Re: [PATCH v2 5/5] virtio-balloon: Add support for providing page hints to host

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 25, 2019 at 10:44:01AM -0400, Nitesh Narayan Lal wrote:
> 
> On 7/24/19 3:02 PM, Michael S. Tsirkin wrote:
> > On Wed, Jul 24, 2019 at 10:05:14AM -0700, Alexander Duyck wrote:
> >> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> >>
> >> Add support for the page hinting feature provided by virtio-balloon.
> >> Hinting 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>
> > Looking at the design, it seems that hinted pages can immediately be
> > reused. I wonder how we can efficiently support this
> > with kvm when poisoning is in effect. Of course we can just
> > ignore the poison. However it seems cleaner to
> > 1. verify page is poisoned with the correct value
> > 2. fill the page with the correct value on fault
> >
> > Requirement 2 requires some kind of madvise that
> > will save the poison e.g. in the VMA.
> >
> > Not a blocker for sure ... 
> >
> >
> >> ---
> >>  drivers/virtio/Kconfig              |    1 +
> >>  drivers/virtio/virtio_balloon.c     |   47 +++++++++++++++++++++++++++++++++++
> >>  include/uapi/linux/virtio_balloon.h |    1 +
> >>  3 files changed, 49 insertions(+)
> >>
> >> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> >> index 078615cf2afc..d45556ae1f81 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_HINTING
> >>  	---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 226fbb995fb0..dee9f8f3ad09 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_hinting.h>
> >>  
> >>  /*
> >>   * Balloon device works in 4K page units.  So each page is pointed to by
> >> @@ -27,6 +28,7 @@
> >>   */
> >>  #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
> >>  #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256
> >> +#define VIRTIO_BALLOON_ARRAY_HINTS_MAX	32
> >>  #define VIRTBALLOON_OOM_NOTIFY_PRIORITY 80
> >>  
> >>  #define VIRTIO_BALLOON_FREE_PAGE_ALLOC_FLAG (__GFP_NORETRY | __GFP_NOWARN | \
> >> @@ -46,6 +48,7 @@ enum virtio_balloon_vq {
> >>  	VIRTIO_BALLOON_VQ_DEFLATE,
> >>  	VIRTIO_BALLOON_VQ_STATS,
> >>  	VIRTIO_BALLOON_VQ_FREE_PAGE,
> >> +	VIRTIO_BALLOON_VQ_HINTING,
> >>  	VIRTIO_BALLOON_VQ_MAX
> >>  };
> >>  
> >> @@ -113,6 +116,10 @@ struct virtio_balloon {
> >>  
> >>  	/* To register a shrinker to shrink memory upon memory pressure */
> >>  	struct shrinker shrinker;
> >> +
> >> +	/* Unused page hinting device */
> >> +	struct virtqueue *hinting_vq;
> >> +	struct page_hinting_dev_info ph_dev_info;
> >>  };
> >>  
> >>  static struct virtio_device_id id_table[] = {
> >> @@ -152,6 +159,22 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> >>  
> >>  }
> >>  
> >> +void virtballoon_page_hinting_react(struct page_hinting_dev_info *ph_dev_info,
> >> +				    unsigned int num_hints)
> >> +{
> >> +	struct virtio_balloon *vb =
> >> +		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
> >> +	struct virtqueue *vq = vb->hinting_vq;
> >> +	unsigned int unused;
> >> +
> >> +	/* We should always be able to add these buffers to an empty queue. */
> >
> > can be an out of memory condition, and then ...
> 
> Do we need an error check here?
> 
> For situations where this fails we should disable hinting completely, maybe?

I would just limit this to vq size, then you know it won't fail.

