Re: [PATCH 2/2] nbd: drop stale partitions on NBD_CLEAR_SOCK

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 06, 2026 at 12:36:58PM +0200, Daan De Meyer via B4 Relay wrote:
> From: Daan De Meyer <daan@amutable.com>
> 
> Commit 267ec4d7223a ("loop: fix partition scan race between udev and
> loop_reread_partitions()") stopped disk_force_media_change() from
> setting GD_NEED_PART_SCAN. Besides requesting a rescan, that bit was what
> removed stale partitions on the next open, as bdev_disk_changed() drops
> all partitions before it consults disk_has_partscan().
> 
> nbd_clear_sock_ioctl() relied on that. It zeroes the capacity through
> nbd_bdev_reset(), but nothing removes the partitions of the disconnected
> device anymore. With the default max_part=16 they linger until the next
> connect sets GD_NEED_PART_SCAN again. With max_part=0 nothing ever sets
> it, so they are never removed at all, even though nbd does not set
> GENHD_FL_NO_PART and partitions can therefore still be added with BLKPG.
> 
> Set GD_NEED_PART_SCAN in nbd_clear_sock_ioctl() so the partitions are
> dropped on the next open. Calling bdev_disk_changed() directly is not an
> option as it needs open_mutex, which nbd_open() acquires under
> config_lock.
> 
> Fixes: 267ec4d7223a ("loop: fix partition scan race between udev and loop_reread_partitions()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daan De Meyer <daan@amutable.com>
> ---
>  drivers/block/nbd.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 8f10762e90ef..cdafe9d39369 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -1612,6 +1612,13 @@ static void nbd_clear_sock_ioctl(struct nbd_device *nbd)
>  	nbd_clear_sock(nbd);
>  	disk_force_media_change(nbd->disk);
>  	nbd_bdev_reset(nbd);
> +	/*
> +	 * Drop the partitions of the disconnected device on the next open. They
> +	 * can exist even with max_part zero as they may have been added manually

Overly long line here.