free_br, &checksum);
+	if (ret)
+		return ret;
+
+	bc->log_sector->count = 0;
+	bc->log_sector->sequence++;
+	ret = add_log_entry(bc, first_br->sector, free_br->sector,
+			    range_size(first_br), checksum);
+	if (ret)
+		return ret;
+
+	set_type(bc, &free_br, BACKUP);
+	return BLK_STS_OK;
+}
+
+static int add_log_entry(struct bow_context *bc, sector_t source, sector_t dest,
+			 unsigned int size, u32 checksum)
+{
+	struct dm_buffer *sector_buffer;
+	u8 *sector;
+
+	if (sizeof(struct log_sector)
+	    + sizeof(struct log_entry) * (bc->log_sector->count + 1)
+		> bc->block_size) {
+		int ret = backup_log_sector(bc);
+
+		if (ret)
+			return ret;
+	}
+
+	sector = dm_bufio_new(bc->bufio, 0, &sector_buffer);
+	if (IS_ERR(sector)) {
+		DMERR("Cannot write boot sector");
+		dm_bufio_release(sector_buffer);
+		return BLK_STS_NOSPC;
+	}
+
+	bc->log_sector->entries[bc->log_sector->count].source = source;
+	bc->log_sector->entries[bc->log_sector->count].dest = dest;
+	bc->log_sector->entries[bc->log_sector->count].size = size;
+	bc->log_sector->entries[bc->log_sector->count].checksum = checksum;
+	bc->log_sector->count++;
+
+	memcpy(sector, bc->log_sector, bc->block_size);
+	dm_bufio_mark_buffer_dirty(sector_buffer);
+	dm_bufio_release(sector_buffer);
+	dm_bufio_write_dirty_buffers(bc->bufio);
+	return BLK_STS_OK;
+}
+
+static int prepare_log(struct bow_context *bc)
+{
+	struct bow_range *free_br, *first_br;
+	struct bvec_iter bi_iter;
+	u32 checksum = 0;
+	int ret;
+
+	/* Carve out first sector as log sector */
+	first_br = container_of(rb_first(&bc->ranges), struct bow_range, node);
+	if (first_br->type != UNCHANGED) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+	if (range_size(first_br) < bc->block_size) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+	bi_iter.bi_sector = 0;
+	bi_iter.bi_size = bc->block_size;
+	ret = split_range(bc, &first_br, &bi_iter);
+	if (ret)
+		return ret;
+	first_br->type = SECTOR0;
+	if (range_size(first_br) != bc->block_size) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+	/* Find free sector for active sector0 reads/writes */
+	free_br = find_free_range(bc);
+	if (!free_br)
+		return BLK_STS_NOSPC;
+	bi_iter.bi_sector = free_br->sector;
+	bi_iter.bi_size = bc->block_size;
+	ret = split_range(bc, &free_br, &bi_iter);
+	if (ret)
+		return ret;
+
+	/* Copy data */
+	ret = copy_data(bc, first_br, free_br, NULL);
+	if (ret)
+		return ret;
+
+	bc->log_sector->sector0 = free_br->sector;
+
+	set_type(bc, &free_br, SECTOR0_CURRENT);
+
+	/* Find free sector to back up original sector zero */
+	free_br = find_free_range(bc);
+	if (!free_br)
+		return BLK_STS_NOSPC;
+	bi_iter.bi_sector = free_br->sector;
+	bi_iter.bi_size = bc->block_size;
+	ret = split_range(bc, &free_br, &bi_iter);
+	if (ret)
+		return ret;
+
+	/* Back up */
+	ret = copy_data(bc, first_br, free_br, &checksum);
+	if (ret)
+		return ret;
+
+	/*
+	 * Set up our replacement boot sector - it will get written when we
+	 * add the first log entry, which we do immediately
+	 */
+	bc->log_sector->magic = MAGIC;
+	bc->log_sector->header_version = HEADER_VERSION;
+	bc->log_sector->header_size = sizeof(*bc->log_sector);
+	bc->log_sector->block_size = bc->block_size;
+	bc->log_sector->count = 0;
+	bc->log_sector->sequence = 0;
+
+	/* Add log entry */
+	ret = add_log_entry(bc, first_br->sector, free_br->sector,
+			    range_size(first_br), checksum);
+	if (ret)
+		return ret;
+
+	set_type(bc, &free_br, BACKUP);
+	return BLK_STS_OK;
+}
+
+static struct bow_range *find_sector0_current(struct bow_context *bc)
+{
