Virtualization - Jan 2018 - [PATCH v25 0/2] Virtio-balloon: support free page reporting

This patch series is separated from the previous "Virtio-balloon
Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,  
implemented by this series enables the virtio-balloon driver to report
hints of guest free pages to the host. It can be used to accelerate live
migration of VMs. Here is an introduction of this usage:

Live migration needs to transfer the VM's memory from the source machine
to the destination round by round. For the 1st round, all the VM's memory
is transferred. From the 2nd round, only the pieces of memory that were
written by the guest (after the 1st round) are transferred. One method
that is popularly used by the hypervisor to track which part of memory is
written is to write-protect all the guest memory.

The second feature enables the optimization of the 1st round memory
transfer - the hypervisor can skip the transfer of guest free pages in the
1st round. It is not concerned that the memory pages are used after they
are given to the hypervisor as a hint of the free pages, because they will
be tracked by the hypervisor and transferred in the next round if they are
used and written.

ChangeLog:
v24->v25:
    - mm: change walk_free_mem_block to return 0 (instead of true) on
          completing the report, and return a non-zero value from the
          callabck, which stops the reporting.
    - virtio-balloon:
        - use enum instead of define for VIRTIO_BALLOON_VQ_INFLATE etc.
        - avoid __virtio_clear_bit when bailing out;
        - a new method to avoid reporting the some cmd id to host twice
        - destroy_workqueue can cancel free page work when the feature is
          negotiated;
        - fail probe when the free page vq size is less than 2.
v23->v24:
    - change feature name VIRTIO_BALLOON_F_FREE_PAGE_VQ to
      VIRTIO_BALLOON_F_FREE_PAGE_HINT
    - kick when vq->num_free < half full, instead of "= half
full"
    - replace BUG_ON with bailing out
    - check vb->balloon_wq in probe(), if null, bail out
    - add a new feature bit for page poisoning
    - solve the corner case that one cmd id being sent to host twice
v22->v23:
    - change to kick the device when the vq is half-way full;
    - open-code batch_free_page_sg into add_one_sg;
    - change cmd_id from "uint32_t" to "__virtio32";
    - reserver one entry in the vq for teh driver to send cmd_id, instead
      of busywaiting for an available entry;
    - add "stop_update" check before queue_work for prudence purpose
for
      now, will have a separate patch to discuss this flag check later;
    - init_vqs: change to put some variables on stack to have simpler
      implementation;
    - add destroy_workqueue(vb->balloon_wq);

v21->v22:
    - add_one_sg: some code and comment re-arrangement
    - send_cmd_id: handle a cornercase

For previous ChangeLog, please reference
https://lwn.net/Articles/743660/

Wei Wang (2):
  mm: support reporting free page blocks
  virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

 drivers/virtio/virtio_balloon.c     | 251 ++++++++++++++++++++++++++++++------
 include/linux/mm.h                  |   6 +
 include/uapi/linux/virtio_balloon.h |   7 +
 mm/page_alloc.c                     |  96 ++++++++++++++
 4 files changed, 324 insertions(+), 36 deletions(-)

-- 
2.7.4

This patch adds support to walk through the free page blocks in the
system and report them via a callback function. Some page blocks may
leave the free list after zone->lock is released, so it is the caller's
responsibility to either detect or prevent the use of such pages.

One use example of this patch is to accelerate live migration by skipping
the transfer of free pages reported from the guest. A popular method used
by the hypervisor to track which part of memory is written during live
migration is to write-protect all the guest memory. So, those pages that
are reported as free pages but are written after the report function
returns will be captured by the hypervisor, and they will be added to the
next round of memory transfer.

