[PATCH 2/2] block: fix the length of a smaller last zone in blkdev_get_zone_info()

[Shashank Mohan Jain]

blkdev_get_zone_info() sets the length of every zone to the zone size
of the device. The last zone of a zoned block device may be smaller
than the others: blk_revalidate_zone_cb() accepts that and remembers
the capacity of that zone in disk->last_zone_capacity. For such a zone
the length reported from the cached information is too large, and
start + len is beyond the end of the device. A conventional last zone
is also given the capacity of a full zone.

Example with a zoned null_blk device of 10 MiB (20480 sectors) with
4 MiB zones, last zone:

  BLKREPORTZONE:       start 16384 len 4096 capacity 4096
  BLKREPORTZONEV2 with
  BLK_ZONE_REP_CACHED: start 16384 len 8192 capacity 4096

Limit the length to what is left of the device. disk_zone_is_last()
still recognises the last zone, as start + len is then equal to the
capacity of the device. The capacity of a conventional zone is equal
to its length, which blk_revalidate_conv_zone() checks, so use the
length there.

Tested with zoned null_blk devices in qemu (all zones sequential, and
two conventional zones followed by sequential ones, both with a smaller
last zone): the start, length and capacity of all zones in the cached
report are the same as in the report from the device with this change.
null_blk cannot create a device with a conventional last zone, so that
case is not tested.

Fixes: f2284eec5053 ("block: introduce blkdev_get_zone_info()")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@xxxxxxxxx>
---
Prepared with Claude Code (Anthropic), model Claude Opus 5.5
(claude-opus-5-5).

 block/blk-zoned.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-zoned.c b/block/blk-zoned.c
index 9555eae91a..ca0950be3e 100644
--- a/block/blk-zoned.c
+++ b/block/blk-zoned.c
@@ -971,7 +971,7 @@ int blkdev_get_zone_info(struct block_device *bdev, sector_t sector,
 	rcu_read_unlock();
 
 	zone->start = sector;
-	zone->len = zone_sectors;
+	zone->len = min(zone_sectors, get_capacity(disk) - sector);
 
 	/*
 	 * If this is a conventional zone, we do not have a zone write plug and
@@ -979,7 +979,7 @@ int blkdev_get_zone_info(struct block_device *bdev, sector_t sector,
 	 */
 	if (zone->cond == BLK_ZONE_COND_NOT_WP) {
 		zone->type = BLK_ZONE_TYPE_CONVENTIONAL;
-		zone->capacity = zone_sectors;
+		zone->capacity = zone->len;
 		zone->wp = ULLONG_MAX;
 		return 0;
 	}
-- 
2.43.0