Linux Virtualization - Apr 2018 - [PATCH v30 2/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

On Wed, Apr 04, 2018 at 12:10:03AM +0800, Wei Wang
wrote:
> Negotiation of the VIRTIO_BALLOON_F_FREE_PAGE_HINT feature indicates the
> support of reporting hints of guest free pages to host via virtio-balloon.
> 
> Host requests the guest to report free page hints by sending a new cmd
> id to the guest via the free_page_report_cmd_id configuration register.
> 
> When the guest starts to report, the first element added to the free page
> vq is the cmd id given by host. When the guest finishes the reporting
> of all the free pages, VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID is added
> to the vq to tell host that the reporting is done. Host polls the free
> page vq after sending the starting cmd id, so the guest doesn't need to
> kick after filling an element to the vq.
> 
> Host may also requests the guest to stop the reporting in advance by
> sending the stop cmd id to the guest via the configuration register.
> 
> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> Signed-off-by: Liang Li <liang.z.li at intel.com>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Cc: Michal Hocko <mhocko at kernel.org>
> ---
>  drivers/virtio/virtio_balloon.c     | 257
+++++++++++++++++++++++++++++++-----
>  include/uapi/linux/virtio_balloon.h |   4 +
>  2 files changed, 225 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index dfe5684..18d24a4 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -51,9 +51,22 @@
>  static struct vfsmount *balloon_mnt;
>  #endif
>  
> +enum virtio_balloon_vq {
> +	VIRTIO_BALLOON_VQ_INFLATE,
> +	VIRTIO_BALLOON_VQ_DEFLATE,
> +	VIRTIO_BALLOON_VQ_STATS,
> +	VIRTIO_BALLOON_VQ_FREE_PAGE,
> +	VIRTIO_BALLOON_VQ_MAX
> +};
> +
>  struct virtio_balloon {
>  	struct virtio_device *vdev;
> -	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
> +	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq, *free_page_vq;
> +
> +	/* Balloon's own wq for cpu-intensive work items */
> +	struct workqueue_struct *balloon_wq;
> +	/* The free page reporting work item submitted to the balloon wq */
> +	struct work_struct report_free_page_work;
>  
>  	/* The balloon servicing is delegated to a freezable workqueue. */
>  	struct work_struct update_balloon_stats_work;
> @@ -63,6 +76,13 @@ struct virtio_balloon {
>  	spinlock_t stop_update_lock;
>  	bool stop_update;
>  
> +	/* The new cmd id received from host */
> +	uint32_t cmd_id_received;
> +	/* The cmd id that is in use */
> +	__virtio32 cmd_id_use;
> +	/* Buffer to store the stop sign */
> +	__virtio32 stop_cmd_id;
> +
>  	/* Waiting for host to ack the pages we released. */
>  	wait_queue_head_t acked;
>  
> @@ -320,17 +340,6 @@ static void stats_handle_request(struct virtio_balloon
*vb)
>  	virtqueue_kick(vq);
>  }
>  
> -static void virtballoon_changed(struct virtio_device *vdev)
> -{
> -	struct virtio_balloon *vb = vdev->priv;
> -	unsigned long flags;
> -
> -	spin_lock_irqsave(&vb->stop_update_lock, flags);
> -	if (!vb->stop_update)
> -		queue_work(system_freezable_wq, &vb->update_balloon_size_work);
> -	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> -}
> -
>  static inline s64 towards_target(struct virtio_balloon *vb)
>  {
>  	s64 target;
> @@ -347,6 +356,34 @@ static inline s64 towards_target(struct virtio_balloon
*vb)
>  	return target - vb->num_pages;
>  }
>  
> +static void virtballoon_changed(struct virtio_device *vdev)
> +{
> +	struct virtio_balloon *vb = vdev->priv;
> +	unsigned long flags;
> +	s64 diff = towards_target(vb);
> +
> +	if (diff) {
> +		spin_lock_irqsave(&vb->stop_update_lock, flags);
> +		if (!vb->stop_update)
> +			queue_work(system_freezable_wq,
> +				   &vb->update_balloon_size_work);
> +		spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> +	}
> +
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +		virtio_cread(vdev, struct virtio_balloon_config,
> +			     free_page_report_cmd_id, &vb->cmd_id_received);
> +		if (vb->cmd_id_received !> +		   
VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID) {
> +			spin_lock_irqsave(&vb->stop_update_lock, flags);
> +			if (!vb->stop_update)
> +				queue_work(vb->balloon_wq,
> +					   &vb->report_free_page_work);
> +			spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> +		}
> +	}
> +}
> +
>  static void update_balloon_size(struct virtio_balloon *vb)
>  {
>  	u32 actual = vb->num_pages;
> @@ -421,42 +458,163 @@ static void update_balloon_size_func(struct
work_struct *work)
>  
>  static int init_vqs(struct virtio_balloon *vb)
>  {
> -	struct virtqueue *vqs[3];
> -	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
> -	static const char * const names[] = { "inflate",
"deflate", "stats" };
> -	int err, nvqs;
> +	struct virtqueue *vqs[VIRTIO_BALLOON_VQ_MAX];
