[dm-devel] [PATCH 4/4] The dm-writecache target

[Mikulas Patocka]

---
 drivers/md/Kconfig         |   24 
 drivers/md/Makefile        |    1 
 drivers/md/dm-writecache.c | 2103 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 2128 insertions(+)

Index: linux-2.6/drivers/md/Kconfig
===================================================================
--- linux-2.6.orig/drivers/md/Kconfig
+++ linux-2.6/drivers/md/Kconfig
@@ -326,6 +326,30 @@ config DM_CACHE_SMQ
          of less memory utilization, improved performance and increased
          adaptability in the face of changing workloads.
 
+config DM_WRITECACHE
+	tristate "Writecache target"
+	depends on BLK_DEV_DM
+	---help---
+	   The writecache target caches writes on persistent memory or SSD.
+	   It is intended for databases or other programs that need extremely
+	   low commit latency.
+
+	   The writecache target doesn't cache reads because reads are supposed
+	   to be cached in standard RAM.
+
+config DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	bool "Handle hardware errors in the writecache target"
+	depends on DM_WRITECACHE
+	default y
+	---help---
+	  This options makes the dm-writecache target test for hardware errors,
+	  so that on hardware error, the device is disabled instead of crashing
+	  the whole kernel. Select 'No', if you have persistent memory based on
+	  standard DRAM. Select 'Yes', if you have persistent memory based on
+	  3D Xpoint or other error-prone technology.
+
+	  If unsure, say Y.
+
 config DM_ERA
        tristate "Era target (EXPERIMENTAL)"
        depends on BLK_DEV_DM
Index: linux-2.6/drivers/md/Makefile
===================================================================
--- linux-2.6.orig/drivers/md/Makefile
+++ linux-2.6/drivers/md/Makefile
@@ -62,6 +62,7 @@ obj-$(CONFIG_DM_ERA)		+= dm-era.o
 obj-$(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
 
 ifeq ($(CONFIG_DM_UEVENT),y)
 dm-mod-objs			+= dm-uevent.o
Index: linux-2.6/drivers/md/dm-writecache.c
===================================================================
--- /dev/null
+++ linux-2.6/drivers/md/dm-writecache.c
@@ -0,0 +1,2103 @@
+#include <linux/device-mapper.h>
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/vmalloc.h>
+#include <linux/kthread.h>
+#include <linux/dm-io.h>
+#include <linux/dm-kcopyd.h>
+#include <linux/dax.h>
+
+#define DM_MSG_PREFIX	"writecache"
+
+/*
+ * Persistent memory is not covered with page structures. Thus, when we need to
+ * write data that is in the persistent memory, we need to copy them to a
+ * temporary location that has struct page. When the macro
+ * WC_COPY_TO_PAGES_BEFORE_WRITING is defined, such copying is performed.
+ *
+ * If the persistent memory were covered with page structures, this macro can be
+ * undefined.
+ */
+#define WC_COPY_TO_PAGES_BEFORE_WRITING
+
+/*
+ * If the architecture doesn't support persistent memory, we can use this driver
+ * in SSD-only mode.
+ */
+#ifndef CONFIG_ARCH_HAS_PMEM_API
+#define WC_ONLY_SSD
+#endif
+
+#define BITMAP_GRANULARITY	65536
+#if BITMAP_GRANULARITY < PAGE_SIZE
+#undef BITMAP_GRANULARITY
+#define BITMAP_GRANULARITY	PAGE_SIZE
+#endif
+
+/* compatibility with the kernel 4.13 */
+#ifndef bio_set_dev
+#define bio_set_dev(bio, dev)	((bio)->bi_bdev = (dev))
+#endif
+
+struct wc_pmem_holder {
+	struct dm_dev *dev;
+	bool vmapped;
+};
+
+static int persistent_memory_claim(struct dm_target *ti, const char *name,
+				   struct wc_pmem_holder *pmem_holder, void **addr, uint64_t *size)
+{
+	int r;
+	loff_t s;
+	long p, da;
+	pfn_t pfn;
+
+	pmem_holder->vmapped = false;
+
+	r = dm_get_device(ti, name, FMODE_READ | FMODE_WRITE, &pmem_holder->dev);
+	if (r)
+		goto err0;
+
+	if (!pmem_holder->dev->dax_dev) {
+		r = -EOPNOTSUPP;
+		goto err1;
+	}
+	s = i_size_read(pmem_holder->dev->bdev->bd_inode);
+	p = s >> PAGE_SHIFT;
+	if (p != s >> PAGE_SHIFT) {
+		r = -EOVERFLOW;
+		goto err1;
+	}
+
+	da = dax_direct_access(pmem_holder->dev->dax_dev, 0, p, addr, &pfn);
+	if (da < 0) {
+		r = da;
+		goto err1;
+	}
+	if (da != p) {
+		long i;
+		pfn_t *pfns;
+		pfns = kvmalloc(p * sizeof(pfn_t), GFP_KERNEL);
+		if (!pfns) {
+			r = -ENOMEM;
+			goto err1;
+		}
+		i = 0;
+		do {
+			long daa;
+			daa = dax_direct_access(pmem_holder->dev->dax_dev, i, p - i, addr, &pfn);
+			if (daa <= 0) {
+				r = daa ? daa : -EINVAL;
+				kvfree(pfns);
+				goto err1;
+			}
+			while (daa--) {
+				pfns[i++] = pfn;
+				pfn.val++;
+			}
+		} while (i < p);
+		*addr = vmap_pfn(pfns, p, VM_MAP, PAGE_KERNEL);
+		kvfree(pfns);
+		if (!*addr) {
+			r = -ENOMEM;
+			goto err1;
+		}
+		pmem_holder->vmapped = true;
+	}
+	*size = s;
+	return 0;
+
+err1:
+	dm_put_device(ti, pmem_holder->dev);
+err0:
+	return r;
+}
+
+static void persistent_memory_release(struct dm_target *ti, struct wc_pmem_holder *pmem_holder,
+				      void *addr, size_t size)
+{
+	if (pmem_holder->vmapped)
+		vunmap(addr);
+	dm_put_device(ti, pmem_holder->dev);
+}
+
+#ifndef WC_COPY_TO_PAGES_BEFORE_WRITING
+
+static struct page *persistent_memory_page(void *addr)
+{
+	return vmalloc_to_page(addr);
+}
+
+static unsigned persistent_memory_page_offset(void *addr)
+{
+	return (unsigned long)addr & (PAGE_SIZE - 1);
+}
+
+static void persistent_memory_flush_cache(void *ptr, size_t size)
+{
+	flush_kernel_vmap_range(ptr, size);
+}
+
+#endif
+
+static void persistent_memory_flush(struct wc_pmem_holder *pmem_holder, void *ptr, size_t size)
+{
+	dax_flush(pmem_holder->dev->dax_dev, ptr, size);
+}
+
+static void persistent_memory_commit_flushed(void)
+{
+	/* the pcommit instruction is gone */
+}
+
+#define MEMORY_SUPERBLOCK_MAGIC		0x23489321
+#define MEMORY_SUPERBLOCK_VERSION	1
+
+struct wc_memory_entry {
+	uint64_t original_sector;
+	uint64_t seq_count;
+};
+
+struct wc_memory_superblock {
+	union {
+		struct {
+			uint32_t magic;
+			uint32_t version;
+			uint32_t block_size;
+			uint32_t pad;
+			uint64_t n_blocks;
+			uint64_t seq_count;
+		};
+		uint8_t padding[64];
+	};
+	struct wc_memory_entry entries[0];
+};
+
+struct wc_entry {
+	struct rb_node rb_node;
+	struct list_head lru;
+	unsigned short wc_list_contiguous;
+	bool write_in_progress
+#if BITS_PER_LONG == 64
+		:1
+#endif
+	;
+	unsigned long index
+#if BITS_PER_LONG == 64
+		:47
+#endif
+	;
+#ifdef CONFIG_DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	uint64_t original_sector;
+	uint64_t seq_count;
+#endif
+};
+
+#ifndef WC_ONLY_SSD
+#define WC_MODE_PMEM(wc)			((wc)->pmem_mode)
+#else
+#define WC_MODE_PMEM(wc)			false
+#endif
+
+struct dm_writecache {
+#ifndef WC_ONLY_SSD
+	bool pmem_mode;
+#endif
+	struct mutex lock;
+	struct rb_root tree;
+	struct list_head lru;
+	struct list_head writeback_start;
+	struct list_head writeback;
+	struct list_head freelist;
+	size_t freelist_size;
+	size_t writeback_size;
+	size_t freelist_high_watermark;
+	size_t freelist_low_watermark;
+	wait_queue_head_t freelist_wait;
+
+	struct dm_target *ti;
+	struct dm_dev *dev;
+	struct dm_dev *ssd_dev;
+	void *memory_map;
+	uint64_t memory_map_size;
+	size_t metadata_sectors;
+	void *block_start;
+	struct wc_entry *entries;
+	unsigned block_size;
+	unsigned char block_size_bits;
+	size_t n_blocks;
+	uint64_t seq_count;
+	int error;
+	bool high_wm_percent_set;
+	bool low_wm_percent_set;
+
+	bool overwrote_committed;
+
+	atomic_t bio_in_progress[2];
+	wait_queue_head_t bio_in_progress_wait[2];
+
+	struct dm_io_client *dm_io;
+
