drivers/virtio/virtio_balloon.c - arm/linux-arm64-legacy - Git at Google

 /*
  * Virtio balloon implementation, inspired by Dor Laor and Marcelo
  * Tosatti's implementations.
  *
  *  Copyright 2008 Rusty Russell IBM Corporation
  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
  *  the Free Software Foundation; either version 2 of the License, or
  *  (at your option) any later version.
  *
  *  This program is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
  *  along with this program; if not, write to the Free Software
  *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  */

 #include <linux/virtio.h>
 #include <linux/virtio_balloon.h>
 #include <linux/swap.h>
 #include <linux/kthread.h>
 #include <linux/freezer.h>
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/module.h>
 #include <linux/balloon_compaction.h>

 /*
  * Balloon device works in 4K page units.  So each page is pointed to by
  * multiple balloon pages.  All memory counters in this driver are in balloon
  * page units.
  */
 #define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
 #define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256

 struct virtio_balloon
 {
 	struct virtio_device *vdev;
 	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;

 	/* Where the ballooning thread waits for config to change. */
 	wait_queue_head_t config_change;

 	/* The thread servicing the balloon. */
 	struct task_struct *thread;

 	/* Waiting for host to ack the pages we released. */
 	wait_queue_head_t acked;

 	/* Number of balloon pages we've told the Host we're not using. */
 	unsigned int num_pages;
 	/*
 	 * The pages we've told the Host we're not using are enqueued
 	 * at vb_dev_info->pages list.
 	 * Each page on this list adds VIRTIO_BALLOON_PAGES_PER_PAGE
 	 * to num_pages above.
 	 */
 	struct balloon_dev_info *vb_dev_info;

 	/* Synchronize access/update to this struct virtio_balloon elements */
 	struct mutex balloon_lock;

 	/* The array of pfns we tell the Host about. */
 	unsigned int num_pfns;
 	u32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];

 	/* Memory statistics */
 	int need_stats_update;
 	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
 };

 static struct virtio_device_id id_table[] = {
 	{ VIRTIO_ID_BALLOON, VIRTIO_DEV_ANY_ID },
 	{ 0 },
 };

 static u32 page_to_balloon_pfn(struct page *page)
 {
 	unsigned long pfn = page_to_pfn(page);

 	BUILD_BUG_ON(PAGE_SHIFT < VIRTIO_BALLOON_PFN_SHIFT);
 	/* Convert pfn from Linux page size to balloon page size. */
 	return pfn * VIRTIO_BALLOON_PAGES_PER_PAGE;
 }

 static struct page *balloon_pfn_to_page(u32 pfn)
 {
 	BUG_ON(pfn % VIRTIO_BALLOON_PAGES_PER_PAGE);
 	return pfn_to_page(pfn / VIRTIO_BALLOON_PAGES_PER_PAGE);
 }

 static void balloon_ack(struct virtqueue *vq)
 {
 	struct virtio_balloon *vb = vq->vdev->priv;

 	wake_up(&vb->acked);
 }

 static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 {
 	struct scatterlist sg;
 	unsigned int len;

 	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);

 	/* We should always be able to add one buffer to an empty queue. */
 	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
 	virtqueue_kick(vq);

 	/* When host has read buffer, this completes via balloon_ack */
 	wait_event(vb->acked, virtqueue_get_buf(vq, &len));
 }

 static void set_page_pfns(u32 pfns[], struct page *page)
 {
 	unsigned int i;

 	/* Set balloon pfns pointing at this page.
 	 * Note that the first pfn points at start of the page. */
 	for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
 		pfns[i] = page_to_balloon_pfn(page) + i;
 }

 static void fill_balloon(struct virtio_balloon *vb, size_t num)
 {
 	struct balloon_dev_info *vb_dev_info = vb->vb_dev_info;

 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));

 	mutex_lock(&vb->balloon_lock);
 	for (vb->num_pfns = 0; vb->num_pfns < num;
 	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
 		struct page *page = balloon_page_enqueue(vb_dev_info);

 		if (!page) {
 			dev_info_ratelimited(&vb->vdev->dev,
 					     "Out of puff! Can't get %u pages\n",
 					     VIRTIO_BALLOON_PAGES_PER_PAGE);
 			/* Sleep for at least 1/5 of a second before retry. */
 			msleep(200);
 			break;
 		}
 		set_page_pfns(vb->pfns + vb->num_pfns, page);
 		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
 		adjust_managed_page_count(page, -1);
 	}

