We got the following warnings from thin_check during thin-pool setup:

  $ thin_check /dev/vdb
  examining superblock
  examining devices tree
    missing devices: [1, 84]
      too few entries in btree_node: 41, expected at least 42 (block 138, 
max_entries = 126)
  examining mapping tree

The phenomenon is the number of entries in one node of details_info tree is
less than (max_entries / 3). And it can be easily reproduced by the following
procedures:

  $ new a thin pool
  $ presume the max entries of details_info tree is 126
  $ new 127 thin devices (e.g. 1~127) to make the root node being full
    and then split
  $ remove the first 43 (e.g. 1~43) thin devices to make the children
    reblance repeatedly
  $ stop the thin pool
  $ thin_check

The root cause is that the B-tree removal procedure in __rebalance2()
doesn't guarantee the invariance: the minimal number of entries in
non-root node should be >= (max_entries / 3).

Simply fix the problem by increasing the rebalance threshold to
make sure 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 valid.

Signed-off-by: Hou Tao <hout...@huawei.com>
---
 drivers/md/persistent-data/dm-btree-remove.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/md/persistent-data/dm-btree-remove.c 
b/drivers/md/persistent-data/dm-btree-remove.c
index 21ea537bd55e..eff04fa23dfa 100644
--- a/drivers/md/persistent-data/dm-btree-remove.c
+++ b/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) {
                /*
-- 
2.22.0


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