[PATCH 4/4] dm-writecache

[PATCH 4/4] dm-writecache

[Mikulas Patocka]

[-- Attachment #1: dm-writecache.patch --]
[-- Type: text/plain, Size: 67238 bytes --]

The dm-writecache target.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 Documentation/device-mapper/writecache.txt |   68 
 drivers/md/Kconfig                         |   11 
 drivers/md/Makefile                        |    1 
 drivers/md/dm-writecache.c                 | 2382 +++++++++++++++++++++++++++++
 4 files changed, 2462 insertions(+)

Index: linux-2.6/drivers/md/Kconfig
===================================================================
--- linux-2.6.orig/drivers/md/Kconfig	2018-05-30 14:50:34.000000000 +0200
+++ linux-2.6/drivers/md/Kconfig	2018-05-30 14:50:34.000000000 +0200
@@ -334,6 +334,17 @@ 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_ERA
        tristate "Era target (EXPERIMENTAL)"
        depends on BLK_DEV_DM
Index: linux-2.6/drivers/md/Makefile
===================================================================
--- linux-2.6.orig/drivers/md/Makefile	2018-05-30 14:50:34.000000000 +0200
+++ linux-2.6/drivers/md/Makefile	2018-05-30 14:50:34.000000000 +0200
@@ -67,6 +67,7 @@ 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	1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/drivers/md/dm-writecache.c	2018-05-30 15:14:46.000000000 +0200
@@ -0,0 +1,2382 @@
+/*
+ * Copyright (C) 2018 Red Hat. 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/vmalloc.h>
+#include <linux/kthread.h>
+#include <linux/swait.h>
+#include <linux/dm-io.h>
+#include <linux/dm-kcopyd.h>
+#include <linux/dax.h>
+#include <linux/pfn_t.h>
+#include <linux/libnvdimm.h>
+
+#define DM_MSG_PREFIX "writecache"
+
+#define HIGH_WATERMARK			50
+#define LOW_WATERMARK			45
+#define MAX_WRITEBACK_JOBS		0
+#define ENDIO_LATENCY			16
+#define WRITEBACK_LATENCY		64
+#define AUTOCOMMIT_BLOCKS_SSD		65536
+#define AUTOCOMMIT_BLOCKS_PMEM		64
+#define AUTOCOMMIT_MSEC			1000
+
+/*
+ * If the architecture doesn't support persistent memory, we can use this driver
+ * in SSD-only mode.
+ */
+#if !defined(CONFIG_ARCH_HAS_PMEM_API) || !defined(CONFIG_DAX)
+#define DM_WRITECACHE_ONLY_SSD
+#endif
+
+//#define WC_MEASURE_LATENCY
+
+#define BITMAP_GRANULARITY	65536
+#if BITMAP_GRANULARITY < PAGE_SIZE
+#undef BITMAP_GRANULARITY
+#define BITMAP_GRANULARITY	PAGE_SIZE
+#endif
+
+#define pmem_assign(dest, src)						\
+do {									\
+	typeof(dest) uniq = (src);					\
+	memcpy_flushcache(&(dest), &uniq, sizeof(dest));		\
+} while (0)
+
+#if defined(__HAVE_ARCH_MEMCPY_MCSAFE) && !defined(DM_WRITECACHE_ONLY_SSD)
+#define DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+#endif
+
+#define MEMORY_SUPERBLOCK_MAGIC		0x23489321
+#define MEMORY_SUPERBLOCK_VERSION	1
+
+struct wc_memory_entry {
+	__le64 original_sector;
+	__le64 seq_count;
+};
+
+struct wc_memory_superblock {
+	union {
+		struct {
+			__le32 magic;
+			__le32 version;
+			__le32 block_size;
+			__le32 pad;
+			__le64 n_blocks;
+			__le64 seq_count;
+		};
+		__le64 padding[8];
+	};
+	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 DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	uint64_t original_sector;
+	uint64_t seq_count;
+#endif
+};
+
+#ifndef DM_WRITECACHE_ONLY_SSD
+#define WC_MODE_PMEM(wc)			(likely((wc)->pmem_mode))
+#define WC_MODE_FUA(wc)				((wc)->writeback_fua)
+#else
+#define WC_MODE_PMEM(wc)			false
+#define WC_MODE_FUA(wc)				false
+#endif
+#define WC_MODE_SORT_FREELIST(wc)		(!WC_MODE_PMEM(wc))
+
+struct dm_writecache {
+#ifndef DM_WRITECACHE_ONLY_SSD
+	bool pmem_mode;
+	bool writeback_fua;
+#endif
+	struct mutex lock;
+	struct rb_root tree;
+	struct list_head lru;
+	union {
+		struct list_head freelist;
+		struct {
+			struct rb_root freetree;
+			struct wc_entry *current_free;
+		};
+	};
+	size_t freelist_size;
+	size_t writeback_size;
+	unsigned uncommitted_blocks;
+	unsigned autocommit_blocks;
+	unsigned max_writeback_jobs;
+	size_t freelist_high_watermark;
+	size_t freelist_low_watermark;
+	struct timer_list autocommit_timer;
+	unsigned long autocommit_jiffies;
+	struct swait_queue_head 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 overwrote_committed;
+	bool memory_vmapped;
+
+	atomic_t bio_in_progress[2];
+	struct swait_queue_head bio_in_progress_wait[2];
+
+	struct dm_io_client *dm_io;
+
+	unsigned writeback_all;
+	struct workqueue_struct *writeback_wq;
+	struct work_struct writeback_work;
+	struct work_struct flush_work;
+
+	struct swait_queue_head 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;
+
+	struct dm_kcopyd_client *dm_kcopyd;
+	unsigned long *dirty_bitmap;
+	unsigned dirty_bitmap_size;
+
+	bool high_wm_percent_set;
+	bool low_wm_percent_set;
+	bool max_writeback_jobs_set;
+	bool autocommit_blocks_set;
+	bool autocommit_time_set;
+	bool writeback_fua_set;
+	bool flush_on_suspend;
+
+#ifdef WC_MEASURE_LATENCY
+	ktime_t lock_acquired_time;
+	ktime_t max_lock_held;
+	ktime_t max_lock_wait;
+	ktime_t max_freelist_wait;
+	ktime_t measure_latency_time;
+	ktime_t max_measure_latency;
+#endif
+};
+
+#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;
+	unsigned page_offset;
+	struct page *page;
+	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;
+	unsigned n_entries;
+	int error;
+};
+
+DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(dm_writecache_throttle,
+					    "A percentage of time allocated for data copying");
+
+static inline void measure_latency_start(struct dm_writecache *wc)
+{
+#ifdef WC_MEASURE_LATENCY
+	wc->measure_latency_time = ktime_get();
+#endif
+}
+
+static inline void measure_latency_end(struct dm_writecache *wc, unsigned long n)
+{
+#ifdef WC_MEASURE_LATENCY
+	ktime_t now = ktime_get();
+	if (now - wc->measure_latency_time > wc->max_measure_latency) {
+		wc->max_measure_latency = now - wc->measure_latency_time;
+		printk(KERN_DEBUG "dm-writecache: measured latency %lld.%03lldus, %lu steps\n",
+		       wc->max_measure_latency / 1000, wc->max_measure_latency % 1000, n);
+	}
+#endif
+}
+
+static void __wc_lock(struct dm_writecache *wc, int line)
+{
+#ifdef WC_MEASURE_LATENCY
+	ktime_t before, after;
+	before = ktime_get();
+#endif
+	mutex_lock(&wc->lock);
+#ifdef WC_MEASURE_LATENCY
+	after = ktime_get();
+	if (unlikely(after - before > wc->max_lock_wait)) {
+		wc->max_lock_wait = after - before;
+		printk(KERN_DEBUG "dm-writecache: waiting for lock for %lld.%03lldus at %d\n",
+		       wc->max_lock_wait / 1000, wc->max_lock_wait % 1000, line);
+		after = ktime_get();
+	}
+	wc->lock_acquired_time = after;
+#endif
+}
+#define wc_lock(wc)	__wc_lock(wc, __LINE__)
+
+static void __wc_unlock(struct dm_writecache *wc, int line)
+{
+#ifdef WC_MEASURE_LATENCY
+	ktime_t now = ktime_get();
+	if (now - wc->lock_acquired_time > wc->max_lock_held) {
+		wc->max_lock_held = now - wc->lock_acquired_time;
+		printk(KERN_DEBUG "dm-writecache: lock held for %lld.%03lldus at %d\n",
+		       wc->max_lock_held / 1000, wc->max_lock_held % 1000, line);
+	}
+#endif
+	mutex_unlock(&wc->lock);
+}
+#define wc_unlock(wc)	__wc_unlock(wc, __LINE__)
+
+#define wc_unlock_long(wc)	mutex_unlock(&wc->lock)
+
+#ifndef DM_WRITECACHE_ONLY_SSD
+static int persistent_memory_claim(struct dm_writecache *wc)
+{
+	int r;
+	loff_t s;
+	long p, da;
+	pfn_t pfn;
+	int id;
+	struct page **pages;
+
+	wc->memory_vmapped = false;
+
+	if (!wc->ssd_dev->dax_dev) {
+		r = -EOPNOTSUPP;
+		goto err1;
+	}
+	s = wc->memory_map_size;
+	p = s >> PAGE_SHIFT;
+	if (!p) {
+		r = -EINVAL;
+		goto err1;
+	}
+	if (p != s >> PAGE_SHIFT) {
+		r = -EOVERFLOW;
+		goto err1;
+	}
+
+	id = dax_read_lock();
+
+	da = dax_direct_access(wc->ssd_dev->dax_dev, 0, p, &wc->memory_map, &pfn);
+	if (da < 0) {
+		wc->memory_map = NULL;
+		r = da;
+		goto err2;
+	}
+	if (!pfn_t_has_page(pfn)) {
+		wc->memory_map = NULL;
+		r = -EOPNOTSUPP;
+		goto err2;
+	}
+#ifdef WC_MEASURE_LATENCY
+	printk(KERN_DEBUG "dm-writecache: device %s, pfn %016llx\n",
+	       wc->ssd_dev->name, pfn.val);
+#endif
+	if (da != p) {
+		long i;
+		wc->memory_map = NULL;
+		pages = kvmalloc(p * sizeof(struct page *), GFP_KERNEL);
+		if (!pages) {
+			r = -ENOMEM;
+			goto err2;
+		}
+		i = 0;
+		do {
+			long daa;
+			void *dummy_addr;
+			daa = dax_direct_access(wc->ssd_dev->dax_dev, i, p - i,
+						&dummy_addr, &pfn);
+			if (daa <= 0) {
+				r = daa ? daa : -EINVAL;
+				goto err3;
+			}
+			if (!pfn_t_has_page(pfn)) {
+				r = -EOPNOTSUPP;
+				goto err3;
+			}
+			while (daa-- && i < p) {
+				pages[i++] = pfn_t_to_page(pfn);
+				pfn.val++;
+			}
+		} while (i < p);
+		wc->memory_map = vmap(pages, p, VM_MAP, PAGE_KERNEL);
+		if (!wc->memory_map) {
+			r = -ENOMEM;
+			goto err3;
+		}
+		kvfree(pages);
+		wc->memory_vmapped = true;
+	}
+
+	dax_read_unlock(id);
+
+	return 0;
+
+err3:
+	kvfree(pages);
+err2:
+	dax_read_unlock(id);
+err1:
+	return r;
+}
+#endif
+