Signed-off-by: Wei Wang <wei.w.wang at intel.com>
Signed-off-by: Liang Li <liang.z.li at intel.com>
Cc: Michal Hocko <mhocko at kernel.org>
Cc: Michael S. Tsirkin <mst at redhat.com>
Acked-by: Michal Hocko <mhocko at kernel.org>
---
 include/linux/mm.h |  6 ++++
 mm/page_alloc.c    | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ea818ff..e65ae2e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1938,6 +1938,12 @@ extern void free_area_init_node(int nid, unsigned long *
zones_size,
 		unsigned long zone_start_pfn, unsigned long *zholes_size);
 extern void free_initmem(void);
 
+extern int walk_free_mem_block(void *opaque,
+			       int min_order,
+			       int (*report_pfn_range)(void *opaque,
+						       unsigned long pfn,
+						       unsigned long num));
+
 /*
  * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK)
  * into the buddy system. The freed pages will be poisoned with pattern
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 76c9688..0f08039 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4899,6 +4899,102 @@ void show_free_areas(unsigned int filter, nodemask_t
*nodemask)
 	show_swap_cache_info();
 }
 
+/*
+ * Walk through a free page list and report the found pfn range via the
+ * callback.
+ *
+ * Return 0 if it completes the reporting. Otherwise, return the Non-zero
+ * value returned from the callback.
+ */
+static int walk_free_page_list(void *opaque,
+			       struct zone *zone,
+			       int order,
+			       enum migratetype mt,
+			       int (*report_pfn_range)(void *,
+						       unsigned long,
+						       unsigned long))
+{
+	struct page *page;
+	struct list_head *list;
+	unsigned long pfn, flags;
+	bool ret = 0;
+
+	spin_lock_irqsave(&zone->lock, flags);
+	list = &zone->free_area[order].free_list[mt];
+	list_for_each_entry(page, list, lru) {
+		pfn = page_to_pfn(page);
+		ret = report_pfn_range(opaque, pfn, 1 << order);
+		if (ret)
+			break;
+	}
+	spin_unlock_irqrestore(&zone->lock, flags);
+
+	return ret;
+}
+
+/**
+ * walk_free_mem_block - Walk through the free page blocks in the system
+ * @opaque: the context passed from the caller
+ * @min_order: the minimum order of free lists to check
+ * @report_pfn_range: the callback to report the pfn range of the free pages
+ *
+ * If the callback returns a non-zero value, stop iterating the list of free
+ * page blocks. Otherwise, continue to report.
+ *
+ * Please note that there are no locking guarantees for the callback and
+ * that the reported pfn range might be freed or disappear after the
+ * callback returns so the caller has to be very careful how it is used.
+ *
+ * The callback itself must not sleep or perform any operations which would
+ * require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC)
+ * or via any lock dependency. It is generally advisable to implement
+ * the callback as simple as possible and defer any heavy lifting to a
+ * different context.
+ *
+ * There is no guarantee that each free range will be reported only once
+ * during one walk_free_mem_block invocation.
+ *
+ * pfn_to_page on the given range is strongly discouraged and if there is
+ * an absolute need for that make sure to contact MM people to discuss
+ * potential problems.
+ *
+ * The function itself might sleep so it cannot be called from atomic
+ * contexts.
+ *
+ * In general low orders tend to be very volatile and so it makes more
+ * sense to query larger ones first for various optimizations which like
+ * ballooning etc... This will reduce the overhead as well.
+ *
+ * Return 0 if it completes the reporting. Otherwise, return the non-zero
+ * value returned from the callback.
+ */
+int walk_free_mem_block(void *opaque,
+			int min_order,
+			int (*report_pfn_range)(void *opaque,
+			unsigned long pfn,
+			unsigned long num))
+{
+	struct zone *zone;
+	int order;
+	enum migratetype mt;
+	bool ret;
+
+	for_each_populated_zone(zone) {
+		for (order = MAX_ORDER - 1; order >= min_order; order--) {
+			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
+				ret = walk_free_page_list(opaque, zone,
+							  order, mt,
+							  report_pfn_range);
+				if (ret)
+					return ret;
+			}
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(walk_free_mem_block);
+
 static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
 {
 	zoneref->zone = zone;
-- 
2.7.4