+	unsigned writeback_all;
+	struct workqueue_struct *writeback_wq;
+	struct work_struct writeback_work;
+
+	wait_queue_head_t endio_thread_wait;
+	struct list_head endio_list;
+	struct task_struct *endio_thread;
+
+	struct task_struct *flush_thread;
+	struct bio *flush_bio;
+	struct completion flush_completion;
+
+	struct bio_set *bio_set;
+	mempool_t *copy_pool;
+#ifdef WC_COPY_TO_PAGES_BEFORE_WRITING
+	mempool_t *page_pool;
+#endif
+	struct wc_pmem_holder pmem_holder;
+	const char *memory_name;
+
+	struct dm_kcopyd_client *dm_kcopyd;
+	unsigned long *dirty_bitmap;
+	unsigned dirty_bitmap_size;
+};
+
+#define WB_LIST_INLINE		16
+
+struct writeback_struct {
+	struct list_head endio_entry;
+	struct dm_writecache *wc;
+	struct wc_entry **wc_list;
+	unsigned wc_list_n;
+#ifdef WC_COPY_TO_PAGES_BEFORE_WRITING
+	unsigned page_offset;
+	struct page *page;
+#endif
+	struct wc_entry *wc_list_inline[WB_LIST_INLINE];
+	struct bio bio;
+};
+
+struct copy_struct {
+	struct list_head endio_entry;
+	struct dm_writecache *wc;
+	struct wc_entry *e;
+	int error;
+};
+
+DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(dm_writecache_throttle,
+					    "A percentage of time allocated for data copying");
+
+static struct wc_memory_superblock *sb(struct dm_writecache *wc)
+{
+	return wc->memory_map;
+}
+
+static struct wc_memory_entry *memory_entry(struct dm_writecache *wc, struct wc_entry *e)
+{
+	if (is_power_of_2(sizeof(struct wc_entry)) && 0)
+		return &sb(wc)->entries[e - wc->entries];
+	else
+		return &sb(wc)->entries[e->index];
+}
+
+static void *memory_data(struct dm_writecache *wc, struct wc_entry *e)
+{
+	return (char *)wc->block_start + (e->index << wc->block_size_bits);
+}
+
+static sector_t cache_sector(struct dm_writecache *wc, struct wc_entry *e)
+{
+	return wc->metadata_sectors +
+		((sector_t)e->index << (wc->block_size_bits - SECTOR_SHIFT));
+}
+
+static uint64_t read_original_sector(struct dm_writecache *wc, struct wc_entry *e)
+{
+#ifdef CONFIG_DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	return e->original_sector;
+#else
+	return le64_to_cpu(memory_entry(wc, e)->original_sector);
+#endif
+}
+
+static uint64_t read_seq_count(struct dm_writecache *wc, struct wc_entry *e)
+{
+#ifdef CONFIG_DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	return e->seq_count;
+#else
+	return le64_to_cpu(memory_entry(wc, e)->seq_count);
+#endif
+}
+
+static void clear_seq_count(struct dm_writecache *wc, struct wc_entry *e)
+{
+#ifdef CONFIG_DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	e->seq_count = -1;
+#endif
+	ACCESS_ONCE(memory_entry(wc, e)->seq_count) = cpu_to_le64(-1);
+}
+
+static void write_original_sector_seq_count(struct dm_writecache *wc, struct wc_entry *e,
+					    uint64_t original_sector, uint64_t seq_count)
+{
+	struct wc_memory_entry *wme;
+#ifdef CONFIG_DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	e->original_sector = original_sector;
+	e->seq_count = seq_count;
+#endif
+	wme = memory_entry(wc, e);
+	ACCESS_ONCE(wme->original_sector) = cpu_to_le64(original_sector);
+	ACCESS_ONCE(wme->seq_count) = cpu_to_le64(seq_count);
+}
+
+#define writecache_error(wc, err, msg, arg...)				\
+do {									\
+	if (!cmpxchg(&(wc)->error, 0, err))				\
+		DMERR(msg, ##arg);					\
+	wake_up(&(wc)->freelist_wait);					\
+} while (0)
+
+#define writecache_has_error(wc)	(unlikely(ACCESS_ONCE((wc)->error)))
+
+static void writecache_flush_all(struct dm_writecache *wc)
+{
+	if (WC_MODE_PMEM(wc))
+		persistent_memory_flush(&wc->pmem_holder, wc->memory_map, wc->memory_map_size);
+	else
+		memset(wc->dirty_bitmap, -1, wc->dirty_bitmap_size);
+}
+
+static void writecache_flush_region(struct dm_writecache *wc, void *ptr, size_t size)
+{
+	if (WC_MODE_PMEM(wc))
+		persistent_memory_flush(&wc->pmem_holder, ptr, size);
+	else
+		__set_bit(((char *)ptr - (char *)wc->memory_map) / BITMAP_GRANULARITY,
+			  wc->dirty_bitmap);
+}
+
+static void writecache_disk_flush(struct dm_writecache *wc, struct dm_dev *dev);
+
+struct io_notify {
+	struct dm_writecache *wc;
+	struct completion c;
+	atomic_t count;
+};
+
+void writecache_notify_io(unsigned long error, void *context)
+{
+	struct io_notify *endio = context;
+
+	if (unlikely(error != 0))
+		writecache_error(endio->wc, -EIO, "error writing metadata");
+	BUG_ON(atomic_read(&endio->count) <= 0);
+	if (atomic_dec_and_test(&endio->count))
+		complete(&endio->c);
+}
+
+static void ssd_commit_flushed(struct dm_writecache *wc)
+{
+	int r;
+	struct dm_io_region region;
+	struct dm_io_request req;
+	struct io_notify endio = {
+		wc,
+		COMPLETION_INITIALIZER_ONSTACK(endio.c),
+		ATOMIC_INIT(1),
+	};
+	unsigned bitmap_bits = wc->dirty_bitmap_size * BITS_PER_LONG;
+	unsigned i = 0;
+
+	while (1) {
+		unsigned j;
+		i = find_next_bit(wc->dirty_bitmap, bitmap_bits, i);
+		if (unlikely(i == bitmap_bits))
+			break;
+		j = find_next_zero_bit(wc->dirty_bitmap, bitmap_bits, i);
+
+		region.bdev = wc->ssd_dev->bdev;
+		region.sector = (sector_t)i * (BITMAP_GRANULARITY >> SECTOR_SHIFT);
+		region.count = (sector_t)(j - i) * (BITMAP_GRANULARITY >> SECTOR_SHIFT);
+
+		if (unlikely(region.sector >= wc->metadata_sectors))
+			break;
+		if (unlikely(region.sector + region.count > wc->metadata_sectors))
+			region.count = wc->metadata_sectors - region.sector;
+
+		atomic_inc(&endio.count);
+		req.bi_op = REQ_OP_WRITE;
+		req.bi_op_flags = REQ_SYNC;
+		req.mem.type = DM_IO_VMA;
+		req.mem.ptr.vma = (char *)wc->memory_map + (size_t)i * BITMAP_GRANULARITY;
+		req.client = wc->dm_io;
+		req.notify.fn = writecache_notify_io;
+		req.notify.context = &endio;
+
+		r = dm_io(&req, 1, &region, NULL);
+		if (unlikely(r)) {
+			/*
+			 * Async dm-io (implied by notify.fn above) won't return an error, but
+			 * if that changes in the future we must catch it: so panic in defense.
+			 */
+			panic(DM_NAME ": " DM_MSG_PREFIX ": dm io error %d", r);
+		}
+		i = j;
+	}
+
+	writecache_notify_io(0, &endio);
+	wait_for_completion_io(&endio.c);
+
+	writecache_disk_flush(wc, wc->ssd_dev);
+
+	memset(wc->dirty_bitmap, 0, wc->dirty_bitmap_size);
+}
+
+static void writecache_commit_flushed(struct dm_writecache *wc)
+{
+	if (WC_MODE_PMEM(wc))
+		persistent_memory_commit_flushed();
+	else
+		ssd_commit_flushed(wc);
+}
+
+static void writecache_disk_flush(struct dm_writecache *wc, struct dm_dev *dev)
+{
+	int r;
+	struct dm_io_region region;
+	struct dm_io_request req;
+
+	region.bdev = dev->bdev;
+	region.sector = 0;
+	region.count = 0;
+	req.bi_op = REQ_OP_WRITE;
+	req.bi_op_flags = REQ_PREFLUSH | REQ_SYNC;
+	req.mem.type = DM_IO_KMEM;
+	req.mem.ptr.addr = NULL;
+	req.client = wc->dm_io;
+	req.notify.fn = NULL;
+
+	r = dm_io(&req, 1, &region, NULL);
+	if (unlikely(r))
+		writecache_error(wc, r, "error flushing metadata: %d", r);
+}
+
+static void writecache_wait_for_ios(struct dm_writecache *wc, int direction)
+{
+	wait_event(wc->bio_in_progress_wait[direction],
+		   !atomic_read(&wc->bio_in_progress[direction]));
+}
+
+#define WFE_RETURN_FOLLOWING	1
+#define WFE_LOWEST_SEQ		2
+
+static struct wc_entry *writecache_find_entry(struct dm_writecache *wc, uint64_t block, int flags)
+{
+	struct wc_entry *e;
+	struct rb_node *node = wc->tree.rb_node;
+
+	if (unlikely(!node))
+		return NULL;
+
+	while (1) {
+		e = container_of(node, struct wc_entry, rb_node);
+		if (read_original_sector(wc, e) == block)
+			break;
+		node = (read_original_sector(wc, e) >= block ?