 	/* Did we get any? */
 	if (vb->num_pfns != 0)
 		tell_host(vb, vb->inflate_vq);
 	mutex_unlock(&vb->balloon_lock);
 }

 static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
 {
 	unsigned int i;

 	/* Find pfns pointing at start of each page, get pages and free them. */
 	for (i = 0; i < num; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
 		struct page *page = balloon_pfn_to_page(pfns[i]);
 		balloon_page_free(page);
 		adjust_managed_page_count(page, 1);
 	}
 }

 static void leak_balloon(struct virtio_balloon *vb, size_t num)
 {
 	struct page *page;
 	struct balloon_dev_info *vb_dev_info = vb->vb_dev_info;

 	/* We can only do one array worth at a time. */
 	num = min(num, ARRAY_SIZE(vb->pfns));

 	mutex_lock(&vb->balloon_lock);
 	for (vb->num_pfns = 0; vb->num_pfns < num;
 	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
 		page = balloon_page_dequeue(vb_dev_info);
 		if (!page)
 			break;
 		set_page_pfns(vb->pfns + vb->num_pfns, page);
 		vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
 	}

 	/*
 	 * Note that if
 	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
 	 * is true, we *have* to do it in this order
 	 */
 	if (vb->num_pfns != 0)
 		tell_host(vb, vb->deflate_vq);
 	mutex_unlock(&vb->balloon_lock);
 	release_pages_by_pfn(vb->pfns, vb->num_pfns);
 }

 static inline void update_stat(struct virtio_balloon *vb, int idx,
 			       u16 tag, u64 val)
 {
 	BUG_ON(idx >= VIRTIO_BALLOON_S_NR);
 	vb->stats[idx].tag = tag;
 	vb->stats[idx].val = val;
 }

 #define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)

 static void update_balloon_stats(struct virtio_balloon *vb)
 {
 	unsigned long events[NR_VM_EVENT_ITEMS];
 	struct sysinfo i;
 	int idx = 0;

 	all_vm_events(events);
 	si_meminfo(&i);

 	update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
 				pages_to_bytes(events[PSWPIN]));
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_OUT,
 				pages_to_bytes(events[PSWPOUT]));
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MAJFLT, events[PGMAJFAULT]);
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MINFLT, events[PGFAULT]);
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMFREE,
 				pages_to_bytes(i.freeram));
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMTOT,
 				pages_to_bytes(i.totalram));
 }

 /*
  * While most virtqueues communicate guest-initiated requests to the hypervisor,
  * the stats queue operates in reverse.  The driver initializes the virtqueue
  * with a single buffer.  From that point forward, all conversations consist of
  * a hypervisor request (a call to this function) which directs us to refill
  * the virtqueue with a fresh stats buffer.  Since stats collection can sleep,
  * we notify our kthread which does the actual work via stats_handle_request().
  */
 static void stats_request(struct virtqueue *vq)
 {
 	struct virtio_balloon *vb = vq->vdev->priv;

 	vb->need_stats_update = 1;
 	wake_up(&vb->config_change);
 }

 static void stats_handle_request(struct virtio_balloon *vb)
 {
 	struct virtqueue *vq;
 	struct scatterlist sg;
 	unsigned int len;

 	vb->need_stats_update = 0;
 	update_balloon_stats(vb);

 	vq = vb->stats_vq;
 	if (!virtqueue_get_buf(vq, &len))
 		return;
 	sg_init_one(&sg, vb->stats, sizeof(vb->stats));
 	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
 	virtqueue_kick(vq);
 }

 static void virtballoon_changed(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;

 	wake_up(&vb->config_change);
 }

 static inline s64 towards_target(struct virtio_balloon *vb)
 {
 	__le32 v;
 	s64 target;

 	virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages, &v);

 	target = le32_to_cpu(v);
 	return target - vb->num_pages;
 }

 static void update_balloon_size(struct virtio_balloon *vb)
 {
 	__le32 actual = cpu_to_le32(vb->num_pages);

 	virtio_cwrite(vb->vdev, struct virtio_balloon_config, actual,
 		      &actual);
 }

 static int balloon(void *_vballoon)
 {
 	struct virtio_balloon *vb = _vballoon;

 	set_freezable();
 	while (!kthread_should_stop()) {
 		s64 diff;

 		try_to_freeze();
 		wait_event_interruptible(vb->config_change,
 					 (diff = towards_target(vb)) != 0
 					 || vb->need_stats_update
 					 || kthread_should_stop()
 					 || freezing(current));
 		if (vb->need_stats_update)
 			stats_handle_request(vb);
 		if (diff > 0)
 			fill_balloon(vb, diff);
 		else if (diff < 0)
 			leak_balloon(vb, -diff);
 		update_balloon_size(vb);