+static void persistent_memory_release(struct dm_writecache *wc)
+{
+	if (wc->memory_vmapped)
+		vunmap(wc->memory_map);
+}
+
+static struct page *persistent_memory_page(void *addr)
+{
+	if (is_vmalloc_addr(addr))
+		return vmalloc_to_page(addr);
+	else
+		return virt_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)
+{
+	if (is_vmalloc_addr(ptr))
+		flush_kernel_vmap_range(ptr, size);
+}
+
+static void persistent_memory_invalidate_cache(void *ptr, size_t size)
+{
+	if (is_vmalloc_addr(ptr))
+		invalidate_kernel_vmap_range(ptr, size);
+}
+
+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 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 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 DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	e->seq_count = -1;
+#endif
+	pmem_assign(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 me;
+#ifdef DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	e->original_sector = original_sector;
+	e->seq_count = seq_count;
+#endif
+	me.original_sector = cpu_to_le64(original_sector);
+	me.seq_count = cpu_to_le64(seq_count);
+	pmem_assign(*memory_entry(wc, e), me);
+}
+
+#define writecache_error(wc, err, msg, arg...)				\
+do {									\
+	if (!cmpxchg(&(wc)->error, 0, err))				\
+		DMERR(msg, ##arg);					\
+	swake_up(&(wc)->freelist_wait);					\
+} while (0)
+
+#define writecache_has_error(wc)	(unlikely(READ_ONCE((wc)->error)))
+
+static void writecache_flush_all_metadata(struct dm_writecache *wc)
+{
+	if (!WC_MODE_PMEM(wc))
+		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))
+		__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;
+};
+
+static 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)
+{
+	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;
+
+		/* writing via async dm-io (implied by notify.fn above) won't return an error */
+	        (void) dm_io(&req, 1, &region, NULL);
+		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))
+		wmb();
+	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.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)
+{
+	swait_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;
+		if (read_original_sector(wc, e) > read_original_sector(wc, ins))
+			node = &parent->rb_left;
+		else
+			node = &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)
+{
+	if (WC_MODE_SORT_FREELIST(wc)) {
+		struct rb_node **node = &wc->freetree.rb_node, *parent = NULL;
+		if (unlikely(!*node))
+			wc->current_free = e;
+		while (*node) {
+			parent = *node;
+			if (&e->rb_node < *node)
+				node = &parent->rb_left;
+			else
+				node = &parent->rb_right;
+		}
+		rb_link_node(&e->rb_node, parent, node);
+		rb_insert_color(&e->rb_node, &wc->freetree);
+	} else {
+		list_add_tail(&e->lru, &wc->freelist);
+	}
+	wc->freelist_size++;
+}
+
+static struct wc_entry *writecache_pop_from_freelist(struct dm_writecache *wc)
+{
+	struct wc_entry *e;
+
+	if (WC_MODE_SORT_FREELIST(wc)) {
+		struct rb_node *next;
+		if (unlikely(!wc->current_free))
+			return NULL;
+		e = wc->current_free;
+		next = rb_next(&e->rb_node);
+		rb_erase(&e->rb_node, &wc->freetree);
+		if (unlikely(!next))
+			next = rb_first(&wc->freetree);
+		wc->current_free = next ? container_of(next, struct wc_entry, rb_node) : NULL;
+	} else {
+		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(swait_active(&wc->freelist_wait)))
+		swake_up(&wc->freelist_wait);
+}
+
+static void __writecache_wait_on_freelist(struct dm_writecache *wc, bool measure, int line)
+{
+	DECLARE_SWAITQUEUE(wait);
+#ifdef WC_MEASURE_LATENCY
+	ktime_t before, after;
+#endif
+
+	prepare_to_swait(&wc->freelist_wait, &wait, TASK_UNINTERRUPTIBLE);
+	wc_unlock(wc);
+#ifdef WC_MEASURE_LATENCY
+	if (measure)
+		before = ktime_get();
+#endif
+	io_schedule();
+	finish_swait(&wc->freelist_wait, &wait);
+#ifdef WC_MEASURE_LATENCY
+	if (measure) {
+		after = ktime_get();
+		if (unlikely(after - before > wc->max_freelist_wait)) {
+			wc->max_freelist_wait = after - before;
+			printk(KERN_DEBUG "dm-writecache: waiting on freelist for %lld.%03lldus at %d\n",
+			       wc->max_freelist_wait / 1000, wc->max_freelist_wait % 1000, line);
+		}
+	}
+#endif
+	wc_lock(wc);
+}
+#define writecache_wait_on_freelist(wc)		__writecache_wait_on_freelist(wc, true, __LINE__)
+#define writecache_wait_on_freelist_long(wc)	__writecache_wait_on_freelist(wc, false, __LINE__)
+
+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->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;
+	bool need_flush_after_free;
+
+	wc->uncommitted_blocks = 0;
+	del_timer(&wc->autocommit_timer);
+
+	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;
+	}
+	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();
+	}
+	writecache_commit_flushed(wc);
+
+	writecache_wait_for_ios(wc, WRITE);
+
+	wc->seq_count++;
+	pmem_assign(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;
+
+	need_flush_after_free = 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);
+				need_flush_after_free = true;
+			}
+		}
+		if (unlikely(e->lru.prev == &wc->lru))
+			break;
+		e = container_of(e->lru.prev, struct wc_entry, lru);
+		cond_resched();
+	}
+
+	if (need_flush_after_free)
+		writecache_commit_flushed(wc);
+}
+
+static void writecache_flush_work(struct work_struct *work)
+{
+	struct dm_writecache *wc = container_of(work, struct dm_writecache, flush_work);
+	wc_lock(wc);
+	writecache_flush(wc);
+	wc_unlock(wc);
+}
+
+static void writecache_autocommit_timer(struct timer_list *t)
+{
+	struct dm_writecache *wc = from_timer(wc, t, autocommit_timer);
+	if (!writecache_has_error(wc))
+		queue_work(wc->writeback_wq, &wc->flush_work);
+}
+
+static void writecache_schedule_autocommit(struct dm_writecache *wc)
+{
+	if (!timer_pending(&wc->autocommit_timer))
+		mod_timer(&wc->autocommit_timer, jiffies + wc->autocommit_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 (wc->writeback_size) {
+		writecache_wait_on_freelist(wc);
+		return true;
+	}
+	return false;
+}
+
+static void writecache_suspend(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+	bool flush_on_suspend;
+
+	del_timer_sync(&wc->autocommit_timer);
+
+	wc_lock(wc);
+	writecache_flush(wc);
+	flush_on_suspend = wc->flush_on_suspend;
+	if (flush_on_suspend) {
+		wc->flush_on_suspend = false;
+		wc->writeback_all++;
+		queue_work(wc->writeback_wq, &wc->writeback_work);
+	}
+	wc_unlock(wc);
+
+	flush_workqueue(wc->writeback_wq);
+
+	wc_lock(wc);
+	if (flush_on_suspend) {
+		wc->writeback_all--;
+	}
+	while (writecache_wait_for_writeback(wc));
+
+	if (WC_MODE_PMEM(wc))
+		persistent_memory_flush_cache(wc->memory_map, wc->memory_map_size);
+
+	writecache_poison_lists(wc);
+
+	wc_unlock_long(wc);
+}
+
+static int writecache_alloc_entries(struct dm_writecache *wc)
+{
+	size_t b;
+	if (wc->entries)
+		return 0;
+	wc->entries = vmalloc(sizeof(struct wc_entry) * wc->n_blocks);
+	if (!wc->entries)
+		return -ENOMEM;
+	for (b = 0; b < wc->n_blocks; b++) {
+		struct wc_entry *e = &wc->entries[b];
+		e->index = b;
+		e->write_in_progress = false;
+	}
+	return 0;
+}
+
+static void writecache_resume(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+	size_t b;
+	bool need_flush = false;
+	__le64 sb_seq_count;
+	int r;
+
+	wc_lock(wc);
+
+	if (WC_MODE_PMEM(wc))
+		persistent_memory_invalidate_cache(wc->memory_map, wc->memory_map_size);
+
+	wc->tree = RB_ROOT;
+	INIT_LIST_HEAD(&wc->lru);
+	if (WC_MODE_SORT_FREELIST(wc)) {
+		wc->freetree = RB_ROOT;
+		wc->current_free = NULL;
+	} else {
+		INIT_LIST_HEAD(&wc->freelist);
+	}
+	wc->freelist_size = 0;
+
+	r = memcpy_mcsafe(&sb_seq_count, &sb(wc)->seq_count, sizeof(uint64_t));
+	if (r) {
+		writecache_error(wc, r, "hardware memory error when reading superblock: %d", r);
+		sb_seq_count = cpu_to_le64(0);
+	}
+	wc->seq_count = le64_to_cpu(sb_seq_count);
+
+#ifdef 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;
+		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 (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:
+				clear_seq_count(wc, e);
+				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 (!old) {
+				writecache_insert_entry(wc, e);
+			} else {
+				if (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_metadata(wc);
+		writecache_commit_flushed(wc);
+	}
+
+	wc_unlock_long(wc);
+}
+
+static int process_flush_mesg(unsigned argc, char **argv, struct dm_writecache *wc)
+{
+	if (argc != 1)
+		return -EINVAL;
+
+	wc_lock(wc);
+	if (dm_suspended(wc->ti)) {
+		wc_unlock(wc);
+		return -EBUSY;
+	}
+	if (writecache_has_error(wc)) {
+		wc_unlock(wc);
+		return -EIO;
+	}
+
+	writecache_flush(wc);
+	wc->writeback_all++;
+	queue_work(wc->writeback_wq, &wc->writeback_work);
+	wc_unlock(wc);
+
+	flush_workqueue(wc->writeback_wq);
+
+	wc_lock(wc);
+	wc->writeback_all--;
+	if (writecache_has_error(wc)) {
+		wc_unlock(wc);
+		return -EIO;
+	}
+	wc_unlock(wc);
+
+	return 0;
+}
+
