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Date:   Tue, 16 Mar 2021 14:13:26 +0000
From:   Christoph Hellwig <hch@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Matteo Croce <mcroce@linux.microsoft.com>,
        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 Gonz?lez <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: <20210316141326.GA37773@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.

And we have some decent infrastructure related to media changes,
grep for disk_events.  I think this needs to plug into that
infrastructure instead of duplicating it.