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 pages 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 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     | 251 ++++++++++++++++++++++++++++++------
 include/uapi/linux/virtio_balloon.h |   7 +
 2 files changed, 222 insertions(+), 36 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index a1fb52c..114985b 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -51,9 +51,22 @@ MODULE_PARM_DESC(oom_pages, "pages to free on
OOM");
 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 +76,11 @@ 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;
+
 	/* Waiting for host to ack the pages we released. */
 	wait_queue_head_t acked;
 
@@ -316,17 +334,6 @@ 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;
@@ -343,6 +350,34 @@ 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;
@@ -417,42 +452,151 @@ 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;
+	int ret = 0;
+
+	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 if the vq is full.
+	 * We are adding one entry each time, which essentially results in no
+	 * memory allocation, so the GFP_KERNEL flag below can be ignored.
+	 * There is always one entry reserved for the cmd id to use.
+	 */
+	if (vq->num_free > 1)
+		ret = virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
+
+	if (vq->num_free == virtqueue_get_vring_size(vq) / 2 ||
+	    vq->num_free <= 2)
+		virtqueue_kick(vq);
+
+	return ret;
+}
+
+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;
+
+	return add_one_sg(vb->free_page_vq, pfn, len);
+}
+
+static int send_cmd_id(struct virtio_balloon *vb, uint32_t cmd_id)
+{
+	struct scatterlist sg;
+	struct virtqueue *vq = vb->free_page_vq;
+	int ret;
+
+	vb->cmd_id_use = cpu_to_virtio32(vb->vdev, cmd_id);
+	sg_init_one(&sg, &vb->cmd_id_use, sizeof(vb->cmd_id_use));
+
+	ret = virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
+	virtqueue_kick(vq);
+
+	return ret;
+}
+
+static void report_free_page_func(struct work_struct *work)
+{
+	struct virtio_balloon *vb;
+	int ret;
+
+	vb = container_of(work, struct virtio_balloon, report_free_page_work);
+
+	/* Start by sending the received cmd id to host with an outbuf */
+	ret = send_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 < 0))
+		goto err;
+
+	/* End by sending a stop id to host with an outbuf */
+	ret = send_cmd_id(vb, VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
+	if (likely(!ret))
+		return;
+err:
+	dev_err(&vb->vdev->dev, "%s failure: free page vq is
broken\n",
+		__func__);
+}
+
 #ifdef CONFIG_BALLOON_COMPACTION
 /*
  * virtballoon_migratepage - perform the balloon page migration on behalf of
@@ -537,6 +681,7 @@ static struct file_system_type balloon_fs = {
 static int virtballoon_probe(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb;
+	__u32 poison_val;
 	int err;
 
 	if (!vdev->config->get) {
@@ -566,18 +711,39 @@ 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 hint.
+		 */
+		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;
+		}
+		INIT_WORK(&vb->report_free_page_work, report_free_page_func);
+		if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) {
+			poison_val = PAGE_POISON;
+			virtio_cwrite(vb->vdev, struct virtio_balloon_config,
+				      poison_val, &poison_val);
+		}
+	}
+
 	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;
@@ -587,7 +753,7 @@ 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
@@ -598,6 +764,9 @@ 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:
@@ -631,6 +800,11 @@ 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)
@@ -674,6 +848,9 @@ static int virtballoon_restore(struct virtio_device *vdev)
 
 static int virtballoon_validate(struct virtio_device *vdev)
 {
+	if (!page_poisoning_enabled())
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
+
 	__virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
 	return 0;
 }
@@ -682,6 +859,8 @@ static unsigned int features[] = {
 	VIRTIO_BALLOON_F_MUST_TELL_HOST,
 	VIRTIO_BALLOON_F_STATS_VQ,
 	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
+	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
+	VIRTIO_BALLOON_F_PAGE_POISON,
 };
 
 static struct virtio_driver virtio_balloon_driver = {
diff --git a/include/uapi/linux/virtio_balloon.h
b/include/uapi/linux/virtio_balloon.h
index 343d7dd..3f97067 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -34,15 +34,22 @@
 #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 */
+#define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
 