> +	vq_callback_t *callbacks[VIRTIO_BALLOON_VQ_MAX];
> +	const char *names[VIRTIO_BALLOON_VQ_MAX];
> +	struct scatterlist sg;
> +	int ret;
>  
>  	/*
> -	 * We expect two virtqueues: inflate and deflate, and
> -	 * optionally stat.
> +	 * Inflateq and deflateq are used unconditionally. The names[]
> +	 * will be NULL if the related feature is not enabled, which will
> +	 * cause no allocation for the corresponding virtqueue in find_vqs.
>  	 */
> -	nvqs = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ) ? 3 :
2;
> -	err = virtio_find_vqs(vb->vdev, nvqs, vqs, callbacks, names, NULL);
> -	if (err)
> -		return err;
> +	callbacks[VIRTIO_BALLOON_VQ_INFLATE] = balloon_ack;
> +	names[VIRTIO_BALLOON_VQ_INFLATE] = "inflate";
> +	callbacks[VIRTIO_BALLOON_VQ_DEFLATE] = balloon_ack;
> +	names[VIRTIO_BALLOON_VQ_DEFLATE] = "deflate";
> +	names[VIRTIO_BALLOON_VQ_STATS] = NULL;
> +	names[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
>  
> -	vb->inflate_vq = vqs[0];
> -	vb->deflate_vq = vqs[1];
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> -		struct scatterlist sg;
> -		unsigned int num_stats;
> -		vb->stats_vq = vqs[2];
> +		names[VIRTIO_BALLOON_VQ_STATS] = "stats";
> +		callbacks[VIRTIO_BALLOON_VQ_STATS] = stats_request;
> +	}
> +
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +		names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "free_page_vq";
> +		callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> +	}
>  
> +	ret = vb->vdev->config->find_vqs(vb->vdev,
VIRTIO_BALLOON_VQ_MAX,
> +					 vqs, callbacks, names, NULL, NULL);
> +	if (ret)
> +		return ret;
> +
> +	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)) {
> +		vb->stats_vq = vqs[VIRTIO_BALLOON_VQ_STATS];
>  		/*
>  		 * Prime this virtqueue with one buffer so the hypervisor can
>  		 * use it to signal us later (it can't be broken yet!).
>  		 */
> -		num_stats = update_balloon_stats(vb);
> -
> -		sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
> -		if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
> -		    < 0)
> -			BUG();
> +		sg_init_one(&sg, vb->stats, sizeof(vb->stats));
> +		ret = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
> +					   GFP_KERNEL);
> +		if (ret) {
> +			dev_warn(&vb->vdev->dev, "%s: add stat_vq
failed\n",
> +				 __func__);
> +			return ret;
> +		}
>  		virtqueue_kick(vb->stats_vq);
>  	}
> +
> +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> +		vb->free_page_vq = vqs[VIRTIO_BALLOON_VQ_FREE_PAGE];
> +
>  	return 0;
>  }
>  
> +static int add_one_sg(struct virtqueue *vq, unsigned long pfn, uint32_t
len)
> +{
> +	struct scatterlist sg;
> +	unsigned int unused;
> +
> +	sg_init_table(&sg, 1);
> +	sg_set_page(&sg, pfn_to_page(pfn), len, 0);
> +
> +	/* Detach all the used buffers from the vq */
> +	while (virtqueue_get_buf(vq, &unused))
> +		;
> +
> +	/*
> +	 * Since this is an optimization feature, losing a couple of free
> +	 * pages to report isn't important. We simply return without adding
> +	 * the page hint if the vq is full.
why not stop scanning of following pages though?
> +	 * We are adding one entry each time, which essentially results in no
> +	 * memory allocation, so the GFP_KERNEL flag below can be ignored.
> +	 * Host works by polling the free page vq for hints after sending the
> +	 * starting cmd id, so the driver doesn't need to kick after filling
> +	 * the vq.
> +	 * Lastly, there is always one entry reserved for the cmd id to use.
> +	 */
> +	if (vq->num_free > 1)
> +		return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> +
> +	return 0;
> +}
> +
> +static int virtio_balloon_send_free_pages(void *opaque, unsigned long pfn,
> +					   unsigned long nr_pages)
> +{
> +	struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
> +	uint32_t len = nr_pages << PAGE_SHIFT;
> +
> +	/*
> +	 * If a stop id or a new cmd id was just received from host, stop
> +	 * the reporting, and return 1 to indicate an active stop.
> +	 */
> +	if (virtio32_to_cpu(vb->vdev, vb->cmd_id_use) !=
vb->cmd_id_received)
> +		return 1;
> +
this access to cmd_id_use and cmd_id_received without locks
bothers me. Pls document why it's safe.
> +	return add_one_sg(vb->free_page_vq, pfn, len);
> +}
> +
> +static int send_start_cmd_id(struct virtio_balloon *vb, uint32_t cmd_id)
> +{
> +	struct scatterlist sg;
> +	struct virtqueue *vq = vb->free_page_vq;
> +
> +	vb->cmd_id_use = cpu_to_virtio32(vb->vdev, cmd_id);
> +	sg_init_one(&sg, &vb->cmd_id_use, sizeof(vb->cmd_id_use));
> +	return virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> +}
> +
> +static int send_stop_cmd_id(struct virtio_balloon *vb)
> +{
> +	struct scatterlist sg;
> +	struct virtqueue *vq = vb->free_page_vq;
> +
> +	sg_init_one(&sg, &vb->stop_cmd_id, sizeof(vb->cmd_id_use));
why the inconsistency?
