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Subject: Re: [PATCH v3 6/6] virtio-balloon: Add support for providing unused
 page reports to host
To:     Alexander Duyck <alexander.duyck@gmail.com>, kvm@vger.kernel.org,
        david@redhat.com, mst@redhat.com, dave.hansen@intel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org
Cc:     yang.zhang.wz@gmail.com, pagupta@redhat.com, riel@surriel.com,
        konrad.wilk@oracle.com, willy@infradead.org,
        lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com,
        pbonzini@redhat.com, dan.j.williams@intel.com,
        alexander.h.duyck@linux.intel.com
References: <20190801222158.22190.96964.stgit@localhost.localdomain>
 <20190801223829.22190.36831.stgit@localhost.localdomain>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Organization: Red Hat Inc,
Message-ID: <1cff09a4-d302-639c-ab08-9d82e5fc1383@redhat.com>
Date:   Mon, 5 Aug 2019 12:00:16 -0400
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On 8/1/19 6:38 PM, 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.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> ---
>  drivers/virtio/Kconfig              |    1 +
>  drivers/virtio/virtio_balloon.c     |   56 +++++++++++++++++++++++++++++++++++
>  include/uapi/linux/virtio_balloon.h |    1 +
>  3 files changed, 58 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 2c19457ab573..971fe924e34f 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
> @@ -37,6 +38,9 @@
>  #define VIRTIO_BALLOON_FREE_PAGE_SIZE \
>  	(1 << (VIRTIO_BALLOON_FREE_PAGE_ORDER + PAGE_SHIFT))
>  
> +/*  limit on the number of pages that can be on the reporting vq */
> +#define VIRTIO_BALLOON_VRING_HINTS_MAX	16
> +
>  #ifdef CONFIG_BALLOON_COMPACTION
>  static struct vfsmount *balloon_mnt;
>  #endif
> @@ -46,6 +50,7 @@ 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
>  };
>  
> @@ -113,6 +118,10 @@ struct virtio_balloon {
>  
>  	/* To register a shrinker to shrink memory upon memory pressure */
>  	struct shrinker shrinker;
> +
> +	/* Unused page reporting device */
> +	struct virtqueue *reporting_vq;
> +	struct page_reporting_dev_info ph_dev_info;
>  };
>  
>  static struct virtio_device_id id_table[] = {
> @@ -152,6 +161,23 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
>  
>  }
>  
> +void virtballoon_unused_page_report(struct page_reporting_dev_info *ph_dev_info,
> +				    unsigned int nents)
> +{
> +	struct virtio_balloon *vb =
> +		container_of(ph_dev_info, struct virtio_balloon, ph_dev_info);
> +	struct virtqueue *vq = vb->reporting_vq;
> +	unsigned int unused;
> +
> +	/* We should always be able to add these buffers to an empty queue. */
> +	virtqueue_add_inbuf(vq, ph_dev_info->sg, nents, vb,
> +			    GFP_NOWAIT | __GFP_NOWARN);


I think you should handle allocation failure here. It is a possibility, isn't?
Maybe return an error or even disable page hinting/reporting?


> +	virtqueue_kick(vq);
> +
> +	/* When host has read buffer, this completes via balloon_ack */
> +	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> +}
> +
>  static void set_page_pfns(struct virtio_balloon *vb,
>  			  __virtio32 pfns[], struct page *page)
>  {
> @@ -476,6 +502,7 @@ 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_REPORTING] = NULL;
>  
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
>  		names[VIRTIO_BALLOON_VQ_STATS] = "stats";
> @@ -487,11 +514,19 @@ static int init_vqs(struct virtio_balloon *vb)
>  		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
>  	}
>  
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> +		names[VIRTIO_BALLOON_VQ_REPORTING] = "reporting_vq";
> +		callbacks[VIRTIO_BALLOON_VQ_REPORTING] = 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_REPORTING))
> +		vb->reporting_vq = vqs[VIRTIO_BALLOON_VQ_REPORTING];
> +
>  	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)) {
> @@ -931,12 +966,30 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  		if (err)
>  			goto out_del_balloon_wq;
>  	}
> +
> +	vb->ph_dev_info.report = virtballoon_unused_page_report;
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING)) {
> +		unsigned int capacity;
> +
> +		capacity = min_t(unsigned int,
> +				 virtqueue_get_vring_size(vb->reporting_vq),
> +				 VIRTIO_BALLOON_VRING_HINTS_MAX);
> +		vb->ph_dev_info.capacity = capacity;
> +
> +		err = page_reporting_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);
> @@ -965,6 +1018,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
> +		page_reporting_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);
> @@ -1034,6 +1089,7 @@ 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_REPORTING,
>  };
>  
>  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..19974392d324 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_REPORTING	5 /* Page reporting virtqueue */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
>
-- 
Thanks
Nitesh