 /* 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;
+	/* Stores PAGE_POISON if page poisoning is in use */
+	__u32 poison_val;
 };
 
 #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
-- 
2.7.4

On Thu, Jan 25, 2018 at 05:14:05PM +0800, Wei Wang
wrote:
> This patch adds support to walk through the free page blocks in the
> system and report them via a callback function. Some page blocks may
> leave the free list after zone->lock is released, so it is the
caller's
> responsibility to either detect or prevent the use of such pages.
> 
> One use example of this patch is to accelerate live migration by skipping
> the transfer of free pages reported from the guest. A popular method used
> by the hypervisor to track which part of memory is written during live
> migration is to write-protect all the guest memory. So, those pages that
> are reported as free pages but are written after the report function
> returns will be captured by the hypervisor, and they will be added to the
> next round of memory transfer.
> 
> Signed-off-by: Wei Wang <wei.w.wang at intel.com>
> Signed-off-by: Liang Li <liang.z.li at intel.com>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Acked-by: Michal Hocko <mhocko at kernel.org>
Commented on v24 that this should be restartable. That comment
still applies.
> ---
>  include/linux/mm.h |  6 ++++
>  mm/page_alloc.c    | 96
++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 102 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ea818ff..e65ae2e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1938,6 +1938,12 @@ extern void free_area_init_node(int nid, unsigned
long * zones_size,
>  		unsigned long zone_start_pfn, unsigned long *zholes_size);
>  extern void free_initmem(void);
>  
> +extern int walk_free_mem_block(void *opaque,
> +			       int min_order,
> +			       int (*report_pfn_range)(void *opaque,
> +						       unsigned long pfn,
> +						       unsigned long num));
> +
>  /*
>   * Free reserved pages within range [PAGE_ALIGN(start), end &
PAGE_MASK)
>   * into the buddy system. The freed pages will be poisoned with pattern
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 76c9688..0f08039 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4899,6 +4899,102 @@ void show_free_areas(unsigned int filter,
nodemask_t *nodemask)
>  	show_swap_cache_info();
>  }
>  
> +/*
> + * Walk through a free page list and report the found pfn range via the
> + * callback.
> + *
> + * Return 0 if it completes the reporting. Otherwise, return the Non-zero
> + * value returned from the callback.
> + */
> +static int walk_free_page_list(void *opaque,
> +			       struct zone *zone,
> +			       int order,
> +			       enum migratetype mt,
> +			       int (*report_pfn_range)(void *,
> +						       unsigned long,
> +						       unsigned long))
> +{
> +	struct page *page;
> +	struct list_head *list;
> +	unsigned long pfn, flags;
> +	bool ret = 0;
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +	list = &zone->free_area[order].free_list[mt];
> +	list_for_each_entry(page, list, lru) {
> +		pfn = page_to_pfn(page);
> +		ret = report_pfn_range(opaque, pfn, 1 << order);
> +		if (ret)
> +			break;
> +	}
> +	spin_unlock_irqrestore(&zone->lock, flags);
> +
> +	return ret;
> +}
> +
> +/**
> + * walk_free_mem_block - Walk through the free page blocks in the system
> + * @opaque: the context passed from the caller
> + * @min_order: the minimum order of free lists to check
> + * @report_pfn_range: the callback to report the pfn range of the free
pages
> + *
> + * If the callback returns a non-zero value, stop iterating the list of
free
> + * page blocks. Otherwise, continue to report.
> + *
> + * Please note that there are no locking guarantees for the callback and
> + * that the reported pfn range might be freed or disappear after the
> + * callback returns so the caller has to be very careful how it is used.
> + *
> + * The callback itself must not sleep or perform any operations which
would
> + * require any memory allocations directly (not even
GFP_NOWAIT/GFP_ATOMIC)
> + * or via any lock dependency. It is generally advisable to implement
> + * the callback as simple as possible and defer any heavy lifting to a
> + * different context.
> + *
> + * There is no guarantee that each free range will be reported only once
> + * during one walk_free_mem_block invocation.
> + *
> + * pfn_to_page on the given range is strongly discouraged and if there is
> + * an absolute need for that make sure to contact MM people to discuss
> + * potential problems.
> + *
> + * The function itself might sleep so it cannot be called from atomic
> + * contexts.
> + *
> + * In general low orders tend to be very volatile and so it makes more
> + * sense to query larger ones first for various optimizations which like
> + * ballooning etc... This will reduce the overhead as well.
> + *
> + * Return 0 if it completes the reporting. Otherwise, return the non-zero
> + * value returned from the callback.
> + */
> +int walk_free_mem_block(void *opaque,
> +			int min_order,
> +			int (*report_pfn_range)(void *opaque,
> +			unsigned long pfn,
> +			unsigned long num))
> +{
> +	struct zone *zone;
> +	int order;
> +	enum migratetype mt;
> +	bool ret;
> +
> +	for_each_populated_zone(zone) {
> +		for (order = MAX_ORDER - 1; order >= min_order; order--) {
> +			for (mt = 0; mt < MIGRATE_TYPES; mt++) {
> +				ret = walk_free_page_list(opaque, zone,
> +							  order, mt,
> +							  report_pfn_range);
> +				if (ret)
> +					return ret;
> +			}
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(walk_free_mem_block);
> +
>  static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref)
>  {
>  	zoneref->zone = zone;
> -- 
> 2.7.4