+static int process_flush_on_suspend_mesg(unsigned argc, char **argv, struct dm_writecache *wc)
+{
+	if (argc != 1)
+		return -EINVAL;
+
+	wc_lock(wc);
+	wc->flush_on_suspend = true;
+	wc_unlock(wc);
+
+	return 0;
+}
+
+static int writecache_message(struct dm_target *ti, unsigned argc, char **argv,
+			      char *result, unsigned maxlen)
+{
+	int r = -EINVAL;
+	struct dm_writecache *wc = ti->private;
+
+	if (!strcasecmp(argv[0], "flush"))
+		r = process_flush_mesg(argc, argv, wc);
+	else if (!strcasecmp(argv[0], "flush_on_suspend"))
+		r = process_flush_on_suspend_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 {
+		struct bio_vec bv = bio_iter_iovec(bio, bio->bi_iter);
+		buf = bvec_kmap_irq(&bv, &flags);
+		size = bv.bv_len;
+		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_flushcache(data, buf, size);
+		}
+
+		bvec_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 = wc->flush_bio;
+
+		if (likely(bio)) {
+			if (bio_op(bio) == REQ_OP_DISCARD)
+				writecache_discard(wc, bio->bi_iter.bi_sector, bio_end_sector(bio));
+			else
+				writecache_flush(wc);
+		}
+
+		set_current_state(TASK_INTERRUPTIBLE);
+		/* for debugging - catch uninitialized use */
+		wc->flush_bio = (void *)0x600 + POISON_POINTER_DELTA;
+		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);
+}
+
+/* FIXME: all the gotos in writecache_map() suggest the need for refactoring */
+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;
+
+	wc_lock(wc);
+
+	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_flush;
+	}
+
+	bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector);
+
+	if (unlikely((((unsigned)bio->bi_iter.bi_sector | bio_sectors(bio)) &
+				(wc->block_size / 512 - 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_end_sector(bio));
+		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_read;
+			} 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);
+			wc->uncommitted_blocks++;
+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);
+				if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks)) {
+					wc->uncommitted_blocks = 0;
+					queue_work(wc->writeback_wq, &wc->flush_work);
+				} else {
+					writecache_schedule_autocommit(wc);
+				}
+				goto unlock_remap;
+			}
+		} while (bio->bi_iter.bi_size);
+
+		if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks)) {
+			writecache_flush(wc);
+		} else {
+			writecache_schedule_autocommit(wc);
+		}
+
+		goto unlock_ok_write;
+	}
+
+unlock_remap_origin:
+	bio_set_dev(bio, wc->dev->bdev);
+	wc_unlock(wc);
+	return DM_MAPIO_REMAPPED;
+
+unlock_remap:
+	/* make sure that writecache_end_io decrements bio_in_progress: */
+	bio->bi_private = (void *)1;
+	atomic_inc(&wc->bio_in_progress[bio_data_dir(bio)]);
+	wc_unlock(wc);
+	return DM_MAPIO_REMAPPED;
+
+unlock_ok_flush:
+#ifdef WC_MEASURE_LATENCY
+	wc_unlock(wc);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+#endif
+
+unlock_ok_read:
+#ifdef WC_MEASURE_LATENCY
+	wc_unlock(wc);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+#endif
+
+unlock_ok_write:
+	wc_unlock(wc);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+
+unlock_error:
+	wc_unlock(wc);
+	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(swait_active(&wc->bio_in_progress_wait[dir])))
+				swake_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;
+
+	raw_spin_lock_irqsave(&wc->endio_thread_wait.lock, flags);
+	list_add_tail(&wb->endio_entry, &wc->endio_list);
+	swake_up_locked(&wc->endio_thread_wait);
+	raw_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;
+
+	c->error = likely(!(read_err | write_err)) ? 0 : -EIO;
+
+	raw_spin_lock_irq(&wc->endio_thread_wait.lock);
+	list_add_tail(&c->endio_entry, &wc->endio_list);
+	swake_up_locked(&wc->endio_thread_wait);
+	raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
+}
+
+static void __writecache_endio_pmem(struct dm_writecache *wc, struct list_head *list)
+{
+	unsigned i;
+	struct writeback_struct *wb;
+	struct wc_entry *e;
+	unsigned long n_walked = 0;
+
+	do {
+		wb = list_entry(list->next, struct writeback_struct, endio_entry);
+		list_del(&wb->endio_entry);
+
+		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;
+			INIT_LIST_HEAD(&e->lru);
+			if (!writecache_has_error(wc))
+				writecache_free_entry(wc, e);
+			BUG_ON(!wc->writeback_size);
+			wc->writeback_size--;
+			n_walked++;
+			if (unlikely(n_walked >= ENDIO_LATENCY)) {
+				writecache_commit_flushed(wc);
+				wc_unlock(wc);
+				wc_lock(wc);
+				n_walked = 0;
+			}
+		} 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;
+		do {
+			BUG_ON(!e->write_in_progress);
+			e->write_in_progress = false;
+			INIT_LIST_HEAD(&e->lru);
+			if (!writecache_has_error(wc))
+				writecache_free_entry(wc, e);
+
+			BUG_ON(!wc->writeback_size);
+			wc->writeback_size--;
+			e++;
+		} while (--c->n_entries);
+		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_SWAITQUEUE(wait);
+		struct list_head list;
+
+		raw_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);
+		__prepare_to_swait(&wc->endio_thread_wait, &wait);
+		raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
+
+		if (unlikely(kthread_should_stop())) {
+			finish_swait(&wc->endio_thread_wait, &wait);
+			break;
+		}
+
+		schedule();
+
+		raw_spin_lock_irq(&wc->endio_thread_wait.lock);
+		__finish_swait(&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);
+		raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
+
+		if (!WC_MODE_FUA(wc))
+			writecache_disk_flush(wc, wc->dev);
+
+		wc_lock(wc);
+
+		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);
+
+		wc_unlock(wc);
+	}
+
+	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;
+	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;
+}
+
+struct writeback_list {
+	struct list_head list;
+	size_t size;
+};
+
+static void __writeback_throttle(struct dm_writecache *wc, struct writeback_list *wbl)
+{
+	if (unlikely(wc->max_writeback_jobs)) {
+		if (READ_ONCE(wc->writeback_size) - wbl->size >= wc->max_writeback_jobs) {
+			wc_lock(wc);
+			while (wc->writeback_size - wbl->size >= wc->max_writeback_jobs) {
+				writecache_wait_on_freelist_long(wc);
+			}
+			wc_unlock(wc);
+		}
+	}
+	cond_resched();
+}
+
+static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeback_list *wbl)
+{
+	struct wc_entry *e, *f;
+	struct bio *bio;
+	struct writeback_struct *wb;
+	unsigned max_pages;
+
+	while (wbl->size) {
+		wbl->size--;
+		e = container_of(wbl->list.prev, struct wc_entry, lru);
+		list_del(&e->lru);
+
+		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);
+		wb->page_offset = PAGE_SIZE;
+		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 (wbl->size && wb->wc_list_n < max_pages) {
+			f = container_of(wbl->list.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;
+			wbl->size--;
+			list_del(&f->lru);
+			wb->wc_list[wb->wc_list_n++] = f;
+			e = f;
+		}
+		bio_set_op_attrs(&wb->bio, REQ_OP_WRITE, WC_MODE_FUA(wc) * REQ_FUA);
+		if (writecache_has_error(wc)) {
+			bio->bi_status = BLK_STS_IOERR;
+			bio_endio(&wb->bio);
+		} else {
+			submit_bio(&wb->bio);
+		}
+
+		__writeback_throttle(wc, wbl);
+	}
+}
+
+static void __writecache_writeback_ssd(struct dm_writecache *wc, struct writeback_list *wbl)
+{
+	struct wc_entry *e, *f;
+	struct dm_io_region from, to;
+	struct copy_struct *c;
+
+	while (wbl->size) {
+		unsigned n_sectors;
+
+		wbl->size--;
+		e = container_of(wbl->list.prev, struct wc_entry, lru);
+		list_del(&e->lru);
+
+		n_sectors = e->wc_list_contiguous << (wc->block_size_bits - SECTOR_SHIFT);
+
+		from.bdev = wc->ssd_dev->bdev;
+		from.sector = cache_sector(wc, e);
+		from.count = n_sectors;
+		to.bdev = wc->dev->bdev;
+		to.sector = read_original_sector(wc, e);
+		to.count = n_sectors;
+
+		c = mempool_alloc(wc->copy_pool, GFP_NOIO);
+		c->wc = wc;
+		c->e = e;
+		c->n_entries = e->wc_list_contiguous;
+
+		while ((n_sectors -= wc->block_size >> SECTOR_SHIFT)) {
+			wbl->size--;
+			f = container_of(wbl->list.prev, struct wc_entry, lru);
+			BUG_ON(f != e + 1);
+			list_del(&f->lru);
+			e = f;
+		}
+
+		dm_kcopyd_copy(wc->dm_kcopyd, &from, 1, &to, 0, writecache_copy_endio, c);
+
+		__writeback_throttle(wc, wbl);
+	}
+}
+
+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;
+	struct writeback_list wbl;
+	unsigned long n_walked;
+
+	wc_lock(wc);
+restart:
+	if (writecache_has_error(wc)) {
+		wc_unlock(wc);
+		return;
+	}
+
+	if (unlikely(wc->writeback_all)) {
+		if (writecache_wait_for_writeback(wc))
+			goto restart;
+	}
+
+	if (wc->overwrote_committed) {
+		writecache_wait_for_ios(wc, WRITE);
+	}
+
+	n_walked = 0;
+	INIT_LIST_HEAD(&skipped);
+	INIT_LIST_HEAD(&wbl.list);
+	wbl.size = 0;
+	while (!list_empty(&wc->lru) &&
+	       (wc->writeback_all ||
+		wc->freelist_size + wc->writeback_size <= wc->freelist_high_watermark)) {
+
+		n_walked++;
+		if (unlikely(n_walked > WRITEBACK_LATENCY) &&
