linux/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 1ULL
#define WRITE_LOG_MAGIC 0x6a736677736872ULL

/*
 * 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)
{
	struct log_writes_c *lc = bio->bi_private;

	if (bio->bi_status) {
		unsigned long flags;

		DMERR("Error writing log block, error=%d", bio->bi_status);
		spin_lock_irqsave(&lc->blocks_lock, flags);
		lc->logging_enabled = false;
		spin_unlock_irqrestore(&lc->blocks_lock, flags);
	}

	bio_free_pages(bio);
	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;
	bio_set_op_attrs(bio, REQ_OP_WRITE, 0);

	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(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++;

	atomic_inc(&lc->io_blocks);
	bio = bio_alloc(GFP_KERNEL, min(block->vec_cnt, BIO_MAX_PAGES));
	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;
	bio_set_op_attrs(bio, REQ_OP_WRITE, 0);

	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(bio);
			bio = bio_alloc(GFP_KERNEL, min(block->vec_cnt - i, BIO_MAX_PAGES));
			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;
			bio_set_op_attrs(bio, REQ_OP_WRITE, 0);

			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(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() &&
			    list_empty(&lc->logging_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;
	int ret;

	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);
	ret = dm_get_device(ti, devname, dm_table_get_mode(ti->table), &lc->dev);
	if (ret) {
		ti->error = "Device lookup failed";
		goto bad;
	}

	logdevname = dm_shift_arg(&as);
	ret = dm_get_device(ti, logdevname, dm_table_get_mode(ti->table),
			    &lc->logdev);
	if (ret) {
		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 (IS_ERR(lc->log_kthread)) {
		ret = PTR_ERR(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_io_data_size = sizeof(struct per_bio_data);
	ti->private = lc;
	return 0;

bad:
	kfree(lc);
	return ret;
}

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_opf & REQ_PREFLUSH);
	bool fua_bio = (bio->bi_opf & REQ_FUA);
	bool discard_bio = (bio_op(bio) == REQ_OP_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 DM_MAPIO_KILL;
	}
	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);
		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 DM_MAPIO_KILL;
		}

		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,
		blk_status_t *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 DM_ENDIO_DONE;
}

/*
 * 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_prepare_ioctl(struct dm_target *ti,
		struct block_device **bdev, fmode_t *mode)
{
	struct log_writes_c *lc = ti->private;
	struct dm_dev *dev = lc->dev;

	*bdev = dev->bdev;
	/*
	 * Only pass ioctls through if the device sizes match exactly.
	 */
	if (ti->len != i_size_read(dev->bdev->bd_inode) >> SECTOR_SHIFT)
		return 1;
	return 0;
}

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,
	.prepare_ioctl = log_writes_prepare_ioctl,
	.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");