+			e->rb_node.rb_left : e->rb_node.rb_right);
+		if (unlikely(!node)) {
+			if (!(flags & WFE_RETURN_FOLLOWING))
+				return NULL;
+			if (read_original_sector(wc, e) >= block) {
+				break;
+			} else {
+				node = rb_next(&e->rb_node);
+				if (unlikely(!node))
+					return NULL;
+				e = container_of(node, struct wc_entry, rb_node);
+				break;
+			}
+		}
+	}
+
+	while (1) {
+		struct wc_entry *e2;
+		if (flags & WFE_LOWEST_SEQ)
+			node = rb_prev(&e->rb_node);
+		else
+			node = rb_next(&e->rb_node);
+		if (!node)
+			return e;
+		e2 = container_of(node, struct wc_entry, rb_node);
+		if (read_original_sector(wc, e2) != block)
+			return e;
+		e = e2;
+	}
+}
+
+static void writecache_insert_entry(struct dm_writecache *wc, struct wc_entry *ins)
+{
+	struct wc_entry *e;
+	struct rb_node **node = &wc->tree.rb_node, *parent = NULL;
+
+	while (*node) {
+		e = container_of(*node, struct wc_entry, rb_node);
+		parent = &e->rb_node;
+		node = read_original_sector(wc, e) > read_original_sector(wc, ins) ?
+			&parent->rb_left : &parent->rb_right;
+	}
+	rb_link_node(&ins->rb_node, parent, node);
+	rb_insert_color(&ins->rb_node, &wc->tree);
+	list_add(&ins->lru, &wc->lru);
+}
+
+static void writecache_unlink(struct dm_writecache *wc, struct wc_entry *e)
+{
+	list_del(&e->lru);
+	rb_erase(&e->rb_node, &wc->tree);
+}
+
+static void writecache_add_to_freelist(struct dm_writecache *wc, struct wc_entry *e)
+{
+	list_add_tail(&e->lru, &wc->freelist);
+	wc->freelist_size++;
+}
+
+struct wc_entry *writecache_pop_from_freelist(struct dm_writecache *wc)
+{
+	struct wc_entry *e;
+
+	if (unlikely(list_empty(&wc->freelist)))
+		return NULL;
+	e = container_of(wc->freelist.next, struct wc_entry, lru);
+	list_del(&e->lru);
+	wc->freelist_size--;
+	if (unlikely(wc->freelist_size <= wc->freelist_high_watermark))
+		queue_work(wc->writeback_wq, &wc->writeback_work);
+
+	return e;
+}
+
+static void writecache_free_entry(struct dm_writecache *wc, struct wc_entry *e)
+{
+	writecache_unlink(wc, e);
+	writecache_add_to_freelist(wc, e);
+	clear_seq_count(wc, e);
+	writecache_flush_region(wc, memory_entry(wc, e), sizeof(struct wc_memory_entry));
+	if (unlikely(waitqueue_active(&wc->freelist_wait)))
+		wake_up(&wc->freelist_wait);
+}
+
+static void writecache_wait_on_freelist(struct dm_writecache *wc)
+{
+	DECLARE_WAITQUEUE(wait, current);
+
+	add_wait_queue(&wc->freelist_wait, &wait);
+	set_current_state(TASK_UNINTERRUPTIBLE);
+	mutex_unlock(&wc->lock);
+	io_schedule();
+	mutex_lock(&wc->lock);
+	remove_wait_queue(&wc->freelist_wait, &wait);
+}
+
+static void writecache_poison_lists(struct dm_writecache *wc)
+{
+	/*
+	 * Catch incorrect access to these values while the device is suspended.
+	 */
+	memset(&wc->tree, -1, sizeof wc->tree);
+	wc->lru.next = LIST_POISON1;
+	wc->lru.prev = LIST_POISON2;
+	wc->writeback_start.next = LIST_POISON1;
+	wc->writeback_start.prev = LIST_POISON2;
+	wc->writeback.next = LIST_POISON1;
+	wc->writeback.prev = LIST_POISON2;
+	wc->freelist.next = LIST_POISON1;
+	wc->freelist.prev = LIST_POISON2;
+}
+
+static void writecache_flush_entry(struct dm_writecache *wc, struct wc_entry *e)
+{
+	writecache_flush_region(wc, memory_entry(wc, e), sizeof(struct wc_memory_entry));
+	if (WC_MODE_PMEM(wc))
+		writecache_flush_region(wc, memory_data(wc, e), wc->block_size);
+}
+
+static bool writecache_entry_is_committed(struct dm_writecache *wc, struct wc_entry *e)
+{
+	return read_seq_count(wc, e) < wc->seq_count;
+}
+
+static void writecache_flush(struct dm_writecache *wc)
+{
+	struct wc_entry *e, *e2;
+	unsigned long j = jiffies;
+
+	if (list_empty(&wc->lru))
+		return;
+
+	e = container_of(wc->lru.next, struct wc_entry, lru);
+	if (writecache_entry_is_committed(wc, e)) {
+		if (wc->overwrote_committed) {
+			writecache_wait_for_ios(wc, WRITE);
+			writecache_disk_flush(wc, wc->ssd_dev);
+			wc->overwrote_committed = false;
+		}
+		return;
+	}
+	j = jiffies - j;
+#ifdef X
+	printk("flush 1 in %u.%03us\n", jiffies_to_msecs(j) / 1000, jiffies_to_msecs(j) % 1000);
+#endif
+	j = jiffies;
+	while (1) {
+		writecache_flush_entry(wc, e);
+		if (unlikely(e->lru.next == &wc->lru))
+			break;
+		e2 = container_of(e->lru.next, struct wc_entry, lru);
+		if (writecache_entry_is_committed(wc, e2))
+			break;
+		e = e2;
+		cond_resched();
+	}
+	j = jiffies - j;
+#ifdef X
+	printk("flush 2 in %u.%03us\n", jiffies_to_msecs(j) / 1000, jiffies_to_msecs(j) % 1000);
+#endif
+	j = jiffies;
+	writecache_commit_flushed(wc);
+
+	j = jiffies - j;
+#ifdef X
+	printk("flush 3 in %u.%03us\n", jiffies_to_msecs(j) / 1000, jiffies_to_msecs(j) % 1000);
+#endif
+	j = jiffies;
+	writecache_wait_for_ios(wc, WRITE);
+	j = jiffies - j;
+#ifdef X
+	printk("flush 4 in %u.%03us\n", jiffies_to_msecs(j) / 1000, jiffies_to_msecs(j) % 1000);
+#endif
+	j = jiffies;
+
+	wc->seq_count++;
+	ACCESS_ONCE(sb(wc)->seq_count) = cpu_to_le64(wc->seq_count);
+	writecache_flush_region(wc, &sb(wc)->seq_count, sizeof sb(wc)->seq_count);
+	writecache_commit_flushed(wc);
+
+	wc->overwrote_committed = false;
+
+	while (1) {
+		/* Free another committed entry with lower seq-count */
+		struct rb_node *rb_node = rb_prev(&e->rb_node);
+
+		if (rb_node) {
+			e2 = container_of(rb_node, struct wc_entry, rb_node);
+			if (read_original_sector(wc, e2) == read_original_sector(wc, e) &&
+			    likely(!e2->write_in_progress))
+				writecache_free_entry(wc, e2);
+		}
+		if (unlikely(e->lru.prev == &wc->lru))
+			break;
+		e = container_of(e->lru.prev, struct wc_entry, lru);
+		cond_resched();
+	}
+	j = jiffies - j;
+#ifdef X
+	printk("flush 5 in %u.%03us\n", jiffies_to_msecs(j) / 1000, jiffies_to_msecs(j) % 1000);
+#endif
+	j = jiffies;
+}
+
+static void writecache_discard(struct dm_writecache *wc, sector_t start, sector_t end)
+{
+	struct wc_entry *e;
+	bool discarded_something = false;
+
+	e = writecache_find_entry(wc, start, WFE_RETURN_FOLLOWING | WFE_LOWEST_SEQ);
+	if (unlikely(!e))
+		return;
+
+	while (read_original_sector(wc, e) < end) {
+		struct rb_node *node = rb_next(&e->rb_node);
+
+		if (likely(!e->write_in_progress)) {
+			if (!discarded_something) {
+				writecache_wait_for_ios(wc, READ);
+				writecache_wait_for_ios(wc, WRITE);
+				discarded_something = true;
+			}
+			writecache_free_entry(wc, e);
+		}
+
+		if (!node)
+			break;
+
+		e = container_of(node, struct wc_entry, rb_node);
+	}
+
+	if (discarded_something)
+		writecache_commit_flushed(wc);
+}
+
+static bool writecache_wait_for_writeback(struct dm_writecache *wc)
+{
+	if (!list_empty(&wc->writeback_start) || !list_empty(&wc->writeback)) {
+		writecache_wait_on_freelist(wc);
+		return true;
+	}
+	return false;
+}
+
+static void writecache_suspend(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+
+	flush_workqueue(wc->writeback_wq);
+
+	mutex_lock(&wc->lock);
+	writecache_flush(wc);
+	while (writecache_wait_for_writeback(wc)) ;
+	mutex_unlock(&wc->lock);
+
+	writecache_poison_lists(wc);
+}
+
+static void writecache_resume(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+	size_t b;
+	bool need_flush = false;
+
+	mutex_lock(&wc->lock);
+
+	wc->tree = RB_ROOT;
