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

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

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