dm: add persistent data library

The persistent-data library offers a re-usable framework for the storage
and management of on-disk metadata in device-mapper targets.

It's used by the thin-provisioning target in the next patch and in an
upcoming hierarchical storage target.

For further information, please read
Documentation/device-mapper/persistent-data.txt

Signed-off-by: Joe Thornber <thornber@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 805 insertions, 0 deletions

diff --git a/drivers/md/persistent-data/dm-btree.c b/drivers/md/persistent-data/dm-btree.c
new file mode 100644
index 000000000000..e0638be53ea4
--- /dev/null
+++ b/drivers/md/persistent-data/dm-btree.c
@@ -0,0 +1,805 @@
+/*
+ * Copyright (C) 2011 Red Hat, Inc.
+ *
+ * This file is released under the GPL.
+ */
+
+#include "dm-btree-internal.h"
+#include "dm-space-map.h"
+#include "dm-transaction-manager.h"
+
+#include <linux/module.h>
+#include <linux/device-mapper.h>
+
+#define DM_MSG_PREFIX "btree"
+
+/*----------------------------------------------------------------
+ * Array manipulation
+ *--------------------------------------------------------------*/
+static void memcpy_disk(void *dest, const void *src, size_t len)
+	__dm_written_to_disk(src)
+{
+	memcpy(dest, src, len);
+	__dm_unbless_for_disk(src);
+}
+
+static void array_insert(void *base, size_t elt_size, unsigned nr_elts,
+			 unsigned index, void *elt)
+	__dm_written_to_disk(elt)
+{
+	if (index < nr_elts)
+		memmove(base + (elt_size * (index + 1)),
+			base + (elt_size * index),
+			(nr_elts - index) * elt_size);
+
+	memcpy_disk(base + (elt_size * index), elt, elt_size);
+}
+
+/*----------------------------------------------------------------*/
+
+/* makes the assumption that no two keys are the same. */
+static int bsearch(struct node *n, uint64_t key, int want_hi)
+{
+	int lo = -1, hi = le32_to_cpu(n->header.nr_entries);
+
+	while (hi - lo > 1) {
+		int mid = lo + ((hi - lo) / 2);
+		uint64_t mid_key = le64_to_cpu(n->keys[mid]);
+
+		if (mid_key == key)
+			return mid;
+
+		if (mid_key < key)
+			lo = mid;
+		else
+			hi = mid;
+	}
+
+	return want_hi ? hi : lo;
+}
+
+int lower_bound(struct node *n, uint64_t key)
+{
+	return bsearch(n, key, 0);
+}
+
+void inc_children(struct dm_transaction_manager *tm, struct node *n,
+		  struct dm_btree_value_type *vt)
+{
+	unsigned i;
+	uint32_t nr_entries = le32_to_cpu(n->header.nr_entries);
+
+	if (le32_to_cpu(n->header.flags) & INTERNAL_NODE)
+		for (i = 0; i < nr_entries; i++)
+			dm_tm_inc(tm, value64(n, i));
+	else if (vt->inc)
+		for (i = 0; i < nr_entries; i++)
+			vt->inc(vt->context,
+				value_ptr(n, i, vt->size));
+}
+
+static int insert_at(size_t value_size, struct node *node, unsigned index,
+		      uint64_t key, void *value)
+		      __dm_written_to_disk(value)
+{
+	uint32_t nr_entries = le32_to_cpu(node->header.nr_entries);
+	__le64 key_le = cpu_to_le64(key);
+
+	if (index > nr_entries ||
+	    index >= le32_to_cpu(node->header.max_entries)) {
+		DMERR("too many entries in btree node for insert");
+		__dm_unbless_for_disk(value);
+		return -ENOMEM;
+	}
+
+	__dm_bless_for_disk(&key_le);
+
+	array_insert(node->keys, sizeof(*node->keys), nr_entries, index, &key_le);
+	array_insert(value_base(node), value_size, nr_entries, index, value);
+	node->header.nr_entries = cpu_to_le32(nr_entries + 1);
+
+	return 0;
+}
+
+/*----------------------------------------------------------------*/
+
+/*
+ * We want 3n entries (for some n).  This works more nicely for repeated
+ * insert remove loops than (2n + 1).
