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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 <mpatocka@redhat.com>
Fixes: 3241b1d3e0aa ("dm: add persistent data library")
Cc: stable@vger.kernel.org

Mikulas Patocka 09a65adc 028ef9c9

+8
+8
drivers/md/persistent-data/dm-btree-remove.c
··· 490 490 491 491 if (le32_to_cpu(n->header.nr_entries) == 1) { 492 492 struct dm_block *child; 493 + int is_shared; 493 494 dm_block_t b = value64(n, 0); 495 + 496 + r = dm_tm_block_is_shared(info->tm, b, &is_shared); 497 + if (r) 498 + return r; 494 499 495 500 r = dm_tm_read_lock(info->tm, b, &btree_node_validator, &child); 496 501 if (r) 497 502 return r; 503 + 504 + if (is_shared) 505 + inc_children(info->tm, dm_block_data(child), vt); 498 506 499 507 memcpy(n, dm_block_data(child), 500 508 dm_bm_block_size(dm_tm_get_bm(info->tm)));