Merge tag 'for-5.5/dm-fixes' of...

Merge tag 'for-5.5/dm-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm

Pull device mapper fixes from Mike Snitzer:

 - Fix DM multipath by restoring full path selector functionality for
   bio-based configurations that don't haave a SCSI device handler.

 - Fix dm-btree removal to ensure non-root btree nodes have at least
   (max_entries / 3) entries. This resolves userspace thin_check
   utility's report of "too few entries in btree_node".

 - Fix both the DM thin-provisioning and dm-clone targets to properly
   flush the data device prior to metadata commit. This resolves the
   potential for inconsistency across a power loss event when the data
   device has a volatile writeback cache.

 - Small documentation fixes to dm-clone and dm-integrity.

* tag 'for-5.5/dm-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm:
  docs: dm-integrity: remove reference to ARC4
  dm thin: Flush data device before committing metadata
  dm thin metadata: Add support for a pre-commit callback
  dm clone: Flush destination device before committing metadata
  dm clone metadata: Use a two phase commit
  dm clone metadata: Track exact changes per transaction
  dm btree: increase rebalance threshold in __rebalance2()
  dm: add dm-clone to the documentation index
  dm mpath: remove harmful bio-based optimization

Documentation/admin-guide/device-mapper/dm-integrity.rst

@@ -144,7 +144,7 @@ journal_crypt:algorithm(:key)	(the key is optional)
 	Encrypt the journal using given algorithm to make sure that the
 	attacker can't read the journal. You can use a block cipher here
 	(such as "cbc(aes)") or a stream cipher (for example "chacha20",
-	"salsa20", "ctr(aes)" or "ecb(arc4)").
+	"salsa20" or "ctr(aes)").
 
 	The journal contains history of last writes to the block device,
 	an attacker reading the journal could see the last sector nubmers

Documentation/admin-guide/device-mapper/index.rst

@@ -8,6 +8,7 @@ Device Mapper
     cache-policies
     cache
     delay
+    dm-clone
     dm-crypt
     dm-dust
     dm-flakey

drivers/md/dm-clone-metadata.c

@@ -67,23 +67,34 @@ struct superblock_disk {
  * To save constantly doing look ups on disk we keep an in core copy of the
  * on-disk bitmap, the region_map.
  *
- * To further reduce metadata I/O overhead we use a second bitmap, the dmap
- * (dirty bitmap), which tracks the dirty words, i.e. longs, of the region_map.
+ * In order to track which regions are hydrated during a metadata transaction,
+ * we use a second set of bitmaps, the dmap (dirty bitmap), which includes two
+ * bitmaps, namely dirty_regions and dirty_words. The dirty_regions bitmap
+ * tracks the regions that got hydrated during the current metadata
+ * transaction. The dirty_words bitmap tracks the dirty words, i.e. longs, of
+ * the dirty_regions bitmap.
+ *
+ * This allows us to precisely track the regions that were hydrated during the
+ * current metadata transaction and update the metadata accordingly, when we
+ * commit the current transaction. This is important because dm-clone should
+ * only commit the metadata of regions that were properly flushed to the
+ * destination device beforehand. Otherwise, in case of a crash, we could end
+ * up with a corrupted dm-clone device.
  *
  * When a region finishes hydrating dm-clone calls
  * dm_clone_set_region_hydrated(), or for discard requests
  * dm_clone_cond_set_range(), which sets the corresponding bits in region_map
  * and dmap.
  *
- * During a metadata commit we scan the dmap for dirty region_map words (longs)
- * and update accordingly the on-disk metadata. Thus, we don't have to flush to
- * disk the whole region_map. We can just flush the dirty region_map words.
+ * During a metadata commit we scan dmap->dirty_words and dmap->dirty_regions
+ * and update the on-disk metadata accordingly. Thus, we don't have to flush to
+ * disk the whole region_map. We can just flush the dirty region_map bits.
  *
- * We use a dirty bitmap, which is smaller than the original region_map, to
- * reduce the amount of memory accesses during a metadata commit. As dm-bitset
- * accesses the on-disk bitmap in 64-bit word granularity, there is no
- * significant benefit in tracking the dirty region_map bits with a smaller
- * granularity.
+ * We use the helper dmap->dirty_words bitmap, which is smaller than the
+ * original region_map, to reduce the amount of memory accesses during a
+ * metadata commit. Moreover, as dm-bitset also accesses the on-disk bitmap in
+ * 64-bit word granularity, the dirty_words bitmap helps us avoid useless disk
+ * accesses.
  *
  * We could update directly the on-disk bitmap, when dm-clone calls either
  * dm_clone_set_region_hydrated() or dm_clone_cond_set_range(), buts this
@@ -92,12 +103,13 @@ struct superblock_disk {
  * e.g., in a hooked overwrite bio's completion routine, and further reduce the
  * I/O completion latency.
  *
- * We maintain two dirty bitmaps. During a metadata commit we atomically swap
- * the currently used dmap with the unused one. This allows the metadata update
- * functions to run concurrently with an ongoing commit.
+ * We maintain two dirty bitmap sets. During a metadata commit we atomically
+ * swap the currently used dmap with the unused one. This allows the metadata
+ * update functions to run concurrently with an ongoing commit.
  */
 struct dirty_map {
 	unsigned long *dirty_words;
+	unsigned long *dirty_regions;
 	unsigned int changed;
 };
 