> +	return virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> +}
> +
> +static void report_free_page_func(struct work_struct *work)
> +{
> +	struct virtio_balloon *vb;
> +	struct virtqueue *vq;
> +	unsigned int unused;
> +	int ret;
> +
> +	vb = container_of(work, struct virtio_balloon, report_free_page_work);
> +	vq = vb->free_page_vq;
> +
> +	/* Start by sending the received cmd id to host with an outbuf */
> +	ret = send_start_cmd_id(vb, vb->cmd_id_received);
> +	if (unlikely(ret))
> +		goto err;
> +
> +	ret = walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
> +	if (unlikely(ret == -EIO))
> +		goto err;
> +
> +	/* End by sending a stop id to host with an outbuf */
> +	ret = send_stop_cmd_id(vb);
> +	if (likely(!ret)) {
> +		/*
> +		 * Ending: make sure all the used buffers have been detached
> +		 * from the vq.
> +		 */
> +		while (vq->num_free != virtqueue_get_vring_size(vq))
> +			virtqueue_get_buf(vq, &unused);
> +		return;
> +	}
> +err:
> +	dev_err(&vb->vdev->dev, "%s: free page vq failure,
ret=%d\n",
> +		__func__, ret);
> +}
> +
>  #ifdef CONFIG_BALLOON_COMPACTION
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf
of
> @@ -570,18 +728,36 @@ static int virtballoon_probe(struct virtio_device
*vdev)
>  	if (err)
>  		goto out_free_vb;
>  
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +		/*
> +		 * There is always one entry reserved for cmd id, so the ring
> +		 * size needs to be at least two to report free page hints.
> +		 */
> +		if (virtqueue_get_vring_size(vb->free_page_vq) < 2)
> +			goto out_free_vb;
> +		vb->balloon_wq = alloc_workqueue("balloon-wq",
> +					WQ_FREEZABLE | WQ_CPU_INTENSIVE, 0);
> +		if (!vb->balloon_wq) {
> +			err = -ENOMEM;
> +			goto out_del_vqs;
> +		}
> +		vb->stop_cmd_id = cpu_to_virtio32(vb->vdev,
> +				VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
> +		INIT_WORK(&vb->report_free_page_work, report_free_page_func);
> +	}
> +
>  	vb->nb.notifier_call = virtballoon_oom_notify;
>  	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
>  	err = register_oom_notifier(&vb->nb);
>  	if (err < 0)
> -		goto out_del_vqs;
> +		goto out_del_balloon_wq;
>  
>  #ifdef CONFIG_BALLOON_COMPACTION
>  	balloon_mnt = kern_mount(&balloon_fs);
>  	if (IS_ERR(balloon_mnt)) {
>  		err = PTR_ERR(balloon_mnt);
>  		unregister_oom_notifier(&vb->nb);
> -		goto out_del_vqs;
> +		goto out_del_balloon_wq;
>  	}
>  
>  	vb->vb_dev_info.migratepage = virtballoon_migratepage;
> @@ -591,7 +767,7 @@ static int virtballoon_probe(struct virtio_device
*vdev)
>  		kern_unmount(balloon_mnt);
>  		unregister_oom_notifier(&vb->nb);
>  		vb->vb_dev_info.inode = NULL;
> -		goto out_del_vqs;
> +		goto out_del_balloon_wq;
>  	}
>  	vb->vb_dev_info.inode->i_mapping->a_ops = &balloon_aops;
>  #endif
> @@ -602,6 +778,9 @@ static int virtballoon_probe(struct virtio_device
*vdev)
>  		virtballoon_changed(vdev);
>  	return 0;
>  
> +out_del_balloon_wq:
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> +		destroy_workqueue(vb->balloon_wq);
>  out_del_vqs:
>  	vdev->config->del_vqs(vdev);
>  out_free_vb:
> @@ -635,6 +814,11 @@ static void virtballoon_remove(struct virtio_device
*vdev)
>  	cancel_work_sync(&vb->update_balloon_size_work);
>  	cancel_work_sync(&vb->update_balloon_stats_work);
>  
> +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +		cancel_work_sync(&vb->report_free_page_work);
> +		destroy_workqueue(vb->balloon_wq);
> +	}
> +
>  	remove_common(vb);
>  #ifdef CONFIG_BALLOON_COMPACTION
>  	if (vb->vb_dev_info.inode)
> @@ -686,6 +870,7 @@ static int virtballoon_validate(struct virtio_device
*vdev)
>  	VIRTIO_BALLOON_F_MUST_TELL_HOST,
>  	VIRTIO_BALLOON_F_STATS_VQ,
>  	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
> +	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
>  };
>  
>  static struct virtio_driver virtio_balloon_driver = {
> diff --git a/include/uapi/linux/virtio_balloon.h
b/include/uapi/linux/virtio_balloon.h
> index 4e8b830..b2d86c2 100644
> --- a/include/uapi/linux/virtio_balloon.h
> +++ b/include/uapi/linux/virtio_balloon.h
> @@ -34,15 +34,19 @@
>  #define VIRTIO_BALLOON_F_MUST_TELL_HOST	0 /* Tell before reclaiming pages
*/
>  #define VIRTIO_BALLOON_F_STATS_VQ	1 /* Memory Stats virtqueue */
>  #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 */
>  
>  /* Size of a PFN in the balloon interface. */
>  #define VIRTIO_BALLOON_PFN_SHIFT 12
>  
> +#define VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID		0
>  struct virtio_balloon_config {
>  	/* Number of pages host wants Guest to give up. */
>  	__u32 num_pages;
>  	/* Number of pages we've actually got in balloon. */
>  	__u32 actual;
> +	/* Free page report command id, readonly by guest */
> +	__u32 free_page_report_cmd_id;
>  };
>  
>  #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
> -- 
> 1.8.3.1