 		/*
 		 * For large balloon changes, we could spend a lot of time
 		 * and always have work to do.  Be nice if preempt disabled.
 		 */
 		cond_resched();
 	}
 	return 0;
 }

 static int init_vqs(struct virtio_balloon *vb)
 {
 	struct virtqueue *vqs[3];
 	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
 	const char *names[] = { "inflate", "deflate", "stats" };
 	int err, nvqs;

 	/*
 	 * We expect two virtqueues: inflate and deflate, and
 	 * optionally stat.
 	 */
 	nvqs = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
 	err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks, names);
 	if (err)
 		return err;

 	vb->inflate_vq = vqs[0];
 	vb->deflate_vq = vqs[1];
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
 		struct scatterlist sg;
 		vb->stats_vq = vqs[2];

 		/*
 		 * Prime this virtqueue with one buffer so the hypervisor can
 		 * use it to signal us later (it can't be broken yet!).
 		 */
 		sg_init_one(&sg, vb->stats, sizeof vb->stats);
 		if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
 		    < 0)
 			BUG();
 		virtqueue_kick(vb->stats_vq);
 	}
 	return 0;
 }

 static const struct address_space_operations virtio_balloon_aops;
 #ifdef CONFIG_BALLOON_COMPACTION
 /*
  * virtballoon_migratepage - perform the balloon page migration on behalf of
  *			     a compation thread.     (called under page lock)
  * @mapping: the page->mapping which will be assigned to the new migrated page.
  * @newpage: page that will replace the isolated page after migration finishes.
  * @page   : the isolated (old) page that is about to be migrated to newpage.
  * @mode   : compaction mode -- not used for balloon page migration.
  *
  * After a ballooned page gets isolated by compaction procedures, this is the
  * function that performs the page migration on behalf of a compaction thread
  * The page migration for virtio balloon is done in a simple swap fashion which
  * follows these two macro steps:
  *  1) insert newpage into vb->pages list and update the host about it;
  *  2) update the host about the old page removed from vb->pages list;
  *
  * This function preforms the balloon page migration task.
  * Called through balloon_mapping->a_ops->migratepage
  */
 static int virtballoon_migratepage(struct address_space *mapping,
 		struct page *newpage, struct page *page, enum migrate_mode mode)
 {
 	struct balloon_dev_info *vb_dev_info = balloon_page_device(page);
 	struct virtio_balloon *vb;
 	unsigned long flags;

 	BUG_ON(!vb_dev_info);

 	vb = vb_dev_info->balloon_device;

 	/*
 	 * In order to avoid lock contention while migrating pages concurrently
 	 * to leak_balloon() or fill_balloon() we just give up the balloon_lock
 	 * this turn, as it is easier to retry the page migration later.
 	 * This also prevents fill_balloon() getting stuck into a mutex
 	 * recursion in the case it ends up triggering memory compaction
 	 * while it is attempting to inflate the ballon.
 	 */
 	if (!mutex_trylock(&vb->balloon_lock))
 		return -EAGAIN;

 	/* balloon's page migration 1st step  -- inflate "newpage" */
 	spin_lock_irqsave(&vb_dev_info->pages_lock, flags);
 	balloon_page_insert(newpage, mapping, &vb_dev_info->pages);
 	vb_dev_info->isolated_pages--;
 	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
 	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
 	set_page_pfns(vb->pfns, newpage);
 	tell_host(vb, vb->inflate_vq);

 	/*
 	 * balloon's page migration 2nd step -- deflate "page"
 	 *
 	 * It's safe to delete page->lru here because this page is at
 	 * an isolated migration list, and this step is expected to happen here
 	 */
 	balloon_page_delete(page);
 	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
 	set_page_pfns(vb->pfns, page);
 	tell_host(vb, vb->deflate_vq);

 	mutex_unlock(&vb->balloon_lock);

 	return MIGRATEPAGE_BALLOON_SUCCESS;
 }

 /* define the balloon_mapping->a_ops callback to allow balloon page migration */
 static const struct address_space_operations virtio_balloon_aops = {
 			.migratepage = virtballoon_migratepage,
 };
 #endif /* CONFIG_BALLOON_COMPACTION */