@@ -115,6 +127,9 @@ struct dm_clone_metadata {
 	struct dirty_map dmap[2];
 	struct dirty_map *current_dmap;
 
+	/* Protected by lock */
+	struct dirty_map *committing_dmap;
+
 	/*
 	 * In core copy of the on-disk bitmap to save constantly doing look ups
 	 * on disk.
@@ -461,34 +476,53 @@ static size_t bitmap_size(unsigned long nr_bits)
 	return BITS_TO_LONGS(nr_bits) * sizeof(long);
 }
 
-static int dirty_map_init(struct dm_clone_metadata *cmd)
+static int __dirty_map_init(struct dirty_map *dmap, unsigned long nr_words,
+			    unsigned long nr_regions)
 {
-	cmd->dmap[0].changed = 0;
-	cmd->dmap[0].dirty_words = kvzalloc(bitmap_size(cmd->nr_words), GFP_KERNEL);
+	dmap->changed = 0;
 
-	if (!cmd->dmap[0].dirty_words) {
-		DMERR("Failed to allocate dirty bitmap");
+	dmap->dirty_words = kvzalloc(bitmap_size(nr_words), GFP_KERNEL);
+	if (!dmap->dirty_words)
+		return -ENOMEM;
+
+	dmap->dirty_regions = kvzalloc(bitmap_size(nr_regions), GFP_KERNEL);
+	if (!dmap->dirty_regions) {
+		kvfree(dmap->dirty_words);
 		return -ENOMEM;
 	}
 
-	cmd->dmap[1].changed = 0;
-	cmd->dmap[1].dirty_words = kvzalloc(bitmap_size(cmd->nr_words), GFP_KERNEL);
+	return 0;
+}
+
+static void __dirty_map_exit(struct dirty_map *dmap)
+{
+	kvfree(dmap->dirty_words);
+	kvfree(dmap->dirty_regions);
+}
+
+static int dirty_map_init(struct dm_clone_metadata *cmd)
+{
+	if (__dirty_map_init(&cmd->dmap[0], cmd->nr_words, cmd->nr_regions)) {
+		DMERR("Failed to allocate dirty bitmap");
+		return -ENOMEM;
+	}
 
