btrfs: zoned: set pseudo max append zone limit in zone emulation mode

The commit 7d7672b ("btrfs: convert count_max_extents() to use
fs_info->max_extent_size") introduced a division by
fs_info->max_extent_size. This max_extent_size is initialized with max
zone append limit size of the device btrfs runs on. However, in zone
emulation mode, the device is not zoned then its zone append limit is
zero. This resulted in zero value of fs_info->max_extent_size and caused
zero division error.

Fix the error by setting non-zero pseudo value to max append zone limit
in zone emulation mode. Set the pseudo value based on max_segments as
suggested in the commit c2ae7b7 ("btrfs: zoned: revive
max_zone_append_bytes").

Fixes: 7d7672b ("btrfs: convert count_max_extents() to use fs_info->max_extent_size")
CC: stable@vger.kernel.org # 5.12+
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/zoned.c

@@ -421,10 +421,19 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device, bool populate_cache)
 	 * since btrfs adds the pages one by one to a bio, and btrfs cannot
 	 * increase the metadata reservation even if it increases the number of
 	 * extents, it is safe to stick with the limit.
+	 *
+	 * With the zoned emulation, we can have non-zoned device on the zoned
+	 * mode. In this case, we don't have a valid max zone append size. So,
+	 * use max_segments * PAGE_SIZE as the pseudo max_zone_append_size.
 	 */
-	zone_info->max_zone_append_size =
-		min_t(u64, (u64)bdev_max_zone_append_sectors(bdev) << SECTOR_SHIFT,
-		      (u64)bdev_max_segments(bdev) << PAGE_SHIFT);
+	if (bdev_is_zoned(bdev)) {
+		zone_info->max_zone_append_size = min_t(u64,
+			(u64)bdev_max_zone_append_sectors(bdev) << SECTOR_SHIFT,
+			(u64)bdev_max_segments(bdev) << PAGE_SHIFT);
+	} else {
+		zone_info->max_zone_append_size =
+			(u64)bdev_max_segments(bdev) << PAGE_SHIFT;
+	}
 	if (!IS_ALIGNED(nr_sectors, zone_sectors))
 		zone_info->nr_zones++;