> 
> >
> >> +	virtqueue_add_inbuf(vq, ph_dev_info->sg, num_hints, vb, GFP_KERNEL);
> >> +	virtqueue_kick(vq);
> > ... this will block forever.
> >
> >> +	/* When host has read buffer, this completes via balloon_ack */
> >> +	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> > However below I suggest limiting capacity which will solve
> > this problem for you.
> >
> >
> >
> >> +}
> >> +
> >>  static void set_page_pfns(struct virtio_balloon *vb,
> >>  			  __virtio32 pfns[], struct page *page)
> >>  {
> >> @@ -476,6 +499,7 @@ 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_HINTING] = NULL;
> >>  
> >>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> >>  		names[VIRTIO_BALLOON_VQ_STATS] = "stats";
> >> @@ -487,11 +511,19 @@ static int init_vqs(struct virtio_balloon *vb)
> >>  		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> >>  	}
> >>  
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
> >> +		names[VIRTIO_BALLOON_VQ_HINTING] = "hinting_vq";
> >> +		callbacks[VIRTIO_BALLOON_VQ_HINTING] = 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_HINTING))
> >> +		vb->hinting_vq = vqs[VIRTIO_BALLOON_VQ_HINTING];
> >> +
> >>  	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)) {
> >> @@ -924,12 +956,24 @@ static int virtballoon_probe(struct virtio_device *vdev)
> >>  		if (err)
> >>  			goto out_del_balloon_wq;
> >>  	}
> >> +
> >> +	vb->ph_dev_info.react = virtballoon_page_hinting_react;
> >> +	vb->ph_dev_info.capacity = VIRTIO_BALLOON_ARRAY_HINTS_MAX;
> > As explained above I think you should limit this by vq size.
> > Otherwise virtqueue add buf might fail.
> > In fact by struct spec reading you need to limit it
> > anyway otherwise it will fail unconditionally.
> > In practice on most hypervisors it will typically work ...
> >
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
> >> +		err = page_hinting_startup(&vb->ph_dev_info);
> >> +		if (err)
> >> +			goto out_unregister_shrinker;
> >> +	}
> >> +
> >>  	virtio_device_ready(vdev);
> >>  
> >>  	if (towards_target(vb))
> >>  		virtballoon_changed(vdev);
> >>  	return 0;
> >>  
> >> +out_unregister_shrinker:
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> >> +		virtio_balloon_unregister_shrinker(vb);
> >>  out_del_balloon_wq:
> >>  	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> >>  		destroy_workqueue(vb->balloon_wq);
> >> @@ -958,6 +1002,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
> >>  {
> >>  	struct virtio_balloon *vb = vdev->priv;
> >>  
> >> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING))
> >> +		page_hinting_shutdown(&vb->ph_dev_info);
> >>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
> >>  		virtio_balloon_unregister_shrinker(vb);
> >>  	spin_lock_irq(&vb->stop_update_lock);
> >> @@ -1027,6 +1073,7 @@ static int virtballoon_validate(struct virtio_device *vdev)
> >>  	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
> >>  	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
> >>  	VIRTIO_BALLOON_F_PAGE_POISON,
> >> +	VIRTIO_BALLOON_F_HINTING,
> >>  };
> >>  
> >>  static struct virtio_driver virtio_balloon_driver = {
> >> diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
> >> index a1966cd7b677..2b0f62814e22 100644
> >> --- a/include/uapi/linux/virtio_balloon.h
> >> +++ b/include/uapi/linux/virtio_balloon.h
> >> @@ -36,6 +36,7 @@
> >>  #define VIRTIO_BALLOON_F_DEFLATE_ON_OOM	2 /* Deflate balloon on OOM */
> >>  #define VIRTIO_BALLOON_F_FREE_PAGE_HINT	3 /* VQ to report free pages */
> >>  #define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
> >> +#define VIRTIO_BALLOON_F_HINTING	5 /* Page hinting virtqueue */
> >>  
> >>  /* Size of a PFN in the balloon interface. */
> >>  #define VIRTIO_BALLOON_PFN_SHIFT 12
> -- 
> Thanks
> Nitesh