On 04/04/2018 02:47 AM, Michael S. Tsirkin wrote:
> On Wed, Apr 04, 2018 at 12:10:03AM +0800, Wei Wang wrote:
>> +static int add_one_sg(struct virtqueue *vq, unsigned long pfn,
uint32_t len)
>> +{
>> +	struct scatterlist sg;
>> +	unsigned int unused;
>> +
>> +	sg_init_table(&sg, 1);
>> +	sg_set_page(&sg, pfn_to_page(pfn), len, 0);
>> +
>> +	/* Detach all the used buffers from the vq */
>> +	while (virtqueue_get_buf(vq, &unused))
>> +		;
>> +
>> +	/*
>> +	 * Since this is an optimization feature, losing a couple of free
>> +	 * pages to report isn't important. We simply return without
adding
>> +	 * the page hint if the vq is full.
> why not stop scanning of following pages though?
Because continuing to send hints is a way to deliver the maximum 
possible hints to host. For example, host may have a delay in taking 
hints at some point, and then it resumes to take hints soon. If the 
driver does not stop when the vq is full, it will be able to put more 
hints to the vq once the vq has available entries to add.

>
>> +	 * We are adding one entry each time, which essentially results in no
>> +	 * memory allocation, so the GFP_KERNEL flag below can be ignored.
>> +	 * Host works by polling the free page vq for hints after sending the
>> +	 * starting cmd id, so the driver doesn't need to kick after
filling
>> +	 * the vq.
>> +	 * Lastly, there is always one entry reserved for the cmd id to use.
>> +	 */
>> +	if (vq->num_free > 1)
>> +		return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
>> +
>> +	return 0;
>> +}
>> +
>> +static int virtio_balloon_send_free_pages(void *opaque, unsigned long
pfn,
>> +					   unsigned long nr_pages)
>> +{
>> +	struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
>> +	uint32_t len = nr_pages << PAGE_SHIFT;
>> +
>> +	/*
>> +	 * If a stop id or a new cmd id was just received from host, stop
>> +	 * the reporting, and return 1 to indicate an active stop.
>> +	 */
>> +	if (virtio32_to_cpu(vb->vdev, vb->cmd_id_use) !=
vb->cmd_id_received)
>> +		return 1;
>> +
> this access to cmd_id_use and cmd_id_received without locks
> bothers me. Pls document why it's safe.
OK. Probably we could add below to the above comments:

cmd_id_use and cmd_id_received don't need to be accessed under locks 
because the reporting does not have to stop immediately before 
cmd_id_received is changed (i.e. when host requests to stop). That is, 
reporting more hints after host requests to stop isn't an issue for this 
optimization feature, because host will simply drop the stale hints next 
time when it needs a new reporting.



>
>> +	return add_one_sg(vb->free_page_vq, pfn, len);
>> +}
>> +
>> +static int send_start_cmd_id(struct virtio_balloon *vb, uint32_t
cmd_id)
>> +{
>> +	struct scatterlist sg;
>> +	struct virtqueue *vq = vb->free_page_vq;
>> +
>> +	vb->cmd_id_use = cpu_to_virtio32(vb->vdev, cmd_id);
>> +	sg_init_one(&sg, &vb->cmd_id_use,
sizeof(vb->cmd_id_use));
>> +	return virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
>> +}
>> +
>> +static int send_stop_cmd_id(struct virtio_balloon *vb)
>> +{
>> +	struct scatterlist sg;
>> +	struct virtqueue *vq = vb->free_page_vq;
>> +
>> +	sg_init_one(&sg, &vb->stop_cmd_id,
sizeof(vb->cmd_id_use));
> why the inconsistency?
Thanks, will make it consistent.

