Re: [PATCH] block: don't set GD_NEED_PART_SCAN if scan partition failed

[Ming Lei]

On Wed, Mar 22, 2023 at 10:47:07AM +0100, Jan Kara wrote:
> On Wed 22-03-23 15:58:35, Ming Lei wrote:
> > On Wed, Mar 22, 2023 at 11:59:26AM +0800, Yu Kuai wrote:
> > > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> > > 
> > > Currently if disk_scan_partitions() failed, GD_NEED_PART_SCAN will still
> > > set, and partition scan will be proceed again when blkdev_get_by_dev()
> > > is called. However, this will cause a problem that re-assemble partitioned
> > > raid device will creat partition for underlying disk.
> > > 
> > > Test procedure:
> > > 
> > > mdadm -CR /dev/md0 -l 1 -n 2 /dev/sda /dev/sdb -e 1.0
> > > sgdisk -n 0:0:+100MiB /dev/md0
> > > blockdev --rereadpt /dev/sda
> > > blockdev --rereadpt /dev/sdb
> > > mdadm -S /dev/md0
> > > mdadm -A /dev/md0 /dev/sda /dev/sdb
> > > 
> > > Test result: underlying disk partition and raid partition can be
> > > observed at the same time
> > > 
> > > Note that this can still happen in come corner cases that
> > > GD_NEED_PART_SCAN can be set for underlying disk while re-assemble raid
> > > device.
> > > 
> > > Fixes: e5cfefa97bcc ("block: fix scan partition for exclusively open device again")
> > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > 
> > The issue still can't be avoided completely, such as, after rebooting,
> > /dev/sda1 & /dev/md0p1 can be observed at the same time. And this one
> > should be underlying partitions scanned before re-assembling raid, I
> > guess it may not be easy to avoid.
> 
> So this was always happening (before my patches, after my patches, and now
> after Yu's patches) and kernel does not have enough information to know
> that sda will become part of md0 device in the future. But mdadm actually
> deals with this as far as I remember and deletes partitions for all devices
> it is assembling the array from (and quick tracing experiment I did
> supports this).

I am testing on Fedora 37, so mdadm v4.2 doesn't delete underlying
partitions before re-assemble.

Also given mdadm or related userspace has to change for avoiding
to scan underlying partitions, just wondering why not let userspace
to tell kernel not do it explicitly?

Thanks,
Ming