+ */
+static uint32_t calc_max_entries(size_t value_size, size_t block_size)
+{
+	uint32_t total, n;
+	size_t elt_size = sizeof(uint64_t) + value_size; /* key + value */
+
+	block_size -= sizeof(struct node_header);
+	total = block_size / elt_size;
+	n = total / 3;		/* rounds down */
+
+	return 3 * n;
+}
+
+int dm_btree_empty(struct dm_btree_info *info, dm_block_t *root)
+{
+	int r;
+	struct dm_block *b;
+	struct node *n;
+	size_t block_size;
+	uint32_t max_entries;
+
+	r = new_block(info, &b);
+	if (r < 0)
+		return r;
+
+	block_size = dm_bm_block_size(dm_tm_get_bm(info->tm));
+	max_entries = calc_max_entries(info->value_type.size, block_size);
+
+	n = dm_block_data(b);
+	memset(n, 0, block_size);
+	n->header.flags = cpu_to_le32(LEAF_NODE);
+	n->header.nr_entries = cpu_to_le32(0);
+	n->header.max_entries = cpu_to_le32(max_entries);
+	n->header.value_size = cpu_to_le32(info->value_type.size);
+
+	*root = dm_block_location(b);
+	return unlock_block(info, b);
+}
+EXPORT_SYMBOL_GPL(dm_btree_empty);
+
+/*----------------------------------------------------------------*/
+
+/*
+ * Deletion uses a recursive algorithm, since we have limited stack space
+ * we explicitly manage our own stack on the heap.
+ */
+#define MAX_SPINE_DEPTH 64
+struct frame {
+	struct dm_block *b;
+	struct node *n;
+	unsigned level;
+	unsigned nr_children;
+	unsigned current_child;
+};
+
+struct del_stack {
+	struct dm_transaction_manager *tm;
+	int top;
+	struct frame spine[MAX_SPINE_DEPTH];
+};
+
+static int top_frame(struct del_stack *s, struct frame **f)
+{
+	if (s->top < 0) {
+		DMERR("btree deletion stack empty");
+		return -EINVAL;
+	}
+
+	*f = s->spine + s->top;
+
+	return 0;
+}
+
+static int unprocessed_frames(struct del_stack *s)
+{
+	return s->top >= 0;
+}
+
+static int push_frame(struct del_stack *s, dm_block_t b, unsigned level)
+{
+	int r;
+	uint32_t ref_count;
+
+	if (s->top >= MAX_SPINE_DEPTH - 1) {
+		DMERR("btree deletion stack out of memory");
+		return -ENOMEM;
+	}
+
+	r = dm_tm_ref(s->tm, b, &ref_count);
+	if (r)
+		return r;
+
+	if (ref_count > 1)
+		/*
+		 * This is a shared node, so we can just decrement it's
+		 * reference counter and leave the children.
+		 */
+		dm_tm_dec(s->tm, b);
+
+	else {
+		struct frame *f = s->spine + ++s->top;
+
+		r = dm_tm_read_lock(s->tm, b, &btree_node_validator, &f->b);
+		if (r) {
+			s->top--;
+			return r;
+		}
+
+		f->n = dm_block_data(f->b);
+		f->level = level;
+		f->nr_children = le32_to_cpu(f->n->header.nr_entries);
+		f->current_child = 0;
+	}
+
+	return 0;
+}
+
+static void pop_frame(struct del_stack *s)
+{
+	struct frame *f = s->spine + s->top--;
+
+	dm_tm_dec(s->tm, dm_block_location(f->b));
+	dm_tm_unlock(s->tm, f->b);
+}
+
+int dm_btree_del(struct dm_btree_info *info, dm_block_t root)
+{
+	int r;
+	struct del_stack *s;
+
+	s = kmalloc(sizeof(*s), GFP_KERNEL);
+	if (!s)
+		return -ENOMEM;
+	s->tm = info->tm;
+	s->top = -1;
+
+	r = push_frame(s, root, 1);
+	if (r)
+		goto out;
+
+	while (unprocessed_frames(s)) {
+		uint32_t flags;
+		struct frame *f;
+		dm_block_t b;
+
+		r = top_frame(s, &f);
+		if (r)
+			goto out;
+