 static int virtballoon_probe(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb;
 	struct address_space *vb_mapping;
 	struct balloon_dev_info *vb_devinfo;
 	int err;

 	vdev->priv = vb = kmalloc(sizeof(*vb), GFP_KERNEL);
 	if (!vb) {
 		err = -ENOMEM;
 		goto out;
 	}

 	vb->num_pages = 0;
 	mutex_init(&vb->balloon_lock);
 	init_waitqueue_head(&vb->config_change);
 	init_waitqueue_head(&vb->acked);
 	vb->vdev = vdev;
 	vb->need_stats_update = 0;

 	vb_devinfo = balloon_devinfo_alloc(vb);
 	if (IS_ERR(vb_devinfo)) {
 		err = PTR_ERR(vb_devinfo);
 		goto out_free_vb;
 	}

 	vb_mapping = balloon_mapping_alloc(vb_devinfo,
 					   (balloon_compaction_check()) ?
 					   &virtio_balloon_aops : NULL);
 	if (IS_ERR(vb_mapping)) {
 		/*
 		 * IS_ERR(vb_mapping) && PTR_ERR(vb_mapping) == -EOPNOTSUPP
 		 * This means !CONFIG_BALLOON_COMPACTION, otherwise we get off.
 		 */
 		err = PTR_ERR(vb_mapping);
 		if (err != -EOPNOTSUPP)
 			goto out_free_vb_devinfo;
 	}

 	vb->vb_dev_info = vb_devinfo;

 	err = init_vqs(vb);
 	if (err)
 		goto out_free_vb_mapping;

 	vb->thread = kthread_run(balloon, vb, "vballoon");
 	if (IS_ERR(vb->thread)) {
 		err = PTR_ERR(vb->thread);
 		goto out_del_vqs;
 	}

 	return 0;

 out_del_vqs:
 	vdev->config->del_vqs(vdev);
 out_free_vb_mapping:
 	balloon_mapping_free(vb_mapping);
 out_free_vb_devinfo:
 	balloon_devinfo_free(vb_devinfo);
 out_free_vb:
 	kfree(vb);
 out:
 	return err;
 }

 static void remove_common(struct virtio_balloon *vb)
 {
 	/* There might be pages left in the balloon: free them. */
 	while (vb->num_pages)
 		leak_balloon(vb, vb->num_pages);
 	update_balloon_size(vb);

 	/* Now we reset the device so we can clean up the queues. */
 	vb->vdev->config->reset(vb->vdev);

 	vb->vdev->config->del_vqs(vb->vdev);
 }

 static void virtballoon_remove(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;

 	kthread_stop(vb->thread);
 	remove_common(vb);
 	balloon_mapping_free(vb->vb_dev_info->mapping);
 	balloon_devinfo_free(vb->vb_dev_info);
 	kfree(vb);
 }

 #ifdef CONFIG_PM_SLEEP
 static int virtballoon_freeze(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;

 	/*
 	 * The kthread is already frozen by the PM core before this
 	 * function is called.
 	 */

 	remove_common(vb);
 	return 0;
 }

 static int virtballoon_restore(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;
 	int ret;

 	ret = init_vqs(vdev->priv);
 	if (ret)
 		return ret;

 	fill_balloon(vb, towards_target(vb));
 	update_balloon_size(vb);
 	return 0;
 }
 #endif

 static unsigned int features[] = {
 	VIRTIO_BALLOON_F_MUST_TELL_HOST,
 	VIRTIO_BALLOON_F_STATS_VQ,
 };

 static struct virtio_driver virtio_balloon_driver = {
 	.feature_table = features,
 	.feature_table_size = ARRAY_SIZE(features),
 	.driver.name =	KBUILD_MODNAME,
 	.driver.owner =	THIS_MODULE,
 	.id_table =	id_table,
 	.probe =	virtballoon_probe,
 	.remove =	virtballoon_remove,
 	.config_changed = virtballoon_changed,
 #ifdef CONFIG_PM_SLEEP
 	.freeze	=	virtballoon_freeze,
 	.restore =	virtballoon_restore,
 #endif
 };

 module_virtio_driver(virtio_balloon_driver);
 MODULE_DEVICE_TABLE(virtio, id_table);
 MODULE_DESCRIPTION("Virtio balloon driver");
 MODULE_LICENSE("GPL");
	/*
	* Virtio balloon implementation, inspired by Dor Laor and Marcelo
	* Tosatti's implementations.
	*
	* Copyright 2008 Rusty Russell IBM Corporation
	*
	* This program is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License as published by
	* the Free Software Foundation; either version 2 of the License, or
	* (at your option) any later version.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program; if not, write to the Free Software
	* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
	*/

