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

From: Lennart Poettering <mzxreary@0pointer.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>,
	Matteo Croce <mcroce@linux.microsoft.com>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, 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
Date: Tue, 20 Apr 2021 22:12:15 +0200	[thread overview]
Message-ID: <YH81n34d2G3C4Re+@gardel-login> (raw)
In-Reply-To: <20210316141326.GA37773@infradead.org>

On Di, 16.03.21 14:13, Christoph Hellwig (hch@infradead.org) wrote:

> 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.

I'd argue this makes sense in one way only, i.e. that whenever the
media_change event is seen the seqnum is implicitly bumped.

I am pretty sure though that loopback devices shouldn't synthesize
media_change events themselves though. There's quite a difference I
would argue between a real media change event caused by external
effect (i.e. humans/hw buttons/sensors) to loop device reuse, which is
exclusively triggered by internal events (i.e. local code). Moreover I
think the loopback subsystem should manage the seqnum on its own,
since it ideally would return the assigned seqnum immediately from the
attachment ioctl, i.e. it shouldn't just be a side-effect of
attachment, but a part of it, if you follow what I mean.

Does that make sense?

Matteo, would it make sense to extend your patch set to bump the
seqnum implicitly on media_change for devices that implement that?

Lennart