+		if (f->current_child >= f->nr_children) {
+			pop_frame(s);
+			continue;
+		}
+
+		flags = le32_to_cpu(f->n->header.flags);
+		if (flags & INTERNAL_NODE) {
+			b = value64(f->n, f->current_child);
+			f->current_child++;
+			r = push_frame(s, b, f->level);
+			if (r)
+				goto out;
+
+		} else if (f->level != (info->levels - 1)) {
+			b = value64(f->n, f->current_child);
+			f->current_child++;
+			r = push_frame(s, b, f->level + 1);
+			if (r)
+				goto out;
+
+		} else {
+			if (info->value_type.dec) {
+				unsigned i;
+
+				for (i = 0; i < f->nr_children; i++)
+					info->value_type.dec(info->value_type.context,
+							     value_ptr(f->n, i, info->value_type.size));
+			}
+			f->current_child = f->nr_children;
+		}
+	}
+
+out:
+	kfree(s);
+	return r;
+}
+EXPORT_SYMBOL_GPL(dm_btree_del);
+
+/*----------------------------------------------------------------*/
+
+static int btree_lookup_raw(struct ro_spine *s, dm_block_t block, uint64_t key,
+			    int (*search_fn)(struct node *, uint64_t),
+			    uint64_t *result_key, void *v, size_t value_size)
+{
+	int i, r;
+	uint32_t flags, nr_entries;
+
+	do {
+		r = ro_step(s, block);
+		if (r < 0)
+			return r;
+
+		i = search_fn(ro_node(s), key);
+
+		flags = le32_to_cpu(ro_node(s)->header.flags);
+		nr_entries = le32_to_cpu(ro_node(s)->header.nr_entries);
+		if (i < 0 || i >= nr_entries)
+			return -ENODATA;
+
+		if (flags & INTERNAL_NODE)
+			block = value64(ro_node(s), i);
+
+	} while (!(flags & LEAF_NODE));
+
+	*result_key = le64_to_cpu(ro_node(s)->keys[i]);
+	memcpy(v, value_ptr(ro_node(s), i, value_size), value_size);
+
+	return 0;
+}
+
+int dm_btree_lookup(struct dm_btree_info *info, dm_block_t root,
+		    uint64_t *keys, void *value_le)
+{
+	unsigned level, last_level = info->levels - 1;
+	int r = -ENODATA;
+	uint64_t rkey;
+	__le64 internal_value_le;
+	struct ro_spine spine;
+
+	init_ro_spine(&spine, info);
+	for (level = 0; level < info->levels; level++) {
+		size_t size;
+		void *value_p;
+
+		if (level == last_level) {
+			value_p = value_le;
+			size = info->value_type.size;
+
+		} else {
+			value_p = &internal_value_le;
+			size = sizeof(uint64_t);
+		}
+
+		r = btree_lookup_raw(&spine, root, keys[level],
+				     lower_bound, &rkey,
+				     value_p, size);
+
+		if (!r) {
+			if (rkey != keys[level]) {
+				exit_ro_spine(&spine);
+				return -ENODATA;
+			}
+		} else {
+			exit_ro_spine(&spine);
+			return r;
+		}
+
+		root = le64_to_cpu(internal_value_le);
+	}
+	exit_ro_spine(&spine);
+
+	return r;
+}
+EXPORT_SYMBOL_GPL(dm_btree_lookup);
+
+/*
+ * Splits a node by creating a sibling node and shifting half the nodes
+ * contents across.  Assumes there is a parent node, and it has room for
+ * another child.
+ *
+ * Before:
+ *	  +--------+
+ *	  | Parent |
+ *	  +--------+
+ *	     |
+ *	     v
+ *	+----------+
+ *	| A ++++++ |
+ *	+----------+
+ *
+ *
+ * After:
+ *		+--------+
+ *		| Parent |
+ *		+--------+
+ *		  |	|
+ *		  v	+------+
+ *	    +---------+	       |
+ *	    | A* +++  |	       v
+ *	    +---------+	  +-------+
+ *			  | B +++ |
+ *			  +-------+
+ *
+ * Where A* is a shadow of A.