-	if (!cmd->dmap[1].dirty_words) {
+	if (__dirty_map_init(&cmd->dmap[1], cmd->nr_words, cmd->nr_regions)) {
 		DMERR("Failed to allocate dirty bitmap");
-		kvfree(cmd->dmap[0].dirty_words);
+		__dirty_map_exit(&cmd->dmap[0]);
 		return -ENOMEM;
 	}
 
 	cmd->current_dmap = &cmd->dmap[0];
+	cmd->committing_dmap = NULL;
 
 	return 0;
 }
 
 static void dirty_map_exit(struct dm_clone_metadata *cmd)
 {
-	kvfree(cmd->dmap[0].dirty_words);
-	kvfree(cmd->dmap[1].dirty_words);
+	__dirty_map_exit(&cmd->dmap[0]);
+	__dirty_map_exit(&cmd->dmap[1]);
 }
 
 static int __load_bitset_in_core(struct dm_clone_metadata *cmd)
@@ -633,21 +667,23 @@ unsigned long dm_clone_find_next_unhydrated_region(struct dm_clone_metadata *cmd
 	return find_next_zero_bit(cmd->region_map, cmd->nr_regions, start);
 }
 
-static int __update_metadata_word(struct dm_clone_metadata *cmd, unsigned long word)
+static int __update_metadata_word(struct dm_clone_metadata *cmd,
+				  unsigned long *dirty_regions,
+				  unsigned long word)
 {
 	int r;
 	unsigned long index = word * BITS_PER_LONG;
 	unsigned long max_index = min(cmd->nr_regions, (word + 1) * BITS_PER_LONG);
 
 	while (index < max_index) {
-		if (test_bit(index, cmd->region_map)) {
+		if (test_bit(index, dirty_regions)) {
 			r = dm_bitset_set_bit(&cmd->bitset_info, cmd->bitset_root,
 					      index, &cmd->bitset_root);
-
 			if (r) {
 				DMERR("dm_bitset_set_bit failed");
 				return r;
 			}
+			__clear_bit(index, dirty_regions);
 		}
 		index++;
 	}
@@ -721,7 +757,7 @@ static int __flush_dmap(struct dm_clone_metadata *cmd, struct dirty_map *dmap)
 		if (word == cmd->nr_words)
 			break;
 
-		r = __update_metadata_word(cmd, word);
+		r = __update_metadata_word(cmd, dmap->dirty_regions, word);
 
 		if (r)
 			return r;
@@ -743,15 +779,17 @@ static int __flush_dmap(struct dm_clone_metadata *cmd, struct dirty_map *dmap)
 	return 0;
 }
 
-int dm_clone_metadata_commit(struct dm_clone_metadata *cmd)
+int dm_clone_metadata_pre_commit(struct dm_clone_metadata *cmd)
 {
-	int r = -EPERM;
+	int r = 0;
 	struct dirty_map *dmap, *next_dmap;
 
 	down_write(&cmd->lock);
 
-	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm))
+	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm)) {
+		r = -EPERM;
 		goto out;
+	}
 
 	/* Get current dirty bitmap */
 	dmap = cmd->current_dmap;
@@ -763,7 +801,7 @@ int dm_clone_metadata_commit(struct dm_clone_metadata *cmd)
 	 * The last commit failed, so we don't have a clean dirty-bitmap to
 	 * use.
 	 */
-	if (WARN_ON(next_dmap->changed)) {
+	if (WARN_ON(next_dmap->changed || cmd->committing_dmap)) {
 		r = -EINVAL;
 		goto out;
 	}
@@ -773,11 +811,33 @@ int dm_clone_metadata_commit(struct dm_clone_metadata *cmd)
 	cmd->current_dmap = next_dmap;
 	spin_unlock_irq(&cmd->bitmap_lock);
 
-	/*
-	 * No one is accessing the old dirty bitmap anymore, so we can flush
-	 * it.
-	 */
-	r = __flush_dmap(cmd, dmap);
+	/* Set old dirty bitmap as currently committing */
+	cmd->committing_dmap = dmap;
+out:
+	up_write(&cmd->lock);
+
+	return r;
+}
+
+int dm_clone_metadata_commit(struct dm_clone_metadata *cmd)
+{
+	int r = -EPERM;
+
+	down_write(&cmd->lock);
+
+	if (cmd->fail_io || dm_bm_is_read_only(cmd->bm))
+		goto out;
+
+	if (WARN_ON(!cmd->committing_dmap)) {
+		r = -EINVAL;
+		goto out;
+	}
+
+	r = __flush_dmap(cmd, cmd->committing_dmap);
+	if (!r) {
+		/* Clear committing dmap */
+		cmd->committing_dmap = NULL;
+	}
 out:
 	up_write(&cmd->lock);
 
