Re: [PATCH v16.1 6/9] virtio-balloon: Add support for providing free page reports to host

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

On Tue, Feb 11, 2020 at 12:03:57PM +0100, David Hildenbrand wrote:
> On 22.01.20 18:43, 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.
> > 
> > Unlike a standard balloon we don't inflate and deflate the pages. Instead
> > we perform the reporting, and once the reporting is completed it is
> > assumed that the page has been dropped from the guest and will be faulted
> > back in the next time the page is accessed.
> > 
> > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > ---
> >  drivers/virtio/Kconfig              |    1 +
> >  drivers/virtio/virtio_balloon.c     |   64 +++++++++++++++++++++++++++++++++++
> >  include/uapi/linux/virtio_balloon.h |    1 +
> >  3 files changed, 66 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 40bb7693e3de..a07b9e18a292 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
> > @@ -47,6 +48,7 @@ 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
> >  };
> >  
> > @@ -114,6 +116,10 @@ struct virtio_balloon {
> >  
> >  	/* To register a shrinker to shrink memory upon memory pressure */
> >  	struct shrinker shrinker;
> > +
> > +	/* Free page reporting device */
> > +	struct virtqueue *reporting_vq;
> > +	struct page_reporting_dev_info pr_dev_info;
> >  };
> >  
> >  static struct virtio_device_id id_table[] = {
> > @@ -153,6 +159,33 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> >  
> >  }
> >  
> > +int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_info,
> > +				   struct scatterlist *sg, unsigned int nents)
> > +{
> > +	struct virtio_balloon *vb =
> > +		container_of(pr_dev_info, struct virtio_balloon, pr_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, sg, nents, vb, GFP_NOWAIT | __GFP_NOWARN);
> > +
> > +	/*
> > +	 * In the extremely unlikely case that something has occurred and we
> > +	 * are able to trigger an error we will simply display a warning
> > +	 * and exit without actually processing the pages.
> > +	 */
> > +	if (WARN_ON_ONCE(err))
> > +		return err;
> > +
> > +	virtqueue_kick(vq);
> > +
> > +	/* When host has read buffer, this completes via balloon_ack */
> > +	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> > +
> > +	return 0;
> > +}
> 
> 
> Did you see the discussion regarding unifying handling of
> inflate/deflate/free_page_hinting_free_page_reporting, requested by
> Michael? I think free page reporting is special and shall be left alone.

Not sure what do you mean by "left alone here". Could you clarify?

> VIRTIO_BALLOON_F_REPORTING is nothing but a more advanced inflate, right
> (sg, inflate based on size - not "virtio pages")?


Not exactly - it's also initiated by guest as opposed to host, and
not guided by the ballon size request set by the host.
And uses a dedicated queue to avoid blocking other functionality ...

I really think this is more like an inflate immediately followed by deflate.



> And you rely on
> deflates not being required before reusing an inflated page.
> 
> I suggest the following:
> 
> /* New interface (+ 2 virtqueues) to inflate/deflate using a SG */
> VIRTIO_BALLOON_F_SG
> /*
>  * No need to deflate when reusing pages (once the inflate request was
>  * processed). Applies to all inflate queues.
>  */
> VIRTIO_BALLOON_F_OPTIONAL_DEFLATE
> 
> And two new virtqueues
> 
> VIRTIO_BALLOON_VQ_INFLATE_SG
> VIRTIO_BALLOON_VQ_DEFLATE_SG
> 
> 
> Your feature would depend on VIRTIO_BALLOON_F_SG &&
> VIRTIO_BALLOON_F_OPTIONAL_DEFLATE. VIRTIO_BALLOON_F_OPTIONAL_DEFLATE
> could be reused to avoid deflating on certain events (e.g., from
> OOM/shrinker).
> 
> Thoughts?

I'd rather wait until we have a usecase and preferably a POC
showing it helps before we add optional deflate ...
For now I personally am fine with just making this go ahead as is,
and imply SG and OPTIONAL_DEFLATE just for this VQ.

Do you feel strongly we need to bring this up to a TC vote?
It means spec patch needs to be written, but it
does not have to be a big patch ...


> -- 
> Thanks,
> 
> David / dhildenb