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

[Alexander Duyck]

On Wed, 2019-07-24 at 15:02 -0400, Michael S. Tsirkin wrote:
> On Wed, Jul 24, 2019 at 10:05:14AM -0700, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxxxx>
> 
> 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 ... 

As per our discussion in the other patch I agree that we should either
ignore the hint/report if page poisoning is enabled, or page poisoning
should result in us poisoning the page when it is faulted back in. I had
assumed we were doing the latter, I didn't realize that is was just
disabling the free page hinting.

> > ---
> >  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 ...
> 
> > +	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.

I wasn't aware that virtqueue_add_inbuf actually performed an allocation.

> > +}
> > +
> >  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 ...

So I would just need to query that via the virtqueue_get_vring_size
function correct? I could probably just set capacity to the minimum of the
HINTS_MAX and that value right?