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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jun 07, 2023 at 12:29:26PM -0400, Demi Marie Obenour wrote:
> 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.

Let's make this a separate patch.

> > > > 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.

Right, but on a very contended system there's no guarantee of when
blkback will pick up the update to "physical-device" and open the
device, so far the block script only writes the physical-device node
and exits.  With the proposed change the block script will also wait
for blkback to react to the physcal-device write, hence making VM
creation slower.

> > > > 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.

Let's see if that's a suitable approach, and we can avoid having to
add an extra node to xenstore.

Thanks, Roger.