+	struct bvec_iter bi_iter;
+
+	bi_iter.bi_sector = bc->log_sector->sector0;
+	bi_iter.bi_size = bc->block_size;
+	return find_first_overlapping_range(&bc->ranges, &bi_iter);
+}
+
+/****** sysfs interface functions ******/
+
+static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
+			  char *buf)
+{
+	struct bow_context *bc = container_of(kobj, struct bow_context,
+					      kobj_holder.kobj);
+
+	return scnprintf(buf, PAGE_SIZE, "%d\n", atomic_read(&bc->state));
+}
+
+static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
+			   const char *buf, size_t count)
+{
+	struct bow_context *bc = container_of(kobj, struct bow_context,
+					      kobj_holder.kobj);
+	enum state state, original_state;
+	int ret;
+
+	state = buf[0] - '0';
+	if (state < TRIM || state > COMMITTED) {
+		DMERR("State value %d out of range", state);
+		return -EINVAL;
+	}
+
+	mutex_lock(&bc->ranges_lock);
+	original_state = atomic_read(&bc->state);
+	if (state != original_state + 1) {
+		DMERR("Invalid state change from %d to %d",
+		      original_state, state);
+		ret = -EINVAL;
+		goto bad;
+	}
+
+	DMINFO("Switching to state %s", state == CHECKPOINT ? "Checkpoint"
+	       : state == COMMITTED ? "Committed" : "Unknown");
+
+	if (state == CHECKPOINT) {
+		ret = prepare_log(bc);
+		if (ret) {
+			DMERR("Failed to switch to checkpoint state");
+			goto bad;
+		}
+	} else if (state == COMMITTED) {
+		struct bow_range *br = find_sector0_current(bc);
+		struct bow_range *sector0_br =
+			container_of(rb_first(&bc->ranges), struct bow_range,
+				     node);
+
+		ret = copy_data(bc, br, sector0_br, 0);
+		if (ret) {
+			DMERR("Failed to switch to committed state");
+			goto bad;
+		}
+	}
+	atomic_inc(&bc->state);
+	ret = count;
+
+bad:
+	mutex_unlock(&bc->ranges_lock);
+	return ret;
+}
+
+static ssize_t free_show(struct kobject *kobj, struct kobj_attribute *attr,
+			  char *buf)
+{
+	struct bow_context *bc = container_of(kobj, struct bow_context,
+					      kobj_holder.kobj);
+	u64 trims_total;
+
+	mutex_lock(&bc->ranges_lock);
+	trims_total = bc->trims_total;
+	mutex_unlock(&bc->ranges_lock);
+
+	return scnprintf(buf, PAGE_SIZE, "%llu\n", trims_total);
+}
+
+static struct kobj_attribute attr_state = __ATTR_RW(state);
+static struct kobj_attribute attr_free = __ATTR_RO(free);
+
+static struct attribute *bow_attrs[] = {
+	&attr_state.attr,
+	&attr_free.attr,
+	NULL
+};
+
+static struct kobj_type bow_ktype = {
+	.sysfs_ops = &kobj_sysfs_ops,
+	.default_attrs = bow_attrs,
+	.release = dm_kobject_release
+};
+
+/****** constructor/destructor ******/
+
+static void dm_bow_dtr(struct dm_target *ti)
+{
+	struct bow_context *bc = (struct bow_context *) ti->private;
+	struct kobject *kobj;
+
+	if (bc->workqueue)
+		destroy_workqueue(bc->workqueue);
+	if (bc->bufio)
+		dm_bufio_client_destroy(bc->bufio);
+
+	kobj = &bc->kobj_holder.kobj;
+	if (kobj->state_initialized) {
+		kobject_put(kobj);
+		wait_for_completion(dm_get_completion_from_kobject(kobj));
+	}
+
+	while (rb_first(&bc->ranges)) {
+		struct bow_range *br = container_of(rb_first(&bc->ranges),
+					      struct bow_range, node);
+
+		rb_erase(&br->node, &bc->ranges);
+		kfree(br);
+	}
+
+	mutex_destroy(&bc->ranges_lock);
+	kfree(bc->log_sector);
+	kfree(bc);
+}
+
+static void dm_bow_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	struct bow_context *bc = ti->private;
+	const unsigned int block_size = bc->block_size;
+
+	limits->logical_block_size =
+		max_t(unsigned int, limits->logical_block_size, block_size);
+	limits->physical_block_size =
+		max_t(unsigned int, limits->physical_block_size, block_size);