On Thu, Jan 25, 2018 at 05:14:06PM +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 pages 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 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     | 251
++++++++++++++++++++++++++++++------
>  include/uapi/linux/virtio_balloon.h |   7 +
>  2 files changed, 222 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c
b/drivers/virtio/virtio_balloon.c
> index a1fb52c..114985b 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -51,9 +51,22 @@ MODULE_PARM_DESC(oom_pages, "pages to free on
OOM");
>  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 +76,11 @@ 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;
> +
>  	/* Waiting for host to ack the pages we released. */
>  	wait_queue_head_t acked;
>  
> @@ -316,17 +334,6 @@ 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;
> @@ -343,6 +350,34 @@ 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;
> @@ -417,42 +452,151 @@ 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;
> +	int ret = 0;
> +
> +	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 if the vq is full.
> +	 * We are adding one entry each time, which essentially results in no
> +	 * memory allocation, so the GFP_KERNEL flag below can be ignored.
> +	 * There is always one entry reserved for the cmd id to use.
> +	 */
> +	if (vq->num_free > 1)
> +		ret = virtqueue_add_inbuf(vq, &sg, 1, vq, GFP_KERNEL);
> +
> +	if (vq->num_free == virtqueue_get_vring_size(vq) / 2 ||
> +	    vq->num_free <= 2)
> +		virtqueue_kick(vq);
> +
> +	return ret;
> +}
> +
> +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;
> +
> +	return add_one_sg(vb->free_page_vq, pfn, len);
> +}
> +
> +static int send_cmd_id(struct virtio_balloon *vb, uint32_t cmd_id)
> +{
> +	struct scatterlist sg;
> +	struct virtqueue *vq = vb->free_page_vq;
> +	int ret;
> +
> +	vb->cmd_id_use = cpu_to_virtio32(vb->vdev, cmd_id);
> +	sg_init_one(&sg, &vb->cmd_id_use, sizeof(vb->cmd_id_use));
> +
> +	ret = virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> +	virtqueue_kick(vq);
> +
> +	return ret;
> +}
> +
> +static void report_free_page_func(struct work_struct *work)
> +{
> +	struct virtio_balloon *vb;
> +	int ret;
> +
> +	vb = container_of(work, struct virtio_balloon, report_free_page_work);
> +
> +	/* Start by sending the received cmd id to host with an outbuf */
> +	ret = send_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 < 0))
> +		goto err;
> +
> +	/* End by sending a stop id to host with an outbuf */
> +	ret = send_cmd_id(vb, VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID);
> +	if (likely(!ret))
> +		return;
> +err:
> +	dev_err(&vb->vdev->dev, "%s failure: free page vq is
broken\n",
> +		__func__);
> +}
> +
So that's very simple, but it only works well if the whole
free list fits in the queue or host processes the queue faster
than the guest. What if it doesn't?
If we had restartability you could just drop the lock
and wait for a vq interrupt to make more progress, which
would be better I think.