@@ -802,6 +862,7 @@ int dm_clone_set_region_hydrated(struct dm_clone_metadata *cmd, unsigned long re
 	dmap = cmd->current_dmap;
 
 	__set_bit(word, dmap->dirty_words);
+	__set_bit(region_nr, dmap->dirty_regions);
 	__set_bit(region_nr, cmd->region_map);
 	dmap->changed = 1;
 
@@ -830,6 +891,7 @@ int dm_clone_cond_set_range(struct dm_clone_metadata *cmd, unsigned long start,
 		if (!test_bit(region_nr, cmd->region_map)) {
 			word = region_nr / BITS_PER_LONG;
 			__set_bit(word, dmap->dirty_words);
+			__set_bit(region_nr, dmap->dirty_regions);
 			__set_bit(region_nr, cmd->region_map);
 			dmap->changed = 1;
 		}

drivers/md/dm-clone-metadata.h

@@ -75,7 +75,23 @@ void dm_clone_metadata_close(struct dm_clone_metadata *cmd);
 
 /*
  * Commit dm-clone metadata to disk.
+ *
+ * We use a two phase commit:
+ *
+ * 1. dm_clone_metadata_pre_commit(): Prepare the current transaction for
+ *    committing. After this is called, all subsequent metadata updates, done
+ *    through either dm_clone_set_region_hydrated() or
+ *    dm_clone_cond_set_range(), will be part of the **next** transaction.
+ *
+ * 2. dm_clone_metadata_commit(): Actually commit the current transaction to
+ *    disk and start a new transaction.
+ *
+ * This allows dm-clone to flush the destination device after step (1) to
+ * ensure that all freshly hydrated regions, for which we are updating the
+ * metadata, are properly written to non-volatile storage and won't be lost in
+ * case of a crash.
  */
+int dm_clone_metadata_pre_commit(struct dm_clone_metadata *cmd);
 int dm_clone_metadata_commit(struct dm_clone_metadata *cmd);
 
 /*
@@ -112,6 +128,7 @@ int dm_clone_metadata_abort(struct dm_clone_metadata *cmd);
  * Switches metadata to a read only mode. Once read-only mode has been entered
  * the following functions will return -EPERM:
  *
+ *   dm_clone_metadata_pre_commit()
  *   dm_clone_metadata_commit()
  *   dm_clone_set_region_hydrated()
  *   dm_clone_cond_set_range()

drivers/md/dm-clone-target.c

@@ -86,6 +86,12 @@ struct clone {
 
 	struct dm_clone_metadata *cmd;
 
+	/*
+	 * bio used to flush the destination device, before committing the
+	 * metadata.
+	 */
+	struct bio flush_bio;
+
 	/* Region hydration hash table */
 	struct hash_table_bucket *ht;
 
@@ -1108,10 +1114,13 @@ static bool need_commit_due_to_time(struct clone *clone)
 /*
  * A non-zero return indicates read-only or fail mode.
  */
-static int commit_metadata(struct clone *clone)
+static int commit_metadata(struct clone *clone, bool *dest_dev_flushed)
 {
 	int r = 0;
 
+	if (dest_dev_flushed)
+		*dest_dev_flushed = false;
+
 	mutex_lock(&clone->commit_lock);
 
 	if (!dm_clone_changed_this_transaction(clone->cmd))
@@ -1122,8 +1131,26 @@ static int commit_metadata(struct clone *clone)
 		goto out;
 	}
 
-	r = dm_clone_metadata_commit(clone->cmd);
+	r = dm_clone_metadata_pre_commit(clone->cmd);
+	if (unlikely(r)) {
+		__metadata_operation_failed(clone, "dm_clone_metadata_pre_commit", r);
+		goto out;
+	}
 
