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dm btree: increase rebalance threshold in __rebalance2() 

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.

Cc: stable@vger.kernel.org
Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
This commit is contained in:
Hou Tao 2019-12-03 19:42:58 +08:00 committed by Mike Snitzer
parent 484e0d2b11
commit 474e559567
1 changed files with 7 additions and 1 deletions
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8
drivers/md/persistent-data/dm-btree-remove.c
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@ -203,7 +203,13 @@ static void __rebalance2(struct dm_btree_info *info, struct btree_node *parent,
struct btree_node *right = r->n; struct btree_node *right = r->n;
uint32_t nr_left = le32_to_cpu(left->header.nr_entries); uint32_t nr_left = le32_to_cpu(left->header.nr_entries);
uint32_t nr_right = le32_to_cpu(right->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) { if (nr_left + nr_right < threshold) {
/* /*