On Mon, Mar 15, 2021 at 08:18:24PM +0000, Matthew Wilcox wrote:

On Mon, Mar 15, 2021 at 09:02:38PM +0100, Matteo Croce wrote:

quoted

From: Matteo Croce <redacted>

Add a sequence number to the disk devices. This number is put in the
uevent so userspace can correlate events when a driver reuses a device,
like the loop one.

Should this be documented as monotonically increasing?  I think this
is actually a media identifier.  Consider (if you will) a floppy disc.
Back when such things were common, it was possible with personal computers
of the era to have multiple floppy discs "in play" and be prompted to
insert them as needed.  So shouldn't it be possible to support something
similar here -- you're really removing the media from the loop device.
With a monotonically increasing number, you're always destroying the
media when you remove it, but in principle, it should be possible to
reinsert the same media and have the same media identifier number.

So ... a lot of devices have UUIDs or similar.  eg:

$ cat /sys/block/nvme0n1/uuid 
e8238fa6-bf53-0001-001b-448b49cec94f

https://linux.die.net/man/8/scsi_id (for scsi)

how about making this way more generic; create an xattr on a file to
store the uuid (if one doesn't already exist) whenever it's used as the
base for a loop device.  then sysfs (or whatever) can report the contents
of that xattr as the unique id.

That can be mostly in userspace -- losetup can create it, and read it.
It can be passed in as the first two current-reserved __u64 entries in
loop_config.  The only kernel change should be creating the sysfs
entry /sys/block/loopN/uuid from those two array entries.