dm: add log writes target

Introduce a new target that is meant for file system developers to test file
system integrity at particular points in the life of a file system.  We capture
all write requests and associated data and log them to a separate device
for later replay.  There is a userspace utility to do this replay.  The
idea behind this is to give file system developers a tool to verify that
the file system is always consistent.
Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Zach Brown <zab@zabbo.net>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Documentation/device-mapper/log-writes.txt

+dm-log-writes
+=============
+
+This target takes 2 devices, one to pass all IO to normally, and one to log all
+of the write operations to.  This is intended for file system developers wishing
+to verify the integrity of metadata or data as the file system is written to.
+There is a log_write_entry written for every WRITE request and the target is
+able to take arbitrary data from userspace to insert into the log.  The data
+that is in the WRITE requests is copied into the log to make the replay happen
+exactly as it happened originally.
+
+Log Ordering
+============
+
+We log things in order of completion once we are sure the write is no longer in
+cache.  This means that normal WRITE requests are not actually logged until the
+next REQ_FLUSH request.  This is to make it easier for userspace to replay the
+log in a way that correlates to what is on disk and not what is in cache, to
+make it easier to detect improper waiting/flushing.
+
+This works by attaching all WRITE requests to a list once the write completes.
+Once we see a REQ_FLUSH request we splice this list onto the request and once
+the FLUSH request completes we log all of the WRITEs and then the FLUSH.  Only
+completed WRITEs, at the time the REQ_FLUSH is issued, are added in order to
+simulate the worst case scenario with regard to power failures.  Consider the
+following example (W means write, C means complete):
+
+W1,W2,W3,C3,C2,Wflush,C1,Cflush
+
+The log would show the following
+
+W3,W2,flush,W1....
+
+Again this is to simulate what is actually on disk, this allows us to detect
+cases where a power failure at a particular point in time would create an
+inconsistent file system.
+
+Any REQ_FUA requests bypass this flushing mechanism and are logged as soon as
+they complete as those requests will obviously bypass the device cache.
+
+Any REQ_DISCARD requests are treated like WRITE requests.  Otherwise we would
+have all the DISCARD requests, and then the WRITE requests and then the FLUSH
+request.  Consider the following example:
+
+WRITE block 1, DISCARD block 1, FLUSH
+
+If we logged DISCARD when it completed, the replay would look like this
+
+DISCARD 1, WRITE 1, FLUSH
+
+which isn't quite what happened and wouldn't be caught during the log replay.
+
+Target interface
+================
+
+i) Constructor
+
+   log-writes <dev_path> <log_dev_path>
+
+   dev_path	: Device that all of the IO will go to normally.
+   log_dev_path : Device where the log entries are written to.
+
+ii) Status
+
+    <#logged entries> <highest allocated sector>
+
+    #logged entries	       : Number of logged entries
+    highest allocated sector   : Highest allocated sector
+
+iii) Messages
+
+    mark <description>
+
+	You can use a dmsetup message to set an arbitrary mark in a log.
+	For example say you want to fsck a file system after every
+	write, but first you need to replay up to the mkfs to make sure
+	we're fsck'ing something reasonable, you would do something like
+	this:
+
+	  mkfs.btrfs -f /dev/mapper/log
+	  dmsetup message log 0 mark mkfs
+	  <run test>
+
+	  This would allow you to replay the log up to the mkfs mark and
+	  then replay from that point on doing the fsck check in the
+	  interval that you want.
+
+	Every log has a mark at the end labeled "dm-log-writes-end".
+
+Userspace component
+===================
+
+There is a userspace tool that will replay the log for you in various ways.
