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Message-ID: <ZICwaWidZxhaGp8v@itl-email>
Date: Wed, 7 Jun 2023 12:29:26 -0400
From: Demi Marie Obenour <demi@...isiblethingslab.com>
To: Roger Pau Monné <roger.pau@...rix.com>
Cc: Jens Axboe <axboe@...nel.dk>, Alasdair Kergon <agk@...hat.com>,
Mike Snitzer <snitzer@...nel.org>, dm-devel@...hat.com,
Marek Marczykowski-Górecki
<marmarek@...isiblethingslab.com>, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v2 16/16] xen-blkback: Inform userspace that device has
been opened
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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