Setting the QCOW_OFLAG_ZERO bit of the L2 entry is forbidden if an
image has subclusters. Instead, the individual 'all zeroes' bits must
be used.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 block/qcow2-cluster.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 70b0aaa00a..207f670c94 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1790,7 +1790,11 @@ static int discard_in_l2_slice(BlockDriverState *bs, uint64_t offset,
 
         /* First remove L2 entries */
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
-        if (!full_discard && s->qcow_version >= 3) {
+        if (has_subclusters(s)) {
+            set_l2_entry(s, l2_slice, l2_index + i, 0);
+            set_l2_bitmap(s, l2_slice, l2_index + i,
+                          full_discard ? 0 : QCOW_L2_BITMAP_ALL_ZEROES);
+        } else if (!full_discard && s->qcow_version >= 3) {
             set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);
         } else {
             set_l2_entry(s, l2_slice, l2_index + i, 0);
-- 
2.20.1

On 22.12.19 12:36, Alberto Garcia wrote:
> Setting the QCOW_OFLAG_ZERO bit of the L2 entry is forbidden if an
> image has subclusters. Instead, the individual 'all zeroes' bits must
> be used.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>