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

[David Hildenbrand]

On 22.01.20 18:43, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> ---
>  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.

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

-- 
Thanks,

David / dhildenb