From 09a65adc7d8bbfce06392cb6d375468e2728ead5 Mon Sep 17 00:00:00 2001 From: Mikulas Patocka Date: Mon, 20 Apr 2026 19:56:44 +0200 Subject: [PATCH] dm-thin: fix metadata refcount underflow There's a bug in dm-thin in the function rebalance_children. If the internal btree node has one entry, the code tries to copy all btree entries from the node's child to the node itself and then decrement the child's reference count. If the child node is shared (it has reference count > 1), we won't free it, so there would be two pointers to each of the grandchildren nodes. But the reference counts of the grandchildren is not increased, thus the reference count doesn't match the number of pointers that point to the grandchildren. This results in "device mapper: space map common: unable to decrement block" errors. Fix this bug by incrementing reference counts on the grandchildren if the btree node is shared. Signed-off-by: Mikulas Patocka Fixes: 3241b1d3e0aa ("dm: add persistent data library") Cc: stable@vger.kernel.org --- drivers/md/persistent-data/dm-btree-remove.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/drivers/md/persistent-data/dm-btree-remove.c b/drivers/md/persistent-data/dm-btree-remove.c index 942cd47eb52d..aeec5b9a1dd5 100644 --- a/drivers/md/persistent-data/dm-btree-remove.c +++ b/drivers/md/persistent-data/dm-btree-remove.c @@ -490,12 +490,20 @@ static int rebalance_children(struct shadow_spine *s, if (le32_to_cpu(n->header.nr_entries) == 1) { struct dm_block *child; + int is_shared; dm_block_t b = value64(n, 0); + r = dm_tm_block_is_shared(info->tm, b, &is_shared); + if (r) + return r; + r = dm_tm_read_lock(info->tm, b, &btree_node_validator, &child); if (r) return r; + if (is_shared) + inc_children(info->tm, dm_block_data(child), vt); + memcpy(n, dm_block_data(child), dm_bm_block_size(dm_tm_get_bm(info->tm)));