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 <houtao1@xxxxxxxxxx>
---
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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