	#include <linux/virtio.h>
	#include <linux/virtio_balloon.h>
	#include <linux/swap.h>
	#include <linux/kthread.h>
	#include <linux/freezer.h>
	#include <linux/delay.h>
	#include <linux/slab.h>
	#include <linux/module.h>
	#include <linux/balloon_compaction.h>

	/*
	* Balloon device works in 4K page units. So each page is pointed to by
	* multiple balloon pages. All memory counters in this driver are in balloon
	* page units.
	*/
	#define VIRTIO_BALLOON_PAGES_PER_PAGE (unsigned)(PAGE_SIZE >> VIRTIO_BALLOON_PFN_SHIFT)
	#define VIRTIO_BALLOON_ARRAY_PFNS_MAX 256

	struct virtio_balloon
	{
	struct virtio_device *vdev;
	struct virtqueue inflate_vq, deflate_vq, *stats_vq;

	/* Where the ballooning thread waits for config to change. */
	wait_queue_head_t config_change;

	/* The thread servicing the balloon. */
	struct task_struct *thread;

	/* Waiting for host to ack the pages we released. */
	wait_queue_head_t acked;

	/* Number of balloon pages we've told the Host we're not using. */
	unsigned int num_pages;
	/*
	* The pages we've told the Host we're not using are enqueued
	* at vb_dev_info->pages list.
	* Each page on this list adds VIRTIO_BALLOON_PAGES_PER_PAGE
	* to num_pages above.
	*/
	struct balloon_dev_info *vb_dev_info;

	/* Synchronize access/update to this struct virtio_balloon elements */
	struct mutex balloon_lock;

	/* The array of pfns we tell the Host about. */
	unsigned int num_pfns;
	u32 pfns[VIRTIO_BALLOON_ARRAY_PFNS_MAX];

	/* Memory statistics */
	int need_stats_update;
	struct virtio_balloon_stat stats[VIRTIO_BALLOON_S_NR];
	};

	static struct virtio_device_id id_table[] = {
	{ VIRTIO_ID_BALLOON, VIRTIO_DEV_ANY_ID },
	{ 0 },
	};

	static u32 page_to_balloon_pfn(struct page *page)
	{
	unsigned long pfn = page_to_pfn(page);

	BUILD_BUG_ON(PAGE_SHIFT < VIRTIO_BALLOON_PFN_SHIFT);
	/* Convert pfn from Linux page size to balloon page size. */
	return pfn * VIRTIO_BALLOON_PAGES_PER_PAGE;
	}

	static struct page *balloon_pfn_to_page(u32 pfn)
	{
	BUG_ON(pfn % VIRTIO_BALLOON_PAGES_PER_PAGE);
	return pfn_to_page(pfn / VIRTIO_BALLOON_PAGES_PER_PAGE);
	}

	static void balloon_ack(struct virtqueue *vq)
	{
	struct virtio_balloon *vb = vq->vdev->priv;

	wake_up(&vb->acked);
	}

	static void tell_host(struct virtio_balloon vb, struct virtqueue vq)
	{
	struct scatterlist sg;
	unsigned int len;

	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);

	/* We should always be able to add one buffer to an empty queue. */
	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
	virtqueue_kick(vq);

	/* When host has read buffer, this completes via balloon_ack */
	wait_event(vb->acked, virtqueue_get_buf(vq, &len));
	}

	static void set_page_pfns(u32 pfns[], struct page *page)
	{
	unsigned int i;

	/* Set balloon pfns pointing at this page.
	* Note that the first pfn points at start of the page. */
	for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
	pfns[i] = page_to_balloon_pfn(page) + i;
	}

	static void fill_balloon(struct virtio_balloon *vb, size_t num)
	{
	struct balloon_dev_info *vb_dev_info = vb->vb_dev_info;

	/* We can only do one array worth at a time. */
	num = min(num, ARRAY_SIZE(vb->pfns));

	mutex_lock(&vb->balloon_lock);
	for (vb->num_pfns = 0; vb->num_pfns < num;
	vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
	struct page *page = balloon_page_enqueue(vb_dev_info);

	if (!page) {
	dev_info_ratelimited(&vb->vdev->dev,
	"Out of puff! Can't get %u pages\n",
	VIRTIO_BALLOON_PAGES_PER_PAGE);
	/* Sleep for at least 1/5 of a second before retry. */
	msleep(200);
	break;
	}
	set_page_pfns(vb->pfns + vb->num_pfns, page);
	vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
	adjust_managed_page_count(page, -1);
	}

