On Di, 16.03.21 14:13, Christoph Hellwig (hch@xxxxxxxxxxxxx) 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@xxxxxxxxxxxxx> > > > > > > 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