Best,
Wei

On Wed, Apr 04, 2018 at 10:07:51AM +0800, Wei Wang
wrote:
> On 04/04/2018 02:47 AM, Michael S. Tsirkin wrote:
> > On Wed, Apr 04, 2018 at 12:10:03AM +0800, Wei Wang wrote:
> > > +static int add_one_sg(struct virtqueue *vq, unsigned long pfn,
uint32_t len)
> > > +{
> > > +	struct scatterlist sg;
> > > +	unsigned int unused;
> > > +
> > > +	sg_init_table(&sg, 1);
> > > +	sg_set_page(&sg, pfn_to_page(pfn), len, 0);
> > > +
> > > +	/* Detach all the used buffers from the vq */
> > > +	while (virtqueue_get_buf(vq, &unused))
> > > +		;
> > > +
> > > +	/*
> > > +	 * Since this is an optimization feature, losing a couple of
free
> > > +	 * pages to report isn't important. We simply return
without adding
> > > +	 * the page hint if the vq is full.
> > why not stop scanning of following pages though?
> 
> Because continuing to send hints is a way to deliver the maximum possible
> hints to host. For example, host may have a delay in taking hints at some
> point, and then it resumes to take hints soon. If the driver does not stop
> when the vq is full, it will be able to put more hints to the vq once the
vq
> has available entries to add.
What this appears to be is just lack of coordination between
host and guest.

But meanwhile you are spending cycles walking the list uselessly.
Instead of trying nilly-willy, the standard thing to do
is to wait for host to consume an entry and proceed.

Coding it up might be tricky, so it's probably acceptable as is
for now, but please replace the justification about with
a TODO entry that we should synchronize with the host.

> 
> > 
> > > +	 * We are adding one entry each time, which essentially results
in no
> > > +	 * memory allocation, so the GFP_KERNEL flag below can be
ignored.
> > > +	 * Host works by polling the free page vq for hints after
sending the
> > > +	 * starting cmd id, so the driver doesn't need to kick
after filling
> > > +	 * the vq.
> > > +	 * Lastly, there is always one entry reserved for the cmd id to
use.
> > > +	 */
> > > +	if (vq->num_free > 1)
> > > +		return virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int virtio_balloon_send_free_pages(void *opaque, unsigned
long pfn,
> > > +					   unsigned long nr_pages)
> > > +{
> > > +	struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
> > > +	uint32_t len = nr_pages << PAGE_SHIFT;
> > > +
> > > +	/*
> > > +	 * If a stop id or a new cmd id was just received from host,
stop
> > > +	 * the reporting, and return 1 to indicate an active stop.
> > > +	 */
> > > +	if (virtio32_to_cpu(vb->vdev, vb->cmd_id_use) !=
vb->cmd_id_received)
> > > +		return 1;
functions returning int should return 0 or -errno on failure,
positive return code should indicate progress.

If you want a boolean, use bool pls.

> > > +
> > this access to cmd_id_use and cmd_id_received without locks
> > bothers me. Pls document why it's safe.
> 
> OK. Probably we could add below to the above comments:
> 
> cmd_id_use and cmd_id_received don't need to be accessed under locks
because
> the reporting does not have to stop immediately before cmd_id_received is
> changed (i.e. when host requests to stop). That is, reporting more hints
> after host requests to stop isn't an issue for this optimization
feature,
> because host will simply drop the stale hints next time when it needs a new
> reporting.
What about the other direction? Can this observe a stale value and
exit erroneously?
> 
> 
> 
> > 
> > > +	return add_one_sg(vb->free_page_vq, pfn, len);
> > > +}
> > > +
> > > +static int send_start_cmd_id(struct virtio_balloon *vb, uint32_t
cmd_id)
> > > +{
> > > +	struct scatterlist sg;
> > > +	struct virtqueue *vq = vb->free_page_vq;
> > > +
> > > +	vb->cmd_id_use = cpu_to_virtio32(vb->vdev, cmd_id);
> > > +	sg_init_one(&sg, &vb->cmd_id_use,
sizeof(vb->cmd_id_use));
> > > +	return virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> > > +}
> > > +
> > > +static int send_stop_cmd_id(struct virtio_balloon *vb)
> > > +{
> > > +	struct scatterlist sg;
> > > +	struct virtqueue *vq = vb->free_page_vq;
> > > +
> > > +	sg_init_one(&sg, &vb->stop_cmd_id,
sizeof(vb->cmd_id_use));
> > why the inconsistency?
> 
> Thanks, will make it consistent.
> 
> Best,
> Wei