+It can be found here: https://github.com/josefbacik/log-writes
+
+Example usage
+=============
+
+Say you want to test fsync on your file system.  You would do something like
+this:
+
+TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc"
+dmsetup create log --table "$TABLE"
+mkfs.btrfs -f /dev/mapper/log
+dmsetup message log 0 mark mkfs
+
+mount /dev/mapper/log /mnt/btrfs-test
+<some test that does fsync at the end>
+dmsetup message log 0 mark fsync
+md5sum /mnt/btrfs-test/foo
+umount /mnt/btrfs-test
+
+dmsetup remove log
+replay-log --log /dev/sdc --replay /dev/sdb --end-mark fsync
+mount /dev/sdb /mnt/btrfs-test
+md5sum /mnt/btrfs-test/foo
+<verify md5sum's are correct>
+
+Another option is to do a complicated file system operation and verify the file
+system is consistent during the entire operation.  You could do this with:
+
+TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc"
+dmsetup create log --table "$TABLE"
+mkfs.btrfs -f /dev/mapper/log
+dmsetup message log 0 mark mkfs
+
+mount /dev/mapper/log /mnt/btrfs-test
+<fsstress to dirty the fs>
+btrfs filesystem balance /mnt/btrfs-test
+umount /mnt/btrfs-test
+dmsetup remove log
+
+replay-log --log /dev/sdc --replay /dev/sdb --end-mark mkfs
+btrfsck /dev/sdb
+replay-log --log /dev/sdc --replay /dev/sdb --start-mark mkfs \
+	--fsck "btrfsck /dev/sdb" --check fua
+
+And that will replay the log until it sees a FUA request, run the fsck command
+and if the fsck passes it will replay to the next FUA, until it is completed or
+the fsck command exists abnormally.

drivers/md/Kconfig

@@ -443,4 +443,20 @@ config DM_SWITCH
 
 	  If unsure, say N.
 
+config DM_LOG_WRITES
+	tristate "Log writes target support"
+	depends on BLK_DEV_DM
+	---help---
+	  This device-mapper target takes two devices, one device to use
+	  normally, one to log all write operations done to the first device.
+	  This is for use by file system developers wishing to verify that
+	  their fs is writing a consitent file system at all times by allowing
+	  them to replay the log in a variety of ways and to check the
+	  contents.
+
+	  To compile this code as a module, choose M here: the module will
+	  be called dm-log-writes.
+
+	  If unsure, say N.
+
 endif # MD

drivers/md/Makefile

@@ -55,6 +55,7 @@ obj-$(CONFIG_DM_CACHE)		+= dm-cache.o
 obj-$(CONFIG_DM_CACHE_MQ)	+= dm-cache-mq.o
 obj-$(CONFIG_DM_CACHE_CLEANER)	+= dm-cache-cleaner.o
 obj-$(CONFIG_DM_ERA)		+= dm-era.o
+obj-$(CONFIG_DM_LOG_WRITES)	+= dm-log-writes.o
 
 ifeq ($(CONFIG_DM_UEVENT),y)
 dm-mod-objs			+= dm-uevent.o

drivers/md/dm-log-writes.c

+/*
+ * Copyright (C) 2014 Facebook. All rights reserved.
+ *
+ * This file is released under the GPL.
+ */
+
+#include <linux/device-mapper.h>
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/blkdev.h>
+#include <linux/bio.h>
+#include <linux/slab.h>
+#include <linux/kthread.h>
+#include <linux/freezer.h>
+
+#define DM_MSG_PREFIX "log-writes"
+
+/*
+ * This target will sequentially log all writes to the target device onto the
+ * log device.  This is helpful for replaying writes to check for fs consistency
+ * at all times.  This target provides a mechanism to mark specific events to
+ * check data at a later time.  So for example you would:
+ *
+ * write data
+ * fsync
+ * dmsetup message /dev/whatever mark mymark
+ * unmount /mnt/test
+ *
+ * Then replay the log up to mymark and check the contents of the replay to
+ * verify it matches what was written.
+ *
+ * We log writes only after they have been flushed, this makes the log describe
+ * close to the order in which the data hits the actual disk, not its cache.  So
+ * for example the following sequence (W means write, C means complete)
+ *
+ * Wa,Wb,Wc,Cc,Ca,FLUSH,FUAd,Cb,CFLUSH,CFUAd
+ *
+ * Would result in the log looking like this:
+ *
+ * c,a,flush,fuad,b,<other writes>,<next flush>
+ *
+ * This is meant to help expose problems where file systems do not properly wait
+ * on data being written before invoking a FLUSH.  FUA bypasses cache so once it
+ * completes it is added to the log as it should be on disk.
+ *
+ * We treat DISCARDs as if they don't bypass cache so that they are logged in
+ * order of completion along with the normal writes.  If we didn't do it this
+ * way we would process all the discards first and then write all the data, when
+ * in fact we want to do the data and the discard in the order that they
+ * completed.
+ */
+#define LOG_FLUSH_FLAG (1 << 0)
+#define LOG_FUA_FLAG (1 << 1)
+#define LOG_DISCARD_FLAG (1 << 2)
+#define LOG_MARK_FLAG (1 << 3)
+
+#define WRITE_LOG_VERSION 1
+#define WRITE_LOG_MAGIC 0x6a736677736872
+
+/*
+ * The disk format for this is braindead simple.
