FAILED: patch "[PATCH] dm-thin: fix metadata refcount underflow" failed to apply to 5.10-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 09a65adc7d8bbfce06392cb6d375468e2728ead5
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026051203-outbreak-showgirl-7530@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 09a65adc7d8bbfce06392cb6d375468e2728ead5 Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@redhat.com>
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 <mpatocka@redhat.com>
Fixes: 3241b1d3e0aa ("dm: add persistent data library")
Cc: stable@vger.kernel.org

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)));