+	INIT_LIST_HEAD(&wc->lru);
+	INIT_LIST_HEAD(&wc->writeback_start);
+	INIT_LIST_HEAD(&wc->writeback);
+	INIT_LIST_HEAD(&wc->freelist);
+	wc->freelist_size = 0;
+
+	for (b = 0; b < wc->n_blocks; b++) {
+		struct wc_entry *e = &wc->entries[b];
+		e->index = b;
+		e->write_in_progress = false;
+	}
+#ifdef CONFIG_DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	for (b = 0; b < wc->n_blocks; b++) {
+		struct wc_entry *e = &wc->entries[b];
+		struct wc_memory_entry wme;
+		int r;
+		if (writecache_has_error(wc)) {
+			e->original_sector = -1;
+			e->seq_count = -1;
+			continue;
+		}
+		r = memcpy_mcsafe(&wme, memory_entry(wc, e), sizeof(struct wc_memory_entry));
+		if (unlikely(r)) {
+			writecache_error(wc, r, "hardware memory error when reading metadata entry %lu: %d", (unsigned long)b, r);
+			e->original_sector = -1;
+			e->seq_count = -1;
+		} else {
+			e->original_sector = le64_to_cpu(wme.original_sector);
+			e->seq_count = le64_to_cpu(wme.seq_count);
+		}
+	}
+#endif
+	for (b = 0; b < wc->n_blocks; b++) {
+		struct wc_entry *e = &wc->entries[b];
+		if (!writecache_entry_is_committed(wc, e)) {
+			if (read_seq_count(wc, e) != -1) {
+erase_this:
+				write_original_sector_seq_count(wc, e, -1, -1);
+				need_flush = true;
+			}
+			writecache_add_to_freelist(wc, e);
+
+		} else {
+			struct wc_entry *old;
+
+			old = writecache_find_entry(wc, read_original_sector(wc, e), 0);
+			if (unlikely(!old)) {
+				writecache_insert_entry(wc, e);
+			} else {
+				if (unlikely(read_seq_count(wc, old) == read_seq_count(wc, e))) {
+					writecache_error(wc, -EINVAL, "two identical entries, position %llu, sector %llu, sequence %llu",
+						 (unsigned long long)b, (unsigned long long)read_original_sector(wc, e),
+						 (unsigned long long)read_seq_count(wc, e));
+				}
+				if (read_seq_count(wc, old) > read_seq_count(wc, e)) {
+					goto erase_this;
+				} else {
+					writecache_free_entry(wc, old);
+					writecache_insert_entry(wc, e);
+					need_flush = true;
+				}
+			}
+		}
+		cond_resched();
+	}
+
+	if (need_flush) {
+		writecache_flush_all(wc);
+		writecache_commit_flushed(wc);
+	}
+
+	mutex_unlock(&wc->lock);
+}
+
+static int process_flush_mesg(unsigned argc, char **argv, struct dm_writecache *wc)
+{
+	uint64_t seq;
+	struct wc_entry *e;
+
+	if (argc != 1)
+		return -EINVAL;
+
+	mutex_lock(&wc->lock);
+	if (unlikely(dm_suspended(wc->ti))) {
+		mutex_unlock(&wc->lock);
+		return -EBUSY;
+	}
+	if (writecache_has_error(wc)) {
+		mutex_unlock(&wc->lock);
+		return -EIO;
+	}
+
+	writecache_flush(wc);
+	seq = wc->seq_count;
+	wc->writeback_all++;
+	mutex_unlock(&wc->lock);
+
+	queue_work(wc->writeback_wq, &wc->writeback_work);
+	flush_workqueue(wc->writeback_wq);
+
+	mutex_lock(&wc->lock);
+	wc->writeback_all--;
+ waited:
+	if (unlikely(dm_suspended(wc->ti))) {
+		mutex_unlock(&wc->lock);
+		return -EBUSY;
+	}
+	if (writecache_has_error(wc)) {
+		mutex_unlock(&wc->lock);
+		return -EIO;
+	}
+	e = NULL;
+	if (!list_empty(&wc->writeback))
+		e = container_of(wc->writeback.prev, struct wc_entry, lru);
+	else if (!list_empty(&wc->writeback_start))
+		e = container_of(wc->writeback_start.prev, struct wc_entry, lru);
+	else if (!list_empty(&wc->lru))
+		e = container_of(wc->lru.prev, struct wc_entry, lru);
+	if (e && read_seq_count(wc, e) < seq) {
+		writecache_wait_on_freelist(wc);
+		goto waited;
+	}
+	mutex_unlock(&wc->lock);
+
+	return 0;
+}
+
+static int writecache_message(struct dm_target *ti, unsigned argc, char **argv)
+{
+	int r = -EINVAL;
+	struct dm_writecache *wc = ti->private;
+
+	if (!strcasecmp(argv[0], "flush"))
+		r = process_flush_mesg(argc, argv, wc);
+	else
+		DMWARN("unrecognised message received: %s", argv[0]);
+
+	return r;
+}
+
+static void bio_copy_block(struct dm_writecache *wc, struct bio *bio, void *data)
+{
+	void *buf;
+	unsigned long flags;
+	unsigned size;
+	int rw = bio_data_dir(bio);
+	unsigned remaining_size = wc->block_size;
+
+	do {
+		buf = bio_kmap_irq(bio, &flags, &size);
+		if (unlikely(size > remaining_size))
+			size = remaining_size;
+
+		if (rw == READ) {
+			int r;
+			r = memcpy_mcsafe(buf, data, size);
+			flush_dcache_page(bio_page(bio));
+			if (unlikely(r)) {
+				writecache_error(wc, r, "hardware memory error when reading data: %d", r);
+				bio->bi_status = BLK_STS_IOERR;
+			}
+		} else {
+			flush_dcache_page(bio_page(bio));
+			memcpy(data, buf, size);
+		}
+
+		bio_kunmap_irq(buf, &flags);
+
+		data = (char *)data + size;
+		remaining_size -= size;
+		bio_advance(bio, size);
+	} while (unlikely(remaining_size));
+}
+
+static int writecache_flush_thread(void *data)
+{
+	struct dm_writecache *wc = data;
+
+	while (!kthread_should_stop()) {
+		struct bio *bio;
+
+		bio = wc->flush_bio;
+		if (unlikely(!bio)) {
+		} else if (bio->bi_opf & REQ_PREFLUSH) {
+			writecache_flush(wc);
+		} else if (bio_op(bio) == REQ_OP_DISCARD) {
+			writecache_discard(wc, bio->bi_iter.bi_sector,
+					   bio->bi_iter.bi_sector + (bio->bi_iter.bi_size >> SECTOR_SHIFT));
+		} else {
+			BUG();
+		}
+
+		set_current_state(TASK_INTERRUPTIBLE);
+		wc->flush_bio = (void *)1;	/* for debugging - catch uninitialized use */
+		complete(&wc->flush_completion);
+
+		schedule();
+	}
+
+	set_current_state(TASK_RUNNING);
+
+	return 0;
+}
+
+static void writecache_offload_bio(struct dm_writecache *wc, struct bio *bio)
+{
+	wc->flush_bio = bio;
+	reinit_completion(&wc->flush_completion);
+	wake_up_process(wc->flush_thread);
+	wait_for_completion_io(&wc->flush_completion);
+}
+
+static int writecache_map(struct dm_target *ti, struct bio *bio)
+{
+	struct wc_entry *e;
+	struct dm_writecache *wc = ti->private;
+
+	bio->bi_private = NULL;
+
+	mutex_lock(&wc->lock);
+
+	if (unlikely(bio->bi_opf & REQ_PREFLUSH)) {
+		if (writecache_has_error(wc))
+			goto unlock_error;
+		if (WC_MODE_PMEM(wc))
+			writecache_flush(wc);
+		else
+			writecache_offload_bio(wc, bio);
+		goto unlock_ok;
+	}
+
+	bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector);
+
+	if (unlikely((bio->bi_iter.bi_sector & (wc->block_size / 512 - 1)) != 0) ||
+	    unlikely((bio->bi_iter.bi_size & (wc->block_size - 1)) != 0)) {
+		DMWARN("I/O is not aligned, sector %llu, size %u, block size %u",
+			(unsigned long long)bio->bi_iter.bi_sector,
+			bio->bi_iter.bi_size, wc->block_size);
+		goto unlock_error;
+	}
+
+	if (unlikely(bio_op(bio) == REQ_OP_DISCARD)) {
+		if (writecache_has_error(wc))
+			goto unlock_error;
+		if (WC_MODE_PMEM(wc))
+			writecache_discard(wc, bio->bi_iter.bi_sector,
+					   bio->bi_iter.bi_sector + (bio->bi_iter.bi_size >> SECTOR_SHIFT));
+		else
+			writecache_offload_bio(wc, bio);
+		goto unlock_remap_origin;
+	}
+
+	if (bio_data_dir(bio) == READ) {
+next_block:
+		e = writecache_find_entry(wc, bio->bi_iter.bi_sector, WFE_RETURN_FOLLOWING);
+		if (e && read_original_sector(wc, e) == bio->bi_iter.bi_sector) {
+			if (WC_MODE_PMEM(wc)) {
+				bio_copy_block(wc, bio, memory_data(wc, e));
+				if (bio->bi_iter.bi_size)
+					goto next_block;
+				goto unlock_ok;
+			} else {
+				dm_accept_partial_bio(bio, wc->block_size >> SECTOR_SHIFT);
