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

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Message-ID: <ZIHyl3skhh/Yy5fe@itl-email>
Date:   Thu, 8 Jun 2023 11:24:06 -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 Thu, Jun 08, 2023 at 12:08:55PM +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.
> 
> Now that I think a bit more about this, how are you planning to handle
> reboot with such devices?  It's fine for loop (because those get
> instantiated by the block script), but likely not with other block
> devices, as on reboot the toolstack will find the block device is
> gone.
> 
> I guess the delete-on-close is only intended to be used for loop
> devices? (or in general block devices that are instantiated by the
> block script itself)

You understand correctly.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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