+ *
+ * At byte 0 we have our super, followed by the following sequence for
+ * nr_entries:
+ *
+ * [   1 sector    ][  entry->nr_sectors ]
+ * [log_write_entry][    data written    ]
+ *
+ * The log_write_entry takes up a full sector so we can have arbitrary length
+ * marks and it leaves us room for extra content in the future.
+ */
+
+/*
+ * Basic info about the log for userspace.
+ */
+struct log_write_super {
+	__le64 magic;
+	__le64 version;
+	__le64 nr_entries;
+	__le32 sectorsize;
+};
+
+/*
+ * sector - the sector we wrote.
+ * nr_sectors - the number of sectors we wrote.
+ * flags - flags for this log entry.
+ * data_len - the size of the data in this log entry, this is for private log
+ * entry stuff, the MARK data provided by userspace for example.
+ */
+struct log_write_entry {
+	__le64 sector;
+	__le64 nr_sectors;
+	__le64 flags;
+	__le64 data_len;
+};
+
+struct log_writes_c {
+	struct dm_dev *dev;
+	struct dm_dev *logdev;
+	u64 logged_entries;
+	u32 sectorsize;
+	atomic_t io_blocks;
+	atomic_t pending_blocks;
+	sector_t next_sector;
+	sector_t end_sector;
+	bool logging_enabled;
+	bool device_supports_discard;
+	spinlock_t blocks_lock;
+	struct list_head unflushed_blocks;
+	struct list_head logging_blocks;
+	wait_queue_head_t wait;
+	struct task_struct *log_kthread;
+};
+
+struct pending_block {
+	int vec_cnt;
+	u64 flags;
+	sector_t sector;
+	sector_t nr_sectors;
+	char *data;
+	u32 datalen;
+	struct list_head list;
+	struct bio_vec vecs[0];
+};
+
+struct per_bio_data {
+	struct pending_block *block;
+};
+
+static void put_pending_block(struct log_writes_c *lc)
+{
+	if (atomic_dec_and_test(&lc->pending_blocks)) {
+		smp_mb__after_atomic();
+		if (waitqueue_active(&lc->wait))
+			wake_up(&lc->wait);
+	}
+}
+
+static void put_io_block(struct log_writes_c *lc)
+{
+	if (atomic_dec_and_test(&lc->io_blocks)) {
+		smp_mb__after_atomic();
+		if (waitqueue_active(&lc->wait))
+			wake_up(&lc->wait);
+	}
+}
+
+static void log_end_io(struct bio *bio, int err)
+{
+	struct log_writes_c *lc = bio->bi_private;
+	struct bio_vec *bvec;
+	int i;
+
+	if (err) {
+		unsigned long flags;
+
+		DMERR("Error writing log block, error=%d", err);
+		spin_lock_irqsave(&lc->blocks_lock, flags);
+		lc->logging_enabled = false;
+		spin_unlock_irqrestore(&lc->blocks_lock, flags);
+	}
+
+	bio_for_each_segment_all(bvec, bio, i)
+		__free_page(bvec->bv_page);
+
+	put_io_block(lc);
+	bio_put(bio);
+}
+
+/*
+ * Meant to be called if there is an error, it will free all the pages
+ * associated with the block.