+				bio_set_dev(bio, wc->ssd_dev->bdev);
+				bio->bi_iter.bi_sector = cache_sector(wc, e);
+				if (!writecache_entry_is_committed(wc, e))
+					writecache_wait_for_ios(wc, WRITE);
+				goto unlock_remap;
+			}
+		} else {
+			if (e) {
+				sector_t next_boundary =
+					read_original_sector(wc, e) - bio->bi_iter.bi_sector;
+				if (next_boundary < bio->bi_iter.bi_size >> SECTOR_SHIFT) {
+					dm_accept_partial_bio(bio, next_boundary);
+				}
+			}
+			goto unlock_remap_origin;
+		}
+	} else {
+		do {
+			if (writecache_has_error(wc))
+				goto unlock_error;
+			e = writecache_find_entry(wc, bio->bi_iter.bi_sector, 0);
+			if (e) {
+				if (!writecache_entry_is_committed(wc, e))
+					goto bio_copy;
+				if (!WC_MODE_PMEM(wc) && !e->write_in_progress) {
+					wc->overwrote_committed = true;
+					goto bio_copy;
+				}
+			}
+			e = writecache_pop_from_freelist(wc);
+			if (unlikely(!e)) {
+				writecache_wait_on_freelist(wc);
+				continue;
+			}
+			write_original_sector_seq_count(wc, e, bio->bi_iter.bi_sector, wc->seq_count);
+			writecache_insert_entry(wc, e);
+bio_copy:
+			if (WC_MODE_PMEM(wc)) {
+				bio_copy_block(wc, bio, memory_data(wc, e));
+			} else {
+				dm_accept_partial_bio(bio, wc->block_size >> SECTOR_SHIFT);
+				bio_set_dev(bio, wc->ssd_dev->bdev);
+				bio->bi_iter.bi_sector = cache_sector(wc, e);
+				goto unlock_remap;
+			}
+		} while (bio->bi_iter.bi_size);
+
+		goto unlock_ok;
+	}
+
+unlock_remap_origin:
+	bio_set_dev(bio, wc->dev->bdev);
+	mutex_unlock(&wc->lock);
+	return DM_MAPIO_REMAPPED;
+
+unlock_remap:
+	bio->bi_private = (void *)1;
+	atomic_inc(&wc->bio_in_progress[bio_data_dir(bio)]);
+	mutex_unlock(&wc->lock);
+	return DM_MAPIO_REMAPPED;
+
+unlock_ok:
+	mutex_unlock(&wc->lock);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+
+unlock_error:
+	mutex_unlock(&wc->lock);
+	bio_io_error(bio);
+	return DM_MAPIO_SUBMITTED;
+}
+
+static int writecache_end_io(struct dm_target *ti, struct bio *bio, blk_status_t *status)
+{
+	struct dm_writecache *wc = ti->private;
+
+	if (bio->bi_private != NULL) {
+		int dir = bio_data_dir(bio);
+		if (atomic_dec_and_test(&wc->bio_in_progress[dir]))
+			if (unlikely(waitqueue_active(&wc->bio_in_progress_wait[dir])))
+				wake_up(&wc->bio_in_progress_wait[dir]);
+	}
+	return 0;
+}
+
+static int writecache_iterate_devices(struct dm_target *ti, iterate_devices_callout_fn fn, void *data)
+{
+	struct dm_writecache *wc = ti->private;
+
+	return fn(ti, wc->dev, 0, ti->len, data);
+}
+
+static void writecache_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	struct dm_writecache *wc = ti->private;
+
+	if (limits->logical_block_size < wc->block_size)
+		limits->logical_block_size = wc->block_size;
+
+	if (limits->physical_block_size < wc->block_size)
+		limits->physical_block_size = wc->block_size;
+
+	if (limits->io_min < wc->block_size)
+		limits->io_min = wc->block_size;
+}
+
+
+static void writecache_writeback_endio(struct bio *bio)
+{
+	struct writeback_struct *wb = container_of(bio, struct writeback_struct, bio);
+	struct dm_writecache *wc = wb->wc;
+	unsigned long flags;
+
+	spin_lock_irqsave(&wc->endio_thread_wait.lock, flags);
+	list_add_tail(&wb->endio_entry, &wc->endio_list);
+	wake_up_locked(&wc->endio_thread_wait);
+	spin_unlock_irqrestore(&wc->endio_thread_wait.lock, flags);
+}
+
+static void writecache_copy_endio(int read_err, unsigned long write_err, void *ptr)
+{
+	struct copy_struct *c = ptr;
+	struct dm_writecache *wc = c->wc;
+	unsigned long flags;
+
+	c->error = likely(!(read_err | write_err)) ? 0 : -EIO;
+
+	spin_lock_irqsave(&wc->endio_thread_wait.lock, flags);	/* !!! TODO: use spin_lock_irq */
+	list_add_tail(&c->endio_entry, &wc->endio_list);
+	wake_up_locked(&wc->endio_thread_wait);
+	spin_unlock_irqrestore(&wc->endio_thread_wait.lock, flags);
+}
+
+static void __writecache_endio_pmem(struct dm_writecache *wc, struct list_head *list)
+{
+	unsigned i;
+	struct writeback_struct *wb;
+	struct wc_entry *e;
+
+	do {
+		wb = list_entry(list->next, struct writeback_struct, endio_entry);
+		list_del(&wb->endio_entry);
+
+#ifdef WC_COPY_TO_PAGES_BEFORE_WRITING
+		{
+			struct bio_vec *bv;
+			bio_for_each_segment_all(bv, &wb->bio, i)
+				mempool_free(bv->bv_page, wc->page_pool);
+		}
+#endif
+		if (unlikely(wb->bio.bi_status != BLK_STS_OK))
+			writecache_error(wc, blk_status_to_errno(wb->bio.bi_status),
+					"write error %d", wb->bio.bi_status);
+		i = 0;
+		do {
+			e = wb->wc_list[i];
+			BUG_ON(!e->write_in_progress);
+			e->write_in_progress = false;
+			if (!writecache_has_error(wc))
+				writecache_free_entry(wc, e);
+			wc->writeback_size--;
+		} while (++i < wb->wc_list_n);
+
+		if (wb->wc_list != wb->wc_list_inline)
+			kfree(wb->wc_list);
+		bio_put(&wb->bio);
+	} while (!list_empty(list));
+}
+
+static void __writecache_endio_ssd(struct dm_writecache *wc, struct list_head *list)
+{
+	struct copy_struct *c;
+	struct wc_entry *e;
+
+	do {
+		c = list_entry(list->next, struct copy_struct, endio_entry);
+		list_del(&c->endio_entry);
+
+		if (unlikely(c->error))
+			writecache_error(wc, c->error, "copy error");
+
+		e = c->e;
+		BUG_ON(!e->write_in_progress);
+		e->write_in_progress = false;
+		if (!writecache_has_error(wc))
+			writecache_free_entry(wc, e);
+
+		wc->writeback_size--;
+		mempool_free(c, wc->copy_pool);
+	} while (!list_empty(list));
+}
+
+static int writecache_endio_thread(void *data)
+{
+	struct dm_writecache *wc = data;
+
+	while (1) {
+		DECLARE_WAITQUEUE(wait, current);
+		struct list_head list;
+
+		spin_lock_irq(&wc->endio_thread_wait.lock);
+continue_locked:
+		if (!list_empty(&wc->endio_list))
+			goto pop_from_list;
+		set_current_state(TASK_INTERRUPTIBLE);
+		__add_wait_queue(&wc->endio_thread_wait, &wait);
+		spin_unlock_irq(&wc->endio_thread_wait.lock);
+
+		if (unlikely(kthread_should_stop())) {
+			set_current_state(TASK_RUNNING);
+			remove_wait_queue(&wc->endio_thread_wait, &wait);
+			break;
+		}
+
+		schedule();
+
+		spin_lock_irq(&wc->endio_thread_wait.lock);
+		__remove_wait_queue(&wc->endio_thread_wait, &wait);
+		goto continue_locked;
+
+pop_from_list:
+		list = wc->endio_list;
+		list.next->prev = list.prev->next = &list;
+		INIT_LIST_HEAD(&wc->endio_list);
+		spin_unlock_irq(&wc->endio_thread_wait.lock);
+
+		writecache_disk_flush(wc, wc->dev);
+
+		mutex_lock(&wc->lock);
+
+		if (WC_MODE_PMEM(wc))
+			__writecache_endio_pmem(wc, &list);
+		else
+			__writecache_endio_ssd(wc, &list);
+
+		writecache_wait_for_ios(wc, READ);
+		writecache_commit_flushed(wc);
+
+		mutex_unlock(&wc->lock);
+	}
+
+	return 0;
+}
+
+static bool wc_add_block(struct writeback_struct *wb, struct wc_entry *e, gfp_t gfp)
+{
+	struct dm_writecache *wc = wb->wc;
+	unsigned block_size = wc->block_size;
+
+#ifndef WC_COPY_TO_PAGES_BEFORE_WRITING
+	void *address = memory_data(wc, e);
+	persistent_memory_flush_cache(address, block_size);
+	return bio_add_page(&wb->bio, persistent_memory_page(address),
+			    block_size, persistent_memory_page_offset(address)) != 0;
+#else
+	int r;
+	if (wb->page_offset == PAGE_SIZE) {
+		wb->page = mempool_alloc(wc->page_pool, gfp);
