Re: [PATCH v1 6/6] virtio-balloon: Add support for aerating memory via hinting

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

On Wed, Jun 19, 2019 at 03:33:38PM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> 
> Add support for aerating memory using the 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>
> ---
>  drivers/virtio/Kconfig              |    1 
>  drivers/virtio/virtio_balloon.c     |  110 ++++++++++++++++++++++++++++++++++-
>  include/uapi/linux/virtio_balloon.h |    1 
>  3 files changed, 108 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> index 023fc3bc01c6..9cdaccf92c3a 100644
> --- a/drivers/virtio/Kconfig
> +++ b/drivers/virtio/Kconfig
> @@ -47,6 +47,7 @@ config VIRTIO_BALLOON
>  	tristate "Virtio balloon driver"
>  	depends on VIRTIO
>  	select MEMORY_BALLOON
> +	select AERATION
>  	---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 44339fc87cc7..91f1e8c9017d 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -18,6 +18,7 @@
>  #include <linux/mm.h>
>  #include <linux/mount.h>
>  #include <linux/magic.h>
> +#include <linux/memory_aeration.h>
>  
>  /*
>   * Balloon device works in 4K page units.  So each page is pointed to by
> @@ -26,6 +27,7 @@
>   */
>  #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 | \
> @@ -45,6 +47,7 @@ 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
>  };
>  
> @@ -54,7 +57,8 @@ enum virtio_balloon_config_read {
>  
>  struct virtio_balloon {
>  	struct virtio_device *vdev;
> -	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
> +	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq,
> +								*hinting_vq;
>  
>  	/* Balloon's own wq for cpu-intensive work items */
>  	struct workqueue_struct *balloon_wq;
> @@ -103,9 +107,21 @@ struct virtio_balloon {
>  	/* Synchronize access/update to this struct virtio_balloon elements */
>  	struct mutex balloon_lock;
>  
> -	/* The array of pfns we tell the Host about. */
> -	unsigned int num_pfns;
> -	__virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
> +
> +	union {
> +		/* The array of pfns we tell the Host about. */
> +		struct {
> +			unsigned int num_pfns;
> +			__virtio32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];
> +		};
> +		/* The array of physical addresses we are hinting on */
> +		struct {
> +			unsigned int num_hints;
> +			__virtio64 hints[VIRTIO_BALLOON_ARRAY_HINTS_MAX];
> +		};
> +	};
> +
> +	struct aerator_dev_info a_dev_info;
>  
>  	/* Memory statistics */
>  	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
> @@ -151,6 +167,68 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
>  
>  }
>  
> +static u64 page_to_hints_pa_order(struct page *page)
> +{
> +	unsigned char order;
> +	dma_addr_t pa;
> +
> +	BUILD_BUG_ON((64 - VIRTIO_BALLOON_PFN_SHIFT) >=
> +		     (1 << VIRTIO_BALLOON_PFN_SHIFT));
> +
> +	/*
> +	 * Record physical page address combined with page order.
> +	 * Order will never exceed 64 - VIRTIO_BALLON_PFN_SHIFT
> +	 * since the size has to fit into a 64b value. So as long
> +	 * as VIRTIO_BALLOON_SHIFT is greater than this combining
> +	 * the two values should be safe.
> +	 */
> +	pa = page_to_phys(page);
> +	order = page_private(page) +
> +		PAGE_SHIFT - VIRTIO_BALLOON_PFN_SHIFT;
> +
> +	return (u64)(pa | order);
> +}
> +
> +void virtballoon_aerator_react(struct aerator_dev_info *a_dev_info)
> +{
> +	struct virtio_balloon *vb = container_of(a_dev_info,
> +						struct virtio_balloon,
> +						a_dev_info);
> +	struct virtqueue *vq = vb->hinting_vq;
> +	struct scatterlist sg;
> +	unsigned int unused;
> +	struct page *page;
> +
> +	mutex_lock(&vb->balloon_lock);
> +
> +	vb->num_hints = 0;
> +
> +	list_for_each_entry(page, &a_dev_info->batch, lru) {
> +		vb->hints[vb->num_hints++] =
> +				cpu_to_virtio64(vb->vdev,
> +						page_to_hints_pa_order(page));
> +	}
> +
> +	/* We shouldn't have been called if there is nothing to process */
> +	if (WARN_ON(vb->num_hints == 0))
> +		goto out;
> +
> +	sg_init_one(&sg, vb->hints,
> +		    sizeof(vb->hints[0]) * vb->num_hints);
> +
> +	/*
> +	 * We should always be able to add one buffer to an
> +	 * empty queue.
> +	 */
> +	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> +	virtqueue_kick(vq);
> +
> +	/* When host has read buffer, this completes via balloon_ack */
> +	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> +out:
> +	mutex_unlock(&vb->balloon_lock);
> +}
> +
>  static void set_page_pfns(struct virtio_balloon *vb,
>  			  __virtio32 pfns[], struct page *page)
>  {
> @@ -475,6 +553,7 @@ 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";
> @@ -486,11 +565,19 @@ 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)) {
> @@ -929,12 +1016,24 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		if (err)
>  			goto out_del_balloon_wq;
>  	}
> +
> +	vb->a_dev_info.react = virtballoon_aerator_react;
> +	vb->a_dev_info.capacity = VIRTIO_BALLOON_ARRAY_HINTS_MAX;
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
> +		err = aerator_startup(&vb->a_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);
> @@ -963,6 +1062,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING))
> +		aerator_shutdown();
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
>  		virtio_balloon_unregister_shrinker(vb);
>  	spin_lock_irq(&vb->stop_update_lock);
> @@ -1032,6 +1133,7 @@ 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



The approach here is very close to what on-demand hinting that is
already upstream does.

This should have resulted in a most of the code being shared
but this does not seem to happen here.

Can we unify the code in some way?
It can still use a separate feature flag, but there are things
I like very much about current hinting code, such as
using s/g instead of passing PFNs in a buffer.

If this doesn't work could you elaborate on why?

-- 
MST