+	bio_reset(&clone->flush_bio);
+	bio_set_dev(&clone->flush_bio, clone->dest_dev->bdev);
+	clone->flush_bio.bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
+
+	r = submit_bio_wait(&clone->flush_bio);
+	if (unlikely(r)) {
+		__metadata_operation_failed(clone, "flush destination device", r);
+		goto out;
+	}
+
+	if (dest_dev_flushed)
+		*dest_dev_flushed = true;
+
+	r = dm_clone_metadata_commit(clone->cmd);
 	if (unlikely(r)) {
 		__metadata_operation_failed(clone, "dm_clone_metadata_commit", r);
 		goto out;
@@ -1194,6 +1221,7 @@ static void process_deferred_bios(struct clone *clone)
 static void process_deferred_flush_bios(struct clone *clone)
 {
 	struct bio *bio;
+	bool dest_dev_flushed;
 	struct bio_list bios = BIO_EMPTY_LIST;
 	struct bio_list bio_completions = BIO_EMPTY_LIST;
 
@@ -1213,7 +1241,7 @@ static void process_deferred_flush_bios(struct clone *clone)
 	    !(dm_clone_changed_this_transaction(clone->cmd) && need_commit_due_to_time(clone)))
 		return;
 
-	if (commit_metadata(clone)) {
+	if (commit_metadata(clone, &dest_dev_flushed)) {
 		bio_list_merge(&bios, &bio_completions);
 
 		while ((bio = bio_list_pop(&bios)))
@@ -1227,8 +1255,17 @@ static void process_deferred_flush_bios(struct clone *clone)
 	while ((bio = bio_list_pop(&bio_completions)))
 		bio_endio(bio);
 
-	while ((bio = bio_list_pop(&bios)))
-		generic_make_request(bio);
+	while ((bio = bio_list_pop(&bios))) {
+		if ((bio->bi_opf & REQ_PREFLUSH) && dest_dev_flushed) {
+			/* We just flushed the destination device as part of
+			 * the metadata commit, so there is no reason to send
+			 * another flush.
+			 */
+			bio_endio(bio);
+		} else {
+			generic_make_request(bio);
+		}
+	}
 }
 
 static void do_worker(struct work_struct *work)
@@ -1400,7 +1437,7 @@ static void clone_status(struct dm_target *ti, status_type_t type,
 
 		/* Commit to ensure statistics aren't out-of-date */
 		if (!(status_flags & DM_STATUS_NOFLUSH_FLAG) && !dm_suspended(ti))
-			(void) commit_metadata(clone);
+			(void) commit_metadata(clone, NULL);
 
 		r = dm_clone_get_free_metadata_block_count(clone->cmd, &nr_free_metadata_blocks);
 
@@ -1834,6 +1871,7 @@ static int clone_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	bio_list_init(&clone->deferred_flush_completions);
 	clone->hydration_offset = 0;
 	atomic_set(&clone->hydrations_in_flight, 0);
+	bio_init(&clone->flush_bio, NULL, 0);
 
 	clone->wq = alloc_workqueue("dm-" DM_MSG_PREFIX, WQ_MEM_RECLAIM, 0);
 	if (!clone->wq) {
@@ -1907,6 +1945,7 @@ static void clone_dtr(struct dm_target *ti)
 	struct clone *clone = ti->private;
 
 	mutex_destroy(&clone->commit_lock);
+	bio_uninit(&clone->flush_bio);
 
 	for (i = 0; i < clone->nr_ctr_args; i++)
 		kfree(clone->ctr_args[i]);
@@ -1961,7 +2000,7 @@ static void clone_postsuspend(struct dm_target *ti)
 	wait_event(clone->hydration_stopped, !atomic_read(&clone->hydrations_in_flight));
 	flush_workqueue(clone->wq);
 
-	(void) commit_metadata(clone);
+	(void) commit_metadata(clone, NULL);
 }
 
 static void clone_resume(struct dm_target *ti)

drivers/md/dm-mpath.c

@@ -599,45 +599,10 @@ static struct pgpath *__map_bio(struct multipath *m, struct bio *bio)
 	return pgpath;
 }
 
