Re: [PATCH v2 16/16] xen-blkback: Inform userspace that device has been opened

From: Demi Marie Obenour <demi@invisiblethingslab.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "Jens Axboe" <axboe@kernel.dk>,
	"Alasdair Kergon" <agk@redhat.com>,
	"Mike Snitzer" <snitzer@kernel.org>,
	dm-devel@redhat.com,
	"Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 16/16] xen-blkback: Inform userspace that device has been opened
Date: Wed, 7 Jun 2023 12:29:26 -0400	[thread overview]
Message-ID: <ZICwaWidZxhaGp8v@itl-email> (raw)
In-Reply-To: <ZIBDgKKDhDj+//Q0@Air-de-Roger>

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On Wed, Jun 07, 2023 at 10:44:48AM +0200, Roger Pau Monné wrote:
> On Tue, Jun 06, 2023 at 01:31:25PM -0400, Demi Marie Obenour wrote:
> > On Tue, Jun 06, 2023 at 11:15:37AM +0200, Roger Pau Monné wrote:
> > > On Tue, May 30, 2023 at 04:31:16PM -0400, Demi Marie Obenour wrote:
> > > > Set "opened" to "0" before the hotplug script is called.  Once the
> > > > device node has been opened, set "opened" to "1".
> > > > 
> > > > "opened" is used exclusively by userspace.  It serves two purposes:
> > > > 
> > > > 1. It tells userspace that the diskseq Xenstore entry is supported.
> > > > 
> > > > 2. It tells userspace that it can wait for "opened" to be set to 1.
> > > >    Once "opened" is 1, blkback has a reference to the device, so
> > > >    userspace doesn't need to keep one.
> > > > 
> > > > Together, these changes allow userspace to use block devices with
> > > > delete-on-close behavior, such as loop devices with the autoclear flag
> > > > set or device-mapper devices with the deferred-remove flag set.
> > > 
> > > There was some work in the past to allow reloading blkback as a
> > > module, it's clear that using delete-on-close won't work if attempting
> > > to reload blkback.
> > 
> > Should blkback stop itself from being unloaded if delete-on-close is in
> > use?
> 
> Hm, maybe.  I guess that's the best we can do right now.

I’ll implement this.

> > > Isn't there some existing way to check whether a device is opened?
> > > (stat syscall maybe?).
> > 
> > Knowing that the device has been opened isn’t enough.  The block script
> > needs to be able to wait for blkback (and not something else) to open
> > the device.  Otherwise it will be confused if the device is opened by
> > e.g. udev.
> 
> Urg, no, the block script cannot wait indefinitely for blkback to open
> the device, as it has an execution timeout.  blkback is free to only
> open the device upon guest frontend connection, and that (when using
> libxl) requires the hotplug scripts execution to be finished so the
> guest can be started.

I’m a bit confused here.  My understanding is that blkdev_get_by_dev()
already opens the device, and that happens in the xenstore watch
handler.  I have tested this with delete-on-close device-mapper devices,
and it does work.

> > > I would like to avoid adding more xenstore blkback state if such
> > > information can be fetched from other methods.
> > 
> > I don’t think it can be, unless the information is passed via a
> > completely different method.  Maybe netlink(7) or ioctl(2)?  Arguably
> > this information should not be stored in Xenstore at all, as it exposes
> > backend implementation details to the frontend.
> 
> Could you maybe use sysfs for this information?

Probably?  This would involve adding a new file in sysfs.

> We have all sorts of crap in xenstore, but it would be best if we can
> see of placing stuff like this in another interface.

Fair.

> Thanks, Roger.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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