+		    likely(!wc->writeback_all) && likely(!dm_suspended(wc->ti))) {
+			queue_work(wc->writeback_wq, &wc->writeback_work);
+			break;
+		}
+
+		e = container_of(wc->lru.prev, struct wc_entry, lru);
+		BUG_ON(e->write_in_progress);
+		if (unlikely(!writecache_entry_is_committed(wc, e))) {
+			writecache_flush(wc);
+		}
+		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);
+				cond_resched();
+				continue;
+			}
+		}
+		wc->writeback_size++;
+		list_del(&e->lru);
+		list_add(&e->lru, &wbl.list);
+		wbl.size++;
+		e->write_in_progress = true;
+		e->wc_list_contiguous = 1;
+
+		f = e;
+
+		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;
+
+			if (!WC_MODE_PMEM(wc)) {
+				if (g != f + 1)
+					break;
+			}
+
+			n_walked++;
+			//if (unlikely(n_walked > WRITEBACK_LATENCY) && likely(!wc->writeback_all))
+			//	break;
+
+			wc->writeback_size++;
+			list_del(&g->lru);
+			list_add(&g->lru, &wbl.list);
+			wbl.size++;
+			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();
+	}
+
+	if (!list_empty(&skipped)) {
+		list_splice_tail(&skipped, &wc->lru);
+		/*
+		 * If we didn't do any progress, we must wait until some
+		 * writeback finishes to avoid burning CPU in a loop
+		 */
+		if (unlikely(!wbl.size))
+			writecache_wait_for_writeback(wc);
+	}
+
+	wc_unlock(wc);
+
+	blk_start_plug(&plug);
+
+	if (WC_MODE_PMEM(wc))
+		__writecache_writeback_pmem(wc, &wbl);
+	else
+		__writecache_writeback_ssd(wc, &wbl);
+
+	blk_finish_plug(&plug);
+
+	if (unlikely(wc->writeback_all)) {
+		wc_lock(wc);
+		while (writecache_wait_for_writeback(wc));
+		wc_unlock(wc);
+	}
+}
+
+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;
+	int r;
+
+	r = calculate_memory_size(wc->memory_map_size, wc->block_size, &wc->n_blocks, NULL);
+	if (r)
+		return r;
+
+	r = writecache_alloc_entries(wc);
+	if (r)
+		return r;
+
+	for (b = 0; b < ARRAY_SIZE(sb(wc)->padding); b++)
+		pmem_assign(sb(wc)->padding[b], cpu_to_le64(0));
+	pmem_assign(sb(wc)->version, cpu_to_le32(MEMORY_SUPERBLOCK_VERSION));
+	pmem_assign(sb(wc)->block_size, cpu_to_le32(wc->block_size));
+	pmem_assign(sb(wc)->n_blocks, cpu_to_le64(wc->n_blocks));
+	pmem_assign(sb(wc)->seq_count, cpu_to_le64(0));
+
+	for (b = 0; b < wc->n_blocks; b++)
+		write_original_sector_seq_count(wc, &wc->entries[b], -1, -1);
+
+	writecache_flush_all_metadata(wc);
+	writecache_commit_flushed(wc);
+	pmem_assign(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);
+
+	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(wc);
+		else
+			vfree(wc->memory_map);
+	}
+
+	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 = HIGH_WATERMARK;
+	int low_wm_percent = LOW_WATERMARK;
+	uint64_t x;
+	struct wc_memory_superblock s;
+
+	static struct dm_arg _args[] = {
+		{0, 10, "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_swait_queue_head(&wc->freelist_wait);
+	timer_setup(&wc->autocommit_timer, writecache_autocommit_timer, 0);
+
+	for (i = 0; i < 2; i++) {
+		atomic_set(&wc->bio_in_progress[i], 0);
+		init_swait_queue_head(&wc->bio_in_progress_wait[i]);
+	}
+
+	wc->dm_io = dm_io_client_create();
+	if (IS_ERR(wc->dm_io)) {
+		r = ERR_PTR(wc->dm_io);
+		ti->error = "Unable to allocate dm-io client";
+		goto bad;
+	}
+
+	wc->writeback_wq = alloc_workqueue("writecache-writeabck", WQ_MEM_RECLAIM, 1);
+	if (!wc->writeback_wq) {
+		r = -ENOMEM;
+		ti->error = "Could not allocate writeback workqueue";
+		goto bad;
+	}
+	INIT_WORK(&wc->writeback_work, writecache_writeback);
+	INIT_WORK(&wc->flush_work, writecache_flush_work);
+
+	init_swait_queue_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 DM_WRITECACHE_ONLY_SSD
+		wc->pmem_mode = false;
+#endif
+	} else if (!strcasecmp(string, "p")) {
+#ifndef DM_WRITECACHE_ONLY_SSD
+		wc->pmem_mode = true;
+		wc->writeback_fua = true;
+#else
+		r = -EOPNOTSUPP;
+		ti->error = "Persistent memory not supported on this architecture";
+		goto bad;
+#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;
+		}
+	}
+
+	/*
+	 * 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;
+
+	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);
+
+#ifndef DM_WRITECACHE_ONLY_SSD
+	if (WC_MODE_PMEM(wc)) {
+		r = persistent_memory_claim(wc);
+		if (r) {
+			ti->error = "Unable to map persistent memory for cache";
+			goto bad;
+		}
+	}
+#endif
+
+	/*
+	 * 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);
+
+	wc->max_writeback_jobs = MAX_WRITEBACK_JOBS;
+	wc->autocommit_blocks = !WC_MODE_PMEM(wc) ? AUTOCOMMIT_BLOCKS_SSD : AUTOCOMMIT_BLOCKS_PMEM;
+	wc->autocommit_jiffies = msecs_to_jiffies(AUTOCOMMIT_MSEC);
+
+	/*
+	 * 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), opt_params--;
+		if (!strcasecmp(string, "high_watermark") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%d%c", &high_wm_percent, &dummy) != 1)
+				goto invalid_optional;
+			if (high_wm_percent < 0 || high_wm_percent > 100)
+				goto invalid_optional;
+			wc->high_wm_percent_set = true;
+		} else if (!strcasecmp(string, "low_watermark") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%d%c", &low_wm_percent, &dummy) != 1)
+				goto invalid_optional;
+			if (low_wm_percent < 0 || low_wm_percent > 100)
+				goto invalid_optional;
+			wc->low_wm_percent_set = true;
+		} else if (!strcasecmp(string, "writeback_jobs") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%u%c", &wc->max_writeback_jobs, &dummy) != 1)
+				goto invalid_optional;
+			wc->max_writeback_jobs_set = true;
+		} else if (!strcasecmp(string, "autocommit_blocks") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%u%c", &wc->autocommit_blocks, &dummy) != 1)
+				goto invalid_optional;
+			wc->autocommit_blocks_set = true;
+		} else if (!strcasecmp(string, "autocommit_time") && opt_params >= 1) {
+			unsigned autocommit_msecs;
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%u%c", &autocommit_msecs, &dummy) != 1)
+				goto invalid_optional;
+			if (autocommit_msecs > 3600000)
+				goto invalid_optional;
+			wc->autocommit_jiffies = jiffies_to_msecs(autocommit_msecs);
+			wc->autocommit_time_set = true;
+		} else if (!strcasecmp(string, "fua")) {
+			if (WC_MODE_PMEM(wc)) {
+#ifndef DM_WRITECACHE_ONLY_SSD
+				wc->writeback_fua = true;
+				wc->writeback_fua_set = true;
+#endif
+			} else goto invalid_optional;
+		} else if (!strcasecmp(string, "nofua")) {
+			if (WC_MODE_PMEM(wc)) {
+#ifndef DM_WRITECACHE_ONLY_SSD
+				wc->writeback_fua = false;
+				wc->writeback_fua_set = true;
+#endif
+			} 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, "dm_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 (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(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);
+
+	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);
+	x += 50;
+	do_div(x, 100);
+	wc->freelist_high_watermark = x;
+	x = (uint64_t)wc->n_blocks * (100 - low_wm_percent);
+	x += 50;
+	do_div(x, 100);
+	wc->freelist_low_watermark = x;
+
+	r = writecache_alloc_entries(wc);
+	if (r) {
+		ti->error = "Cannot allocate memory";
+		goto bad;
+	}
+
+	ti->num_flush_bios = 1;
+	ti->flush_supported = true;
+	ti->num_discard_bios = 1;
+
+	if (WC_MODE_PMEM(wc))
+		persistent_memory_flush_cache(wc->memory_map, wc->memory_map_size);
+
+	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:
+		DMEMIT("%c %s %s %u ", WC_MODE_PMEM(wc) ? 'p' : 's',
+				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;
+		if (wc->max_writeback_jobs_set)
+			extra_args += 2;
+		if (wc->autocommit_blocks_set)
+			extra_args += 2;
+		if (wc->autocommit_time_set)
+			extra_args += 2;
+#ifndef DM_WRITECACHE_ONLY_SSD
+		if (wc->writeback_fua_set)
+			extra_args++;
+#endif
+		DMEMIT("%u", extra_args);
+		if (wc->high_wm_percent_set) {
+			x = (uint64_t)wc->freelist_high_watermark * 100;
+			x += wc->n_blocks / 2;
+			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;
+			x += wc->n_blocks / 2;
+			do_div(x, (size_t)wc->n_blocks);
+			DMEMIT(" low_watermark %u", 100 - (unsigned)x);
+		}
+		if (wc->max_writeback_jobs_set) {
+			DMEMIT(" writeback_jobs %u", wc->max_writeback_jobs);
+		}
+		if (wc->autocommit_blocks_set) {
+			DMEMIT(" autocommit_blocks %u", wc->autocommit_blocks);
+		}
+		if (wc->autocommit_time_set) {
+			DMEMIT(" autocommit_time %u", jiffies_to_msecs(wc->autocommit_jiffies));
+		}
+#ifndef DM_WRITECACHE_ONLY_SSD
+		if (wc->writeback_fua_set) {
+			DMEMIT(" %sfua", wc->writeback_fua ? "" : "no");
+		}
+#endif
+		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 <dm-devel@redhat.com>");
+MODULE_LICENSE("GPL");
Index: linux-2.6/Documentation/device-mapper/writecache.txt
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/Documentation/device-mapper/writecache.txt	2018-05-30 14:50:34.000000000 +0200
@@ -0,0 +1,68 @@
+The writecache target caches writes on persistent memory or on SSD. It
+doesn't cache reads because reads are supposed to be cached in page cache
+in normal RAM.
