Re: [PATCH v5 6/6] virtio-balloon: Add support for providing unused page reports to host

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Sep 3, 2019 at 12:32 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Aug 12, 2019 at 02:33:56PM -0700, 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     |   65 +++++++++++++++++++++++++++++++++++
> >  include/uapi/linux/virtio_balloon.h |    1 +
> >  3 files changed, 67 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..52f9eeda1877 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 +161,32 @@ 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, err;
> > +
> > +     /* We should always be able to add these buffers to an empty queue. */
> > +     err = virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
> > +                               GFP_NOWAIT | __GFP_NOWARN);
> > +
> > +     /*
> > +      * In the extremely unlikely case that something has changed and we
> > +      * are able to trigger an error we will simply display a warning
> > +      * and exit without actually processing the pages.
> > +      */
> > +     if (WARN_ON(err))
> > +             return;
> > +
> > +     virtqueue_kick(vq);
> > +
> > +     /* When host has read buffer, this completes via balloon_ack */
> > +     wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> > +}
> > +
> >  static void set_page_pfns(struct virtio_balloon *vb,
> >                         __virtio32 pfns[], struct page *page)
> >  {
> > @@ -476,6 +511,7 @@ 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 +523,19 @@ 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;
> > +     }
> > +
> >       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)) {
> > @@ -931,12 +975,30 @@ static int virtballoon_probe(struct virtio_device *vdev)
> >               if (err)
> >                       goto out_del_balloon_wq;
> >       }
> > +
> > +     vb->ph_dev_info.report = virtballoon_unused_page_report;
> > +     if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> > +             unsigned int capacity;
> > +
> > +             capacity = min_t(unsigned int,
> > +                              virtqueue_get_vring_size(vb->reporting_vq) - 1,
> > +                              VIRTIO_BALLOON_VRING_HINTS_MAX);
>
> Hmm why - 1 exactly?
> This might end up being 0 in the unusual configuration of vq size 1.
> Also, VIRTIO_BALLOON_VRING_HINTS_MAX is a power of 2 but
> virtqueue_get_vring_size(vb->reporting_vq) - 1 won't
> be if we are using split rings - donnu if that matters.

Is a vq size of 1 valid? Does that mean you can use that 1 descriptor?

Odds are I probably misunderstood the ring config in the other hinting
implementation. Looking it over now I guess it was adding one
additional entry for a command header and that was why it was
reserving one additional slot. I can update the code to drop the "- 1"
if the ring is capable of being fully utilized.

Thanks.

- Alex