doc: Update rcu_assign_pointer() definition in whatisRCU.txt

[linux/fpc-iii.git] / 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_error) {
                unsigned long flags;

                DMERR("Error writing log block, error=%d", bio->bi_error);
                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() &&
                            !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;
        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 -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);
                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_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");