linux-kernel - Re: [PATCH -next 1/5] block: add disk sequence number

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Message-ID: <20210315201824.GB2577561@casper.infradead.org>
Date:   Mon, 15 Mar 2021 20:18:24 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Lennart Poettering <lennart@...ttering.net>,
        Luca Boccassi <bluca@...ian.org>, Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Damien Le Moal <damien.lemoal@....com>,
        Tejun Heo <tj@...nel.org>,
        Javier González <javier@...igon.com>,
        Niklas Cassel <niklas.cassel@....com>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH -next 1/5] block: add disk sequence number

On Mon, Mar 15, 2021 at 09:02:38PM +0100, Matteo Croce wrote:
> From: Matteo Croce <mcroce@...rosoft.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.