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

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Message-ID: <ZIGot1mn+lChK4q8@Air-de-Roger>
Date:   Thu, 8 Jun 2023 12:08:55 +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, 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)

Thanks, Roger.