+	limits->io_min = max_t(unsigned int, limits->io_min, block_size);
+
+	if (limits->max_discard_sectors == 0) {
+		limits->discard_granularity = 1 << 12;
+		limits->max_hw_discard_sectors = 1 << 15;
+		limits->max_discard_sectors = 1 << 15;
+		bc->forward_trims = false;
+	} else {
+		limits->discard_granularity = 1 << 12;
+		bc->forward_trims = true;
+	}
+}
+
+static int dm_bow_ctr_optional(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	struct bow_context *bc = ti->private;
+	struct dm_arg_set as;
+	static const struct dm_arg _args[] = {
+		{0, 1, "Invalid number of feature args"},
+	};
+	unsigned int opt_params;
+	const char *opt_string;
+	int err;
+	char dummy;
+
+	as.argc = argc;
+	as.argv = argv;
+
+	err = dm_read_arg_group(_args, &as, &opt_params, &ti->error);
+	if (err)
+		return err;
+
+	while (opt_params--) {
+		opt_string = dm_shift_arg(&as);
+		if (!opt_string) {
+			ti->error = "Not enough feature arguments";
+			return -EINVAL;
+		}
+
+		if (sscanf(opt_string, "block_size:%u%c",
+					&bc->block_size, &dummy) == 1) {
+			if (bc->block_size < SECTOR_SIZE ||
+			    bc->block_size > 4096 ||
+			    !is_power_of_2(bc->block_size)) {
+				ti->error = "Invalid block_size";
+				return -EINVAL;
+			}
+		} else {
+			ti->error = "Invalid feature arguments";
+			return -EINVAL;
+		}
+	}
+
+	return 0;
+}
+
+static int dm_bow_ctr(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	struct bow_context *bc;
+	struct bow_range *br;
+	int ret;
+	struct mapped_device *md = dm_table_get_md(ti->table);
+
+	if (argc < 1) {
+		ti->error = "Invalid argument count";
+		return -EINVAL;
+	}
+
+	bc = kzalloc(sizeof(*bc), GFP_KERNEL);
+	if (!bc) {
+		ti->error = "Cannot allocate bow context";
+		return -ENOMEM;
+	}
+
+	ti->num_flush_bios = 1;
+	ti->num_discard_bios = 1;
+	ti->num_write_same_bios = 1;
+	ti->private = bc;
+
+	ret = dm_get_device(ti, argv[0], dm_table_get_mode(ti->table),
+			    &bc->dev);
+	if (ret) {
+		ti->error = "Device lookup failed";
+		goto bad;
+	}
+
+	bc->block_size =
+		bdev_get_queue(bc->dev->bdev)->limits.logical_block_size;
+	if (argc > 1) {
+		ret = dm_bow_ctr_optional(ti, argc - 1, &argv[1]);
+		if (ret)
+			goto bad;
+	}
+
+	bc->block_shift = ilog2(bc->block_size);
+	bc->log_sector = kzalloc(bc->block_size, GFP_KERNEL);
+	if (!bc->log_sector) {
+		ti->error = "Cannot allocate log sector";
+		goto bad;
+	}
+
+	init_completion(&bc->kobj_holder.completion);
+	ret = kobject_init_and_add(&bc->kobj_holder.kobj, &bow_ktype,
+				   &disk_to_dev(dm_disk(md))->kobj, "%s",
+				   "bow");
+	if (ret) {
+		ti->error = "Cannot create sysfs node";
+		goto bad;
+	}
+
+	mutex_init(&bc->ranges_lock);
+	bc->ranges = RB_ROOT;
+	bc->bufio = dm_bufio_client_create(bc->dev->bdev, bc->block_size, 1, 0,
+					   NULL, NULL);
+	if (IS_ERR(bc->bufio)) {
+		ti->error = "Cannot initialize dm-bufio";
+		ret = PTR_ERR(bc->bufio);
+		bc->bufio = NULL;
+		goto bad;
+	}
+
+	bc->workqueue = alloc_workqueue("dm-bow",
+					WQ_CPU_INTENSIVE | WQ_MEM_RECLAIM
+					| WQ_UNBOUND, num_online_cpus());
+	if (!bc->workqueue) {
+		ti->error = "Cannot allocate workqueue";
+		ret = -ENOMEM;
+		goto bad;
+	}
+
+	INIT_LIST_HEAD(&bc->trimmed_list);
+
+	br = kzalloc(sizeof(*br), GFP_KERNEL);
+	if (!br) {
+		ti->error = "Cannot allocate ranges";
+		ret = -ENOMEM;
+		goto bad;
+	}
+
+	br->sector = ti->len;
+	br->type = TOP;
+	rb_link_node(&br->node, NULL, &bc->ranges.rb_node);
+	rb_insert_color(&br->node, &bc->ranges);
+
+	br = kzalloc(sizeof(*br), GFP_KERNEL);
+	if (!br) {
+		ti->error = "Cannot allocate ranges";
