Date:   Mon, 13 Nov 2017 19:32:29 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org,
        akpm@...ux-foundation.org, mawilcox@...rosoft.com,
        david@...hat.com, penguin-kernel@...ove.SAKURA.ne.jp,
        cornelia.huck@...ibm.com, mgorman@...hsingularity.net,
        aarcange@...hat.com, amit.shah@...hat.com, pbonzini@...hat.com,
        willy@...radead.org, liliang.opensource@...il.com,
        yang.zhang.wz@...il.com, quan.xu@...yun.com,
        Nitesh Narayan Lal <nilal@...hat.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

You should Cc Nitesh who is working on a related feature.

On Mon, Nov 13, 2017 at 06:34:48PM +0800, Wei Wang wrote:
> Ping for comments, thanks.
> 
> On 11/03/2017 04:13 PM, Wei Wang wrote:
> > Negotiation of the VIRTIO_BALLOON_F_FREE_PAGE_VQ feature indicates the
> > support of reporting hints of guest free pages to the host via
> > virtio-balloon. The host requests the guest to report the free pages by
> > sending commands via the virtio-balloon configuration registers.
> > 
> > When the guest starts to report, the first element added to the free page
> > vq is a sequence id of the start reporting command. The id is given by
> > the host, and it indicates whether the following free pages correspond
> > to the command. For example, the host may stop the report and start again
> > with a new command id. The obsolete pages for the previous start command
> > can be detected by the id dismatching on the host. The id is added to the
> > vq using an output buffer, and the free pages are added to the vq using
> > input buffer.
> > 
> > Here are some explainations about the added configuration registers:
> > - host2guest_cmd: a register used by the host to send commands to the
> > guest.
> > - guest2host_cmd: written by the guest to ACK to the host about the
> > commands that have been received. The host will clear the corresponding
> > bits on the host2guest_cmd register. The guest also uses this register
> > to send commands to the host (e.g. when finish free page reporting).

I am not sure what is the role of guest2host_cmd. Reporting of
the correct cmd id seems sufficient indication that guest
received the start command. Not getting any more seems sufficient
to detect stop.


> > - free_page_cmd_id: the sequence id of the free page report command
> > given by the host.
> > 
> > Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> > Signed-off-by: Liang Li <liang.z.li@...el.com>
> > Cc: Michael S. Tsirkin <mst@...hat.com>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > ---
> >   drivers/virtio/virtio_balloon.c     | 234 ++++++++++++++++++++++++++++++++----
> >   include/uapi/linux/virtio_balloon.h |  11 ++
> >   2 files changed, 223 insertions(+), 22 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index b31fc25..4087f04 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -55,7 +55,12 @@ static struct vfsmount *balloon_mnt;
> >   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;
> > @@ -65,6 +70,10 @@ struct virtio_balloon {
> >   	spinlock_t stop_update_lock;
> >   	bool stop_update;
> > +	/* Stop reporting free pages */
> > +	bool report_free_page_stop;

I would revert logic here: bool report_free_page;

> > +	uint32_t free_page_cmd_id;
> > +
> >   	/* Waiting for host to ack the pages we released. */
> >   	wait_queue_head_t acked;
> > @@ -191,6 +200,30 @@ static void send_balloon_page_sg(struct virtio_balloon *vb,
> >   		kick_and_wait(vq, vb->acked);
> >   }
> > +static void send_free_page_sg(struct virtqueue *vq, void *addr, uint32_t size)
> > +{
> > +	int err = 0;
> > +	unsigned int len;
> > +
> > +	/* Detach all the used buffers from the vq */
> > +	while (virtqueue_get_buf(vq, &len))
> > +		;
> > +
> > +	/*
> > +	 * Since this is an optimization feature, losing a couple of free
> > +	 * pages to report isn't important. We simply resturn without adding
> > +	 * the page if the vq is full.
> > +	 */
> > +	if (vq->num_free) {
> > +		err = add_one_sg(vq, addr, size);
> > +		BUG_ON(err);
> > +	}
> > +
> > +	/* Batch till the vq is full */
> > +	if (!vq->num_free)
> > +		virtqueue_kick(vq);
> > +}
> > +
> >   /*
> >    * Send balloon pages in sgs to host. The balloon pages are recorded in the
> >    * page xbitmap. Each bit in the bitmap corresponds to a page of PAGE_SIZE.
> > @@ -495,9 +528,8 @@ static void stats_handle_request(struct virtio_balloon *vb)
> >   	virtqueue_kick(vq);
> >   }
> > -static void virtballoon_changed(struct virtio_device *vdev)
> > +static void virtballoon_cmd_balloon_memory(struct virtio_balloon *vb)
> >   {
> > -	struct virtio_balloon *vb = vdev->priv;
> >   	unsigned long flags;
> >   	spin_lock_irqsave(&vb->stop_update_lock, flags);
> > @@ -506,6 +538,50 @@ static void virtballoon_changed(struct virtio_device *vdev)
> >   	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> >   }
> > +static void virtballoon_cmd_report_free_page_start(struct virtio_balloon *vb)
> > +{
> > +	unsigned long flags;
> > +
> > +	vb->report_free_page_stop = false;

this flag is used a lot outside any locks. Why is this safe?
Please add some comments explaining access to this flag.