-static struct pgpath *__map_bio_fast(struct multipath *m, struct bio *bio)
-{
-	struct pgpath *pgpath;
-	unsigned long flags;
-
-	/* Do we need to select a new pgpath? */
-	/*
-	 * FIXME: currently only switching path if no path (due to failure, etc)
-	 * - which negates the point of using a path selector
-	 */
-	pgpath = READ_ONCE(m->current_pgpath);
-	if (!pgpath)
-		pgpath = choose_pgpath(m, bio->bi_iter.bi_size);
-
-	if (!pgpath) {
-		if (test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) {
-			/* Queue for the daemon to resubmit */
-			spin_lock_irqsave(&m->lock, flags);
-			bio_list_add(&m->queued_bios, bio);
-			spin_unlock_irqrestore(&m->lock, flags);
-			queue_work(kmultipathd, &m->process_queued_bios);
-
-			return ERR_PTR(-EAGAIN);
-		}
-		return NULL;
-	}
-
-	return pgpath;
-}
-
 static int __multipath_map_bio(struct multipath *m, struct bio *bio,
 			       struct dm_mpath_io *mpio)
 {
-	struct pgpath *pgpath;
-
-	if (!m->hw_handler_name)
-		pgpath = __map_bio_fast(m, bio);
-	else
-		pgpath = __map_bio(m, bio);
+	struct pgpath *pgpath = __map_bio(m, bio);
 
 	if (IS_ERR(pgpath))
 		return DM_MAPIO_SUBMITTED;

drivers/md/dm-thin-metadata.c

@@ -188,6 +188,15 @@ struct dm_pool_metadata {
 	unsigned long flags;
 	sector_t data_block_size;
 
+	/*
+	 * Pre-commit callback.
+	 *
+	 * This allows the thin provisioning target to run a callback before
+	 * the metadata are committed.
+	 */
+	dm_pool_pre_commit_fn pre_commit_fn;
+	void *pre_commit_context;
+
 	/*
 	 * We reserve a section of the metadata for commit overhead.
 	 * All reported space does *not* include this.
@@ -826,6 +835,14 @@ static int __commit_transaction(struct dm_pool_metadata *pmd)
 	if (unlikely(!pmd->in_service))
 		return 0;
 
+	if (pmd->pre_commit_fn) {
+		r = pmd->pre_commit_fn(pmd->pre_commit_context);
+		if (r < 0) {
+			DMERR("pre-commit callback failed");
+			return r;
+		}
+	}
+
 	r = __write_changed_details(pmd);
 	if (r < 0)
 		return r;
@@ -892,6 +909,8 @@ struct dm_pool_metadata *dm_pool_metadata_open(struct block_device *bdev,
 	pmd->in_service = false;
 	pmd->bdev = bdev;
 	pmd->data_block_size = data_block_size;
+	pmd->pre_commit_fn = NULL;
+	pmd->pre_commit_context = NULL;
 
 	r = __create_persistent_data_objects(pmd, format_device);
 	if (r) {
@@ -2044,6 +2063,16 @@ int dm_pool_register_metadata_threshold(struct dm_pool_metadata *pmd,
 	return r;
 }
 
+void dm_pool_register_pre_commit_callback(struct dm_pool_metadata *pmd,
+					  dm_pool_pre_commit_fn fn,
+					  void *context)
+{
+	pmd_write_lock_in_core(pmd);
+	pmd->pre_commit_fn = fn;
+	pmd->pre_commit_context = context;
+	pmd_write_unlock(pmd);
+}
+
 int dm_pool_metadata_set_needs_check(struct dm_pool_metadata *pmd)
 {
 	int r = -EINVAL;

drivers/md/dm-thin-metadata.h

@@ -230,6 +230,13 @@ bool dm_pool_metadata_needs_check(struct dm_pool_metadata *pmd);
  */
 void dm_pool_issue_prefetches(struct dm_pool_metadata *pmd);
 
+/* Pre-commit callback */
+typedef int (*dm_pool_pre_commit_fn)(void *context);
+
+void dm_pool_register_pre_commit_callback(struct dm_pool_metadata *pmd,
+					  dm_pool_pre_commit_fn fn,
+					  void *context);
+
 /*----------------------------------------------------------------*/
 
