Re: [Devel] [PATCH RHEL10 COMMIT] drivers/md/dm-qcow2: allow shared L1 entries during merge from RO image

[Konstantin Khorenko <khorenko@virtuozzo.com>]

The commit is pushed to "branch-rh10-6.12.0-211.39.1.16.x.vz10-ovz" and will appear at git@bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.11.vz10
------>
commit 08f42e9ddb46a95e8cf467d93248fb985e55b9b8
Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
Date:   Thu Aug 27 19:06:17 2026 +0300

    drivers/md/dm-qcow2: allow shared L1 entries during merge from RO image
    
    For RO images with internal snapshots L1 entries are shared.
    Write-mode metadata parsing stops at such entries to avoid
    modifying a shared L2 table, which made prepare_backward_merge()
    see the cluster as unallocated and skip it with
    "nothing to merge": the merged result would silently lose all
    data under shared L1 entries.
    Allow such scenario in parse_l1() and ease WARN in
    prepare_backward_merge().
    
    https://virtuozzo.atlassian.net/browse/VSTOR-138288
    Feature: dm-qcow2: block device over QCOW2 files driver
    Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
    Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
    Reviewed-by: Vasileios Almpanis <vasileios.almpanis@virtuozzo.com>
    Reviewed-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 drivers/md/dm-qcow2-map.c | 18 ++++++++++++++++--
 drivers/md/dm-qcow2.h     |  5 +++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 3154cae955916..c5a79a51942ae 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -1726,6 +1726,14 @@ static bool qio_is_fully_alloced(struct qcow2 *qcow2, struct qio *qio,
 	return !(subclus_mask & ~alloced_mask);
 }
 
+static bool qio_may_modify_image(struct qcow2 *qcow2, struct qio *qio)
+{
+	if (qcow2_file_is_writable(qcow2))
+		return true;
+
+	return !fake_merge_qio(qio);
+}
+
 static loff_t parse_l1(struct qcow2 *qcow2, struct qcow2_map *map,
 		       struct qio **qio, bool write)
 
@@ -1762,7 +1770,8 @@ static loff_t parse_l1(struct qcow2 *qcow2, struct qcow2_map *map,
 		goto out;
 	if (delay_if_dirty(qcow2, l1->md, l1->index_in_page, qio))
 		goto out;
-	if (write && map->clu_is_cow)
+	/* Don't refuse L1 parse for merge qios with readonly disks */
+	if (write && map->clu_is_cow && qio_may_modify_image(qcow2, *qio))
 		goto out; /* Avoid to return pos */
 
 	ret = pos;
@@ -2061,6 +2070,10 @@ static int parse_metadata(struct qcow2 *qcow2, struct qio **qio,
 	      qio_discard_unmaps_cluster(qcow2, *qio, map)))
 		return 0;
 
+	/* Don't need refcount table if we don't modify the image */
+	if (!qio_may_modify_image(qcow2, *qio))
+		return 0;
+
 	/* Now refcounters table/block */
 	ret = qcow2_handle_r1r2_maps(qcow2, pos, qio, &map->r1,
 			       &map->r2, map->compressed);
@@ -2732,7 +2745,8 @@ static int prepare_backward_merge(struct qcow2 *qcow2, struct qio **qio,
 	int ret;
 
 	if (!map->data_clu_alloced) {
-		WARN_ON_ONCE(map->clu_is_cow); /* Strange COW at L1 */
+		/* 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)) {
 			/* Nothing is to merge */
 			goto endio;
diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
index 0f006f1ae48cc..d9b8c38e093f5 100644
--- a/drivers/md/dm-qcow2.h
+++ b/drivers/md/dm-qcow2.h
@@ -460,6 +460,11 @@ static inline bool qcow2_wants_check(struct qcow2_target *tgt)
 	return !!(tgt->md_writeback_error|tgt->truncate_error);
 }
 
+static inline bool qcow2_file_is_writable(struct qcow2 *qcow2)
+{
+	return qcow2->file->f_mode & FMODE_WRITE;
+}
+
 static inline void remap_to_clu(struct qcow2 *qcow2, struct qio *qio, loff_t clu_pos)
 {
 	qio->bi_iter.bi_sector &= (to_sector(qcow2->clu_size) - 1);
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