Ack. Thank you.
On Tue, Dec 03, 2019 at 07:42:58PM +0800, Hou Tao wrote:
> 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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