> > +	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 virtballoon_changed(struct virtio_device *vdev)
> > +{
> > +	struct virtio_balloon *vb = vdev->priv;
> > +	u32 host2guest_cmd, guest2host_cmd = 0;
> > +
> > +	if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {
> > +		virtballoon_cmd_balloon_memory(vb);
> > +		return;

This might be a handy feature: one can disable balloon without
hot-unplug. But I would use a separate feature flag to
control it.

> > +	}
> > +
> > +	virtio_cread(vb->vdev, struct virtio_balloon_config, host2guest_cmd,
> > +		     &host2guest_cmd);
> > +
> > +	if (host2guest_cmd & VIRTIO_BALLOON_CMD_BALLOON_MEMORY) {
> > +		virtballoon_cmd_balloon_memory(vb);
> > +		guest2host_cmd |= VIRTIO_BALLOON_CMD_BALLOON_MEMORY;
> > +	}
> > +
> > +	if (host2guest_cmd & VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_START) {
> > +		virtballoon_cmd_report_free_page_start(vb);
> > +		guest2host_cmd |= VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_START;
> > +	}
> > +
> > +	if (host2guest_cmd & VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_STOP) {
> > +		vb->report_free_page_stop = true;
> > +		guest2host_cmd |= VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_STOP;
> > +	}

I am not sure why free page has start+stop but e.g. balloon has a single
bit. In fact I would really just use command id. When it changes, we
know a new report is needed.

> > +
> > +	/* Ack to the host about the commands that have been received */
> > +	virtio_cwrite(vb->vdev, struct virtio_balloon_config, guest2host_cmd,
> > +		      &guest2host_cmd);

So the same register is used to ack stop command and to signal
end of report. This seems buggy.

> > +}
> > +
> >   static inline s64 towards_target(struct virtio_balloon *vb)
> >   {
> >   	s64 target;
> > @@ -597,42 +673,147 @@ static void update_balloon_size_func(struct work_struct *work)
> >   		queue_work(system_freezable_wq, work);
> >   }
> > -static int init_vqs(struct virtio_balloon *vb)
> > +static bool virtio_balloon_send_free_pages(void *opaque, unsigned long pfn,
> > +					   unsigned long nr_pages)
> >   {
> > -	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 virtio_balloon *vb = (struct virtio_balloon *)opaque;
> > +	void *addr = (void *)pfn_to_kaddr(pfn);

How do we know all free pages have a kaddr?

> > +	uint32_t len = nr_pages << PAGE_SHIFT;
> > +
> > +	if (vb->report_free_page_stop)
> > +		return false;
> > +
> > +	send_free_page_sg(vb->free_page_vq, addr, len);
> > +	return true;
> > +}
> > +
> > +static void report_free_page_end(struct virtio_balloon *vb)
> > +{
> > +	u32 cmd = VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_STOP;
> >   	/*
> > -	 * We expect two virtqueues: inflate and deflate, and
> > -	 * optionally stat.
> > +	 * The host may have already requested to stop the reporting before we
> > +	 * finish, so no need to notify the host in this case.
> >   	 */
> > -	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 (vb->report_free_page_stop)
> > +		return;
> > +	vb->report_free_page_stop = true;
> > +
> > +	virtio_cwrite(vb->vdev, struct virtio_balloon_config, guest2host_cmd,
> > +		      &cmd);

Wouldn't it be easier to add a buffer in the queue?

