Re: [PATCH 0/2] loop, nbd: drop partitions left behind by the GD_NEED_PART_SCAN removal

[Christian Brauner]

On Thu, Aug 06, 2026 at 12:36:56PM +0200, Daan De Meyer via B4 Relay wrote:
> Commit 267ec4d7223a ("loop: fix partition scan race between udev and
> loop_reread_partitions()") stopped disk_force_media_change() from setting
> GD_NEED_PART_SCAN. The caller audit in that commit only considered the bit
> as a request to rescan partitions, but it did more than that:
> bdev_disk_changed() drops every entry in disk->part_tbl before it consults
> disk_has_partscan(), and only the re-adding half is gated on partition
> scanning being enabled. The lazy scan on the next open was therefore also
> the only thing removing partitions from a device that has partitions but
> no partition scanning.
> 
> Loop devices without LO_FLAGS_PARTSCAN are exactly such devices, and they
> are not unusual. bdev_add_partition() only rejects GENHD_FL_NO_PART disks,
> so BLKPG_ADD_PARTITION works while GD_SUPPRESS_PART_SCAN is set, and
> parted, libfdisk and systemd all fall back to BLKPG when BLKRRPART fails
> with -EINVAL, which is what a loop device without LO_FLAGS_PARTSCAN
> returns.
> 
> Commit c4f4c0fc551c ("loop: remove manually added partitions on detach")
> fixed the detach path after this was reported as loop devices picking up
> partition devices from a previously built image:
> 
>   https://bugs.debian.org/1141434
> 
> These two patches fix the two remaining places that relied on the same
> lazy cleanup, LOOP_CHANGE_FD and NBD_CLEAR_SOCK.
> 
> Both were tested with the blktests loop and nbd groups on 7.2-rc5. New
> cases covering the two paths fail without these patches and pass with
> them; they will be submitted to blktests separately.
> 
> Signed-off-by: Daan De Meyer <daan@xxxxxxxxxxxx>

Reviewed-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>