+ */
+static void free_pending_block(struct log_writes_c *lc,
+			       struct pending_block *block)
+{
+	int i;
+
+	for (i = 0; i < block->vec_cnt; i++) {
+		if (block->vecs[i].bv_page)
+			__free_page(block->vecs[i].bv_page);
+	}
+	kfree(block->data);
+	kfree(block);
+	put_pending_block(lc);
+}
+
+static int write_metadata(struct log_writes_c *lc, void *entry,
+			  size_t entrylen, void *data, size_t datalen,
+			  sector_t sector)
+{
+	struct bio *bio;
+	struct page *page;
+	void *ptr;
+	size_t ret;
+
+	bio = bio_alloc(GFP_KERNEL, 1);
+	if (!bio) {
+		DMERR("Couldn't alloc log bio");
+		goto error;
+	}
+	bio->bi_iter.bi_size = 0;
+	bio->bi_iter.bi_sector = sector;
+	bio->bi_bdev = lc->logdev->bdev;
+	bio->bi_end_io = log_end_io;
+	bio->bi_private = lc;
+	set_bit(BIO_UPTODATE, &bio->bi_flags);
+
+	page = alloc_page(GFP_KERNEL);
+	if (!page) {
+		DMERR("Couldn't alloc log page");
+		bio_put(bio);
+		goto error;
+	}
+
+	ptr = kmap_atomic(page);
+	memcpy(ptr, entry, entrylen);
+	if (datalen)
+		memcpy(ptr + entrylen, data, datalen);
+	memset(ptr + entrylen + datalen, 0,
+	       lc->sectorsize - entrylen - datalen);
+	kunmap_atomic(ptr);
+
+	ret = bio_add_page(bio, page, lc->sectorsize, 0);
+	if (ret != lc->sectorsize) {
+		DMERR("Couldn't add page to the log block");
+		goto error_bio;
+	}
+	submit_bio(WRITE, bio);
+	return 0;
+error_bio:
+	bio_put(bio);
+	__free_page(page);
+error:
+	put_io_block(lc);
+	return -1;
+}
+
+static int log_one_block(struct log_writes_c *lc,
+			 struct pending_block *block, sector_t sector)
+{
+	struct bio *bio;
+	struct log_write_entry entry;
+	size_t ret;
+	int i;
+
+	entry.sector = cpu_to_le64(block->sector);
+	entry.nr_sectors = cpu_to_le64(block->nr_sectors);
+	entry.flags = cpu_to_le64(block->flags);
+	entry.data_len = cpu_to_le64(block->datalen);
+	if (write_metadata(lc, &entry, sizeof(entry), block->data,
+			   block->datalen, sector)) {
+		free_pending_block(lc, block);
+		return -1;
+	}
+
+	if (!block->vec_cnt)
+		goto out;
+	sector++;
+
+	bio = bio_alloc(GFP_KERNEL, block->vec_cnt);
+	if (!bio) {
+		DMERR("Couldn't alloc log bio");
+		goto error;
+	}
+	atomic_inc(&lc->io_blocks);
+	bio->bi_iter.bi_size = 0;
+	bio->bi_iter.bi_sector = sector;
+	bio->bi_bdev = lc->logdev->bdev;
+	bio->bi_end_io = log_end_io;
+	bio->bi_private = lc;
+	set_bit(BIO_UPTODATE, &bio->bi_flags);
+
+	for (i = 0; i < block->vec_cnt; i++) {
+		/*
+		 * The page offset is always 0 because we allocate a new page
+		 * for every bvec in the original bio for simplicity sake.
+		 */
+		ret = bio_add_page(bio, block->vecs[i].bv_page,
+				   block->vecs[i].bv_len, 0);
+		if (ret != block->vecs[i].bv_len) {
+			atomic_inc(&lc->io_blocks);
+			submit_bio(WRITE, bio);
+			bio = bio_alloc(GFP_KERNEL, block->vec_cnt - i);
+			if (!bio) {
+				DMERR("Couldn't alloc log bio");
+				goto error;
+			}
+			bio->bi_iter.bi_size = 0;
+			bio->bi_iter.bi_sector = sector;
+			bio->bi_bdev = lc->logdev->bdev;
+			bio->bi_end_io = log_end_io;
+			bio->bi_private = lc;
+			set_bit(BIO_UPTODATE, &bio->bi_flags);
+
+			ret = bio_add_page(bio, block->vecs[i].bv_page,
+					   block->vecs[i].bv_len, 0);
+			if (ret != block->vecs[i].bv_len) {
+				DMERR("Couldn't add page on new bio?");
+				bio_put(bio);
+				goto error;
+			}
+		}
+		sector += block->vecs[i].bv_len >> SECTOR_SHIFT;
+	}
+	submit_bio(WRITE, bio);
+out:
+	kfree(block->data);
+	kfree(block);
+	put_pending_block(lc);
+	return 0;
+error:
+	free_pending_block(lc, block);
+	put_io_block(lc);
+	return -1;
+}
+
+static int log_super(struct log_writes_c *lc)
+{
+	struct log_write_super super;
+
+	super.magic = cpu_to_le64(WRITE_LOG_MAGIC);
+	super.version = cpu_to_le64(WRITE_LOG_VERSION);
+	super.nr_entries = cpu_to_le64(lc->logged_entries);
+	super.sectorsize = cpu_to_le32(lc->sectorsize);
+
+	if (write_metadata(lc, &super, sizeof(super), NULL, 0, 0)) {
+		DMERR("Couldn't write super");
+		return -1;
+	}
+
+	return 0;
+}
+
+static inline sector_t logdev_last_sector(struct log_writes_c *lc)
+{
+	return i_size_read(lc->logdev->bdev->bd_inode) >> SECTOR_SHIFT;
+}
+
+static int log_writes_kthread(void *arg)
+{
+	struct log_writes_c *lc = (struct log_writes_c *)arg;
+	sector_t sector = 0;
+
+	while (!kthread_should_stop()) {
+		bool super = false;
+		bool logging_enabled;
+		struct pending_block *block = NULL;
+		int ret;
+
+		spin_lock_irq(&lc->blocks_lock);
+		if (!list_empty(&lc->logging_blocks)) {
+			block = list_first_entry(&lc->logging_blocks,
+						 struct pending_block, list);
+			list_del_init(&block->list);
+			if (!lc->logging_enabled)
+				goto next;
+
+			sector = lc->next_sector;
+			if (block->flags & LOG_DISCARD_FLAG)
+				lc->next_sector++;
+			else
+				lc->next_sector += block->nr_sectors + 1;
+
+			/*
+			 * Apparently the size of the device may not be known
+			 * right away, so handle this properly.