+		if (unlikely(!wb->page))
+			return false;
+		wb->page_offset = 0;
+	}
+
+	r = memcpy_mcsafe((char *)page_address(wb->page) + wb->page_offset, memory_data(wc, e), block_size);
+	if (unlikely(r))
+		writecache_error(wc, r, "hardware memory error when attempting to writeback block %lu: %d", (unsigned long)e->index, r);
+
+	if (unlikely(!bio_add_page(&wb->bio, wb->page, block_size, wb->page_offset))) {
+		if (!wb->page_offset) {
+			mempool_free(wb->page, wc->page_pool);
+			wb->page_offset = PAGE_SIZE;
+		}
+		return false;
+	}
+	wb->page_offset += block_size;
+	return true;
+#endif
+}
+
+static void __writecache_writeback_pmem(struct dm_writecache *wc)
+{
+	struct wc_entry *e, *f;
+	struct bio *bio;
+	struct writeback_struct *wb;
+	unsigned max_pages;
+
+	while (!list_empty(&wc->writeback_start)) {
+		e = container_of(wc->writeback_start.prev, struct wc_entry, lru);
+		list_del(&e->lru);
+		list_add(&e->lru, &wc->writeback);
+
+		mutex_unlock(&wc->lock);
+
+		max_pages = e->wc_list_contiguous;
+
+		bio = bio_alloc_bioset(GFP_NOIO, max_pages, wc->bio_set);
+		wb = container_of(bio, struct writeback_struct, bio);
+		wb->wc = wc;
+		wb->bio.bi_end_io = writecache_writeback_endio;
+		bio_set_dev(&wb->bio, wc->dev->bdev);
+		wb->bio.bi_iter.bi_sector = read_original_sector(wc, e);
+#ifdef WC_COPY_TO_PAGES_BEFORE_WRITING
+		wb->page_offset = PAGE_SIZE;
+#endif
+		if (max_pages > WB_LIST_INLINE) {
+			wb->wc_list = kmalloc(max_pages * sizeof(struct wc_entry *),
+					      GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
+			if (unlikely(!wb->wc_list))
+				goto use_inline_list;
+		} else {
+use_inline_list:
+			wb->wc_list = wb->wc_list_inline;
+			max_pages = WB_LIST_INLINE;
+		}
+
+		BUG_ON(!wc_add_block(wb, e, GFP_NOIO));
+
+		wb->wc_list[0] = e;
+		wb->wc_list_n = 1;
+
+		while (!list_empty(&wc->writeback_start) && wb->wc_list_n < max_pages) {
+			f = container_of(wc->writeback_start.prev, struct wc_entry, lru);
+			if (read_original_sector(wc, f) !=
+			    read_original_sector(wc, e) + (wc->block_size >> SECTOR_SHIFT))
+				break;
+			if (!wc_add_block(wb, f, GFP_NOWAIT | __GFP_NOWARN))
+				break;
+			list_del(&f->lru);
+			list_add(&f->lru, &wc->writeback);
+			wb->wc_list[wb->wc_list_n++] = f;
+			e = f;
+		}
+		bio_set_op_attrs(&wb->bio, REQ_OP_WRITE, 0);
+		if (writecache_has_error(wc)) {
+			bio->bi_status = BLK_STS_IOERR;
+			bio_endio(&wb->bio);
+		} else {
+			submit_bio(&wb->bio);
+		}
+		cond_resched();
+
+		mutex_lock(&wc->lock);
+	}
+}
+
+static void __writecache_writeback_ssd(struct dm_writecache *wc)
+{
+	struct wc_entry *e;
+	struct dm_io_region from, to;
+	struct copy_struct *c;
+
+	while (!list_empty(&wc->writeback_start)) {
+		e = container_of(wc->writeback_start.prev, struct wc_entry, lru);
+		list_del(&e->lru);
+		list_add(&e->lru, &wc->writeback);
+		mutex_unlock(&wc->lock);
+
+		from.bdev = wc->ssd_dev->bdev;
+		from.sector = cache_sector(wc, e);
+		from.count = wc->block_size >> SECTOR_SHIFT;
+		to.bdev = wc->dev->bdev;
+		to.sector = read_original_sector(wc, e);
+		to.count = wc->block_size >> SECTOR_SHIFT;
+
+		c = mempool_alloc(wc->copy_pool, GFP_NOIO);
+		c->wc = wc;
+		c->e = e;
+
+		dm_kcopyd_copy(wc->dm_kcopyd, &from, 1, &to, 0, writecache_copy_endio, c);
+
+		mutex_lock(&wc->lock);
+	}
+}
+
+static void writecache_writeback(struct work_struct *work)
+{
+	struct dm_writecache *wc = container_of(work, struct dm_writecache, writeback_work);
+	struct blk_plug plug;
+	struct wc_entry *e, *f, *g;
+	struct rb_node *node, *next_node;
+	struct list_head skipped;
+#ifdef X
+	unsigned long n_walked;
+	unsigned long j;
+#endif
+
+	mutex_lock(&wc->lock);
+restart:
+	if (writecache_has_error(wc) || unlikely(dm_suspended(wc->ti))) {
+		mutex_unlock(&wc->lock);
+		return;
+	}
+
+	if (unlikely(wc->writeback_all)) {
+		if (writecache_wait_for_writeback(wc))
+			goto restart;
+	}
+
+	if (wc->overwrote_committed)
+		writecache_wait_for_ios(wc, WRITE);
+
+#ifdef X
+	n_walked = 0;
+	j = jiffies;
+#endif
+	INIT_LIST_HEAD(&skipped);
+	while (!list_empty(&wc->lru) &&
+	       (wc->writeback_all ||
+		wc->freelist_size + wc->writeback_size <= wc->freelist_high_watermark)) {
+#ifdef X
+		n_walked++;
+#endif
+		e = container_of(wc->lru.prev, struct wc_entry, lru);
+		BUG_ON(e->write_in_progress);
+		if (unlikely(!writecache_entry_is_committed(wc, e))) {
+#ifdef X
+			unsigned long j2 = jiffies;
+#endif
+			writecache_flush(wc);
+#ifdef X
+			j2 = jiffies - j2;
+			printk("flush in %u.%03us\n", jiffies_to_msecs(j2) / 1000, jiffies_to_msecs(j2) % 1000);
+#endif
+		}
+		node = rb_prev(&e->rb_node);
+		if (node) {
+			f = container_of(node, struct wc_entry, rb_node);
+			if (unlikely(read_original_sector(wc, f) ==
+				     read_original_sector(wc, e))) {
+				BUG_ON(!f->write_in_progress);
+				list_del(&e->lru);
+				list_add(&e->lru, &skipped);
+				continue;
+			}
+		}
+		wc->writeback_size++;
+		list_del(&e->lru);
+		list_add(&e->lru, &wc->writeback_start);
+		e->write_in_progress = true;
+		e->wc_list_contiguous = 1;
+
+		f = e;
+
+		if (!WC_MODE_PMEM(wc)) {
+			cond_resched();
+			continue;
+		}
+
+		/* Only coalesce if we are on pmem */
+		while (1) {
+			next_node = rb_next(&f->rb_node);
+			if (unlikely(!next_node))
+				break;
+			g = container_of(next_node, struct wc_entry, rb_node);
+			if (read_original_sector(wc, g) ==
+			    read_original_sector(wc, f)) {
+				f = g;
+				continue;
+			}
+			if (read_original_sector(wc, g) !=
+			    read_original_sector(wc, f) + (wc->block_size >> SECTOR_SHIFT))
+				break;
+			if (unlikely(g->write_in_progress))
+				break;
+			if (unlikely(!writecache_entry_is_committed(wc, g)))
+				break;
+
+			wc->writeback_size++;
+			list_del(&g->lru);
+			list_add(&g->lru, &wc->writeback_start);
+			g->write_in_progress = true;
+			g->wc_list_contiguous = BIO_MAX_PAGES;
+			f = g;
+			e->wc_list_contiguous++;
+			if (unlikely(e->wc_list_contiguous == BIO_MAX_PAGES))
+				break;
+		}
+		cond_resched();
+	}
+
+	list_splice_tail(&skipped, &wc->lru);
+
+#ifdef X
+	j = jiffies - j;
+	printk("walked %lu in %u.%03us\n", n_walked, jiffies_to_msecs(j) / 1000, jiffies_to_msecs(j) % 1000);
+#endif
+	mutex_unlock(&wc->lock);
+
+	blk_start_plug(&plug);
+	mutex_lock(&wc->lock);
+
+	if (WC_MODE_PMEM(wc))
+		__writecache_writeback_pmem(wc);
+	else
+		__writecache_writeback_ssd(wc);
+
+	mutex_unlock(&wc->lock);
+	blk_finish_plug(&plug);
+}
+
+static int calculate_memory_size(uint64_t device_size, unsigned block_size,
+				 size_t *n_blocks_p, size_t *n_metadata_blocks_p)
+{
+	uint64_t n_blocks, offset;
+	struct wc_entry e;
+
+	n_blocks = device_size;
+	do_div(n_blocks, block_size + sizeof(struct wc_memory_entry));
+
+	while (1) {
+		if (!n_blocks)
+			return -ENOSPC;
+		/* Verify the following entries[n_blocks] won't overflow */
+		if (n_blocks >= (size_t)-sizeof(struct wc_memory_superblock) / sizeof(struct wc_memory_entry))
+			return -EFBIG;
+		offset = offsetof(struct wc_memory_superblock, entries[n_blocks]);
+		offset = (offset + block_size - 1) & ~(uint64_t)(block_size - 1);
+		if (offset + n_blocks * block_size <= device_size)
