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

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Date:   Wed, 7 Jun 2023 10:44:48 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Demi Marie Obenour <demi@...isiblethingslab.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 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.

> > 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 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?

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.

Thanks, Roger.