+			 */
+			if (!lc->end_sector)
+				lc->end_sector = logdev_last_sector(lc);
+			if (lc->end_sector &&
+			    lc->next_sector >= lc->end_sector) {
+				DMERR("Ran out of space on the logdev");
+				lc->logging_enabled = false;
+				goto next;
+			}
+			lc->logged_entries++;
+			atomic_inc(&lc->io_blocks);
+
+			super = (block->flags & (LOG_FUA_FLAG | LOG_MARK_FLAG));
+			if (super)
+				atomic_inc(&lc->io_blocks);
+		}
+next:
+		logging_enabled = lc->logging_enabled;
+		spin_unlock_irq(&lc->blocks_lock);
+		if (block) {
+			if (logging_enabled) {
+				ret = log_one_block(lc, block, sector);
+				if (!ret && super)
+					ret = log_super(lc);
+				if (ret) {
+					spin_lock_irq(&lc->blocks_lock);
+					lc->logging_enabled = false;
+					spin_unlock_irq(&lc->blocks_lock);
+				}
+			} else
+				free_pending_block(lc, block);
+			continue;
+		}
+
+		if (!try_to_freeze()) {
+			set_current_state(TASK_INTERRUPTIBLE);
+			if (!kthread_should_stop() &&
+			    !atomic_read(&lc->pending_blocks))
+				schedule();
+			__set_current_state(TASK_RUNNING);
+		}
+	}
+	return 0;
+}
+
+/*
+ * Construct a log-writes mapping:
+ * log-writes <dev_path> <log_dev_path>
+ */
+static int log_writes_ctr(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	struct log_writes_c *lc;
+	struct dm_arg_set as;
+	const char *devname, *logdevname;
+
+	as.argc = argc;
+	as.argv = argv;
+
+	if (argc < 2) {
+		ti->error = "Invalid argument count";
+		return -EINVAL;
+	}
+
+	lc = kzalloc(sizeof(struct log_writes_c), GFP_KERNEL);
+	if (!lc) {
+		ti->error = "Cannot allocate context";
+		return -ENOMEM;
+	}
+	spin_lock_init(&lc->blocks_lock);
+	INIT_LIST_HEAD(&lc->unflushed_blocks);
+	INIT_LIST_HEAD(&lc->logging_blocks);
+	init_waitqueue_head(&lc->wait);
+	lc->sectorsize = 1 << SECTOR_SHIFT;
+	atomic_set(&lc->io_blocks, 0);
+	atomic_set(&lc->pending_blocks, 0);
+
+	devname = dm_shift_arg(&as);
+	if (dm_get_device(ti, devname, dm_table_get_mode(ti->table), &lc->dev)) {
+		ti->error = "Device lookup failed";
+		goto bad;
+	}
+
+	logdevname = dm_shift_arg(&as);
+	if (dm_get_device(ti, logdevname, dm_table_get_mode(ti->table), &lc->logdev)) {
+		ti->error = "Log device lookup failed";
+		dm_put_device(ti, lc->dev);
+		goto bad;
+	}
+
+	lc->log_kthread = kthread_run(log_writes_kthread, lc, "log-write");
+	if (!lc->log_kthread) {
+		ti->error = "Couldn't alloc kthread";
+		dm_put_device(ti, lc->dev);
+		dm_put_device(ti, lc->logdev);
+		goto bad;
+	}
+
+	/* We put the super at sector 0, start logging at sector 1 */
+	lc->next_sector = 1;
+	lc->logging_enabled = true;
+	lc->end_sector = logdev_last_sector(lc);
+	lc->device_supports_discard = true;
+
+	ti->num_flush_bios = 1;
+	ti->flush_supported = true;