+
+When the device is constructed, the first sector should be zeroed or the
+first sector should contain valid superblock from previous invocation.
+
+Constructor parameters:
+1. type of the cache device - "p" or "s"
+	p - persistent memory
+	s - SSD
+2. the underlying device that will be cached
+3. the cache device
+4. block size (4096 is recommended; the maximum block size is the page
+   size)
+5. the number of optional parameters (the parameters with an argument
+   count as two)
+	high_watermark n	(default: 50)
+		start writeback when the number of used blocks reach this
+		watermark
+	low_watermark x		(default: 45)
+		stop writeback when the number of used blocks drops below
+		this watermark
+	writeback_jobs n	(default: unlimited)
+		limit the number of blocks that are in flight during
+		writeback. Setting this value reduces writeback
+		throughput, but it may improve latency of read requests
+	autocommit_blocks n	(default: 64 for pmem, 65536 for ssd)
+		when the application writes this amount of blocks without
+		issuing the FLUSH request, the blocks are automatically
+		commited
+	autocommit_time ms	(default: 1000)
+		autocommit time in milliseconds. The data is automatically
+		commited if this time passes and no FLUSH request is
+		received
+	fua			(by default on)
+		applicable only to persistent memory - use the FUA flag
+		when writing data from persistent memory back to the
+		underlying device
+	nofua
+		applicable only to persistent memory - don't use the FUA
+		flag when writing back data and send the FLUSH request
+		afterwards
+		- some underlying devices perform better with fua, some
+		  with nofua. The user should test it
+
+Status:
+1. error indicator - 0 if there was no error, otherwise error number
+2. the number of blocks
+3. the number of free blocks
+4. the number of blocks under writeback
+
+Messages:
+	flush
+		flush the cache device. The message returns successfully
+		if the cache device was flushed without an error
+	flush_on_suspend
+		flush the cache device on next suspend. Use this message
+		when you are going to remove the cache device. The proper
+		sequence for removing the cache device is:
+		1. send the "flush_on_suspend" message
+		2. load an inactive table with a linear target that maps
+		   to the underlying device
+		3. suspend the device
+		4. ask for status and verify that there are no errors
+		5. resume the device, so that it will use the linear
+		   target
+		6. the cache device is now inactive and it can be deleted

[PATCH 0/4] dm-writecache

[Mikulas Patocka]

Hi

Here I'm sending patches for the dm-writecache target. It is for kernel 
4.14-rc8.

Mikulas

[PATCH 4/4] dm-writecache

[Mikulas Patocka]

The dm-writecache target.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/Kconfig         |   11 
 drivers/md/Makefile        |    1 
 drivers/md/dm-writecache.c | 2356 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 2368 insertions(+)

Index: linux-2.6/drivers/md/Kconfig
===================================================================
--- linux-2.6.orig/drivers/md/Kconfig
+++ linux-2.6/drivers/md/Kconfig
@@ -326,6 +326,17 @@ 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_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
@@ -63,6 +63,7 @@ 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 +1,2356 @@
+#include <linux/device-mapper.h>
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/vmalloc.h>
+#include <linux/kthread.h>
+#include <linux/swait.h>
+#include <linux/delay.h>
+#include <linux/dm-io.h>
+#include <linux/dm-kcopyd.h>
+#include <linux/dax.h>
+#include <linux/pfn_t.h>
+
+#define DM_MSG_PREFIX	"writecache"
+
+#define HIGH_WATERMARK			50
+#define LOW_WATERMARK			45
+#define MAX_WRITEBACK_JOBS		1024
+#define ENDIO_LATENCY			16
+#define WRITEBACK_LATENCY		64
+#define AUTOCOMMIT_BLOCKS_SSD		65536
+#define AUTOCOMMIT_BLOCKS_PMEM		64
+#define AUTOCOMMIT_MSEC			1000
+
+/*
+ * If the architecture doesn't support persistent memory, we can use this driver
+ * in SSD-only mode.
+ */
+#ifndef CONFIG_ARCH_HAS_PMEM_API
+#define DM_WRITECACHE_ONLY_SSD
+#endif
+
+//#define WC_MEASURE_LATENCY
+
+#define BITMAP_GRANULARITY	65536
+#if BITMAP_GRANULARITY < PAGE_SIZE
+#undef BITMAP_GRANULARITY
+#define BITMAP_GRANULARITY	PAGE_SIZE
+#endif
+
+/*
+ * The clwb instruction is very slow on Broadwell, so we use non-temporal
+ * stores instead.
+ */
+#ifdef CONFIG_X86_64
+#define NT_STORE(dest, src)	asm ("movnti %1, %0" : "=m"(dest) : "r"(src))
+#define FLUSH_RANGE(dax, ptr, size)  do { } while (0)
+#define COMMIT_FLUSHED()	wmb()
+#else
+#define NT_STORE(dest, src)	ACCESS_ONCE(dest) = (src)
+#define FLUSH_RANGE		dax_flush
+#define COMMIT_FLUSHED()	do { } while (0)
+#endif
+
+#if defined(__HAVE_ARCH_MEMCPY_MCSAFE) && !defined(DM_WRITECACHE_ONLY_SSD)
+#define DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+#endif
+
+struct wc_pmem_holder {
+	struct dm_dev *dev;
+	bool vmapped;
+	bool page_backed;
+};
+
+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;
+	int id;
+
+	pmem_holder->vmapped = false;
+	pmem_holder->page_backed = true;
+
+	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) {
+		r = -EINVAL;
+		goto err1;
+	}
+	if (p != s >> PAGE_SHIFT) {
+		r = -EOVERFLOW;
+		goto err1;
+	}
+
+	id = dax_read_lock();
+
+	da = dax_direct_access(pmem_holder->dev->dax_dev, 0, p, addr, &pfn);
+	if (da < 0) {
+		r = da;
+		goto err2;
+	}
+#ifdef WC_MEASURE_LATENCY
+	printk(KERN_DEBUG "dm-writecache: device %s, pfn %016llx\n", pmem_holder->dev->name, pfn.val);
+#endif
+	if (da == p) {
+		pmem_holder->page_backed &= pfn_t_has_page(pfn);
+	} else {
+		long i;
+		pfn_t *pfns;
+		pfns = kvmalloc(p * sizeof(pfn_t), GFP_KERNEL);
+		if (!pfns) {
+			r = -ENOMEM;
+			goto err2;
+		}
+		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 err2;
+			}
+			pmem_holder->page_backed &= pfn_t_has_page(pfn);
+			while (daa-- && i < p) {
+				pfns[i++] = pfn;
+				pfn.val++;
+			}
+		} while (i < p);
+		*addr = vmap_pfn(pfns, p, VM_MAP, PAGE_KERNEL);
+		kvfree(pfns);
+		if (!*addr) {
+			r = -ENOMEM;
+			goto err2;
+		}
+		pmem_holder->vmapped = true;
+	}
+	*size = s;
+
+	dax_read_unlock(id);
+#ifdef WC_MEASURE_LATENCY
+	printk(KERN_DEBUG "dm-writecache: device %s, pages %d\n", pmem_holder->dev->name, pmem_holder->page_backed);
+#endif
+
+	return 0;
+
+err2:
+	dax_read_unlock(id);
+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);
+}
+
+static struct page *persistent_memory_page(void *addr)
+{
+	if (is_vmalloc_addr(addr))
+		return vmalloc_to_page(addr);
+	else
+		return virt_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)
+{
+	if (is_vmalloc_addr(ptr))
+		flush_kernel_vmap_range(ptr, size);
+}
+
+static void persistent_memory_flush(struct wc_pmem_holder *pmem_holder, void *ptr, size_t size)
+{
+	FLUSH_RANGE(pmem_holder->dev->dax_dev, ptr, size);
+}
+
+static void persistent_memory_commit_flushed(void)
+{
+	COMMIT_FLUSHED();
+}
+
+#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;
+		};
+		uint64_t padding[8];
+	};
+	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 DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	uint64_t original_sector;
+	uint64_t seq_count;
+#endif
+};
+
+#ifndef DM_WRITECACHE_ONLY_SSD
+#define WC_MODE_PMEM(wc)			((wc)->pmem_mode)
+#else
+#define WC_MODE_PMEM(wc)			false
+#endif
+
+struct dm_writecache {
+#ifndef DM_WRITECACHE_ONLY_SSD
+	bool pmem_mode;
+#endif
+	struct mutex lock;
+	struct rb_root tree;
+	struct list_head lru;
+	struct list_head freelist;
+	size_t freelist_size;
+	size_t writeback_size;
+	unsigned uncommitted_blocks;
+	unsigned autocommit_blocks;
+	unsigned max_writeback_jobs;
+	size_t freelist_high_watermark;
+	size_t freelist_low_watermark;
+	struct timer_list autocommit_timer;
+	unsigned long autocommit_jiffies;
+	struct swait_queue_head 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 overwrote_committed;
+
+	atomic_t bio_in_progress[2];
+	struct swait_queue_head bio_in_progress_wait[2];
+
+	struct dm_io_client *dm_io;
+
+	unsigned writeback_all;