+ */
+static int btree_split_sibling(struct shadow_spine *s, dm_block_t root,
+			       unsigned parent_index, uint64_t key)
+{
+	int r;
+	size_t size;
+	unsigned nr_left, nr_right;
+	struct dm_block *left, *right, *parent;
+	struct node *ln, *rn, *pn;
+	__le64 location;
+
+	left = shadow_current(s);
+
+	r = new_block(s->info, &right);
+	if (r < 0)
+		return r;
+
+	ln = dm_block_data(left);
+	rn = dm_block_data(right);
+
+	nr_left = le32_to_cpu(ln->header.nr_entries) / 2;
+	nr_right = le32_to_cpu(ln->header.nr_entries) - nr_left;
+
+	ln->header.nr_entries = cpu_to_le32(nr_left);
+
+	rn->header.flags = ln->header.flags;
+	rn->header.nr_entries = cpu_to_le32(nr_right);
+	rn->header.max_entries = ln->header.max_entries;
+	rn->header.value_size = ln->header.value_size;
+	memcpy(rn->keys, ln->keys + nr_left, nr_right * sizeof(rn->keys[0]));
+
+	size = le32_to_cpu(ln->header.flags) & INTERNAL_NODE ?
+		sizeof(uint64_t) : s->info->value_type.size;
+	memcpy(value_ptr(rn, 0, size), value_ptr(ln, nr_left, size),
+	       size * nr_right);
+
+	/*
+	 * Patch up the parent
+	 */
+	parent = shadow_parent(s);
+
+	pn = dm_block_data(parent);
+	location = cpu_to_le64(dm_block_location(left));
+	__dm_bless_for_disk(&location);
+	memcpy_disk(value_ptr(pn, parent_index, sizeof(__le64)),
+		    &location, sizeof(__le64));
+
+	location = cpu_to_le64(dm_block_location(right));
+	__dm_bless_for_disk(&location);
+
+	r = insert_at(sizeof(__le64), pn, parent_index + 1,
+		      le64_to_cpu(rn->keys[0]), &location);
+	if (r)
+		return r;
+
+	if (key < le64_to_cpu(rn->keys[0])) {
+		unlock_block(s->info, right);
+		s->nodes[1] = left;
+	} else {
+		unlock_block(s->info, left);
+		s->nodes[1] = right;
+	}
+
+	return 0;
+}
+
+/*
+ * Splits a node by creating two new children beneath the given node.
+ *
+ * Before:
+ *	  +----------+
+ *	  | A ++++++ |
+ *	  +----------+
+ *
+ *
+ * After:
+ *	+------------+
+ *	| A (shadow) |
+ *	+------------+
+ *	    |	|
+ *   +------+	+----+
+ *   |		     |
+ *   v		     v
+ * +-------+	 +-------+
+ * | B +++ |	 | C +++ |
+ * +-------+	 +-------+
+ */
+static int btree_split_beneath(struct shadow_spine *s, uint64_t key)
+{
+	int r;
+	size_t size;
+	unsigned nr_left, nr_right;
+	struct dm_block *left, *right, *new_parent;
+	struct node *pn, *ln, *rn;
+	__le64 val;
+
+	new_parent = shadow_current(s);
+
+	r = new_block(s->info, &left);
+	if (r < 0)
+		return r;
+
+	r = new_block(s->info, &right);
+	if (r < 0) {
+		/* FIXME: put left */
+		return r;
+	}
+
+	pn = dm_block_data(new_parent);
+	ln = dm_block_data(left);
+	rn = dm_block_data(right);
+
+	nr_left = le32_to_cpu(pn->header.nr_entries) / 2;
+	nr_right = le32_to_cpu(pn->header.nr_entries) - nr_left;
+
+	ln->header.flags = pn->header.flags;
+	ln->header.nr_entries = cpu_to_le32(nr_left);
+	ln->header.max_entries = pn->header.max_entries;
+	ln->header.value_size = pn->header.value_size;
+
+	rn->header.flags = pn->header.flags;
+	rn->header.nr_entries = cpu_to_le32(nr_right);
+	rn->header.max_entries = pn->header.max_entries;
+	rn->header.value_size = pn->header.value_size;
+
+	memcpy(ln->keys, pn->keys, nr_left * sizeof(pn->keys[0]));
+	memcpy(rn->keys, pn->keys + nr_left, nr_right * sizeof(pn->keys[0]));
+
+	size = le32_to_cpu(pn->header.flags) & INTERNAL_NODE ?