> > +}
> > +
> > +static void report_free_page_cmd_id(struct virtio_balloon *vb)
> > +{
> > +	struct scatterlist sg;
> > +	int err;
> > +
> > +	virtio_cread(vb->vdev, struct virtio_balloon_config, free_page_cmd_id,
> > +		     &vb->free_page_cmd_id);
> > +	sg_init_one(&sg, &vb->free_page_cmd_id, sizeof(uint32_t));
> > +	err = virtqueue_add_outbuf(vb->free_page_vq, &sg, 1,
> > +				   &vb->free_page_cmd_id, GFP_KERNEL);
> > +	BUG_ON(err);
> > +}
> > +
> > +static void report_free_page(struct work_struct *work)
> > +{
> > +	struct virtio_balloon *vb;
> > +
> > +	vb = container_of(work, struct virtio_balloon, report_free_page_work);
> > +	report_free_page_cmd_id(vb);
> > +	walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
> > +	/*
> > +	 * The last few free page blocks that were added may not reach the
> > +	 * batch size, but need a kick to notify the device to handle them.
> > +	 */
> > +	virtqueue_kick(vb->free_page_vq);
> > +	report_free_page_end(vb);
> > +}
> > +
> > +static int init_vqs(struct virtio_balloon *vb)
> > +{
> > +	struct virtqueue **vqs;
> > +	vq_callback_t **callbacks;
> > +	const char **names;
> > +	struct scatterlist sg;
> > +	int i, nvqs, err = -ENOMEM;
> > +
> > +	/* Inflateq and deflateq are used unconditionally */
> > +	nvqs = 2;
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ))
> > +		nvqs++;
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ))
> > +		nvqs++;
> > +
> > +	/* Allocate space for find_vqs parameters */
> > +	vqs = kcalloc(nvqs, sizeof(*vqs), GFP_KERNEL);
> > +	if (!vqs)
> > +		goto err_vq;
> > +	callbacks = kmalloc_array(nvqs, sizeof(*callbacks), GFP_KERNEL);
> > +	if (!callbacks)
> > +		goto err_callback;
> > +	names = kmalloc_array(nvqs, sizeof(*names), GFP_KERNEL);
> > +	if (!names)
> > +		goto err_names;
> > +
> > +	callbacks[0] = balloon_ack;
> > +	names[0] = "inflate";
> > +	callbacks[1] = balloon_ack;
> > +	names[1] = "deflate";
> > +
> > +	i = 2;
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> > +		callbacks[i] = stats_request;
> > +		names[i] = "stats";
> > +		i++;
> > +	}
> > +
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {
> > +		callbacks[i] = NULL;
> > +		names[i] = "free_page_vq";
> > +	}
> > +
> > +	err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks, names,
> > +					 NULL, NULL);
> >   	if (err)
> > -		return err;
> > +		goto err_find;
> >   	vb->inflate_vq = vqs[0];
> >   	vb->deflate_vq = vqs[1];
> > +	i = 2;
> >   	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> > -		struct scatterlist sg;
> > -		unsigned int num_stats;
> > -		vb->stats_vq = vqs[2];
> > -
> > +		vb->stats_vq = vqs[i++];
> >   		/*
> >   		 * 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);
> > +		sg_init_one(&sg, vb->stats, sizeof(vb->stats));
> >   		if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
> > -		    < 0)
> > -			BUG();
> > +		    < 0) {
> > +			dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
> > +				 __func__);
> > +			goto err_find;
> > +		}
> >   		virtqueue_kick(vb->stats_vq);
> >   	}
> > +
> > +	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ))
> > +		vb->free_page_vq = vqs[i];
> > +
> > +	kfree(names);
> > +	kfree(callbacks);
> > +	kfree(vqs);
> >   	return 0;
> > +
> > +err_find:
> > +	kfree(names);
> > +err_names:
> > +	kfree(callbacks);
> > +err_callback:
> > +	kfree(vqs);
> > +err_vq:
> > +	return err;
> >   }
> >   #ifdef CONFIG_BALLOON_COMPACTION
> > @@ -761,6 +942,13 @@ static int virtballoon_probe(struct virtio_device *vdev)
> >   	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_SG))
> >   		xb_init(&vb->page_xb);
> > +	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {
> > +		vb->balloon_wq = alloc_workqueue("balloon-wq",
> > +					WQ_FREEZABLE | WQ_CPU_INTENSIVE, 0);
> > +		INIT_WORK(&vb->report_free_page_work, report_free_page);
> > +		vb->report_free_page_stop = true;
> > +	}
> > +
> >   	vb->nb.notifier_call = virtballoon_oom_notify;
> >   	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
> >   	err = register_oom_notifier(&vb->nb);
> > @@ -825,6 +1013,7 @@ static void virtballoon_remove(struct virtio_device *vdev)
> >   	spin_unlock_irq(&vb->stop_update_lock);
> >   	cancel_work_sync(&vb->update_balloon_size_work);
> >   	cancel_work_sync(&vb->update_balloon_stats_work);
> > +	cancel_work_sync(&vb->report_free_page_work);
> >   	remove_common(vb);
> >   #ifdef CONFIG_BALLOON_COMPACTION
> > @@ -878,6 +1067,7 @@ static unsigned int features[] = {
> >   	VIRTIO_BALLOON_F_STATS_VQ,
> >   	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
> >   	VIRTIO_BALLOON_F_SG,
> > +	VIRTIO_BALLOON_F_FREE_PAGE_VQ,
> >   };
> >   static struct virtio_driver virtio_balloon_driver = {
> > diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
> > index 37780a7..b758484 100644
> > --- a/include/uapi/linux/virtio_balloon.h
> > +++ b/include/uapi/linux/virtio_balloon.h
> > @@ -35,15 +35,26 @@
> >   #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_SG		3 /* Use sg instead of PFN lists */
> > +#define VIRTIO_BALLOON_F_FREE_PAGE_VQ	4 /* VQ to report free pages */
> >   /* Size of a PFN in the balloon interface. */
> >   #define VIRTIO_BALLOON_PFN_SHIFT 12
> > +#define	VIRTIO_BALLOON_CMD_BALLOON_MEMORY		(1 << 0)
> > +#define	VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_START	(1 << 1)
> > +#define	VIRTIO_BALLOON_CMD_REPORT_FREE_PAGE_STOP	(1 << 2)
> > +
> >   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;
> > +	/* Host-to-guest command, readonly by guest */
> > +	__u32 host2guest_cmd;
> > +	/* Sequence id of the free_page report command, readonly by guest */
> > +	__u32 free_page_cmd_id;
> > +	/* Guest-to-host command */
> > +	__u32 guest2host_cmd;
> >   };
> >   #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */

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