Re: [RFC][Patch v12 2/2] virtio-balloon: interface to support free page reporting

[Cornelia Huck]

On Wed, 14 Aug 2019 07:47:40 -0400
Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:

> On 8/14/19 6:29 AM, Cornelia Huck wrote:
> > On Mon, 12 Aug 2019 09:12:35 -0400
> > Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
> >  
> >> Enables the kernel to negotiate VIRTIO_BALLOON_F_REPORTING feature with
> >> the host. If it is available and page_reporting_flag is set to true,
> >> page_reporting is enabled and its callback is configured along with
> >> the max_pages count which indicates the maximum number of pages that
> >> can be isolated and reported at a time. Currently, only free pages of
> >> order >= (MAX_ORDER - 2) are reported. To prevent any false OOM
> >> max_pages count is set to 16.
> >>
> >> By default page_reporting feature is enabled and gets loaded as soon
> >> as the virtio-balloon driver is loaded. However, it could be disabled
> >> by writing the page_reporting_flag which is a virtio-balloon parameter.
> >>
> >> Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
> >> ---
> >>  drivers/virtio/Kconfig              |  1 +
> >>  drivers/virtio/virtio_balloon.c     | 64 ++++++++++++++++++++++++++++-
> >>  include/uapi/linux/virtio_balloon.h |  1 +
> >>  3 files changed, 65 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> >> index 226fbb995fb0..defec00d4ee2 100644
> >> --- a/drivers/virtio/virtio_balloon.c
> >> +++ b/drivers/virtio/virtio_balloon.c  
> > (...)
> >  
> >> +static void virtballoon_page_reporting_setup(struct virtio_balloon *vb)
> >> +{
> >> +	struct device *dev = &vb->vdev->dev;
> >> +	int err;
> >> +
> >> +	vb->page_reporting_conf.report = virtballoon_report_pages;
> >> +	vb->page_reporting_conf.max_pages = PAGE_REPORTING_MAX_PAGES;
> >> +	err = page_reporting_enable(&vb->page_reporting_conf);
> >> +	if (err < 0) {
> >> +		dev_err(dev, "Failed to enable reporting, err = %d\n", err);
> >> +		page_reporting_flag = false;  
> > Should we clear the feature bit in this case as well?  
> 
> I think yes.

Eww, I didn't recall that we don't call the ->probe callback until
after feature negotiation has finished, so scratch that particular idea.

For what reasons may page_reporting_enable() fail? Does it make sense
to fail probing the device in that case? And does it make sense to
re-try later (i.e. leave page_reporting_flag set)?

> If I am not wrong then in a case where page reporting setup fails for some
> reason and at a later point the user wants to re-enable it then for that balloon
> driver has to be reloaded.
> Which would mean re-negotiation of the feature bit.

Re-negotiation actually already happens if a driver is unbound and
rebound.

> 
> >  
> >> +		vb->page_reporting_conf.report = NULL;
> >> +		vb->page_reporting_conf.max_pages = 0;
> >> +		return;
> >> +	}
> >> +}
> >> +
> >>  static void set_page_pfns(struct virtio_balloon *vb,
> >>  			  __virtio32 pfns[], struct page *page)
> >>  {
> >> @@ -476,6 +524,7 @@ static int init_vqs(struct virtio_balloon *vb)
> >>  	names[VIRTIO_BALLOON_VQ_DEFLATE] = "deflate";
> >>  	names[VIRTIO_BALLOON_VQ_STATS] = NULL;
> >>  	names[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> >> +	names[VIRTIO_BALLOON_VQ_REPORTING] = NULL;
> >>  
> >>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> >>  		names[VIRTIO_BALLOON_VQ_STATS] = "stats";
> >> @@ -487,11 +536,18 @@ static int init_vqs(struct virtio_balloon *vb)
> >>  		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> >>  	}
> >>  
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> >> +		names[VIRTIO_BALLOON_VQ_REPORTING] = "reporting_vq";
> >> +		callbacks[VIRTIO_BALLOON_VQ_REPORTING] = balloon_ack;  
> > Do we even want to try to set up the reporting queue if reporting has
> > been disabled via module parameter? Might make more sense to not even
> > negotiate the feature bit in that case.  
> 
> True.
> I think this should be replaced with something like (page_reporting_flag &&
> virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)).

Yes.

Is page_reporting_flag supposed to be changeable on the fly? The only
way to really turn off the feature bit from the driver is to not pass
in the feature in the features table; we could provide two different
tables depending on the flag if it were static.

> 
> >  
> >> +	}
> >>  	err = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
> >>  					 vqs, callbacks, names, NULL, NULL);
> >>  	if (err)
> >>  		return err;
> >>  
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
> >> +		vb->reporting_vq = vqs[VIRTIO_BALLOON_VQ_REPORTING];
> >> +
> >>  	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
> >>  	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> >>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> >> @@ -924,6 +980,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
> >>  		if (err)
> >>  			goto out_del_balloon_wq;
> >>  	}
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING) &&
> >> +	    page_reporting_flag)
> >> +		virtballoon_page_reporting_setup(vb);  
> > In that case, you'd only need to check for the feature bit here.  
> 
> Why is that?
> I think both the checks should be present here as we need both the conditions to
> be true to enable page reporting.

Yeah, because we can't clear the feature bit if the flag is not set.

> However, the order should be reversed because of the reason you mentioned earlier.
> 
> >  
> >>  	virtio_device_ready(vdev);
> >>  
> >>  	if (towards_target(vb))