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:   Mon, 16 Dec 2019 17:23:44 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     SeongJae Park <sjpark@...zon.com>, axboe@...nel.dk,
        konrad.wilk@...cle.com, roger.pau@...rix.com
Cc:     pdurrant@...zon.com, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, xen-devel@...ts.xenproject.org,
        sj38.park@...il.com
Subject: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a
 memory pressure is detected

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.
> 
> 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.

Wouldn't a get_device() before calling the callback and a put_device()
afterwards avoid that problem?


Juergen