[Devel] [PATCH RHEL10 COMMIT] dm-qcow2: scan lower delta in one pass over empty L1 entry

[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 7af917e3954e3085a2c0aed3d3d6cf9db7797cf8
Author: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
Date:   Wed Aug 19 03:07:43 2026 +0300

    dm-qcow2: scan lower delta in one pass over empty L1 entry
    
    When an L1 entry is absent and there is a backing image, the seek
    code descends into the lower delta one cluster at a time.
    
    Change advance_and_spawn_lower_seek_qio() to take limit instead of
    a size. Pass bigger range if L1 entry is empty.
    
    On a 16G image (128K clusters, extended L2) with no L1 entries over a
    backing image with data at 15G, warm-cache SEEK_DATA improves from
    40.2 ms to 0.01 ms.
    
    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, 15 insertions(+), 14 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index 43188ceebc9af..879d2f1827006 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -4439,26 +4439,19 @@ static inline void seek_qio_next_clu(struct qio *qio, struct qcow2_map *map)
 	seek_qio_set_sector(qio, bi_sector);
 }
 
-static struct qio *advance_and_spawn_lower_seek_qio(struct qio *old_qio, u32 size)
+static struct qio *advance_and_spawn_lower_seek_qio(struct qio *old_qio, loff_t end)
 {
+	struct qcow2 *lower = old_qio->qcow2->lower;
 	struct qio *new_qio;
-	loff_t start;
 
-	start = to_bytes(old_qio->bi_iter.bi_sector);
+	if (end > lower->hdr.size)
+		end = lower->hdr.size;
 
-	if (start + size > old_qio->qcow2->lower->hdr.size)
-		size = old_qio->qcow2->lower->hdr.size - start;
-
-	new_qio = alloc_seek_qio(old_qio->qcow2->lower, old_qio, start + size);
+	new_qio = alloc_seek_qio(lower, old_qio, end);
 	if (!new_qio)
 		return NULL;
 
-	if (old_qio->bi_iter.bi_size == size) {
-		seek_qio_next_clu(old_qio, NULL);
-	} else {
-		old_qio->bi_iter.bi_sector += to_sector(size);
-		old_qio->bi_iter.bi_size -= size;
-	}
+	seek_qio_set_sector(old_qio, to_sector(end));
 
 	return new_qio;
 }
@@ -4506,8 +4499,16 @@ static int qcow2_llseek_hole_qio(struct qio *qio, int whence, loff_t *result)
 			 */
 			if (to_bytes(qio->bi_iter.bi_sector) < qio->qcow2->lower->hdr.size) {
 				struct qio *new_qio;
+				loff_t end;
+
+				if (!(map.level & L2_LEVEL))
+					end = min_t(loff_t,
+						    to_bytes(get_next_l2(qio)),
+						    SEEK_QIO_DATA(qio)->lim);
+				else
+					end = to_bytes(qio->bi_iter.bi_sector) + size;
 
-				new_qio = advance_and_spawn_lower_seek_qio(qio, size);
+				new_qio = advance_and_spawn_lower_seek_qio(qio, end);
 				if (!new_qio) {
 					ret = -ENOMEM;
 					break;