+	ti->num_discard_bios = 1;
+	ti->discards_supported = true;
+	ti->per_bio_data_size = sizeof(struct per_bio_data);
+	ti->private = lc;
+	return 0;
+
+bad:
+	kfree(lc);
+	return -EINVAL;
+}
+
+static int log_mark(struct log_writes_c *lc, char *data)
+{
+	struct pending_block *block;
+	size_t maxsize = lc->sectorsize - sizeof(struct log_write_entry);
+
+	block = kzalloc(sizeof(struct pending_block), GFP_KERNEL);
+	if (!block) {
+		DMERR("Error allocating pending block");
+		return -ENOMEM;
+	}
+
+	block->data = kstrndup(data, maxsize, GFP_KERNEL);
+	if (!block->data) {
+		DMERR("Error copying mark data");
+		kfree(block);
+		return -ENOMEM;
+	}
+	atomic_inc(&lc->pending_blocks);
+	block->datalen = strlen(block->data);
+	block->flags |= LOG_MARK_FLAG;
+	spin_lock_irq(&lc->blocks_lock);
+	list_add_tail(&block->list, &lc->logging_blocks);
+	spin_unlock_irq(&lc->blocks_lock);
+	wake_up_process(lc->log_kthread);
+	return 0;
+}
+
+static void log_writes_dtr(struct dm_target *ti)
+{
+	struct log_writes_c *lc = ti->private;
+
+	spin_lock_irq(&lc->blocks_lock);
+	list_splice_init(&lc->unflushed_blocks, &lc->logging_blocks);
+	spin_unlock_irq(&lc->blocks_lock);
+
+	/*
+	 * This is just nice to have since it'll update the super to include the
+	 * unflushed blocks, if it fails we don't really care.
+	 */
+	log_mark(lc, "dm-log-writes-end");
+	wake_up_process(lc->log_kthread);
+	wait_event(lc->wait, !atomic_read(&lc->io_blocks) &&
+		   !atomic_read(&lc->pending_blocks));
+	kthread_stop(lc->log_kthread);
+
+	WARN_ON(!list_empty(&lc->logging_blocks));
+	WARN_ON(!list_empty(&lc->unflushed_blocks));
+	dm_put_device(ti, lc->dev);
+	dm_put_device(ti, lc->logdev);
+	kfree(lc);
+}
+
+static void normal_map_bio(struct dm_target *ti, struct bio *bio)
+{
+	struct log_writes_c *lc = ti->private;
+
+	bio->bi_bdev = lc->dev->bdev;
+}
+
+static int log_writes_map(struct dm_target *ti, struct bio *bio)
+{
+	struct log_writes_c *lc = ti->private;
+	struct per_bio_data *pb = dm_per_bio_data(bio, sizeof(struct per_bio_data));
+	struct pending_block *block;
+	struct bvec_iter iter;
+	struct bio_vec bv;
+	size_t alloc_size;
+	int i = 0;
+	bool flush_bio = (bio->bi_rw & REQ_FLUSH);
+	bool fua_bio = (bio->bi_rw & REQ_FUA);
+	bool discard_bio = (bio->bi_rw & REQ_DISCARD);
+
+	pb->block = NULL;
+
+	/* Don't bother doing anything if logging has been disabled */
+	if (!lc->logging_enabled)
+		goto map_bio;
+
+	/*
+	 * Map reads as normal.
+	 */
+	if (bio_data_dir(bio) == READ)
+		goto map_bio;
+
+	/* No sectors and not a flush?  Don't care */
+	if (!bio_sectors(bio) && !flush_bio)
+		goto map_bio;
+
+	/*
+	 * Discards will have bi_size set but there's no actual data, so just
+	 * allocate the size of the pending block.
