Re: [PATCH 3/3] xen-blkfront: dynamic configuration of per-vbd resources

[Bob Liu <bob.liu@oracle.com>]

On 07/22/2016 03:45 PM, Roger Pau Monné wrote:
> On Thu, Jul 21, 2016 at 06:08:05PM +0800, Bob Liu wrote:
>>
>> On 07/21/2016 04:57 PM, Roger Pau Monné wrote:
..[snip]..
>>>> +
>>>> +static ssize_t dynamic_reconfig_device(struct blkfront_info *info, ssize_t count)
>>>> +{
>>>> +	unsigned int i;
>>>> +	int err = -EBUSY;
>>>> +
>>>> +	/*
>>>> +	 * Make sure no migration in parallel, device lock is actually a
>>>> +	 * mutex.
>>>> +	 */
>>>> +	if (!device_trylock(&info->xbdev->dev)) {
>>>> +		pr_err("Fail to acquire dev:%s lock, may be in migration.\n",
>>>> +			dev_name(&info->xbdev->dev));
>>>> +		return err;
>>>> +	}
>>>> +
>>>> +	/*
>>>> +	 * Prevent new requests and guarantee no uncompleted reqs.
>>>> +	 */
>>>> +	blk_mq_freeze_queue(info->rq);
>>>> +	if (part_in_flight(&info->gd->part0))
>>>> +		goto out;
>>>> +
>>>> +	/*
>>>> +	 * Front 				Backend
>>>> +	 * Switch to XenbusStateClosed
>>>> +	 *					frontend_changed():
>>>> +	 *					 case XenbusStateClosed:
>>>> +	 *						xen_blkif_disconnect()
>>>> +	 *						Switch to XenbusStateClosed
>>>> +	 * blkfront_resume():
>>>> +	 *					frontend_changed():
>>>> +	 *						reconnect
>>>> +	 * Wait until XenbusStateConnected
>>>> +	 */
>>>> +	info->reconfiguring = true;
>>>> +	xenbus_switch_state(info->xbdev, XenbusStateClosed);
>>>> +
>>>> +	/* Poll every 100ms, 1 minute timeout. */
>>>> +	for (i = 0; i < 600; i++) {
>>>> +		/*
>>>> +		 * Wait backend enter XenbusStateClosed, blkback_changed()
>>>> +		 * will clear reconfiguring.
>>>> +		 */
>>>> +		if (!info->reconfiguring)
>>>> +			goto resume;
>>>> +		schedule_timeout_interruptible(msecs_to_jiffies(100));
>>>> +	}
>>>
>>> Instead of having this wait, could you just set info->reconfiguring = 1, set 
>>> the frontend state to XenbusStateClosed and mimic exactly what a resume from 
>>> suspension does? blkback_changed would have to set the frontend state to 
>>> InitWait when it detects that the backend has switched to Closed, and call 
>>> blkfront_resume.
>>
>>
>> I think that won't work.
>> In the real "resume" case, the power management system will trigger all ->resume() path.
>> But there is no place for dynamic configuration.
> 
> Hello,
> 
> I think it should be possible to set info->reconfiguring and wait for the 
> backend to switch to state Closed, at that point we should call blkif_resume 
> (from blkback_changed) and the backend will follow the reconection.
> 

Okay, I get your point. Yes, that's an option.

But this will make 'dynamic configuration' to be async, I'm worry about the end-user will get panic.
E.g
A end-user "echo <new value> > /sys/devices/vbd-xxx/max_indirect_segs",
but then the device will be Closed and disappeared, the user have to wait for a random time so that the device can resume.

-- 
Regards,
-Bob