+			break;
+		n_blocks--;
+	}
+
+	/* check if the bit field overflows */
+	e.index = n_blocks;
+	if (e.index != n_blocks)
+		return -EFBIG;
+
+	if (n_blocks_p)
+		*n_blocks_p = n_blocks;
+	if (n_metadata_blocks_p)
+		*n_metadata_blocks_p = offset >> __ffs(block_size);
+	return 0;
+}
+
+static int init_memory(struct dm_writecache *wc)
+{
+	size_t b, n_blocks;
+	int r;
+
+	r = calculate_memory_size(wc->memory_map_size, wc->block_size, &n_blocks, NULL);
+	if (r)
+		return r;
+
+	memset(sb(wc), 0, sizeof(struct wc_memory_superblock));
+	sb(wc)->version = cpu_to_le32(MEMORY_SUPERBLOCK_VERSION);
+	sb(wc)->block_size = cpu_to_le32(wc->block_size);
+	sb(wc)->n_blocks = cpu_to_le64(n_blocks);
+	sb(wc)->seq_count = cpu_to_le64(0);
+
+	for (b = 0; b < n_blocks; b++) {
+		sb(wc)->entries[b].original_sector = cpu_to_le64(-1);
+		sb(wc)->entries[b].seq_count = cpu_to_le64(-1);
+	}
+
+	writecache_flush_all(wc);
+	writecache_commit_flushed(wc);
+	sb(wc)->magic = cpu_to_le32(MEMORY_SUPERBLOCK_MAGIC);
+	writecache_flush_region(wc, &sb(wc)->magic, sizeof sb(wc)->magic);
+	writecache_commit_flushed(wc);
+
+	return 0;
+}
+
+static void writecache_dtr(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+
+	if (!wc)
+		return;
+
+	if (wc->endio_thread)
+		kthread_stop(wc->endio_thread);
+
+	if (wc->flush_thread)
+		kthread_stop(wc->flush_thread);
+
+#ifdef WC_COPY_TO_PAGES_BEFORE_WRITING
+	mempool_destroy(wc->page_pool);
+#endif
+
+	if (wc->bio_set)
+		bioset_free(wc->bio_set);
+
+	mempool_destroy(wc->copy_pool);
+
+	if (wc->writeback_wq)
+		destroy_workqueue(wc->writeback_wq);
+
+	if (wc->dev)
+		dm_put_device(ti, wc->dev);
+
+	if (wc->ssd_dev)
+		dm_put_device(ti, wc->ssd_dev);
+
+	if (wc->entries)
+		vfree(wc->entries);
+
+	if (wc->memory_map) {
+		if (WC_MODE_PMEM(wc))
+			persistent_memory_release(ti, &wc->pmem_holder,
+						  wc->memory_map, wc->memory_map_size);
+		else
+			vfree(wc->memory_map);
+	}
+
+	kfree(wc->memory_name);
+
+	if (wc->dm_kcopyd)
+		dm_kcopyd_client_destroy(wc->dm_kcopyd);
+
+	if (wc->dm_io)
+		dm_io_client_destroy(wc->dm_io);
+
+	if (wc->dirty_bitmap)
+		vfree(wc->dirty_bitmap);
+
+	kfree(wc);
+}
+
+static int writecache_ctr(struct dm_target *ti, unsigned argc, char **argv)
+{
+	struct dm_writecache *wc;
+	struct dm_arg_set as;
+	const char *string;
+	unsigned opt_params;
+	size_t offset, data_size;
+	int i, r;
+	char dummy;
+	int high_wm_percent = 95;
+	int low_wm_percent = 90;
+	uint64_t x;
+	struct wc_memory_superblock s;
+
+	static struct dm_arg _args[] = {
+		{0, 4, "Invalid number of feature args"},
+	};
+
+	as.argc = argc;
+	as.argv = argv;
+
+	wc = kzalloc(sizeof(struct dm_writecache), GFP_KERNEL);
+	if (!wc) {
+		ti->error = "Cannot allocate writecache structure";
+		r = -ENOMEM;
+		goto bad;
+	}
+	ti->private = wc;
+	wc->ti = ti;
+
+	mutex_init(&wc->lock);
+	writecache_poison_lists(wc);
+	init_waitqueue_head(&wc->freelist_wait);
+
+	for (i = 0; i < 2; i++) {
+		atomic_set(&wc->bio_in_progress[i], 0);
+		init_waitqueue_head(&wc->bio_in_progress_wait[i]);
+	}
+
+	wc->dm_io = dm_io_client_create();
+	if (!wc->dm_io) {
+		r = -ENOMEM;
+		ti->error = "Unable to allocate dm-io client";
+		goto bad;
+	}
+
+	wc->writeback_wq = alloc_workqueue("writecache-writeabck",
+					   WQ_MEM_RECLAIM | WQ_UNBOUND, 1);
+	if (!wc->writeback_wq) {
+		r = -ENOMEM;
+		ti->error = "Could not allocate writeback workqueue";
+		goto bad;
+	}
+	INIT_WORK(&wc->writeback_work, writecache_writeback);
+
+	init_waitqueue_head(&wc->endio_thread_wait);
+	INIT_LIST_HEAD(&wc->endio_list);
+	wc->endio_thread = kthread_create(writecache_endio_thread, wc, "writecache_endio");
+	if (IS_ERR(wc->endio_thread)) {
+		r = PTR_ERR(wc->endio_thread);
+		wc->endio_thread = NULL;
+		ti->error = "Couldn't spawn endio thread";
+		goto bad;
+	}
+	wake_up_process(wc->endio_thread);
+
+	/*
+	 * Parse the mode (pmem or ssd)
+	 */
+	string = dm_shift_arg(&as);
+	if (!string)
+		goto bad_arguments;
+
+	if (!strcasecmp(string, "s")) {
+#ifndef WC_ONLY_SSD
+		wc->pmem_mode = false;
+#endif
+	} else if (!strcasecmp(string, "p")) {
+#ifndef WC_ONLY_SSD
+		wc->pmem_mode = true;
+#else
+		r = -EOPNOTSUPP;
+		ti->error = "Persistent memory not supported on this architecture";
+#endif
+	} else {
+		goto bad_arguments;
+	}
+
+	if (WC_MODE_PMEM(wc)) {
+		wc->bio_set = bioset_create(BIO_POOL_SIZE,
+					    offsetof(struct writeback_struct, bio),
+					    BIOSET_NEED_BVECS);
+		if (!wc->bio_set) {
+			r = -ENOMEM;
+			ti->error = "Could not allocate bio set";
+			goto bad;
+		}
+	} else {
+		wc->copy_pool = mempool_create_kmalloc_pool(1, sizeof(struct copy_struct));
+		if (!wc->copy_pool) {
+			r = -ENOMEM;
+			ti->error = "Could not allocate mempool";
+			goto bad;
+		}
+	}
+#ifdef WC_COPY_TO_PAGES_BEFORE_WRITING
+	if (WC_MODE_PMEM(wc)) {
+		wc->page_pool = mempool_create_page_pool(BIO_POOL_SIZE, 0);
+		if (!wc->page_pool) {
+			r = -ENOMEM;
+			ti->error = "Could not allocate page mempool";
+			goto bad;
+		}
+	}
+#endif
+	/*
+	 * Parse the origin data device
+	 */
+	string = dm_shift_arg(&as);
+	if (!string)
+		goto bad_arguments;
+	r = dm_get_device(ti, string, dm_table_get_mode(ti->table), &wc->dev);
+	if (r) {
+		ti->error = "Origin data device lookup failed";
+		goto bad;
+	}
+
+	/*
+	 * Parse cache data device (be it pmem or ssd)
+	 */
+	string = dm_shift_arg(&as);
+	if (!string)
+		goto bad_arguments;
+	if (WC_MODE_PMEM(wc)) {
+		wc->memory_name = kstrdup(string, GFP_KERNEL);
+		if (!wc->memory_name) {
+			r = -ENOMEM;
+			ti->error = "Could not allocate string";
+			goto bad;
+		}
+
+		r = persistent_memory_claim(ti, wc->memory_name, &wc->pmem_holder,
+					    &wc->memory_map, &wc->memory_map_size);
+		if (r) {
+			ti->error = "Unable to map persistent memory for cache";
+			goto bad;
+		}
+	} else {
+		r = dm_get_device(ti, string, dm_table_get_mode(ti->table), &wc->ssd_dev);
+		if (r) {
+			ti->error = "Cache data device lookup failed";
+			goto bad;
+		}
+		wc->memory_map_size = i_size_read(wc->ssd_dev->bdev->bd_inode);
+	}
+
+	/*
+	 * Parse the cache block size
+	 */
+	string = dm_shift_arg(&as);
+	if (!string)
+		goto bad_arguments;
+	if (sscanf(string, "%u%c", &wc->block_size, &dummy) != 1 ||
+	    wc->block_size < 512 || wc->block_size > PAGE_SIZE ||
+	    (wc->block_size & (wc->block_size - 1))) {
+		r = -EINVAL;
+		ti->error = "Invalid block size";
+		goto bad;
+	}
+	wc->block_size_bits = __ffs(wc->block_size);
+
+	/*
+	 * Parse optional arguments
+	 */
+	r = dm_read_arg_group(_args, &as, &opt_params, &ti->error);
+	if (r)
+		goto bad;
+
+	while (opt_params) {
+		string = dm_shift_arg(&as);
+		if (!strcasecmp(string, "high-watermark")) {
+			string = dm_shift_arg(&as);
+			if (!string)
+				goto invalid_optional;
+			if (sscanf(string, "%d%%%c", &high_wm_percent, &dummy) == 1) {
+				if (high_wm_percent < 0 || high_wm_percent > 100)
+					goto invalid_optional;
+				wc->high_wm_percent_set = true;
+			} else
+				goto invalid_optional;
+		} else if (!strcasecmp(string, "low-watermark")) {
+			string = dm_shift_arg(&as);
+			if (!string)
+				goto invalid_optional;