	/* Did we get any? */
	if (vb->num_pfns != 0)
	tell_host(vb, vb->inflate_vq);
	mutex_unlock(&vb->balloon_lock);
	}

	static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
	{
	unsigned int i;

	/* Find pfns pointing at start of each page, get pages and free them. */
	for (i = 0; i < num; i += VIRTIO_BALLOON_PAGES_PER_PAGE) {
	struct page *page = balloon_pfn_to_page(pfns[i]);
	balloon_page_free(page);
	adjust_managed_page_count(page, 1);
	}
	}

	static void leak_balloon(struct virtio_balloon *vb, size_t num)
	{
	struct page *page;
	struct balloon_dev_info *vb_dev_info = vb->vb_dev_info;

	/* We can only do one array worth at a time. */
	num = min(num, ARRAY_SIZE(vb->pfns));

	mutex_lock(&vb->balloon_lock);
	for (vb->num_pfns = 0; vb->num_pfns < num;
	vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
	page = balloon_page_dequeue(vb_dev_info);
	if (!page)
	break;
	set_page_pfns(vb->pfns + vb->num_pfns, page);
	vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
	}

	/*
	* Note that if
	* virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
	* is true, we have to do it in this order
	*/
	if (vb->num_pfns != 0)
	tell_host(vb, vb->deflate_vq);
	mutex_unlock(&vb->balloon_lock);
	release_pages_by_pfn(vb->pfns, vb->num_pfns);
	}

	static inline void update_stat(struct virtio_balloon *vb, int idx,
	u16 tag, u64 val)
	{
	BUG_ON(idx >= VIRTIO_BALLOON_S_NR);
	vb->stats[idx].tag = tag;
	vb->stats[idx].val = val;
	}

	#define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)

	static void update_balloon_stats(struct virtio_balloon *vb)
	{
	unsigned long events[NR_VM_EVENT_ITEMS];
	struct sysinfo i;
	int idx = 0;

	all_vm_events(events);
	si_meminfo(&i);

	update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
	pages_to_bytes(events[PSWPIN]));
	update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_OUT,
	pages_to_bytes(events[PSWPOUT]));
	update_stat(vb, idx++, VIRTIO_BALLOON_S_MAJFLT, events[PGMAJFAULT]);
	update_stat(vb, idx++, VIRTIO_BALLOON_S_MINFLT, events[PGFAULT]);
	update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMFREE,
	pages_to_bytes(i.freeram));
	update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMTOT,
	pages_to_bytes(i.totalram));
	}

	/*
	* While most virtqueues communicate guest-initiated requests to the hypervisor,
	* the stats queue operates in reverse. The driver initializes the virtqueue
	* with a single buffer. From that point forward, all conversations consist of
	* a hypervisor request (a call to this function) which directs us to refill
	* the virtqueue with a fresh stats buffer. Since stats collection can sleep,
	* we notify our kthread which does the actual work via stats_handle_request().
	*/
	static void stats_request(struct virtqueue *vq)
	{
	struct virtio_balloon *vb = vq->vdev->priv;

	vb->need_stats_update = 1;
	wake_up(&vb->config_change);
	}

	static void stats_handle_request(struct virtio_balloon *vb)
	{
	struct virtqueue *vq;
	struct scatterlist sg;
	unsigned int len;

	vb->need_stats_update = 0;
	update_balloon_stats(vb);

	vq = vb->stats_vq;
	if (!virtqueue_get_buf(vq, &len))
	return;
	sg_init_one(&sg, vb->stats, sizeof(vb->stats));
	virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
	virtqueue_kick(vq);
	}

	static void virtballoon_changed(struct virtio_device *vdev)
	{
	struct virtio_balloon *vb = vdev->priv;

	wake_up(&vb->config_change);
	}

	static inline s64 towards_target(struct virtio_balloon *vb)
	{
	__le32 v;
	s64 target;

	virtio_cread(vb->vdev, struct virtio_balloon_config, num_pages, &v);

	target = le32_to_cpu(v);
	return target - vb->num_pages;
	}

	static void update_balloon_size(struct virtio_balloon *vb)
	{
	__le32 actual = cpu_to_le32(vb->num_pages);