>  #ifdef CONFIG_BALLOON_COMPACTION
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf
of
> @@ -537,6 +681,7 @@ static struct file_system_type balloon_fs = {
>  static int virtballoon_probe(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb;
> +	__u32 poison_val;
>  	int err;
>  
>  	if (!vdev->config->get) {
> @@ -566,18 +711,39 @@ 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 hint.
> +		 */
> +		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;
> +		}
> +		INIT_WORK(&vb->report_free_page_work, report_free_page_func);
> +		if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON)) {
> +			poison_val = PAGE_POISON;
> +			virtio_cwrite(vb->vdev, struct virtio_balloon_config,
> +				      poison_val, &poison_val);
> +		}
> +	}
> +
>  	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;
> @@ -587,7 +753,7 @@ 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
> @@ -598,6 +764,9 @@ 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:
> @@ -631,6 +800,11 @@ 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)
> @@ -674,6 +848,9 @@ static int virtballoon_restore(struct virtio_device
*vdev)
>  
>  static int virtballoon_validate(struct virtio_device *vdev)
>  {
> +	if (!page_poisoning_enabled())
> +		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
> +
>  	__virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
>  	return 0;
>  }
> @@ -682,6 +859,8 @@ static unsigned int features[] = {
>  	VIRTIO_BALLOON_F_MUST_TELL_HOST,
>  	VIRTIO_BALLOON_F_STATS_VQ,
>  	VIRTIO_BALLOON_F_DEFLATE_ON_OOM,
> +	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
> +	VIRTIO_BALLOON_F_PAGE_POISON,
>  };
>  
>  static struct virtio_driver virtio_balloon_driver = {
> diff --git a/include/uapi/linux/virtio_balloon.h
b/include/uapi/linux/virtio_balloon.h
> index 343d7dd..3f97067 100644
> --- a/include/uapi/linux/virtio_balloon.h
> +++ b/include/uapi/linux/virtio_balloon.h
> @@ -34,15 +34,22 @@
>  #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 */
> +#define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
>  
>  /* 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;
> +	/* Stores PAGE_POISON if page poisoning is in use */
> +	__u32 poison_val;
>  };
>  
>  #define VIRTIO_BALLOON_S_SWAP_IN  0   /* Amount of memory swapped in */
> -- 
> 2.7.4

