dm persistent data: fix btree rebalancing after remove

When we remove an entry from a node we sometimes rebalance with it's
two neighbours.  This wasn't being done correctly; in some cases
entries have to move all the way from the right neighbour to the left
neighbour, or vice versa.  This patch pretty much re-writes the
balancing code to fix it.

This code is barely used currently; only when you delete a thin
device, and then only if you have hundreds of them in the same pool.
Once we have discard support, which removes mappings, this will be used
much more heavily.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

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

@@ -128,18 +128,9 @@ static void delete_at(struct node *n, unsigned index)
 	n->header.nr_entries = cpu_to_le32(nr_entries - 1);
 }
 
-static unsigned del_threshold(struct node *n)
-{
-	return le32_to_cpu(n->header.max_entries) / 3;
-}
-
 static unsigned merge_threshold(struct node *n)
 {
-	/*
-	 * The extra one is because we know we're potentially going to
-	 * delete an entry.
-	 */
-	return 2 * (le32_to_cpu(n->header.max_entries) / 3) + 1;
+	return le32_to_cpu(n->header.max_entries) / 3;
 }
 
 struct child {
@@ -188,6 +179,15 @@ static int exit_child(struct dm_btree_info *info, struct child *c)
 
 static void shift(struct node *left, struct node *right, int count)
 {
+	uint32_t nr_left = le32_to_cpu(left->header.nr_entries);
+	uint32_t nr_right = le32_to_cpu(right->header.nr_entries);
+	uint32_t max_entries = le32_to_cpu(left->header.max_entries);
+	uint32_t r_max_entries = le32_to_cpu(right->header.max_entries);
+
+	BUG_ON(max_entries != r_max_entries);
+	BUG_ON(nr_left - count > max_entries);
+	BUG_ON(nr_right + count > max_entries);
+
 	if (!count)
 		return;
 
@@ -199,13 +199,8 @@ static void shift(struct node *left, struct node *right, int count)
 		node_shift(right, count);
 	}
 
-	left->header.nr_entries =
-		cpu_to_le32(le32_to_cpu(left->header.nr_entries) - count);
-	BUG_ON(le32_to_cpu(left->header.nr_entries) > le32_to_cpu(left->header.max_entries));
-
-	right->header.nr_entries =
-		cpu_to_le32(le32_to_cpu(right->header.nr_entries) + count);
-	BUG_ON(le32_to_cpu(right->header.nr_entries) > le32_to_cpu(right->header.max_entries));
+	left->header.nr_entries = cpu_to_le32(nr_left - count);
+	right->header.nr_entries = cpu_to_le32(nr_right + count);
 }
 
 static void __rebalance2(struct dm_btree_info *info, struct node *parent,
@@ -215,8 +210,9 @@ static void __rebalance2(struct dm_btree_info *info, struct node *parent,
 	struct 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;
 
-	if (nr_left + nr_right <= merge_threshold(left)) {
+	if (nr_left + nr_right < threshold) {
 		/*
 		 * Merge
 		 */
@@ -234,9 +230,6 @@ static void __rebalance2(struct dm_btree_info *info, struct node *parent,
 		 * Rebalance.
 		 */
 		unsigned target_left = (nr_left + nr_right) / 2;
-		unsigned shift_ = nr_left - target_left;
-		BUG_ON(le32_to_cpu(left->header.max_entries) <= nr_left - shift_);
-		BUG_ON(le32_to_cpu(right->header.max_entries) <= nr_right + shift_);
 		shift(left, right, nr_left - target_left);
 		*key_ptr(parent, r->index) = right->keys[0];
 	}
@@ -272,6 +265,84 @@ static int rebalance2(struct shadow_spine *s, struct dm_btree_info *info,
 	return exit_child(info, &right);
 }
 