+		ret = -ENOMEM;
+		goto bad;
+	}
+
+	br->sector = 0;
+	br->type = UNCHANGED;
+	rb_link_node(&br->node, bc->ranges.rb_node,
+		     &bc->ranges.rb_node->rb_left);
+	rb_insert_color(&br->node, &bc->ranges);
+
+	ti->discards_supported = true;
+
+	return 0;
+
+bad:
+	dm_bow_dtr(ti);
+	return ret;
+}
+
+/****** Handle writes ******/
+
+static int prepare_unchanged_range(struct bow_context *bc, struct bow_range *br,
+				   struct bvec_iter *bi_iter,
+				   bool record_checksum)
+{
+	struct bow_range *backup_br;
+	struct bvec_iter backup_bi;
+	sector_t log_source, log_dest;
+	unsigned int log_size;
+	u32 checksum = 0;
+	int ret;
+	int original_type;
+	sector_t sector0;
+
+	/* Find a free range */
+	backup_br = find_free_range(bc);
+	if (!backup_br)
+		return BLK_STS_NOSPC;
+
+	/* Carve out a backup range. This may be smaller than the br given */
+	backup_bi.bi_sector = backup_br->sector;
+	backup_bi.bi_size = min(range_size(backup_br), (u64) bi_iter->bi_size);
+	ret = split_range(bc, &backup_br, &backup_bi);
+	if (ret)
+		return ret;
+
+	/*
+	 * Carve out a changed range. This will not be smaller than the backup
+	 * br since the backup br is smaller than the source range and iterator
+	 */
+	bi_iter->bi_size = backup_bi.bi_size;
+	ret = split_range(bc, &br, bi_iter);
+	if (ret)
+		return ret;
+	if (range_size(br) != range_size(backup_br)) {
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+
+
+	/* Copy data over */
+	ret = copy_data(bc, br, backup_br, record_checksum ? &checksum : NULL);
+	if (ret)
+		return ret;
+
+	/* Add an entry to the log */
+	log_source = br->sector;
+	log_dest = backup_br->sector;
+	log_size = range_size(br);
+
+	/*
+	 * Set the types. Note that since set_type also amalgamates ranges
+	 * we have to set both sectors to their final type before calling
+	 * set_type on either
+	 */
+	original_type = br->type;
+	sector0 = backup_br->sector;
+	bc->trims_total -= range_size(backup_br);
+	if (backup_br->type == TRIMMED)
+		list_del(&backup_br->trimmed_list);
+	backup_br->type = br->type == SECTOR0_CURRENT ? SECTOR0_CURRENT
+						      : BACKUP;
+	br->type = CHANGED;
+	set_type(bc, &backup_br, backup_br->type);
+
+	/*
+	 * Add the log entry after marking the backup sector, since adding a log
+	 * can cause another backup
+	 */
+	ret = add_log_entry(bc, log_source, log_dest, log_size, checksum);
+	if (ret) {
+		br->type = original_type;
+		return ret;
+	}
+
+	/* Now it is safe to mark this backup successful */
+	if (original_type == SECTOR0_CURRENT)
+		bc->log_sector->sector0 = sector0;
+
+	set_type(bc, &br, br->type);
+	return ret;
+}
+
+static int prepare_free_range(struct bow_context *bc, struct bow_range *br,
+			      struct bvec_iter *bi_iter)
+{
+	int ret;
+
+	ret = split_range(bc, &br, bi_iter);
+	if (ret)
+		return ret;
+	set_type(bc, &br, CHANGED);
+	return BLK_STS_OK;
+}
+
+static int prepare_changed_range(struct bow_context *bc, struct bow_range *br,
+				 struct bvec_iter *bi_iter)
+{
+	/* Nothing to do ... */
+	return BLK_STS_OK;
+}
+
+static int prepare_one_range(struct bow_context *bc,
+			     struct bvec_iter *bi_iter)
+{
+	struct bow_range *br = find_first_overlapping_range(&bc->ranges,
+							    bi_iter);
+	switch (br->type) {
+	case CHANGED:
+		return prepare_changed_range(bc, br, bi_iter);
+
+	case TRIMMED:
+		return prepare_free_range(bc, br, bi_iter);
+
+	case UNCHANGED:
+	case BACKUP:
+		return prepare_unchanged_range(bc, br, bi_iter, true);
+
+	/*
+	 * We cannot track the checksum for the active sector0, since it
+	 * may change at any point.
