dm btree remove: fix a bug when rebalancing nodes after removal

Commit 4c7e309340ff ("dm btree remove: fix bug in redistribute3") wasn't
a complete fix for redistribute3().

The redistribute3 function takes 3 btree nodes and shares out the entries
evenly between them.  If the three nodes in total contained
(MAX_ENTRIES * 3) - 1 entries between them then this was erroneously getting
rebalanced as (MAX_ENTRIES - 1) on the left and right, and (MAX_ENTRIES + 1) in
the center.

Fix this issue by being more careful about calculating the target number
of entries for the left and right nodes.

Unit tested in userspace using this program:
https://github.com/jthornber/redistribute3-test/blob/master/redistribute3_t.c

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

1 files changed, 11 insertions, 6 deletions

diff --git a/drivers/md/persistent-data/dm-btree-remove.c b/drivers/md/persistent-data/dm-btree-remove.c
index 421a36c593e3..2e4c4cb79e4d 100644
--- a/drivers/md/persistent-data/dm-btree-remove.c
+++ b/drivers/md/persistent-data/dm-btree-remove.c
@@ -301,11 +301,16 @@ static void redistribute3(struct dm_btree_info *info, struct btree_node *parent,
 {
         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);
+        unsigned total = nr_left + nr_center + nr_right;
+        unsigned target_right = total / 3;
+        unsigned remainder = (target_right * 3) != total;
+        unsigned target_left = target_right + remainder;
+
+        BUG_ON(target_left > max_entries);
+        BUG_ON(target_right > max_entries);
 
         if (nr_left < nr_right) {
-                s = nr_left - target;
+                s = nr_left - target_left;
 
                 if (s < 0 && nr_center < -s) {
                         /* not enough in central node */
@@ -316,10 +321,10 @@ static void redistribute3(struct dm_btree_info *info, struct btree_node *parent,
                 } else
                         shift(left, center, s);
 
-                shift(center, right, target - nr_right);
+                shift(center, right, target_right - nr_right);
 
         } else {
-                s = target - nr_right;
+                s = target_right - nr_right;
                 if (s > 0 && nr_center < s) {
                         /* not enough in central node */
                         shift(center, right, nr_center);
@@ -329,7 +334,7 @@ static void redistribute3(struct dm_btree_info *info, struct btree_node *parent,
                 } else
                         shift(center, right, s);
 
-                shift(left, center, nr_left - target);
+                shift(left, center, nr_left - target_left);
         }
 
         *key_ptr(parent, c->index) = center->keys[0];