 #endif

drivers/md/dm-thin.c

@@ -328,6 +328,7 @@ struct pool_c {
 	dm_block_t low_water_blocks;
 	struct pool_features requested_pf; /* Features requested during table load */
 	struct pool_features adjusted_pf;  /* Features used after adjusting for constituent devices */
+	struct bio flush_bio;
 };
 
 /*
@@ -2383,8 +2384,16 @@ static void process_deferred_bios(struct pool *pool)
 	while ((bio = bio_list_pop(&bio_completions)))
 		bio_endio(bio);
 
-	while ((bio = bio_list_pop(&bios)))
-		generic_make_request(bio);
+	while ((bio = bio_list_pop(&bios))) {
+		/*
+		 * The data device was flushed as part of metadata commit,
+		 * so complete redundant flushes immediately.
+		 */
+		if (bio->bi_opf & REQ_PREFLUSH)
+			bio_endio(bio);
+		else
+			generic_make_request(bio);
+	}
 }
 
 static void do_worker(struct work_struct *ws)
@@ -3115,6 +3124,7 @@ static void pool_dtr(struct dm_target *ti)
 	__pool_dec(pt->pool);
 	dm_put_device(ti, pt->metadata_dev);
 	dm_put_device(ti, pt->data_dev);
+	bio_uninit(&pt->flush_bio);
 	kfree(pt);
 
 	mutex_unlock(&dm_thin_pool_table.mutex);
@@ -3180,6 +3190,29 @@ static void metadata_low_callback(void *context)
 	dm_table_event(pool->ti->table);
 }
 
+/*
+ * We need to flush the data device **before** committing the metadata.
+ *
+ * This ensures that the data blocks of any newly inserted mappings are
+ * properly written to non-volatile storage and won't be lost in case of a
+ * crash.
+ *
+ * Failure to do so can result in data corruption in the case of internal or
+ * external snapshots and in the case of newly provisioned blocks, when block
+ * zeroing is enabled.
+ */
+static int metadata_pre_commit_callback(void *context)
+{
+	struct pool_c *pt = context;
+	struct bio *flush_bio = &pt->flush_bio;
+
+	bio_reset(flush_bio);
+	bio_set_dev(flush_bio, pt->data_dev->bdev);
+	flush_bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;
+
+	return submit_bio_wait(flush_bio);
+}
+
 static sector_t get_dev_size(struct block_device *bdev)
 {
 	return i_size_read(bdev->bd_inode) >> SECTOR_SHIFT;
@@ -3348,6 +3381,7 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
 	pt->data_dev = data_dev;
 	pt->low_water_blocks = low_water_blocks;
 	pt->adjusted_pf = pt->requested_pf = pf;
+	bio_init(&pt->flush_bio, NULL, 0);
 	ti->num_flush_bios = 1;
 
 	/*
@@ -3374,6 +3408,10 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
 	if (r)
 		goto out_flags_changed;
 
+	dm_pool_register_pre_commit_callback(pt->pool->pmd,
+					     metadata_pre_commit_callback,
+					     pt);
+
 	pt->callbacks.congested_fn = pool_is_congested;
 	dm_table_add_target_callbacks(ti->table, &pt->callbacks);
 

drivers/md/persistent-data/dm-btree-remove.c

@@ -203,7 +203,13 @@ static void __rebalance2(struct dm_btree_info *info, struct btree_node *parent,
 	struct btree_node *right = r->n;
 	uint32_t nr_left = le32_to_cpu(left->header.nr_entries);
 	uint32_t nr_right = le32_to_cpu(right->header.nr_entries);
-	unsigned threshold = 2 * merge_threshold(left) + 1;
+	/*
+	 * Ensure the number of entries in each child will be greater
+	 * than or equal to (max_entries / 3 + 1), so no matter which
+	 * child is used for removal, the number will still be not
+	 * less than (max_entries / 3).
+	 */
+	unsigned int threshold = 2 * (merge_threshold(left) + 1);
 
 	if (nr_left + nr_right < threshold) {
 		/*