+	 */
+	case SECTOR0_CURRENT:
+		return prepare_unchanged_range(bc, br, bi_iter, false);
+
+	case SECTOR0:	/* Handled in the dm_bow_map */
+	case TOP:	/* Illegal - top is off the end of the device */
+	default:
+		WARN_ON(1);
+		return BLK_STS_IOERR;
+	}
+}
+
+struct write_work {
+	struct work_struct work;
+	struct bow_context *bc;
+	struct bio *bio;
+};
+
+static void bow_write(struct work_struct *work)
+{
+	struct write_work *ww = container_of(work, struct write_work, work);
+	struct bow_context *bc = ww->bc;
+	struct bio *bio = ww->bio;
+	struct bvec_iter bi_iter = bio->bi_iter;
+	int ret = BLK_STS_OK;
+
+	kfree(ww);
+
+	mutex_lock(&bc->ranges_lock);
+	do {
+		ret = prepare_one_range(bc, &bi_iter);
+		bi_iter.bi_sector += bi_iter.bi_size / SECTOR_SIZE;
+		bi_iter.bi_size = bio->bi_iter.bi_size
+			- (bi_iter.bi_sector - bio->bi_iter.bi_sector)
+			  * SECTOR_SIZE;
+	} while (!ret && bi_iter.bi_size);
+
+	mutex_unlock(&bc->ranges_lock);
+
+	if (!ret) {
+		bio_set_dev(bio, bc->dev->bdev);
+		submit_bio(bio);
+	} else {
+		DMERR("Write failure with error %d", -ret);
+		bio->bi_status = ret;
+		bio_endio(bio);
+	}
+}
+
+static int queue_write(struct bow_context *bc, struct bio *bio)
+{
+	struct write_work *ww = kmalloc(sizeof(*ww), GFP_NOIO | __GFP_NORETRY
+					| __GFP_NOMEMALLOC | __GFP_NOWARN);
+	if (!ww) {
+		DMERR("Failed to allocate write_work");
+		return -ENOMEM;
+	}
+
+	INIT_WORK(&ww->work, bow_write);
+	ww->bc = bc;
+	ww->bio = bio;
+	queue_work(bc->workqueue, &ww->work);
+	return DM_MAPIO_SUBMITTED;
+}
+
+static int handle_sector0(struct bow_context *bc, struct bio *bio)
+{
+	int ret = DM_MAPIO_REMAPPED;
+
+	if (bio->bi_iter.bi_size > bc->block_size) {
+		struct bio * split = bio_split(bio,
+					       bc->block_size >> SECTOR_SHIFT,
+					       GFP_NOIO,
+					       &fs_bio_set);
+		if (!split) {
+			DMERR("Failed to split bio");
+			bio->bi_status = BLK_STS_RESOURCE;
+			bio_endio(bio);
+			return DM_MAPIO_SUBMITTED;
+		}
+
+		bio_chain(split, bio);
+		split->bi_iter.bi_sector = bc->log_sector->sector0;
+		bio_set_dev(split, bc->dev->bdev);
+		submit_bio(split);
+
+		if (bio_data_dir(bio) == WRITE)
+			ret = queue_write(bc, bio);
+	} else {
+		bio->bi_iter.bi_sector = bc->log_sector->sector0;
+	}
+
+	return ret;
+}
+
+static int add_trim(struct bow_context *bc, struct bio *bio)
+{
+	struct bow_range *br;
+	struct bvec_iter bi_iter = bio->bi_iter;
+
+	DMDEBUG("add_trim: %llu, %u",
+		(unsigned long long)bio->bi_iter.bi_sector,
+		bio->bi_iter.bi_size);
+
+	do {
+		br = find_first_overlapping_range(&bc->ranges, &bi_iter);
+
+		switch (br->type) {
+		case UNCHANGED:
+			if (!split_range(bc, &br, &bi_iter))
+				set_type(bc, &br, TRIMMED);
+			break;
+
+		case TRIMMED:
+			/* Nothing to do */
+			break;
+
+		default:
+			/* No other case is legal in TRIM state */
+			WARN_ON(true);
+			break;
+		}
+
+		bi_iter.bi_sector += bi_iter.bi_size / SECTOR_SIZE;
+		bi_iter.bi_size = bio->bi_iter.bi_size
