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

[JÃrgen GroÃ]

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.
>
> 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