	virtio_cwrite(vb->vdev, struct virtio_balloon_config, actual,
	&actual);
	}

	static int balloon(void *_vballoon)
	{
	struct virtio_balloon *vb = _vballoon;

	set_freezable();
	while (!kthread_should_stop()) {
	s64 diff;

	try_to_freeze();
	wait_event_interruptible(vb->config_change,
	(diff = towards_target(vb)) != 0
	\|\| vb->need_stats_update
	\|\| kthread_should_stop()
	\|\| freezing(current));
	if (vb->need_stats_update)
	stats_handle_request(vb);
	if (diff > 0)
	fill_balloon(vb, diff);
	else if (diff < 0)
	leak_balloon(vb, -diff);
	update_balloon_size(vb);

	/*
	* For large balloon changes, we could spend a lot of time
	* and always have work to do. Be nice if preempt disabled.
	*/
	cond_resched();
	}
	return 0;
	}

	static int init_vqs(struct virtio_balloon *vb)
	{
	struct virtqueue *vqs[3];
	vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request };
	const char *names[] = { "inflate", "deflate", "stats" };
	int err, nvqs;

	/*
	* We expect two virtqueues: inflate and deflate, and
	* optionally stat.
	*/
	nvqs = virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
	err = vb->vdev->config->find_vqs(vb->vdev, nvqs, vqs, callbacks, names);
	if (err)
	return err;

	vb->inflate_vq = vqs[0];
	vb->deflate_vq = vqs[1];
	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
	struct scatterlist sg;
	vb->stats_vq = vqs[2];

	/*
	* Prime this virtqueue with one buffer so the hypervisor can
	* use it to signal us later (it can't be broken yet!).
	*/
	sg_init_one(&sg, vb->stats, sizeof vb->stats);
	if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
	< 0)
	BUG();
	virtqueue_kick(vb->stats_vq);
	}
	return 0;
	}

	static const struct address_space_operations virtio_balloon_aops;
	#ifdef CONFIG_BALLOON_COMPACTION
	/*
	* virtballoon_migratepage - perform the balloon page migration on behalf of
	* a compation thread. (called under page lock)
	* @mapping: the page->mapping which will be assigned to the new migrated page.
	* @newpage: page that will replace the isolated page after migration finishes.
	* @page : the isolated (old) page that is about to be migrated to newpage.
	* @mode : compaction mode -- not used for balloon page migration.
	*
	* After a ballooned page gets isolated by compaction procedures, this is the
	* function that performs the page migration on behalf of a compaction thread
	* The page migration for virtio balloon is done in a simple swap fashion which
	* follows these two macro steps:
	* 1) insert newpage into vb->pages list and update the host about it;
	* 2) update the host about the old page removed from vb->pages list;
	*
	* This function preforms the balloon page migration task.
	* Called through balloon_mapping->a_ops->migratepage
	*/
	static int virtballoon_migratepage(struct address_space *mapping,
	struct page newpage, struct page page, enum migrate_mode mode)
	{
	struct balloon_dev_info *vb_dev_info = balloon_page_device(page);
	struct virtio_balloon *vb;
	unsigned long flags;

	BUG_ON(!vb_dev_info);

	vb = vb_dev_info->balloon_device;

	/*
	* In order to avoid lock contention while migrating pages concurrently
	* to leak_balloon() or fill_balloon() we just give up the balloon_lock
	* this turn, as it is easier to retry the page migration later.
	* This also prevents fill_balloon() getting stuck into a mutex
	* recursion in the case it ends up triggering memory compaction
	* while it is attempting to inflate the ballon.
	*/
	if (!mutex_trylock(&vb->balloon_lock))
	return -EAGAIN;

	/* balloon's page migration 1st step -- inflate "newpage" */
	spin_lock_irqsave(&vb_dev_info->pages_lock, flags);
	balloon_page_insert(newpage, mapping, &vb_dev_info->pages);
	vb_dev_info->isolated_pages--;
	spin_unlock_irqrestore(&vb_dev_info->pages_lock, flags);
	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
	set_page_pfns(vb->pfns, newpage);
	tell_host(vb, vb->inflate_vq);