+			- (bi_iter.bi_sector - bio->bi_iter.bi_sector)
+			  * SECTOR_SIZE;
+
+	} while (bi_iter.bi_size);
+
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+}
+
+static int remove_trim(struct bow_context *bc, struct bio *bio)
+{
+	struct bow_range *br;
+	struct bvec_iter bi_iter = bio->bi_iter;
+
+	DMDEBUG("remove_trim: %llu, %u",
+		(unsigned long long)bio->bi_iter.bi_sector,
+		bio->bi_iter.bi_size);
+
+	do {
+		br = find_first_overlapping_range(&bc->ranges, &bi_iter);
+
+		switch (br->type) {
+		case UNCHANGED:
+			/* Nothing to do */
+			break;
+
+		case TRIMMED:
+			if (!split_range(bc, &br, &bi_iter))
+				set_type(bc, &br, UNCHANGED);
+			break;
+
+		default:
+			/* No other case is legal in TRIM state */
+			WARN_ON(true);
+			break;
+		}
+
+		bi_iter.bi_sector += bi_iter.bi_size / SECTOR_SIZE;
+		bi_iter.bi_size = bio->bi_iter.bi_size
+			- (bi_iter.bi_sector - bio->bi_iter.bi_sector)
+			  * SECTOR_SIZE;
+
+	} while (bi_iter.bi_size);
+
+	return DM_MAPIO_REMAPPED;
+}
+
+int remap_unless_illegal_trim(struct bow_context *bc, struct bio *bio)
+{
+	if (!bc->forward_trims && bio_op(bio) == REQ_OP_DISCARD) {
+		bio->bi_status = BLK_STS_NOTSUPP;
+		bio_endio(bio);
+		return DM_MAPIO_SUBMITTED;
+	} else {
+		bio_set_dev(bio, bc->dev->bdev);
+		return DM_MAPIO_REMAPPED;
+	}
+}
+
+/****** dm interface ******/
+
+static int dm_bow_map(struct dm_target *ti, struct bio *bio)
+{
+	int ret = DM_MAPIO_REMAPPED;
+	struct bow_context *bc = ti->private;
+
+	if (likely(bc->state.counter == COMMITTED))
+		return remap_unless_illegal_trim(bc, bio);
+
+	if (bio_data_dir(bio) == READ && bio->bi_iter.bi_sector != 0)
+		return remap_unless_illegal_trim(bc, bio);
+
+	if (atomic_read(&bc->state) != COMMITTED) {
+		enum state state;
+
+		mutex_lock(&bc->ranges_lock);
+		state = atomic_read(&bc->state);
+		if (state == TRIM) {
+			if (bio_op(bio) == REQ_OP_DISCARD)
+				ret = add_trim(bc, bio);
+			else if (bio_data_dir(bio) == WRITE)
+				ret = remove_trim(bc, bio);
+			else
+				/* pass-through */;
+		} else if (state == CHECKPOINT) {
+			if (bio->bi_iter.bi_sector == 0)
+				ret = handle_sector0(bc, bio);
+			else if (bio_data_dir(bio) == WRITE)
+				ret = queue_write(bc, bio);
+			else
+				/* pass-through */;
+		} else {
+			/* pass-through */
+		}
+		mutex_unlock(&bc->ranges_lock);
+	}
+
+	if (ret == DM_MAPIO_REMAPPED)
+		return remap_unless_illegal_trim(bc, bio);
+
+	return ret;
+}
+
+static void dm_bow_tablestatus(struct dm_target *ti, char *result,
+			       unsigned int maxlen)
+{
+	char *end = result + maxlen;
+	struct bow_context *bc = ti->private;
+	struct rb_node *i;
+	int trimmed_list_length = 0;
+	int trimmed_range_count = 0;
+	struct bow_range *br;
+
+	if (maxlen == 0)
+		return;
+	result[0] = 0;
+
+	list_for_each_entry(br, &bc->trimmed_list, trimmed_list)
+		if (br->type == TRIMMED) {
+			++trimmed_list_length;
+		} else {
+			scnprintf(result, end - result,
+				  "ERROR: non-trimmed entry in trimmed_list");
