linux-kernel - Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected

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Date:   Tue, 17 Dec 2019 07:23:12 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     SeongJae Park <sj38.park@...il.com>
Cc:     axboe@...nel.dk, konrad.wilk@...cle.com, roger.pau@...rix.com,
        linux-block@...r.kernel.org, xen-devel@...ts.xenproject.org,
        pdurrant@...zon.com, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a
 memory pressure is detected

On 16.12.19 20:48, SeongJae Park wrote:
> On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote:
> 
>> On 16.12.19 17:15, SeongJae Park wrote:
>>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@...zon.com> wrote:
>>>
>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@...zon.com> wrote:
>>>>
>>>>> From: SeongJae Park <sjpark@...zon.de>
>>>>>
>>> [...]
>>>>> --- a/drivers/block/xen-blkback/xenbus.c
>>>>> +++ b/drivers/block/xen-blkback/xenbus.c
>>>>> @@ -824,6 +824,24 @@ static void frontend_changed(struct xenbus_device *dev,
>>>>>    }
>>>>>    
>>>>>    
>>>>> +/* Once a memory pressure is detected, squeeze free page pools for a while. */
>>>>> +static unsigned int buffer_squeeze_duration_ms = 10;
>>>>> +module_param_named(buffer_squeeze_duration_ms,
>>>>> +		buffer_squeeze_duration_ms, int, 0644);
>>>>> +MODULE_PARM_DESC(buffer_squeeze_duration_ms,
>>>>> +"Duration in ms to squeeze pages buffer when a memory pressure is detected");
>>>>> +
>>>>> +/*
>>>>> + * Callback received when the memory pressure is detected.
>>>>> + */
>>>>> +static void reclaim_memory(struct xenbus_device *dev)
>>>>> +{
>>>>> +	struct backend_info *be = dev_get_drvdata(&dev->dev);
>>>>> +
>>>>> +	be->blkif->buffer_squeeze_end = jiffies +
>>>>> +		msecs_to_jiffies(buffer_squeeze_duration_ms);
>>>>
>>>> This callback might race with 'xen_blkbk_probe()'.  The race could result in
>>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links
>>>> 'be' to the 'dev'.  Please _don't merge_ this patch now!
>>>>
>>>> I will do more test and share results.  Meanwhile, if you have any opinion,
>>>> please let me know.
> 
> I reduced system memory and attached bunch of devices in short time so that
> memory pressure occurs while device attachments are ongoing.  Under this
> circumstance, I was able to see the race.
> 
>>>
>>> Not only '->blkif', but 'be' itself also coule be a NULL.  As similar
>>> concurrency issues could be in other drivers in their way, I suggest to change
>>> the reclaim callback ('->reclaim_memory') to be called for each driver instead
>>> of each device.  Then, each driver could be able to deal with its concurrency
>>> issues by itself.
>>
>> Hmm, I don't like that. This would need to be changed back in case we
>> add per-guest quota.
> 
> Extending this callback in that way would be still not too hard.  We could use
> the argument to the callback.  I would keep the argument of the callback to
> 'struct device *' as is, and will add a comment saying 'NULL' value of the
> argument means every devices.  As an example, xenbus would pass NULL-ending
> array of the device pointers that need to free its resources.
> 
> After seeing this race, I am now also thinking it could be better to delegate
> detailed control of each device to its driver, as some drivers have some
> complicated and unique relation with its devices.
> 
>>
>> Wouldn't a get_device() before calling the callback and a put_device()
>> afterwards avoid that problem?
> 
> I didn't used the reference count manipulation operations because other similar
> parts also didn't.  But, if there is no implicit reference count guarantee, it
> seems those operations are indeed necessary.
> 
> That said, as get/put operations only adjust the reference count, those will
> not make the callback to wait until the linking of the 'backend' and 'blkif' to
> the device (xen_blkbk_probe()) is finished.  Thus, the race could still happen.
> Or, am I missing something?

No, I think we need a xenbus lock per device which will need to be
taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the
callback.


Juergen