+			if (sscanf(string, "%d%%%c", &low_wm_percent, &dummy) == 1) {
+				if (low_wm_percent < 0 || low_wm_percent > 100)
+					goto invalid_optional;
+				wc->low_wm_percent_set = true;
+			} else
+				goto invalid_optional;
+		} else {
+invalid_optional:
+			r = -EINVAL;
+			ti->error = "Invalid optional argument";
+			goto bad;
+		}
+	}
+
+	if (!WC_MODE_PMEM(wc)) {
+		struct dm_io_region region;
+		struct dm_io_request req;
+		size_t n_blocks, n_metadata_blocks;
+		uint64_t n_bitmap_bits;
+
+		init_completion(&wc->flush_completion);
+		wc->flush_thread = kthread_create(writecache_flush_thread, wc, "writecache_flush");
+		if (IS_ERR(wc->flush_thread)) {
+			r = PTR_ERR(wc->flush_thread);
+			wc->flush_thread = NULL;
+			ti->error = "Couldn't spawn endio thread";
+			goto bad;
+		}
+		writecache_offload_bio(wc, NULL);
+
+		r = calculate_memory_size(wc->memory_map_size, wc->block_size,
+					  &n_blocks, &n_metadata_blocks);
+		if (r) {
+			ti->error = "Invalid device size";
+			goto bad;
+		}
+
+		n_bitmap_bits = (((uint64_t)n_metadata_blocks << wc->block_size_bits) + BITMAP_GRANULARITY - 1) / BITMAP_GRANULARITY;
+		/* this is limitation of test_bit functions */
+		if (n_bitmap_bits > 1U << 31) {
+			r = -EFBIG;
+			ti->error = "Invalid device size";
+		}
+
+		wc->memory_map = vmalloc(n_metadata_blocks << wc->block_size_bits);
+		if (!wc->memory_map) {
+			r = -ENOMEM;
+			ti->error = "Unable to allocate memory for metadata";
+			goto bad;
+		}
+
+		wc->dm_kcopyd = dm_kcopyd_client_create(&dm_kcopyd_throttle);
+		if (!wc->dm_kcopyd) {
+			r = -ENOMEM;
+			ti->error = "Unable to allocate dm-kcopyd client";
+			goto bad;
+		}
+
+		wc->metadata_sectors = n_metadata_blocks << (wc->block_size_bits - SECTOR_SHIFT);
+		wc->dirty_bitmap_size = (n_bitmap_bits + BITS_PER_LONG - 1) / BITS_PER_LONG * sizeof(unsigned long);
+		wc->dirty_bitmap = vzalloc(wc->dirty_bitmap_size);
+		if (!wc->dirty_bitmap) {
+			r = -ENOMEM;
+			ti->error = "Unable to allocate dirty bitmap";
+			goto bad;
+		}
+
+		region.bdev = wc->ssd_dev->bdev;
+		region.sector = 0;
+		region.count = wc->metadata_sectors;
+		req.bi_op = REQ_OP_READ;
+		req.bi_op_flags = REQ_SYNC;
+		req.mem.type = DM_IO_VMA;
+		req.mem.ptr.vma = (char *)wc->memory_map;
+		req.client = wc->dm_io;
+		req.notify.fn = NULL;
+
+		r = dm_io(&req, 1, &region, NULL);
+		if (unlikely(r)) {
+			ti->error = "Unable to read metadata";
+			goto bad;
+		}
+	}
+
+	r = memcpy_mcsafe(&s, sb(wc), sizeof(struct wc_memory_superblock));
+	if (r) {
+		ti->error = "Hardware memory error when reading superblock";
+		goto bad;
+	}
+	if (!le32_to_cpu(le32_to_cpu(s.magic)) && !le32_to_cpu(s.version)) {
+		r = init_memory(wc);
+		if (r) {
+			ti->error = "Unable to initialize device";
+			goto bad;
+		}
+		r = memcpy_mcsafe(&s, sb(wc), sizeof(struct wc_memory_superblock));
+		if (r) {
+			ti->error = "Hardware memory error when reading superblock";
+			goto bad;
+		}
+	}
+
+	if (le32_to_cpu(s.magic) != MEMORY_SUPERBLOCK_MAGIC) {
+		ti->error = "Invalid magic in the superblock";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (le32_to_cpu(s.version) != MEMORY_SUPERBLOCK_VERSION) {
+		ti->error = "Invalid version in the superblock";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	if (le32_to_cpu(s.block_size) != wc->block_size) {
+		ti->error = "Block size does not match superblock";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	wc->n_blocks = le64_to_cpu(s.n_blocks);
+	wc->seq_count = le64_to_cpu(s.seq_count);
+
+	offset = wc->n_blocks * sizeof(struct wc_memory_entry);
+	if (offset / sizeof(struct wc_memory_entry) != le64_to_cpu(sb(wc)->n_blocks)) {
+overflow:
+		ti->error = "Overflow in size calculation";
+		r = -EINVAL;
+		goto bad;
+	}
+	offset += sizeof(struct wc_memory_superblock);
+	if (offset < sizeof(struct wc_memory_superblock))
+		goto overflow;
+	offset = (offset + wc->block_size - 1) & ~(size_t)(wc->block_size - 1);
+	data_size = wc->n_blocks * (size_t)wc->block_size;
+	if (!offset || (data_size / wc->block_size != wc->n_blocks) ||
+	    (offset + data_size < offset))
+		goto overflow;
+	if (offset + data_size > wc->memory_map_size) {
+		ti->error = "Memory area is too small";
+		r = -EINVAL;
+		goto bad;
+	}
+
+	wc->metadata_sectors = offset >> SECTOR_SHIFT;
+	wc->block_start = (char *)sb(wc) + offset;
+
+	x = (uint64_t)wc->n_blocks * (100 - high_wm_percent);
+	do_div(x, 100);
+	wc->freelist_high_watermark = x;
+	x = (uint64_t)wc->n_blocks * (100 - low_wm_percent);
+	do_div(x, 100);
+	wc->freelist_low_watermark = x;
+
+	wc->entries = vmalloc(sizeof(struct wc_entry) * wc->n_blocks);
+	if (unlikely(!wc->entries)) {
+		ti->error = "Cannot allocate memory";
+		r = -ENOMEM;
+		goto bad;
+	}
+
+	ti->num_flush_bios = 1;
+	ti->flush_supported = true;
+	ti->num_discard_bios = 1;
+
+	return 0;
+
+bad_arguments:
+	r = -EINVAL;
+	ti->error = "Bad arguments";
+bad:
+	writecache_dtr(ti);
+	return r;
+}
+
+static void writecache_status(struct dm_target *ti, status_type_t type,
+			      unsigned status_flags, char *result, unsigned maxlen)
+{
+	struct dm_writecache *wc = ti->private;
+	unsigned extra_args;
+	unsigned sz = 0;
+	uint64_t x;
+
+	switch (type) {
+	case STATUSTYPE_INFO:
+		DMEMIT("%ld %llu %llu %llu", writecache_has_error(wc), (unsigned long long)wc->n_blocks,
+		       (unsigned long long)wc->freelist_size, (unsigned long long)wc->writeback_size);
+		break;
+	case STATUSTYPE_TABLE:
+		if (WC_MODE_PMEM(wc))
+			DMEMIT("p %s %s %u ", wc->dev->name, wc->memory_name, wc->block_size);
+		else
+			DMEMIT("s %s %s %u ", wc->dev->name, wc->ssd_dev->name, wc->block_size);
+		extra_args = 0;
+		if (wc->high_wm_percent_set)
+			extra_args += 2;
+		if (wc->low_wm_percent_set)
+			extra_args += 2;
+		DMEMIT("%u", extra_args);
+		if (wc->high_wm_percent_set) {
+			x = (uint64_t)wc->freelist_high_watermark * 100;
+			do_div(x, (size_t)wc->n_blocks);
+			DMEMIT(" high-watermark %u%%", 100 - (unsigned)x);
+		}
+		if (wc->low_wm_percent_set) {
+			x = (uint64_t)wc->freelist_low_watermark * 100;
+			do_div(x, (size_t)wc->n_blocks);
+			DMEMIT(" low-watermark %u%%", 100 - (unsigned)x);
+		}
+		break;
+	}
+}
+
+static struct target_type writecache_target = {
+	.name			= "writecache",
+	.version		= {1, 0, 0},
+	.module			= THIS_MODULE,
+	.ctr			= writecache_ctr,
+	.dtr			= writecache_dtr,
+	.status			= writecache_status,
+	.postsuspend		= writecache_suspend,
+	.resume			= writecache_resume,
+	.message		= writecache_message,
+	.map			= writecache_map,
+	.end_io			= writecache_end_io,
+	.iterate_devices	= writecache_iterate_devices,
+	.io_hints		= writecache_io_hints,
+};
+
+static int __init dm_writecache_init(void)
+{
+	int r;
+
+	r = dm_register_target(&writecache_target);
+	if (r < 0) {
+		DMERR("register failed %d", r);
+		return r;
+	}
+
+	return 0;
+}
+
+static void __exit dm_writecache_exit(void)
+{
+	dm_unregister_target(&writecache_target);
+}
+
+module_init(dm_writecache_init);
+module_exit(dm_writecache_exit);
+
+MODULE_DESCRIPTION(DM_NAME " writecache target");
+MODULE_AUTHOR("Mikulas Patocka <mpatocka at redhat.com>");
+MODULE_LICENSE("GPL");