[Xen-devel] Re: [PATCH] xend: Fix vbd/tapdisk device destruction

[Ryan O'Connor <rjo@xxxxxxxxx>]

Hi Kan,

On Wed, Jun 24, 2009 at 01:35:51PM +0900, Masaki Kanno wrote:
Content-Description: Mail message body
> Hi,
> 
> When I repeated creating and shutting off a guest domain, I detected 
> that processes of tapdisk2 were left.  Then I saw the following error 
> message in xend.log.
<snip> 
> [2009-06-22 14:36:21 3626] DEBUG (XendDomainInfo:2221) Removing vbd/769
> [2009-06-22 14:36:21 3626] DEBUG (XendDomainInfo:1137) 
> XendDomainInfo.destroyDevice: deviceClass = tap, device = vbd/769
> [2009-06-22 14:36:21 3626] ERROR (XendDomainInfo:2228) Device release failed: 
> vm2; tap; vbd/769
> Traceback (most recent call last):
>   File "usr/lib/python2.4/site-packages/xen/xend/XendDomainInfo.py", line 
> 2222, in _releaseDevices
>     self.destroyDevice(true_devclass, dev, False);
>   File "usr/lib/python2.4/site-packages/xen/xend/XendDomainInfo.py", line 
> 1154, in destroyDevice
>     path = self.getDeviceController(deviceClass).readBackend(dev, 
> \047params\047)
>   File "usr/lib/python2.4/site-packages/xen/xend/server/DevController.py", 
> line 467, in readBackend
>     raise VmError("Device %s not connected" % devid)
> VmError: Device 769 not connected
> 
> 
> This patch solves the problem.  And the patch solves the detected 
> problem by Ryan.

Your attached patch does indeed solve this problem, however it breaks
save/restore for blktap1. Taking a few steps back, I'm not sure
hard-coding the device path to device/vbd is the right way to fix this.
Xend seems to trust that devices in /vm/<uuid>/<deivceClass>/ actually
belong to <devClass>.

I think this points to a bigger problem. Bllktap2 devices need their
own device controller for their entire life-cycle, but xend currently
treats the blktap2 device as a vbd after it is created. As a result we
have blktap2 specific code in DevController, and we switch between using
the vbd and tap controller on the blktap2 device. Further, if we want to
get save/restore working for blktap2 we'll need to add more
blktap2-specific code to xend (to store and retrieve the original
uname).

For these reasons I don't think blktap2's current approach of creating a
vanilla vbd device will work in the long run. Instead I think we need a
dedicated blktap2 controller class. I have a patch that does just this,
which I will post later today after a bit more testing.

Thanks,
Ryan
-- 
 Ryan O'Connor <rjo@xxxxxxxxx>

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