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

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Date:	Fri, 22 Jul 2016 17:43:32 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
CC:	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	konrad.wilk@...cle.com, jgross@...e.com
Subject: Re: [PATCH 3/3] xen-blkfront: dynamic configuration of per-vbd resources


On 07/22/2016 05:34 PM, Roger Pau Monné wrote:
> On Fri, Jul 22, 2016 at 04:17:48PM +0800, Bob Liu wrote:
>>
>> 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.
> 
> That should not happen, AFAICT on migration the device never dissapears. 

Oh, yes.

> alloc_disk and friends should not be called on resume from migration (see 
> the switch in blkfront_connect, you should take the BLKIF_STATE_SUSPENDED 
> path for the reconfiguration).
> 

What about if the end-user starts I/O immediately after writing new value to /sys?
But the resume is still in progress.

-- 
Regards,
-Bob