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

[SeongJae Park <sj38.park@xxxxxxxxx>]

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@xxxxxxxxxx> wrote:
> > 
> >> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@xxxxxxxxxx> wrote:
> >>
> >>> From: SeongJae Park <sjpark@xxxxxxxxx>
> >>>
> > [...]
> >>> --- 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?

I also modified the code to do 'get_device()' and 'put_device()' as you
suggested and did test, but the race was still reproducible.


Thanks,
SeongJae Park

> 
> 
> Juergen

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