+	struct workqueue_struct *writeback_wq;
+	struct work_struct writeback_work;
+	struct work_struct flush_work;
+
+	struct swait_queue_head 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;
+	mempool_t *page_pool;
+	struct wc_pmem_holder pmem_holder;
+	const char *memory_name;
+
+	struct dm_kcopyd_client *dm_kcopyd;
+	unsigned long *dirty_bitmap;
+	unsigned dirty_bitmap_size;
+
+	bool high_wm_percent_set;
+	bool low_wm_percent_set;
+	bool max_writeback_jobs_set;
+	bool autocommit_blocks_set;
+	bool autocommit_time_set;
+
+#ifdef WC_MEASURE_LATENCY
+	ktime_t lock_acquired_time;
+	ktime_t max_lock_held;
+	ktime_t max_lock_wait;
+	ktime_t max_freelist_wait;
+	ktime_t measure_latency_time;
+	ktime_t max_measure_latency;
+#endif
+};
+
+#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;
+	unsigned page_offset;
+	struct page *page;
+	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;
+	unsigned n_entries;
+	int error;
+};
+
+DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(dm_writecache_throttle,
+					    "A percentage of time allocated for data copying");
+
+static inline void measure_latency_start(struct dm_writecache *wc)
+{
+#ifdef WC_MEASURE_LATENCY
+	wc->measure_latency_time = ktime_get();
+#endif
+}
+
+static inline void measure_latency_end(struct dm_writecache *wc, unsigned long n)
+{
+#ifdef WC_MEASURE_LATENCY
+	ktime_t now = ktime_get();
+	if (now - wc->measure_latency_time > wc->max_measure_latency) {
+		wc->max_measure_latency = now - wc->measure_latency_time;
+		printk(KERN_DEBUG "dm-writecache: measured latency %lld.%03lldus, %lu steps\n", wc->max_measure_latency / 1000, wc->max_measure_latency % 1000, n);
+	}
+#endif
+}
+
+static void __wc_lock(struct dm_writecache *wc, int line)
+{
+#ifdef WC_MEASURE_LATENCY
+	ktime_t before, after;
+	before = ktime_get();
+#endif
+	mutex_lock(&wc->lock);
+#ifdef WC_MEASURE_LATENCY
+	after = ktime_get();
+	if (unlikely(after - before > wc->max_lock_wait)) {
+		wc->max_lock_wait = after - before;
+		printk(KERN_DEBUG "dm-writecache: waiting for lock for %lld.%03lldus at %d\n", wc->max_lock_wait / 1000, wc->max_lock_wait % 1000, line);
+		after = ktime_get();
+	}
+	wc->lock_acquired_time = after;
+#endif
+}
+#define wc_lock(wc)	__wc_lock(wc, __LINE__)
+
+static void __wc_unlock(struct dm_writecache *wc, int line)
+{
+#ifdef WC_MEASURE_LATENCY
+	ktime_t now = ktime_get();
+	if (now - wc->lock_acquired_time > wc->max_lock_held) {
+		wc->max_lock_held = now - wc->lock_acquired_time;
+		printk(KERN_DEBUG "dm-writecache: lock held for %lld.%03lldus at %d\n", wc->max_lock_held / 1000, wc->max_lock_held % 1000, line);
+	}
+#endif
+	mutex_unlock(&wc->lock);
+}
+#define wc_unlock(wc)	__wc_unlock(wc, __LINE__)
+
+#define wc_unlock_long(wc)	mutex_unlock(&wc->lock)
+
+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 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 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 DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	e->seq_count = -1;
+#endif
+	NT_STORE(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 DM_WRITECACHE_HANDLE_HARDWARE_ERRORS
+	e->original_sector = original_sector;
+	e->seq_count = seq_count;
+#endif
+	wme = memory_entry(wc, e);
+	NT_STORE(wme->original_sector, cpu_to_le64(original_sector));
+	NT_STORE(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);					\
+	swake_up(&(wc)->freelist_wait);					\
+} while (0)
+
+#define writecache_has_error(wc)	(unlikely(ACCESS_ONCE((wc)->error)))
+
+static void writecache_flush_all_metadata(struct dm_writecache *wc)
+{
+	if (WC_MODE_PMEM(wc)) {
+		persistent_memory_flush(&wc->pmem_holder, sb(wc),
+			offsetof(struct wc_memory_superblock, entries[wc->n_blocks]));
+	} 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)
+{
+	swait_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);
+		//prefetch(node);
+		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(swait_active(&wc->freelist_wait)))
+		swake_up(&wc->freelist_wait);
+}
+
+static void __writecache_wait_on_freelist(struct dm_writecache *wc, bool measure, int line)
+{
+	DECLARE_SWAITQUEUE(wait);
+#ifdef WC_MEASURE_LATENCY
+	ktime_t before, after;
+#endif
+
+	prepare_to_swait(&wc->freelist_wait, &wait, TASK_UNINTERRUPTIBLE);
+	wc_unlock(wc);
+#ifdef WC_MEASURE_LATENCY
+	if (measure)
+		before = ktime_get();
+#endif
+	io_schedule();
+	finish_swait(&wc->freelist_wait, &wait);
+#ifdef WC_MEASURE_LATENCY
+	if (measure) {
+		after = ktime_get();
+		if (unlikely(after - before > wc->max_freelist_wait)) {
+			wc->max_freelist_wait = after - before;
+			printk(KERN_DEBUG "dm-writecache: waiting on freelist for %lld.%03lldus at %d\n", wc->max_freelist_wait / 1000, wc->max_freelist_wait % 1000, line);
+		}
+	}
+#endif
+	wc_lock(wc);
+}
+#define writecache_wait_on_freelist(wc)		__writecache_wait_on_freelist(wc, true, __LINE__)
+#define writecache_wait_on_freelist_long(wc)	__writecache_wait_on_freelist(wc, false, __LINE__)
+
+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->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;
+	bool need_flush_after_free;
+
+	wc->uncommitted_blocks = 0;
+	del_timer(&wc->autocommit_timer);
+
+	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;
+	}
+	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();
+	}
+	writecache_commit_flushed(wc);
+
+	writecache_wait_for_ios(wc, WRITE);
+
+	wc->seq_count++;
+	NT_STORE(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;
+
+	need_flush_after_free = 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);
+				need_flush_after_free = true;
+			}
+		}
+		if (unlikely(e->lru.prev == &wc->lru))
+			break;
+		e = container_of(e->lru.prev, struct wc_entry, lru);
+		cond_resched();
+	}
+
+	if (need_flush_after_free)
+		writecache_commit_flushed(wc);
+}
+
+static void writecache_flush_work(struct work_struct *work)
+{
+	struct dm_writecache *wc = container_of(work, struct dm_writecache, flush_work);
+	wc_lock(wc);
+	writecache_flush(wc);
+	wc_unlock(wc);
+}
+
+static void writecache_autocommit_timer(unsigned long data)
+{
+	struct dm_writecache *wc = (struct dm_writecache *)data;
+	if (!writecache_has_error(wc))
+		queue_work(wc->writeback_wq, &wc->flush_work);
+}
+
+static void writecache_schedule_autocommit(struct dm_writecache *wc)
+{
+	if (!timer_pending(&wc->autocommit_timer))
+		mod_timer(&wc->autocommit_timer, jiffies + wc->autocommit_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 (wc->writeback_size) {
+		writecache_wait_on_freelist(wc);
+		return true;
+	}
+	return false;
+}
+
+static void writecache_suspend(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+
+	del_timer_sync(&wc->autocommit_timer);
+
+	flush_workqueue(wc->writeback_wq);
+
+	wc_lock(wc);
+	writecache_flush(wc);
+	while (writecache_wait_for_writeback(wc));
+	while (wc->writeback_all) {
+		wc_unlock_long(wc);
+		msleep(1);
+		wc_lock(wc);
+	}
+	wc_unlock_long(wc);
+
+	writecache_poison_lists(wc);
+}
+
+static int writecache_alloc_entries(struct dm_writecache *wc)
+{
+	size_t b;
+	if (wc->entries)
+		return 0;
+	wc->entries = vmalloc(sizeof(struct wc_entry) * wc->n_blocks);
+	if (!wc->entries)
+		return -ENOMEM;
+	for (b = 0; b < wc->n_blocks; b++) {
+		struct wc_entry *e = &wc->entries[b];
+		e->index = b;
+		e->write_in_progress = false;
+	}
+	return 0;
+}
+
+static void writecache_resume(struct dm_target *ti)
+{
+	struct dm_writecache *wc = ti->private;
+	size_t b;
+	bool need_flush = false;
+	uint64_t sb_seq_count;
+	int r;
+
+	wc_lock(wc);
+
+	wc->tree = RB_ROOT;
+	INIT_LIST_HEAD(&wc->lru);
+	INIT_LIST_HEAD(&wc->freelist);
+	wc->freelist_size = 0;
+
+	r = memcpy_mcsafe(&sb_seq_count, &sb(wc)->seq_count, sizeof(uint64_t));
+	if (r) {
+		writecache_error(wc, r, "hardware memory error when reading superblock: %d", r);
+		sb_seq_count = cpu_to_le64(0);
+	}
+	wc->seq_count = le64_to_cpu(sb_seq_count);
+
+#ifdef 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;
+		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 (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:
+				clear_seq_count(wc, e);
+				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 (!old) {
+				writecache_insert_entry(wc, e);
+			} else {
+				if (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_metadata(wc);
+		writecache_commit_flushed(wc);
+	}
+
+	wc_unlock_long(wc);
+}
+
+static int process_flush_mesg(unsigned argc, char **argv, struct dm_writecache *wc)
+{
+	uint64_t seq;
+
+	if (argc != 1)
+		return -EINVAL;
+
+	wc_lock(wc);
+	if (dm_suspended(wc->ti)) {
+		wc_unlock(wc);
+		return -EBUSY;
+	}
+	if (writecache_has_error(wc)) {
+		wc_unlock(wc);
+		return -EIO;
+	}
+
+	writecache_flush(wc);
+	seq = wc->seq_count;
+	wc->writeback_all++;
+	wc_unlock(wc);
+
+	queue_work(wc->writeback_wq, &wc->writeback_work);
+	flush_workqueue(wc->writeback_wq);
+
+	wc_lock(wc);
+	wc->writeback_all--;
+	if (dm_suspended(wc->ti)) {
+		wc_unlock(wc);
+		return -EBUSY;
+	}
+	if (writecache_has_error(wc)) {
+		wc_unlock(wc);
+		return -EIO;
+	}
+	wc_unlock(wc);
+
+	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_flushcache(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_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 {
+			writecache_flush(wc);
+		}
+
+		set_current_state(TASK_INTERRUPTIBLE);
+		wc->flush_bio = (void *)0x600 + POISON_POINTER_DELTA;	/* 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;
+
+	wc_lock(wc);
+
+	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_flush;
+	}
+
+	bio->bi_iter.bi_sector = dm_target_offset(ti, bio->bi_iter.bi_sector);
+
+	if (unlikely((((unsigned)bio->bi_iter.bi_sector | bio_sectors(bio)) &
+				(wc->block_size / 512 - 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_read;
+			} 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);
+			wc->uncommitted_blocks++;
+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);
+				if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks)) {
+					wc->uncommitted_blocks = 0;
+					queue_work(wc->writeback_wq, &wc->flush_work);
+				} else {
+					writecache_schedule_autocommit(wc);
+				}
+				goto unlock_remap;
+			}
+		} while (bio->bi_iter.bi_size);
+
+		if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks)) {
+			writecache_flush(wc);
+		} else {
+			writecache_schedule_autocommit(wc);
+		}
+
+		goto unlock_ok_write;
+	}
+
+unlock_remap_origin:
+	bio_set_dev(bio, wc->dev->bdev);
+	wc_unlock(wc);
+	return DM_MAPIO_REMAPPED;
+
+unlock_remap:
+	bio->bi_private = (void *)1;	/* make sure that writecache_end_io decrements bio_in_progress */
+	atomic_inc(&wc->bio_in_progress[bio_data_dir(bio)]);
+	wc_unlock(wc);
+	return DM_MAPIO_REMAPPED;
+
+unlock_ok_flush:
+#ifdef WC_MEASURE_LATENCY
+	wc_unlock(wc);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+#endif
+
+unlock_ok_read:
+#ifdef WC_MEASURE_LATENCY
+	wc_unlock(wc);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+#endif
+
+unlock_ok_write:
+	wc_unlock(wc);
+	bio_endio(bio);
+	return DM_MAPIO_SUBMITTED;
+
+unlock_error:
+	wc_unlock(wc);
+	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(swait_active(&wc->bio_in_progress_wait[dir])))
+				swake_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;
+
+	raw_spin_lock_irqsave(&wc->endio_thread_wait.lock, flags);
+	list_add_tail(&wb->endio_entry, &wc->endio_list);
+	swake_up_locked(&wc->endio_thread_wait);
+	raw_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;
+
+	c->error = likely(!(read_err | write_err)) ? 0 : -EIO;
+
+	raw_spin_lock_irq(&wc->endio_thread_wait.lock);
+	list_add_tail(&c->endio_entry, &wc->endio_list);
+	swake_up_locked(&wc->endio_thread_wait);
+	raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
+}
+
+static void __writecache_endio_pmem(struct dm_writecache *wc, struct list_head *list)
+{
+	unsigned i;
+	struct writeback_struct *wb;
+	struct wc_entry *e;
+	unsigned long n_walked = 0;
+
+	do {
+		wb = list_entry(list->next, struct writeback_struct, endio_entry);
+		list_del(&wb->endio_entry);
+
+		if (!wc->pmem_holder.page_backed) {
+			struct bio_vec *bv;
+			bio_for_each_segment_all(bv, &wb->bio, i)
+				mempool_free(bv->bv_page, wc->page_pool);
+		}
+
+		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;
+			INIT_LIST_HEAD(&e->lru);
+			if (!writecache_has_error(wc))
+				writecache_free_entry(wc, e);
+			BUG_ON(!wc->writeback_size);
+			wc->writeback_size--;
+			n_walked++;
+			if (unlikely(n_walked >= ENDIO_LATENCY)) {
+				writecache_commit_flushed(wc);
+				wc_unlock(wc);
+				wc_lock(wc);
+				n_walked = 0;
+			}
+		} 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;
+		do {
+			BUG_ON(!e->write_in_progress);
+			e->write_in_progress = false;
+			INIT_LIST_HEAD(&e->lru);
+			if (!writecache_has_error(wc))
+				writecache_free_entry(wc, e);
+
+			BUG_ON(!wc->writeback_size);
+			wc->writeback_size--;
+			e++;
+		} while (--c->n_entries);
+		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_SWAITQUEUE(wait);
+		struct list_head list;
+
+		raw_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);
+		__prepare_to_swait(&wc->endio_thread_wait, &wait);
+		raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
+
+		if (unlikely(kthread_should_stop())) {
+			finish_swait(&wc->endio_thread_wait, &wait);
+			break;
+		}
+
+		schedule();
+
+		raw_spin_lock_irq(&wc->endio_thread_wait.lock);
+		__finish_swait(&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);
+		raw_spin_unlock_irq(&wc->endio_thread_wait.lock);
+
+		writecache_disk_flush(wc, wc->dev);
+
+		wc_lock(wc);
+
+		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);
+
+		wc_unlock(wc);
+	}
+
+	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;
+	int r;
+
+	if (wc->pmem_holder.page_backed) {
+		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;
+	}
+
+	if (wb->page_offset == PAGE_SIZE) {
+		wb->page = mempool_alloc(wc->page_pool, gfp);
+		if (unlikely(!wb->page))
+			return false;
+		wb->page_offset = 0;
+	}
+
+#if 1
+	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);
+#else
+	memcpy_flushcache((char *)page_address(wb->page) + wb->page_offset, memory_data(wc, e), block_size);
+#endif
+
+	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;
+}
+
+struct writeback_list {
+	struct list_head list;
+	size_t size;
+};
+
+static void __writeback_throttle(struct dm_writecache *wc, struct writeback_list *wbl)
+{
+	if (ACCESS_ONCE(wc->writeback_size) - wbl->size >= wc->max_writeback_jobs) {
+		wc_lock(wc);
+		while (wc->writeback_size - wbl->size >= wc->max_writeback_jobs) {
+			writecache_wait_on_freelist_long(wc);
+		}
+		wc_unlock(wc);
+	}
+	cond_resched();
+}
+
+static void __writecache_writeback_pmem(struct dm_writecache *wc, struct writeback_list *wbl)
+{
+	struct wc_entry *e, *f;
+	struct bio *bio;
+	struct writeback_struct *wb;
+	unsigned max_pages;
+
+	while (wbl->size) {
+		wbl->size--;
+		e = container_of(wbl->list.prev, struct wc_entry, lru);
+		list_del(&e->lru);
+
+		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);
+		wb->page_offset = PAGE_SIZE;
+		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 (wbl->size && wb->wc_list_n < max_pages) {
+			f = container_of(wbl->list.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;
+			wbl->size--;
+			list_del(&f->lru);
+			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);
+		}
+
+		__writeback_throttle(wc, wbl);
+
+	}
+}
+
+static void __writecache_writeback_ssd(struct dm_writecache *wc, struct writeback_list *wbl)
+{
+	struct wc_entry *e, *f;
+	struct dm_io_region from, to;
+	struct copy_struct *c;
+
+	while (wbl->size) {
+		unsigned n_sectors;
+
+		wbl->size--;
+		e = container_of(wbl->list.prev, struct wc_entry, lru);
+		list_del(&e->lru);
+
+		n_sectors = e->wc_list_contiguous << (wc->block_size_bits - SECTOR_SHIFT);
+
+		from.bdev = wc->ssd_dev->bdev;
+		from.sector = cache_sector(wc, e);
+		from.count = n_sectors;
+		to.bdev = wc->dev->bdev;
+		to.sector = read_original_sector(wc, e);
+		to.count = n_sectors;
+
+		c = mempool_alloc(wc->copy_pool, GFP_NOIO);
+		c->wc = wc;
+		c->e = e;
+		c->n_entries = e->wc_list_contiguous;
+
+		while ((n_sectors -= wc->block_size >> SECTOR_SHIFT)) {
