[Devel] [PATCH VZ10 v2 09/10] drivers/md/dm-qcow2: allow shared L1 entries during merge from RO image

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

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>
---
 drivers/md/dm-qcow2-cmd.c |  2 --
 drivers/md/dm-qcow2-map.c | 18 ++++++++++++++++--
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-qcow2-cmd.c b/drivers/md/dm-qcow2-cmd.c
index f2c8f51e17af6..fa3762c58372b 100644
--- a/drivers/md/dm-qcow2-cmd.c
+++ b/drivers/md/dm-qcow2-cmd.c
@@ -266,8 +266,6 @@ static int qcow2_merge_backward_start(struct qcow2_target *tgt, int efd, u32 dep
 		return -ENOENT;
 	if (!qcow2_file_is_writable(lower))
 		return -EACCES;
-	if (!qcow2_file_is_writable(qcow2) && qcow2->hdr.nb_snapshots)
-		return -EACCES;
 	if (qcow2->clu_size != lower->clu_size)
 		return -EOPNOTSUPP;
 	if (lower->hdr.size < qcow2->hdr.size)
diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index ebe68e8d0d3d8..baa6778f1686c 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);
@@ -2749,7 +2762,8 @@ static int prepare_backward_merge(struct qcow2 *qcow2, struct qio **qio,
 	}
 
 	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;
-- 
2.43.5