+	 */
+	if (discard_bio)
+		alloc_size = sizeof(struct pending_block);
+	else
+		alloc_size = sizeof(struct pending_block) + sizeof(struct bio_vec) * bio_segments(bio);
+
+	block = kzalloc(alloc_size, GFP_NOIO);
+	if (!block) {
+		DMERR("Error allocating pending block");
+		spin_lock_irq(&lc->blocks_lock);
+		lc->logging_enabled = false;
+		spin_unlock_irq(&lc->blocks_lock);
+		return -ENOMEM;
+	}
+	INIT_LIST_HEAD(&block->list);
+	pb->block = block;
+	atomic_inc(&lc->pending_blocks);
+
+	if (flush_bio)
+		block->flags |= LOG_FLUSH_FLAG;
+	if (fua_bio)
+		block->flags |= LOG_FUA_FLAG;
+	if (discard_bio)
+		block->flags |= LOG_DISCARD_FLAG;
+
+	block->sector = bio->bi_iter.bi_sector;
+	block->nr_sectors = bio_sectors(bio);
+
+	/* We don't need the data, just submit */
+	if (discard_bio) {
+		WARN_ON(flush_bio || fua_bio);
+		if (lc->device_supports_discard)
+			goto map_bio;
+		bio_endio(bio, 0);
+		return DM_MAPIO_SUBMITTED;
+	}
+
+	/* Flush bio, splice the unflushed blocks onto this list and submit */
+	if (flush_bio && !bio_sectors(bio)) {
+		spin_lock_irq(&lc->blocks_lock);
+		list_splice_init(&lc->unflushed_blocks, &block->list);
+		spin_unlock_irq(&lc->blocks_lock);
+		goto map_bio;
+	}
+
+	/*
+	 * We will write this bio somewhere else way later so we need to copy
+	 * the actual contents into new pages so we know the data will always be
+	 * there.
+	 *
+	 * We do this because this could be a bio from O_DIRECT in which case we
+	 * can't just hold onto the page until some later point, we have to
+	 * manually copy the contents.
+	 */
+	bio_for_each_segment(bv, bio, iter) {
+		struct page *page;
+		void *src, *dst;
+
+		page = alloc_page(GFP_NOIO);
+		if (!page) {
+			DMERR("Error allocing page");
+			free_pending_block(lc, block);
+			spin_lock_irq(&lc->blocks_lock);
+			lc->logging_enabled = false;
+			spin_unlock_irq(&lc->blocks_lock);
+			return -ENOMEM;
+		}
+
+		src = kmap_atomic(bv.bv_page);
+		dst = kmap_atomic(page);
+		memcpy(dst, src + bv.bv_offset, bv.bv_len);
+		kunmap_atomic(dst);
+		kunmap_atomic(src);
+		block->vecs[i].bv_page = page;
+		block->vecs[i].bv_len = bv.bv_len;
+		block->vec_cnt++;
+		i++;
+	}
+
+	/* Had a flush with data in it, weird */
+	if (flush_bio) {
+		spin_lock_irq(&lc->blocks_lock);
+		list_splice_init(&lc->unflushed_blocks, &block->list);
+		spin_unlock_irq(&lc->blocks_lock);
+	}
+map_bio:
+	normal_map_bio(ti, bio);
+	return DM_MAPIO_REMAPPED;
+}
+
+static int normal_end_io(struct dm_target *ti, struct bio *bio, int error)
+{
+	struct log_writes_c *lc = ti->private;
+	struct per_bio_data *pb = dm_per_bio_data(bio, sizeof(struct per_bio_data));
+
+	if (bio_data_dir(bio) == WRITE && pb->block) {
+		struct pending_block *block = pb->block;
+		unsigned long flags;
+
+		spin_lock_irqsave(&lc->blocks_lock, flags);
+		if (block->flags & LOG_FLUSH_FLAG) {
+			list_splice_tail_init(&block->list, &lc->logging_blocks);
+			list_add_tail(&block->list, &lc->logging_blocks);
+			wake_up_process(lc->log_kthread);
+		} else if (block->flags & LOG_FUA_FLAG) {
+			list_add_tail(&block->list, &lc->logging_blocks);
+			wake_up_process(lc->log_kthread);
+		} else
+			list_add_tail(&block->list, &lc->unflushed_blocks);
+		spin_unlock_irqrestore(&lc->blocks_lock, flags);
+	}
+
+	return error;
+}
+
+/*
+ * INFO format: <logged entries> <highest allocated sector>
+ */
+static void log_writes_status(struct dm_target *ti, status_type_t type,
+			      unsigned status_flags, char *result,
+			      unsigned maxlen)
+{
+	unsigned sz = 0;
+	struct log_writes_c *lc = ti->private;
+
+	switch (type) {
+	case STATUSTYPE_INFO:
+		DMEMIT("%llu %llu", lc->logged_entries,
+		       (unsigned long long)lc->next_sector - 1);
+		if (!lc->logging_enabled)
+			DMEMIT(" logging_disabled");
+		break;
+
+	case STATUSTYPE_TABLE:
+		DMEMIT("%s %s", lc->dev->name, lc->logdev->name);
+		break;
+	}
+}
+
+static int log_writes_ioctl(struct dm_target *ti, unsigned int cmd,
+			    unsigned long arg)
+{
+	struct log_writes_c *lc = ti->private;
+	struct dm_dev *dev = lc->dev;
+	int r = 0;
+
+	/*
+	 * Only pass ioctls through if the device sizes match exactly.