+			wbl->size--;
+			f = container_of(wbl->list.prev, struct wc_entry, lru);
+			BUG_ON(f != e + 1);
+			list_del(&f->lru);
+			e = f;
+		}
+
+		dm_kcopyd_copy(wc->dm_kcopyd, &from, 1, &to, 0, writecache_copy_endio, c);
+
+		__writeback_throttle(wc, wbl);
+	}
+}
+
+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;
+	struct writeback_list wbl;
+	unsigned long n_walked;
+
+	wc_lock(wc);
+restart:
+	if (writecache_has_error(wc) || unlikely(dm_suspended(wc->ti))) {
+		wc_unlock(wc);
+		return;
+	}
+
+	if (unlikely(wc->writeback_all)) {
+		if (writecache_wait_for_writeback(wc))
+			goto restart;
+	}
+
+	if (wc->overwrote_committed) {
+		writecache_wait_for_ios(wc, WRITE);
+	}
+
+	n_walked = 0;
+	INIT_LIST_HEAD(&skipped);
+	INIT_LIST_HEAD(&wbl.list);
+	wbl.size = 0;
+	while (!list_empty(&wc->lru) &&
+	       (wc->writeback_all ||
+		wc->freelist_size + wc->writeback_size <= wc->freelist_high_watermark)) {
+
+		n_walked++;
+		if (unlikely(n_walked > WRITEBACK_LATENCY) && likely(!wc->writeback_all)) {
+			queue_work(wc->writeback_wq, &wc->writeback_work);
+			break;
+		}
+
+		e = container_of(wc->lru.prev, struct wc_entry, lru);
+		BUG_ON(e->write_in_progress);
+		if (unlikely(!writecache_entry_is_committed(wc, e))) {
+			writecache_flush(wc);
+		}
+		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);
+				cond_resched();
+				continue;
+			}
+		}
+		wc->writeback_size++;
+		list_del(&e->lru);
+		list_add(&e->lru, &wbl.list);
+		wbl.size++;
+		e->write_in_progress = true;
+		e->wc_list_contiguous = 1;
+
+		f = e;
+
+		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;
+
+			if (!WC_MODE_PMEM(wc)) {
+				if (g != f + 1)
+					break;
+			}
+
+			n_walked++;
+			//if (unlikely(n_walked > WRITEBACK_LATENCY) && likely(!wc->writeback_all))
+			//	break;
+
+			wc->writeback_size++;
+			list_del(&g->lru);
+			list_add(&g->lru, &wbl.list);
+			wbl.size++;
+			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();
+	}
+
+	if (!list_empty(&skipped)) {
+		list_splice_tail(&skipped, &wc->lru);
+		/*
+		 * If we didn't do any progress, we must wait until some
+		 * writeback finishes to avoid burning CPU in a loop
+		 */
+		if (unlikely(!wbl.size))
+			writecache_wait_for_writeback(wc);
+	}
+
+	wc_unlock(wc);
+
+	blk_start_plug(&plug);
+
+	if (WC_MODE_PMEM(wc))
+		__writecache_writeback_pmem(wc, &wbl);
+	else
+		__writecache_writeback_ssd(wc, &wbl);
+
+	blk_finish_plug(&plug);
+
+	if (unlikely(wc->writeback_all)) {
+		wc_lock(wc);
+		while (writecache_wait_for_writeback(wc));
+		wc_unlock(wc);
+	}
+}
+
+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;
+	int r;
+
+	r = calculate_memory_size(wc->memory_map_size, wc->block_size, &wc->n_blocks, NULL);
+	if (r)
+		return r;
+
+	r = writecache_alloc_entries(wc);
+	if (r)
+		return r;
+
+	for (b = 0; b < ARRAY_SIZE(sb(wc)->padding); b++)
+		NT_STORE(sb(wc)->padding[b], cpu_to_le64(0));
+	NT_STORE(sb(wc)->version, cpu_to_le32(MEMORY_SUPERBLOCK_VERSION));
+	NT_STORE(sb(wc)->block_size, cpu_to_le32(wc->block_size));
+	NT_STORE(sb(wc)->n_blocks, cpu_to_le64(wc->n_blocks));
+	NT_STORE(sb(wc)->seq_count, cpu_to_le64(0));
+
+	for (b = 0; b < wc->n_blocks; b++)
+		write_original_sector_seq_count(wc, &wc->entries[b], -1, -1);
+
+	writecache_flush_all_metadata(wc);
+	writecache_commit_flushed(wc);
+	NT_STORE(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);
+
+	mempool_destroy(wc->page_pool);
+
+	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 = HIGH_WATERMARK;
+	int low_wm_percent = LOW_WATERMARK;
+	uint64_t x;
+	struct wc_memory_superblock s;
+
+	static struct dm_arg _args[] = {
+		{0, 10, "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_swait_queue_head(&wc->freelist_wait);
+	setup_timer(&wc->autocommit_timer, writecache_autocommit_timer, (unsigned long)wc);
+
+	for (i = 0; i < 2; i++) {
+		atomic_set(&wc->bio_in_progress[i], 0);
+		init_swait_queue_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, 1);
+	if (!wc->writeback_wq) {
+		r = -ENOMEM;
+		ti->error = "Could not allocate writeback workqueue";
+		goto bad;
+	}
+	INIT_WORK(&wc->writeback_work, writecache_writeback);
+	INIT_WORK(&wc->flush_work, writecache_flush_work);
+
+	init_swait_queue_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 DM_WRITECACHE_ONLY_SSD
+		wc->pmem_mode = false;
+#endif
+	} else if (!strcasecmp(string, "p")) {
+#ifndef DM_WRITECACHE_ONLY_SSD
+		wc->pmem_mode = true;
+#else
+		r = -EOPNOTSUPP;
+		ti->error = "Persistent memory not supported on this architecture";
+		goto bad;
+#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;
+		}
+	}
+
+	/*
+	 * 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;
+		}
+
+		if (!wc->pmem_holder.page_backed) {
+			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;
+			}
+		}
+	} 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);
+
+	wc->max_writeback_jobs = MAX_WRITEBACK_JOBS;
+	wc->autocommit_blocks = !WC_MODE_PMEM(wc) ? AUTOCOMMIT_BLOCKS_SSD : AUTOCOMMIT_BLOCKS_PMEM;
+	wc->autocommit_jiffies = msecs_to_jiffies(AUTOCOMMIT_MSEC);
+
+	/*
+	 * 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), opt_params--;
+		if (!strcasecmp(string, "high_watermark") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%d%%%c", &high_wm_percent, &dummy) != 1)
+				goto invalid_optional;
+			if (high_wm_percent < 0 || high_wm_percent > 100)
+				goto invalid_optional;
+			wc->high_wm_percent_set = true;
+		} else if (!strcasecmp(string, "low_watermark") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%d%%%c", &low_wm_percent, &dummy) != 1)
+				goto invalid_optional;
+			if (low_wm_percent < 0 || low_wm_percent > 100)
+				goto invalid_optional;
+			wc->low_wm_percent_set = true;
+		} else if (!strcasecmp(string, "writeback_jobs") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%u%c", &wc->max_writeback_jobs, &dummy) != 1)
+				goto invalid_optional;
+			if (!wc->max_writeback_jobs)
+				goto invalid_optional;
+			wc->max_writeback_jobs_set = true;
+		} else if (!strcasecmp(string, "autocommit_blocks") && opt_params >= 1) {
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%u%c", &wc->autocommit_blocks, &dummy) != 1)
+				goto invalid_optional;
+			wc->autocommit_blocks_set = true;
+		} else if (!strcasecmp(string, "autocommit_time") && opt_params >= 1) {
+			unsigned autocommit_msecs;
+			string = dm_shift_arg(&as), opt_params--;
+			if (sscanf(string, "%u%c", &autocommit_msecs, &dummy) != 1)
+				goto invalid_optional;
+			if (autocommit_msecs > 3600000)
+				goto invalid_optional;
+			wc->autocommit_jiffies = jiffies_to_msecs(autocommit_msecs);
+			wc->autocommit_time_set = true;
+		} 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 (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);
+
+	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);
+	x += 50;
+	do_div(x, 100);
+	wc->freelist_high_watermark = x;
+	x = (uint64_t)wc->n_blocks * (100 - low_wm_percent);
+	x += 50;
+	do_div(x, 100);
+	wc->freelist_low_watermark = x;
+
+	r = writecache_alloc_entries(wc);
+	if (r) {
+		ti->error = "Cannot allocate memory";
+		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;
+		if (wc->max_writeback_jobs_set)
+			extra_args += 2;
+		if (wc->autocommit_blocks_set)
+			extra_args += 2;
+		if (wc->autocommit_time_set)
+			extra_args += 2;
+		DMEMIT("%u", extra_args);
+		if (wc->high_wm_percent_set) {
+			x = (uint64_t)wc->freelist_high_watermark * 100;
+			x += wc->n_blocks / 2;
+			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;
+			x += wc->n_blocks / 2;
+			do_div(x, (size_t)wc->n_blocks);
+			DMEMIT(" low_watermark %u%%", 100 - (unsigned)x);
+		}
+		if (wc->max_writeback_jobs_set) {
+			DMEMIT(" writeback_jobs %u", wc->max_writeback_jobs);
+		}
+		if (wc->autocommit_blocks_set) {
+			DMEMIT(" autocommit_blocks %u", wc->autocommit_blocks);
+		}
+		if (wc->autocommit_time_set) {
+			DMEMIT(" autocommit_time %u", jiffies_to_msecs(wc->autocommit_jiffies));
+		}
+		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@redhat.com>");
+MODULE_LICENSE("GPL");