[Devel] [PATCH RHEL10 COMMIT] dm-qcow2: keep seek parse window within limit

[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 at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.9.vz10
------>
commit 1c541ce04d8b6336ee269b95e52ad8bf02cc954d
Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
Date:   Wed Aug 19 03:07:40 2026 +0300

    dm-qcow2: keep seek parse window within limit
    
    seek_qio_next_clu() always sets the window size to the whole
    cluster, so the window may cross the scan limit. The top level qio
    then parses metadata past the virtual disk end if the disk size is
    not cluster aligned, and SEEK_HOLE may report "sector + size" past
    the limit.
    This is especially painful for lower delta seeks, since at top
    level this is luckily alleviated by blkdev_llseek_wrapper().
    
    Add seek_qio_set_sector(), which sets bi_sector and also clamps
    bi_size if needed.
    
    https://virtuozzo.atlassian.net/browse/VSTOR-139407
    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>
---
 drivers/md/dm-qcow2-map.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 542c7949321e5..342e4af688509 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -4352,6 +4352,16 @@ struct qio_data_llseek_hole {
 
 #define SEEK_QIO_DATA(qio) ((struct qio_data_llseek_hole *)qio->data)
 
+static void seek_qio_set_sector(struct qio *qio, sector_t bi_sector)
+{
+	loff_t pos = to_bytes(bi_sector);
+	loff_t end = round_up(pos + 1, qio->qcow2->clu_size);
+
+	qio->bi_iter.bi_sector = bi_sector;
+	end = min_t(loff_t, end, SEEK_QIO_DATA(qio)->lim);
+	qio->bi_iter.bi_size = end > pos ? end - pos : 0;
+}
+
 static struct qio *alloc_seek_qio(struct qcow2 *qcow2, struct qio *parent, loff_t new_lim)
 {
 	struct qio_data_llseek_hole *data;
@@ -4376,12 +4386,7 @@ static struct qio *alloc_seek_qio(struct qcow2 *qcow2, struct qio *parent, loff_
 
 	if (parent) {
 		data->higher = parent;
-		qio->bi_iter.bi_sector = parent->bi_iter.bi_sector;
-
-		if (to_bytes(qio->bi_iter.bi_sector) + parent->bi_iter.bi_size > new_lim)
-			qio->bi_iter.bi_size = new_lim - to_bytes(qio->bi_iter.bi_sector);
-		else
-			qio->bi_iter.bi_size = parent->bi_iter.bi_size;
+		seek_qio_set_sector(qio, parent->bi_iter.bi_sector);
 	}
 
 	return qio;
@@ -4422,16 +4427,18 @@ static inline sector_t get_next_clu(struct qio *qio)
 
 static inline void seek_qio_next_clu(struct qio *qio, struct qcow2_map *map)
 {
+	sector_t bi_sector;
+
 	/*
 	 * Whole L2 table is unmapped - skip to next l2 table,
 	 * but only if there is no backing image
 	 */
 	if (map && !(map->level & L2_LEVEL) && !qio->qcow2->lower)
-		qio->bi_iter.bi_sector = get_next_l2(qio);
+		bi_sector = get_next_l2(qio);
 	else
-		qio->bi_iter.bi_sector = get_next_clu(qio);
+		bi_sector = get_next_clu(qio);
 
-	qio->bi_iter.bi_size = qio->qcow2->clu_size;
+	seek_qio_set_sector(qio, bi_sector);
 }
 
 static struct qio *advance_and_spawn_lower_seek_qio(struct qio *old_qio, u32 size)
@@ -4567,9 +4574,7 @@ loff_t qcow2_llseek_hole(struct dm_target *ti, loff_t offset, int whence)
 	if (!qio)
 		return -ENOMEM;
 
-	qio->bi_iter.bi_sector = to_sector(offset);
-	qio->bi_iter.bi_size = qcow2->clu_size -
-			       to_bytes(qio->bi_iter.bi_sector) % qcow2->clu_size;
+	seek_qio_set_sector(qio, to_sector(offset));
 
 	ret = qcow2_llseek_hole_qio(qio, whence, &result);
 	/* In case of error remap ENXIO as it have special meaning for llseek */