Re: [PATCH RFC] btrfs: pick device with lowest devt for show_devname

[Anand Jain <anand.jain@oracle.com>]

On 11/3/23 04:26, David Sterba wrote:
> On Thu, Nov 02, 2023 at 07:10:48PM +0800, Anand Jain wrote:
>> In a non-single-device Btrfs filesystem, if Btrfs is already mounted and
>> if you run the command 'mount -a,' it will fail and the command
>> 'umount <device>' also fails. As below:
>>
>> ----------------
>> $ cat /etc/fstab | grep btrfs
>> UUID=12345678-1234-1234-1234-123456789abc /btrfs btrfs defaults,nofail 0 0
>>
>> $ mkfs.btrfs -qf --uuid 12345678-1234-1234-1234-123456789abc /dev/sda2 /dev/sda1
>> $ mount --verbose -a
>> /                        : ignored
>> /btrfs                   : successfully mounted
>>
>> $ ls -l /dev/disk/by-uuid | grep 12345678-1234-1234-1234-123456789abc
>> lrwxrwxrwx 1 root root 10 Nov  2 17:43 12345678-1234-1234-1234-123456789abc -> ../../sda1
>>
>> $ cat /proc/self/mounts | grep btrfs
>> /dev/sda2 /btrfs btrfs rw,relatime,space_cache=v2,subvolid=5,subvol=/ 0 0
>>
>> $ findmnt --df /btrfs
>> SOURCE    FSTYPE SIZE  USED AVAIL USE% TARGET
>> /dev/sda2 btrfs    2G  5.8M  1.8G   0% /btrfs
>>
>> $ mount --verbose -a
>> /                        : ignored
>> mount: /btrfs: /dev/sda1 already mounted or mount point busy.
>> $echo $?
>> 32
>>
>> $ umount /dev/sda1
>> umount: /dev/sda1: not mounted.
>> $ echo $?
>> 32
>> ----------------
>>
>> I assume (RFC) this is because '/dev/disk/by-uuid,' '/proc/self/mounts,'
>> and 'findmnt' do not all reference the same device, resulting in the
>> 'mount -a' and 'umount' failures. However, an empirically found solution
>> is to align them using a rule, such as the disk with the lowest 'devt,'
>> for a multi-device Btrfs filesystem.
>>
>> I'm not yet sure (RFC) how to create a udev rule to point to the disk with
>> the lowest 'devt,' as this kernel patch does, and I believe it is
>> possible.
>>
>> And this would ensure that '/proc/self/mounts,' 'findmnt,' and
>> '/dev/disk/by-uuid' all reference the same device.
>>
>> After applying this patch, the above test passes. Unfortunately,
>> /dev/disk/by-uuid also points to the lowest 'devt' by chance, even though
>> no rule has been set as of now. As shown below.
> 
> Does this mean the devid of the device shown in /proc/self/mount won't
> be the lowest? Here the devid is the logical device number, while devt
> is some internal identifier or at least not something I'd consider a
> good identifier from user perspective.
> 
> The lowest devid has been there for a long time so I'd consider this as
> behaviour change which can potentially break things.

It's not the lowest devid, but rather the latest_bdev since commit
6605fd2f394b ('btrfs: use latest_dev in btrfs_show_devname').

We need a rule for choosing a device in a multi-device filesystem that
works both inside and outside the kernel. The major-minor (devt) is the
only consistent option.

OR Any other ideas?

Thanks, Anand