Re: [Devel] [PATCH RHEL10 COMMIT] drivers/md/dm-qcow2: set L2_READS_ALL_ZEROES after some discards

[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 1e432a95dec2de142feca1838b1ebd4dfe3379ba
Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
Date:   Thu Aug 27 19:06:14 2026 +0300

    drivers/md/dm-qcow2: set L2_READS_ALL_ZEROES after some discards
    
    Discard does not guarantee zero data. Therefore, to safely erase
    data, users may write zeroes and then discard. Imagine we have
    a backing file. If we write zeroes and do the discard, next read
    will give the stale data from backing image.
    Probably this is rather an edge case, but to be sure let's just
    set L2 entry to L2_READS_ALL_ZEROES so the device looks more
    consistent to users.
    
    Feature: dm-qcow2: block device over QCOW2 files driver
    https://virtuozzo.atlassian.net/browse/VSTOR-139406
    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 | 24 +++++++++++++++++++++---
 drivers/md/dm-qcow2.h     |  1 +
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 90229cdb5b006..0a164c6f56c2d 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -1309,6 +1309,7 @@ static void revert_l_entries_update(struct qcow2 *qcow2, struct wb_desc *wbd)
 	struct page *pe_page;
 	bool skip_odd;
 	u64 pos, old;
+	bool cleared;
 	int i, ret;
 
 	skip_odd = qcow2->ext_l2 && wbd->lx_level == L2_LEVEL;
@@ -1317,6 +1318,8 @@ static void revert_l_entries_update(struct qcow2 *qcow2, struct wb_desc *wbd)
 	for_each_set_bit(i, wbd->changed_indexes, LX_INDEXES_PER_PAGE) {
 		pos = get_u64_from_be_page(wbd->md->page, i);
 
+		cleared = !(pos & ~(u64)L2_READS_ALL_ZEROES);
+
 		/* Here we restore prealloced and compressed clu mappings */
 		pe_page = wbd->pe_page;
 		if (pe_page) { /* Only L2 has this. */
@@ -1330,7 +1333,7 @@ static void revert_l_entries_update(struct qcow2 *qcow2, struct wb_desc *wbd)
 		set_u64_to_be_page(wbd->md->page, i, 0);
 		if (skip_odd && (i & 1))
 			continue; /* pos contains ext_l2 part of L2 entry */
-		if (!pos)
+		if (cleared)
 			continue; /* no cluster was allocated */
 
 		spin_unlock(&qcow2->md_pages_lock);
@@ -3368,14 +3371,18 @@ static bool qio_discard_updates_metadata(struct qcow2 *qcow2, struct qio *qio,
  *
  * Discard covering the whole cluster replaces the entry and unuses
  * the discarded (or COW source) cluster.
+ * If the backing is present, set 'reads as zeroes' to avoid exposing
+ * stale data.
  */
 static int prepare_cluster_discard(struct qcow2 *qcow2, struct qio **qio,
 				   struct qcow2_map *map)
 {
+	bool zeroes = maybe_mapped_in_lower_delta(qcow2, *qio);
 	u32 index_in_page = map->l2.index_in_page;
 	struct md_page *md = map->l2.md;
 	loff_t unuse_pos, unuse_end;
 	struct qio_ext *ext;
+	u64 new_ext_l2 = 0;
 	int ret;
 
 	WARN_ON_ONCE(!(map->level & L2_LEVEL) || !map->data_clu_alloced);
@@ -3399,6 +3406,8 @@ static int prepare_cluster_discard(struct qcow2 *qcow2, struct qio **qio,
 		unuse_pos = map->data_clu_pos;
 		unuse_end = map->data_clu_pos + qcow2->clu_size;
 	}
+	if (zeroes && qcow2->ext_l2)
+		new_ext_l2 = (u64)U32_MAX << 32;
 
 	ret = prepare_l_entry_replace(qcow2, map, *qio, md, index_in_page,
 				      unuse_pos, unuse_end, L2_LEVEL);
@@ -3408,6 +3417,10 @@ static int prepare_cluster_discard(struct qcow2 *qcow2, struct qio **qio,
 	ext = (*qio)->ext;
 	ext->lx_md = md;
 
+	ext->new_ext_l2 = new_ext_l2;
+	if (zeroes && !qcow2->ext_l2)
+		ext->set_all_zeroes = true;
+
 	(*qio)->flags |= QIO_IS_DISCARD_FL;
 	return 1;
 }
@@ -4053,6 +4066,7 @@ static void process_indexes_write(struct qcow2 *qcow2,
 	struct qio *qio;
 	bool discard;
 	u32 arg_mask;
+	u64 entry;
 	int ret;
 
 	while (1) {
@@ -4076,11 +4090,15 @@ static void process_indexes_write(struct qcow2 *qcow2,
 			goto set_ext_l2;
 		}
 
+		entry = ext->allocated_clu_pos;
+		if (unlikely(ext->set_all_zeroes))
+			entry = L2_READS_ALL_ZEROES;
+
 		/* XXX: check prealloced_pos ==> revert */
 		ret = prepare_l_entry_update(qcow2, qio, lx_md,
 					     ext->lx_index_in_page,
-					     &ext->allocated_clu_pos,
-					     arg_mask, ext->lx_level);
+					     &entry, arg_mask,
+					     ext->lx_level);
 		if (ret < 0) {
 			qio->bi_status = errno_to_blk_status(ret);
 			qio_endio(qio);
diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
index 8a24e04130e4d..0f006f1ae48cc 100644
--- a/drivers/md/dm-qcow2.h
+++ b/drivers/md/dm-qcow2.h
@@ -312,6 +312,7 @@ struct qio_ext {
 	u64 new_ext_l2;
 	u32 cow_mask;
 	bool only_set_ext_l2:1;
+	bool set_all_zeroes:1;
 
 	u8 lx_level;
 
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