+	 */
+	if (ti->len != i_size_read(dev->bdev->bd_inode) >> SECTOR_SHIFT)
+		r = scsi_verify_blk_ioctl(NULL, cmd);
+
+	return r ? : __blkdev_driver_ioctl(dev->bdev, dev->mode, cmd, arg);
+}
+
+static int log_writes_merge(struct dm_target *ti, struct bvec_merge_data *bvm,
+			    struct bio_vec *biovec, int max_size)
+{
+	struct log_writes_c *lc = ti->private;
+	struct request_queue *q = bdev_get_queue(lc->dev->bdev);
+
+	if (!q->merge_bvec_fn)
+		return max_size;
+
+	bvm->bi_bdev = lc->dev->bdev;
+	bvm->bi_sector = dm_target_offset(ti, bvm->bi_sector);
+
+	return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
+}
+
+static int log_writes_iterate_devices(struct dm_target *ti,
+				      iterate_devices_callout_fn fn,
+				      void *data)
+{
+	struct log_writes_c *lc = ti->private;
+
+	return fn(ti, lc->dev, 0, ti->len, data);
+}
+
+/*
+ * Messages supported:
+ *   mark <mark data> - specify the marked data.
+ */
+static int log_writes_message(struct dm_target *ti, unsigned argc, char **argv)
+{
+	int r = -EINVAL;
+	struct log_writes_c *lc = ti->private;
+
+	if (argc != 2) {
+		DMWARN("Invalid log-writes message arguments, expect 2 arguments, got %d", argc);
+		return r;
+	}
+
+	if (!strcasecmp(argv[0], "mark"))
+		r = log_mark(lc, argv[1]);
+	else
+		DMWARN("Unrecognised log writes target message received: %s", argv[0]);
+
+	return r;
+}
+
+static void log_writes_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	struct log_writes_c *lc = ti->private;
+	struct request_queue *q = bdev_get_queue(lc->dev->bdev);
+
+	if (!q || !blk_queue_discard(q)) {
+		lc->device_supports_discard = false;
+		limits->discard_granularity = 1 << SECTOR_SHIFT;
+		limits->max_discard_sectors = (UINT_MAX >> SECTOR_SHIFT);
+	}
+}
+
+static struct target_type log_writes_target = {
+	.name   = "log-writes",
+	.version = {1, 0, 0},
+	.module = THIS_MODULE,
+	.ctr    = log_writes_ctr,
+	.dtr    = log_writes_dtr,
+	.map    = log_writes_map,
+	.end_io = normal_end_io,
+	.status = log_writes_status,
+	.ioctl	= log_writes_ioctl,
+	.merge	= log_writes_merge,
+	.message = log_writes_message,
+	.iterate_devices = log_writes_iterate_devices,
+	.io_hints = log_writes_io_hints,
+};
+
+static int __init dm_log_writes_init(void)
+{
+	int r = dm_register_target(&log_writes_target);
+
+	if (r < 0)
+		DMERR("register failed %d", r);
+
+	return r;
+}
+
+static void __exit dm_log_writes_exit(void)
+{
+	dm_unregister_target(&log_writes_target);
+}
+
+module_init(dm_log_writes_init);
+module_exit(dm_log_writes_exit);
+
+MODULE_DESCRIPTION(DM_NAME " log writes target");
+MODULE_AUTHOR("Josef Bacik <jbacik@fb.com>");
+MODULE_LICENSE("GPL");