+/*
+ * We dump as many entries from center as possible into left, then the rest
+ * in right, then rebalance2.  This wastes some cpu, but I want something
+ * simple atm.
+ */
+static void delete_center_node(struct dm_btree_info *info, struct node *parent,
+			       struct child *l, struct child *c, struct child *r,
+			       struct node *left, struct node *center, struct node *right,
+			       uint32_t nr_left, uint32_t nr_center, uint32_t nr_right)
+{
+	uint32_t max_entries = le32_to_cpu(left->header.max_entries);
+	unsigned shift = min(max_entries - nr_left, nr_center);
+
+	BUG_ON(nr_left + shift > max_entries);
+	node_copy(left, center, -shift);
+	left->header.nr_entries = cpu_to_le32(nr_left + shift);
+
+	if (shift != nr_center) {
+		shift = nr_center - shift;
+		BUG_ON((nr_right + shift) > max_entries);
+		node_shift(right, shift);
+		node_copy(center, right, shift);
+		right->header.nr_entries = cpu_to_le32(nr_right + shift);
+	}
+	*key_ptr(parent, r->index) = right->keys[0];
+
+	delete_at(parent, c->index);
+	r->index--;
+
+	dm_tm_dec(info->tm, dm_block_location(c->block));
+	__rebalance2(info, parent, l, r);
+}
+
+/*
+ * Redistributes entries among 3 sibling nodes.
+ */
+static void redistribute3(struct dm_btree_info *info, struct node *parent,
+			  struct child *l, struct child *c, struct child *r,
+			  struct node *left, struct node *center, struct node *right,
+			  uint32_t nr_left, uint32_t nr_center, uint32_t nr_right)
+{
+	int s;
+	uint32_t max_entries = le32_to_cpu(left->header.max_entries);
+	unsigned target = (nr_left + nr_center + nr_right) / 3;
+	BUG_ON(target > max_entries);
+
+	if (nr_left < nr_right) {
+		s = nr_left - target;
+
+		if (s < 0 && nr_center < -s) {
+			/* not enough in central node */
+			shift(left, center, nr_center);
+			s = nr_center - target;
+			shift(left, right, s);
+			nr_right += s;
+		} else
+			shift(left, center, s);
+
+		shift(center, right, target - nr_right);
+
+	} else {
+		s = target - nr_right;
+		if (s > 0 && nr_center < s) {
+			/* not enough in central node */
+			shift(center, right, nr_center);
+			s = target - nr_center;
+			shift(left, right, s);
+			nr_left -= s;
+		} else
+			shift(center, right, s);
+
+		shift(left, center, nr_left - target);
+	}
+
+	*key_ptr(parent, c->index) = center->keys[0];
+	*key_ptr(parent, r->index) = right->keys[0];
+}
+
 static void __rebalance3(struct dm_btree_info *info, struct node *parent,
 			 struct child *l, struct child *c, struct child *r)
 {
@@ -282,62 +353,18 @@ static void __rebalance3(struct dm_btree_info *info, struct node *parent,
 	uint32_t nr_left = le32_to_cpu(left->header.nr_entries);
 	uint32_t nr_center = le32_to_cpu(center->header.nr_entries);
 	uint32_t nr_right = le32_to_cpu(right->header.nr_entries);
-	uint32_t max_entries = le32_to_cpu(left->header.max_entries);
 
-	unsigned target;
+	unsigned threshold = merge_threshold(left) * 4 + 1;
 
 	BUG_ON(left->header.max_entries != center->header.max_entries);
 	BUG_ON(center->header.max_entries != right->header.max_entries);
 
-	if (((nr_left + nr_center + nr_right) / 2) < merge_threshold(center)) {
-		/*
-		 * Delete center node:
-		 *
-		 * We dump as many entries from center as possible into
-		 * left, then the rest in right, then rebalance2.  This
-		 * wastes some cpu, but I want something simple atm.
-		 */
-		unsigned shift = min(max_entries - nr_left, nr_center);
-
-		BUG_ON(nr_left + shift > max_entries);
-		node_copy(left, center, -shift);
-		left->header.nr_entries = cpu_to_le32(nr_left + shift);
-
-		if (shift != nr_center) {
-			shift = nr_center - shift;
-			BUG_ON((nr_right + shift) >= max_entries);
-			node_shift(right, shift);
-			node_copy(center, right, shift);
-			right->header.nr_entries = cpu_to_le32(nr_right + shift);
-		}
-		*key_ptr(parent, r->index) = right->keys[0];
-
-		delete_at(parent, c->index);
-		r->index--;
-
-		dm_tm_dec(info->tm, dm_block_location(c->block));
-		__rebalance2(info, parent, l, r);
-
-		return;
-	}
-
-	/*
-	 * Rebalance
-	 */
-	target = (nr_left + nr_center + nr_right) / 3;
-	BUG_ON(target > max_entries);
-
-	/*
-	 * Adjust the left node
-	 */
-	shift(left, center, nr_left - target);
-
-	/*
-	 * Adjust the right node
-	 */
-	shift(center, right, target - nr_right);
-	*key_ptr(parent, c->index) = center->keys[0];
-	*key_ptr(parent, r->index) = right->keys[0];
+	if ((nr_left + nr_center + nr_right) < threshold)
+		delete_center_node(info, parent, l, c, r, left, center, right,
+				   nr_left, nr_center, nr_right);
+	else
+		redistribute3(info, parent, l, c, r, left, center, right,
+			      nr_left, nr_center, nr_right);
 }
 
 static int rebalance3(struct shadow_spine *s, struct dm_btree_info *info,
@@ -441,9 +468,6 @@ static int rebalance_children(struct shadow_spine *s,
 	if (r)
 		return r;
 
-	if (child_entries > del_threshold(n))
-		return 0;
-
 	has_left_sibling = i > 0;
 	has_right_sibling = i < (le32_to_cpu(n->header.nr_entries) - 1);