[Devel] [PATCH RHEL10 COMMIT] drivers/md/dm-qcow2: respect zeroes during merge

[Konstantin Khorenko]

The commit is pushed to "branch-rh10-6.12.0-211.39.1.16.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.11.vz10
------>
commit 21c4cf6aff361f9183ed73540de82c24294c7995
Author: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Date:   Thu Aug 27 19:06:19 2026 +0300

    drivers/md/dm-qcow2: respect zeroes during merge
    
    The merge machinery used to skip unallocated clusters, therefore
    breaking all clusters with zero bits and non-zero backing.
    Technically this bug was present even before discard changes, as
    qcow2 image could have 'reads as zero' bits before being inserted
    into dm-qcow2.
    Properly commit all zero clusters (albeit with data for now).
    
    https://virtuozzo.atlassian.net/browse/VSTOR-138288
    Feature: dm-qcow2: block device over QCOW2 files driver
    Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
    Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
    Reviewed-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
    Reviewed-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 drivers/md/dm-qcow2-map.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 53cb11242bf0d..78ee7cf5e8817 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -2744,6 +2744,13 @@ static void requeue_if_ok(struct qcow2_target *tgt, struct qio *unused,
 	qcow2_dispatch_qios(qio->qcow2, qio, NULL);
 }
 
+static bool clu_zeroes_present(struct qcow2 *qcow2, struct qcow2_map *map)
+{
+	if (qcow2->ext_l2)
+		return (map->ext_l2 >> 32) != 0;
+	return map->all_zeroes;
+}
+
 static int prepare_backward_merge(struct qcow2 *qcow2, struct qio **qio,
 				  struct qcow2_map *map, bool write)
 {
@@ -2761,7 +2768,7 @@ static int prepare_backward_merge(struct qcow2 *qcow2, struct qio **qio,
 		}
 	}
 
-	if (!map->data_clu_alloced) {
+	if (!map->data_clu_alloced && !clu_zeroes_present(qcow2, map)) {
 		/* Strange COW at L1, except the merge from RO image */
 		WARN_ON_ONCE(map->clu_is_cow && qio_may_modify_image(qcow2, *qio));
 		if (fake_merge_qio(*qio)) {
@@ -2808,7 +2815,7 @@ static int prepare_backward_merge(struct qcow2 *qcow2, struct qio **qio,
 		goto endio;
 	}
 
-	if (!map->clu_is_cow) {
+	if (!map->clu_is_cow && map->data_clu_alloced) {
 		/* Forced set these to unuse them after discard */
 		(*qio)->ext->unuse_clu_pos = map->data_clu_pos;
 		(*qio)->ext->unuse_clu_end = map->data_clu_pos + qcow2->clu_size;