[Devel] [PATCH VZ10 v2 4/7] drivers/md/dm-qcow2: fix seek skip to the next L2 table

[Andrey Zhadchenko]

get_next_l2() adds the number of remaining clusters to the current
sector, so for a position which is not cluster aligned (e.g. the
llseek offset itself) the result overshoots the L2 table boundary by
the in-cluster offset.

Compute the boundary by simple round_up to the size covered by a
single L2 table.

Also allow skipping l2 if we have lower delta but range does not
exist there (e.g. past the delta end).

https://virtuozzo.atlassian.net/browse/VSTOR-139407
Feature: dm-qcow2: block device over QCOW2 files driver
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
---
v2:
 - changed l2 calculation to a simplier round_up()
 - expanded l2 skip for non-existing backing. Updated commit
message

 drivers/md/dm-qcow2-map.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/md/dm-qcow2-map.c b/drivers/md/dm-qcow2-map.c
index d6d15b04215c6..43188ceebc9af 100644
--- a/drivers/md/dm-qcow2-map.c
+++ b/drivers/md/dm-qcow2-map.c
@@ -4405,12 +4405,9 @@ static struct qio *free_seek_qio_ret_higher(struct qio *qio)
 static inline sector_t get_next_l2(struct qio *qio)
 {
 	struct qcow2 *qcow2 = qio->qcow2;
-	loff_t start, add;
+	u64 l2_cover = to_sector((u64)qcow2->clu_size * qcow2->l2_entries);
 
-	start = to_bytes(qio->bi_iter.bi_sector);
-	add = qcow2->l2_entries - (start / qcow2->clu_size) % qcow2->l2_entries;
-
-	return qio->bi_iter.bi_sector + (qcow2->clu_size / to_bytes(1)) * add;
+	return round_up(qio->bi_iter.bi_sector + 1, l2_cover);
 }
 
 static inline sector_t get_next_clu(struct qio *qio)
@@ -4431,9 +4428,10 @@ static inline void seek_qio_next_clu(struct qio *qio, struct qcow2_map *map)
 
 	/*
 	 * Whole L2 table is unmapped - skip to next l2 table,
-	 * but only if there is no backing image
+	 * but only if there is no backing data
 	 */
-	if (map && !(map->level & L2_LEVEL) && !qio->qcow2->lower)
+	if (map && !(map->level & L2_LEVEL) &&
+	    !maybe_mapped_in_lower_delta(qio->qcow2, qio))
 		bi_sector = get_next_l2(qio);
 	else
 		bi_sector = get_next_clu(qio);
-- 
2.43.5