On Thu, Jan 25, 2018 at 05:14:04PM +0800, Wei Wang
wrote:
> This patch series is separated from the previous "Virtio-balloon
> Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,
> implemented by this series enables the virtio-balloon driver to report
> hints of guest free pages to the host. It can be used to accelerate live
> migration of VMs. Here is an introduction of this usage:
> 
> Live migration needs to transfer the VM's memory from the source
machine
> to the destination round by round. For the 1st round, all the VM's
memory
> is transferred. From the 2nd round, only the pieces of memory that were
> written by the guest (after the 1st round) are transferred. One method
> that is popularly used by the hypervisor to track which part of memory is
> written is to write-protect all the guest memory.
> 
> The second feature enables the optimization of the 1st round memory
> transfer - the hypervisor can skip the transfer of guest free pages in the
> 1st round. It is not concerned that the memory pages are used after they
> are given to the hypervisor as a hint of the free pages, because they will
> be tracked by the hypervisor and transferred in the next round if they are
> used and written.
Could you post performance numbers please?
> ChangeLog:
> v24->v25:
>     - mm: change walk_free_mem_block to return 0 (instead of true) on
>           completing the report, and return a non-zero value from the
>           callabck, which stops the reporting.
>     - virtio-balloon:
>         - use enum instead of define for VIRTIO_BALLOON_VQ_INFLATE etc.
>         - avoid __virtio_clear_bit when bailing out;
>         - a new method to avoid reporting the some cmd id to host twice
>         - destroy_workqueue can cancel free page work when the feature is
>           negotiated;
>         - fail probe when the free page vq size is less than 2.
> v23->v24:
>     - change feature name VIRTIO_BALLOON_F_FREE_PAGE_VQ to
>       VIRTIO_BALLOON_F_FREE_PAGE_HINT
>     - kick when vq->num_free < half full, instead of "= half
full"
>     - replace BUG_ON with bailing out
>     - check vb->balloon_wq in probe(), if null, bail out
>     - add a new feature bit for page poisoning
>     - solve the corner case that one cmd id being sent to host twice
> v22->v23:
>     - change to kick the device when the vq is half-way full;
>     - open-code batch_free_page_sg into add_one_sg;
>     - change cmd_id from "uint32_t" to "__virtio32";
>     - reserver one entry in the vq for teh driver to send cmd_id, instead
>       of busywaiting for an available entry;
>     - add "stop_update" check before queue_work for prudence
purpose for
>       now, will have a separate patch to discuss this flag check later;
>     - init_vqs: change to put some variables on stack to have simpler
>       implementation;
>     - add destroy_workqueue(vb->balloon_wq);
> 
> v21->v22:
>     - add_one_sg: some code and comment re-arrangement
>     - send_cmd_id: handle a cornercase
> 
> For previous ChangeLog, please reference
> https://lwn.net/Articles/743660/
> 
> Wei Wang (2):
>   mm: support reporting free page blocks
>   virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT
> 
>  drivers/virtio/virtio_balloon.c     | 251
++++++++++++++++++++++++++++++------
>  include/linux/mm.h                  |   6 +
>  include/uapi/linux/virtio_balloon.h |   7 +
>  mm/page_alloc.c                     |  96 ++++++++++++++
>  4 files changed, 324 insertions(+), 36 deletions(-)
> 
> -- 
> 2.7.4

On 02/02/2018 03:15 AM, Michael S. Tsirkin wrote:
> On Thu, Jan 25, 2018 at 05:14:04PM +0800, Wei Wang wrote:
>> This patch series is separated from the previous "Virtio-balloon
>> Enhancement" series. The new feature,
VIRTIO_BALLOON_F_FREE_PAGE_HINT,
>> implemented by this series enables the virtio-balloon driver to report
>> hints of guest free pages to the host. It can be used to accelerate
live
>> migration of VMs. Here is an introduction of this usage:
>>
>> Live migration needs to transfer the VM's memory from the source
machine
>> to the destination round by round. For the 1st round, all the VM's
memory
>> is transferred. From the 2nd round, only the pieces of memory that were
>> written by the guest (after the 1st round) are transferred. One method
>> that is popularly used by the hypervisor to track which part of memory
is
>> written is to write-protect all the guest memory.
>>
>> The second feature enables the optimization of the 1st round memory
>> transfer - the hypervisor can skip the transfer of guest free pages in
the
>> 1st round. It is not concerned that the memory pages are used after
they
>> are given to the hypervisor as a hint of the free pages, because they
will
>> be tracked by the hypervisor and transferred in the next round if they
are
>> used and written.
> Could you post performance numbers please?
Yes, it was posted here https://lkml.org/lkml/2018/1/25/698

I just changed the host side to poll the vq so that we don't need kick 
in the driver, it works pretty well. I'll test a little bit more and 
post out a new version with new performance numbers attached in the 
cover letter.

Best,
Wei