+		sizeof(__le64) : s->info->value_type.size;
+	memcpy(value_ptr(ln, 0, size), value_ptr(pn, 0, size), nr_left * size);
+	memcpy(value_ptr(rn, 0, size), value_ptr(pn, nr_left, size),
+	       nr_right * size);
+
+	/* new_parent should just point to l and r now */
+	pn->header.flags = cpu_to_le32(INTERNAL_NODE);
+	pn->header.nr_entries = cpu_to_le32(2);
+	pn->header.max_entries = cpu_to_le32(
+		calc_max_entries(sizeof(__le64),
+				 dm_bm_block_size(
+					 dm_tm_get_bm(s->info->tm))));
+	pn->header.value_size = cpu_to_le32(sizeof(__le64));
+
+	val = cpu_to_le64(dm_block_location(left));
+	__dm_bless_for_disk(&val);
+	pn->keys[0] = ln->keys[0];
+	memcpy_disk(value_ptr(pn, 0, sizeof(__le64)), &val, sizeof(__le64));
+
+	val = cpu_to_le64(dm_block_location(right));
+	__dm_bless_for_disk(&val);
+	pn->keys[1] = rn->keys[0];
+	memcpy_disk(value_ptr(pn, 1, sizeof(__le64)), &val, sizeof(__le64));
+
+	/*
+	 * rejig the spine.  This is ugly, since it knows too
+	 * much about the spine
+	 */
+	if (s->nodes[0] != new_parent) {
+		unlock_block(s->info, s->nodes[0]);
+		s->nodes[0] = new_parent;
+	}
+	if (key < le64_to_cpu(rn->keys[0])) {
+		unlock_block(s->info, right);
+		s->nodes[1] = left;
+	} else {
+		unlock_block(s->info, left);
+		s->nodes[1] = right;
+	}
+	s->count = 2;
+
+	return 0;
+}
+
+static int btree_insert_raw(struct shadow_spine *s, dm_block_t root,
+			    struct dm_btree_value_type *vt,
+			    uint64_t key, unsigned *index)
+{
+	int r, i = *index, top = 1;
+	struct node *node;
+
+	for (;;) {
+		r = shadow_step(s, root, vt);
+		if (r < 0)
+			return r;
+
+		node = dm_block_data(shadow_current(s));
+
+		/*
+		 * We have to patch up the parent node, ugly, but I don't
+		 * see a way to do this automatically as part of the spine
+		 * op.
+		 */
+		if (shadow_has_parent(s) && i >= 0) { /* FIXME: second clause unness. */
+			__le64 location = cpu_to_le64(dm_block_location(shadow_current(s)));
+
+			__dm_bless_for_disk(&location);
+			memcpy_disk(value_ptr(dm_block_data(shadow_parent(s)), i, sizeof(uint64_t)),
+				    &location, sizeof(__le64));
+		}
+
+		node = dm_block_data(shadow_current(s));
+
+		if (node->header.nr_entries == node->header.max_entries) {
+			if (top)
+				r = btree_split_beneath(s, key);
+			else
+				r = btree_split_sibling(s, root, i, key);
+
+			if (r < 0)
+				return r;
+		}
+
+		node = dm_block_data(shadow_current(s));
+
+		i = lower_bound(node, key);
+
+		if (le32_to_cpu(node->header.flags) & LEAF_NODE)
+			break;
+
+		if (i < 0) {
+			/* change the bounds on the lowest key */
+			node->keys[0] = cpu_to_le64(key);
+			i = 0;
+		}
+
+		root = value64(node, i);
+		top = 0;
+	}
+
+	if (i < 0 || le64_to_cpu(node->keys[i]) != key)
+		i++;
+
+	*index = i;
+	return 0;
+}
+
+static int insert(struct dm_btree_info *info, dm_block_t root,
+		  uint64_t *keys, void *value, dm_block_t *new_root,
+		  int *inserted)
+		  __dm_written_to_disk(value)
+{
+	int r, need_insert;
+	unsigned level, index = -1, last_level = info->levels - 1;
+	dm_block_t block = root;
+	struct shadow_spine spine;
+	struct node *n;
+	struct dm_btree_value_type le64_type;
+
+	le64_type.context = NULL;
+	le64_type.size = sizeof(__le64);
+	le64_type.inc = NULL;
+	le64_type.dec = NULL;
+	le64_type.equal = NULL;
+
+	init_shadow_spine(&spine, info);
+
+	for (level = 0; level < (info->levels - 1); level++) {
+		r = btree_insert_raw(&spine, block, &le64_type, keys[level], &index);
+		if (r < 0)
+			goto bad;
+
