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Date:   Mon, 15 Mar 2021 21:04:52 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Matteo Croce <mcroce@linux.microsoft.com>
Cc:     linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Lennart Poettering <lennart@poettering.net>,
        Luca Boccassi <bluca@debian.org>, Jens Axboe <axboe@kernel.dk>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        Tejun Heo <tj@kernel.org>,
        Javier =?iso-8859-1?Q?Gonz=E1lez?= <javier@javigon.com>,
        Niklas Cassel <niklas.cassel@wdc.com>,
        Johannes Thumshirn <johannes.thumshirn@wdc.com>,
        Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH -next 1/5] block: add disk sequence number
Message-ID: <20210315210452.GC2577561@casper.infradead.org>
References: <20210315200242.67355-1-mcroce@linux.microsoft.com>
 <20210315200242.67355-2-mcroce@linux.microsoft.com>
 <20210315201824.GB2577561@casper.infradead.org>
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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:
> > From: Matteo Croce <mcroce@microsoft.com>
> > 
> > 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.