Re: [PATCH v3 6/6] virtio-balloon: Add support for providing unused page reports to host

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Mon, 2019-08-05 at 12:00 -0400, Nitesh Narayan Lal wrote:
> On 8/1/19 6:38 PM, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > 
> > Add support for the page reporting feature provided by virtio-balloon.
> > Reporting differs from the regular balloon functionality in that is is
> > much less durable than a standard memory balloon. Instead of creating a
> > list of pages that cannot be accessed the pages are only inaccessible
> > while they are being indicated to the virtio interface. Once the
> > interface has acknowledged them they are placed back into their respective
> > free lists and are once again accessible by the guest system.
> > 
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> >  drivers/virtio/Kconfig              |    1 +
> >  drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
> >  include/uapi/linux/virtio_balloon.h |    1 +
> >  3 files changed, 58 insertions(+)
> > 
> > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> > index 078615cf2afc..4b2dd8259ff5 100644
> > --- a/drivers/virtio/Kconfig
> > +++ b/drivers/virtio/Kconfig
> > @@ -58,6 +58,7 @@ config VIRTIO_BALLOON
> >  	tristate "Virtio balloon driver"
> >  	depends on VIRTIO
> >  	select MEMORY_BALLOON
> > +	select PAGE_REPORTING
> >  	---help---
> >  	 This driver supports increasing and decreasing the amount
> >  	 of memory within a KVM guest.
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 2c19457ab573..971fe924e34f 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/mount.h>
> >  #include <linux/magic.h>
> >  #include <linux/pseudo_fs.h>
> > +#include <linux/page_reporting.h>
> >  
> >  /*
> >   * Balloon device works in 4K page units.  So each page is pointed to by
> > @@ -37,6 +38,9 @@
> >  #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
> >  	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
> >  
> > +/*  limit on the number of pages that can be on the reporting vq */
> > +#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
> > +
> >  #ifdef CONFIG_BALLOON_COMPACTION
> >  static struct vfsmount *balloon_mnt;
> >  #endif
> > @@ -46,6 +50,7 @@ enum virtio_balloon_vq {
> >  	VIRTIO_BALLOON_VQ_DEFLATE,
> >  	VIRTIO_BALLOON_VQ_STATS,
> >  	VIRTIO_BALLOON_VQ_FREE_PAGE,
> > +	VIRTIO_BALLOON_VQ_REPORTING,
> >  	VIRTIO_BALLOON_VQ_MAX
> >  };
> >  
> > @@ -113,6 +118,10 @@ struct virtio_balloon {
> >  
> >  	/* To register a shrinker to shrink memory upon memory pressure */
> >  	struct shrinker shrinker;
> > +
> > +	/* Unused page reporting device */
> > +	struct virtqueue *reporting_vq;
> > +	struct page_reporting_dev_info ph_dev_info;
> >  };
> >  
> >  static struct virtio_device_id id_table[] = {
> > @@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> >  
> >  }
> >  
> > +void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
> > +				    unsigned int nents)
> > +{
> > +	struct virtio_balloon *vb =
> > +		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
> > +	struct virtqueue *vq = vb->reporting_vq;
> > +	unsigned int unused;
> > +
> > +	/* We should always be able to add these buffers to an empty queue. */
> > +	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
> > +			    GFP_NOWAIT | __GFP_NOWARN);
> 
> I think you should handle allocation failure here. It is a possibility, isn't?
> Maybe return an error or even disable page hinting/reporting?
> 

I don't think it is an issue I have to worry about. Specifically I am
limiting the size of the scatterlist based on the size of the vq. As such
I will never exceed the size and should be able to use it to store the
scatterlist directly.