+			return;
+		}
+
+	if (!rb_first(&bc->ranges)) {
+		scnprintf(result, end - result, "ERROR: Empty ranges");
+		return;
+	}
+
+	if (container_of(rb_first(&bc->ranges), struct bow_range, node)
+	    ->sector) {
+		scnprintf(result, end - result,
+			 "ERROR: First range does not start at sector 0");
+		return;
+	}
+
+	for (i = rb_first(&bc->ranges); i; i = rb_next(i)) {
+		struct bow_range *br = container_of(i, struct bow_range, node);
+
+		result += scnprintf(result, end - result, "%s: %llu",
+				    readable_type[br->type],
+				    (unsigned long long)br->sector);
+		if (result >= end)
+			return;
+
+		result += scnprintf(result, end - result, "\n");
+		if (result >= end)
+			return;
+
+		if (br->type == TRIMMED)
+			++trimmed_range_count;
+
+		if (br->type == TOP) {
+			if (br->sector != ti->len) {
+				scnprintf(result, end - result,
+					 "\nERROR: Top sector is incorrect");
+			}
+
+			if (&br->node != rb_last(&bc->ranges)) {
+				scnprintf(result, end - result,
+					  "\nERROR: Top sector is not last");
+			}
+
+			break;
+		}
+
+		if (!rb_next(i)) {
+			scnprintf(result, end - result,
+				  "\nERROR: Last range not of type TOP");
+			return;
+		}
+
+		if (br->sector > range_top(br)) {
+			scnprintf(result, end - result,
+				  "\nERROR: sectors out of order");
+			return;
+		}
+	}
+
+	if (trimmed_range_count != trimmed_list_length)
+		scnprintf(result, end - result,
+			  "\nERROR: not all trimmed ranges in trimmed list");
+}
+
+static void dm_bow_status(struct dm_target *ti, status_type_t type,
+			  unsigned int status_flags, char *result,
+			  unsigned int maxlen)
+{
+	switch (type) {
+	case STATUSTYPE_INFO:
+		if (maxlen)
+			result[0] = 0;
+		break;
+
+	case STATUSTYPE_TABLE:
+		dm_bow_tablestatus(ti, result, maxlen);
+		break;
+	}
+}
+
+int dm_bow_prepare_ioctl(struct dm_target *ti, struct block_device **bdev)
+{
+	struct bow_context *bc = ti->private;
+	struct dm_dev *dev = bc->dev;
+
+	*bdev = dev->bdev;
+	/* Only pass ioctls through if the device sizes match exactly. */
+	return ti->len != i_size_read(dev->bdev->bd_inode) >> SECTOR_SHIFT;
+}
+
+static int dm_bow_iterate_devices(struct dm_target *ti,
+				  iterate_devices_callout_fn fn, void *data)
+{
+	struct bow_context *bc = ti->private;
+
+	return fn(ti, bc->dev, 0, ti->len, data);
+}
+
+static struct target_type bow_target = {
+	.name   = "bow",
+	.version = {1, 2, 0},
+	.module = THIS_MODULE,
+	.ctr    = dm_bow_ctr,
+	.dtr    = dm_bow_dtr,
+	.map    = dm_bow_map,
+	.status = dm_bow_status,
+	.prepare_ioctl  = dm_bow_prepare_ioctl,
+	.iterate_devices = dm_bow_iterate_devices,
+	.io_hints = dm_bow_io_hints,
+};
+
+int __init dm_bow_init(void)
+{
+	int r = dm_register_target(&bow_target);
+
+	if (r < 0)
+		DMERR("registering bow failed %d", r);
+	return r;
+}
+
+void dm_bow_exit(void)
+{
+	dm_unregister_target(&bow_target);
+}
+
+MODULE_LICENSE("GPL");
+
+module_init(dm_bow_init);
+module_exit(dm_bow_exit);