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

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 7/24/19 3:02 PM, Michael S. Tsirkin wrote:
> On Wed, Jul 24, 2019 at 10:05:14AM -0700, Alexander Duyck wrote:
>> From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>
>> 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@linux.intel.com>
> 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
Once VIRTIO_BALLOON_F_PAGE_POISON user side support is available.
Can't we just use that at the time of initialization?
> Requirement 2 requires some kind of madvise that
> will save the poison e.g. in the VMA.
>
> Not a blocker for sure ... 
>
>
>> ---
>>  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.
>
>
>
>> +}
>> +
>>  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 ...
>
>> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING)) {
>> +		err = page_hinting_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);
>> @@ -958,6 +1002,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
>>  {
>>  	struct virtio_balloon *vb = vdev->priv;
>>  
>> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_HINTING))
>> +		page_hinting_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);
>> @@ -1027,6 +1073,7 @@ 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
-- 
Thanks
Nitesh