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SubjectRe: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected
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On Tue, 17 Dec 2019 09:30:32 +0100 SeongJae Park <sjpark@amazon.com> wrote:

> On Tue, 17 Dec 2019 09:16:47 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
>
> > On 17.12.19 08:59, SeongJae Park wrote:
> > > On Tue, 17 Dec 2019 07:23:12 +0100 "Jürgen Groß" <jgross@suse.com> wrote:
> > >
> > >> 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@amazon.com> wrote:
> > >>>>>
> > >>>>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote:
> > >>>>>>
> > >>>>>>> From: SeongJae Park <sjpark@amazon.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.
> > >
> > > I also agree that locking should be used at last. But, as each driver manages
> > > its devices and resources in their way, it could have its unique race
> > > conditions. And, each unique race condition might have its unique efficient
> > > way to synchronize it. Therefore, I think the synchronization should be done
> > > by each driver, not by xenbus and thus we should make the callback to be called
> > > per-driver.
> >
> > xenbus controls creation and removing of devices, so applying locking
> > at xenbus level is the right thing to do in order to avoid races with
> > device removal.
> >
> > In case a backend has further synchronization requirements those have to
> > be handled at backend level, of course.
> >
> > In the end you'll need the xenbus level locking anyway in order to avoid
> > a race when the last backend specific device is just being removed when
> > the callback is about to be called for that device. Or you'd need to
> > call try_get_module() before calling into each backend...
>
> Agreed. Thank you for your kind explanation of your concerns.

Just posted the v11 patchset[1], which is based on your idea and passed my
test.

[1] https://lore.kernel.org/xen-devel/20191217160748.693-1-sjpark@amazon.com/


Thanks,
SeongJae Park

>
>
> Thanks,
> SeongJae Park
>
> >
> >
> > Juergen

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