+		n = dm_block_data(shadow_current(&spine));
+		need_insert = ((index >= le32_to_cpu(n->header.nr_entries)) ||
+			       (le64_to_cpu(n->keys[index]) != keys[level]));
+
+		if (need_insert) {
+			dm_block_t new_tree;
+			__le64 new_le;
+
+			r = dm_btree_empty(info, &new_tree);
+			if (r < 0)
+				goto bad;
+
+			new_le = cpu_to_le64(new_tree);
+			__dm_bless_for_disk(&new_le);
+
+			r = insert_at(sizeof(uint64_t), n, index,
+				      keys[level], &new_le);
+			if (r)
+				goto bad;
+		}
+
+		if (level < last_level)
+			block = value64(n, index);
+	}
+
+	r = btree_insert_raw(&spine, block, &info->value_type,
+			     keys[level], &index);
+	if (r < 0)
+		goto bad;
+
+	n = dm_block_data(shadow_current(&spine));
+	need_insert = ((index >= le32_to_cpu(n->header.nr_entries)) ||
+		       (le64_to_cpu(n->keys[index]) != keys[level]));
+
+	if (need_insert) {
+		if (inserted)
+			*inserted = 1;
+
+		r = insert_at(info->value_type.size, n, index,
+			      keys[level], value);
+		if (r)
+			goto bad_unblessed;
+	} else {
+		if (inserted)
+			*inserted = 0;
+
+		if (info->value_type.dec &&
+		    (!info->value_type.equal ||
+		     !info->value_type.equal(
+			     info->value_type.context,
+			     value_ptr(n, index, info->value_type.size),
+			     value))) {
+			info->value_type.dec(info->value_type.context,
+					     value_ptr(n, index, info->value_type.size));
+		}
+		memcpy_disk(value_ptr(n, index, info->value_type.size),
+			    value, info->value_type.size);
+	}
+
+	*new_root = shadow_root(&spine);
+	exit_shadow_spine(&spine);
+
+	return 0;
+
+bad:
+	__dm_unbless_for_disk(value);
+bad_unblessed:
+	exit_shadow_spine(&spine);
+	return r;
+}
+
+int dm_btree_insert(struct dm_btree_info *info, dm_block_t root,
+		    uint64_t *keys, void *value, dm_block_t *new_root)
+		    __dm_written_to_disk(value)
+{
+	return insert(info, root, keys, value, new_root, NULL);
+}
+EXPORT_SYMBOL_GPL(dm_btree_insert);
+
+int dm_btree_insert_notify(struct dm_btree_info *info, dm_block_t root,
+			   uint64_t *keys, void *value, dm_block_t *new_root,
+			   int *inserted)
+			   __dm_written_to_disk(value)
+{
+	return insert(info, root, keys, value, new_root, inserted);
+}
+EXPORT_SYMBOL_GPL(dm_btree_insert_notify);
+
+/*----------------------------------------------------------------*/
+
+static int find_highest_key(struct ro_spine *s, dm_block_t block,
+			    uint64_t *result_key, dm_block_t *next_block)
+{
+	int i, r;
+	uint32_t flags;
+
+	do {
+		r = ro_step(s, block);
+		if (r < 0)
+			return r;
+
+		flags = le32_to_cpu(ro_node(s)->header.flags);
+		i = le32_to_cpu(ro_node(s)->header.nr_entries);
+		if (!i)
+			return -ENODATA;
+		else
+			i--;
+
+		*result_key = le64_to_cpu(ro_node(s)->keys[i]);
+		if (next_block || flags & INTERNAL_NODE)
+			block = value64(ro_node(s), i);
+
+	} while (flags & INTERNAL_NODE);
+
+	if (next_block)
+		*next_block = block;
+	return 0;
+}
+
+int dm_btree_find_highest_key(struct dm_btree_info *info, dm_block_t root,
+			      uint64_t *result_keys)
+{
+	int r = 0, count = 0, level;
+	struct ro_spine spine;
+
+	init_ro_spine(&spine, info);
+	for (level = 0; level < info->levels; level++) {
+		r = find_highest_key(&spine, root, result_keys + level,
+				     level == info->levels - 1 ? NULL : &root);
+		if (r == -ENODATA) {
+			r = 0;
+			break;
+
+		} else if (r)
+			break;
+
+		count++;
+	}
+	exit_ro_spine(&spine);
+
+	return r ? r : count;
+}
+EXPORT_SYMBOL_GPL(dm_btree_find_highest_key);