	/*
	* balloon's page migration 2nd step -- deflate "page"
	*
	* It's safe to delete page->lru here because this page is at
	* an isolated migration list, and this step is expected to happen here
	*/
	balloon_page_delete(page);
	vb->num_pfns = VIRTIO_BALLOON_PAGES_PER_PAGE;
	set_page_pfns(vb->pfns, page);
	tell_host(vb, vb->deflate_vq);

	mutex_unlock(&vb->balloon_lock);

	return MIGRATEPAGE_BALLOON_SUCCESS;
	}

	/* define the balloon_mapping->a_ops callback to allow balloon page migration */
	static const struct address_space_operations virtio_balloon_aops = {
	.migratepage = virtballoon_migratepage,
	};
	#endif /* CONFIG_BALLOON_COMPACTION */

	static int virtballoon_probe(struct virtio_device *vdev)
	{
	struct virtio_balloon *vb;
	struct address_space *vb_mapping;
	struct balloon_dev_info *vb_devinfo;
	int err;

	vdev->priv = vb = kmalloc(sizeof(*vb), GFP_KERNEL);
	if (!vb) {
	err = -ENOMEM;
	goto out;
	}

	vb->num_pages = 0;
	mutex_init(&vb->balloon_lock);
	init_waitqueue_head(&vb->config_change);
	init_waitqueue_head(&vb->acked);
	vb->vdev = vdev;
	vb->need_stats_update = 0;

	vb_devinfo = balloon_devinfo_alloc(vb);
	if (IS_ERR(vb_devinfo)) {
	err = PTR_ERR(vb_devinfo);
	goto out_free_vb;
	}

	vb_mapping = balloon_mapping_alloc(vb_devinfo,
	(balloon_compaction_check()) ?
	&virtio_balloon_aops : NULL);
	if (IS_ERR(vb_mapping)) {
	/*
	* IS_ERR(vb_mapping) && PTR_ERR(vb_mapping) == -EOPNOTSUPP
	* This means !CONFIG_BALLOON_COMPACTION, otherwise we get off.
	*/
	err = PTR_ERR(vb_mapping);
	if (err != -EOPNOTSUPP)
	goto out_free_vb_devinfo;
	}

	vb->vb_dev_info = vb_devinfo;

	err = init_vqs(vb);
	if (err)
	goto out_free_vb_mapping;

	vb->thread = kthread_run(balloon, vb, "vballoon");
	if (IS_ERR(vb->thread)) {
	err = PTR_ERR(vb->thread);
	goto out_del_vqs;
	}

	return 0;

	out_del_vqs:
	vdev->config->del_vqs(vdev);
	out_free_vb_mapping:
	balloon_mapping_free(vb_mapping);
	out_free_vb_devinfo:
	balloon_devinfo_free(vb_devinfo);
	out_free_vb:
	kfree(vb);
	out:
	return err;
	}

	static void remove_common(struct virtio_balloon *vb)
	{
	/* There might be pages left in the balloon: free them. */
	while (vb->num_pages)
	leak_balloon(vb, vb->num_pages);
	update_balloon_size(vb);

	/* Now we reset the device so we can clean up the queues. */
	vb->vdev->config->reset(vb->vdev);

	vb->vdev->config->del_vqs(vb->vdev);
	}

	static void virtballoon_remove(struct virtio_device *vdev)
	{
	struct virtio_balloon *vb = vdev->priv;

	kthread_stop(vb->thread);
	remove_common(vb);
	balloon_mapping_free(vb->vb_dev_info->mapping);
	balloon_devinfo_free(vb->vb_dev_info);
	kfree(vb);
	}

	#ifdef CONFIG_PM_SLEEP
	static int virtballoon_freeze(struct virtio_device *vdev)
	{
	struct virtio_balloon *vb = vdev->priv;

	/*
	* The kthread is already frozen by the PM core before this
	* function is called.
	*/

	remove_common(vb);
	return 0;
	}

	static int virtballoon_restore(struct virtio_device *vdev)
	{
	struct virtio_balloon *vb = vdev->priv;
	int ret;

	ret = init_vqs(vdev->priv);
	if (ret)
	return ret;

	fill_balloon(vb, towards_target(vb));
	update_balloon_size(vb);
	return 0;
	}
	#endif

	static unsigned int features[] = {
	VIRTIO_BALLOON_F_MUST_TELL_HOST,
	VIRTIO_BALLOON_F_STATS_VQ,
	};

	static struct virtio_driver virtio_balloon_driver = {
	.feature_table = features,
	.feature_table_size = ARRAY_SIZE(features),
	.driver.name = KBUILD_MODNAME,
	.driver.owner = THIS_MODULE,
	.id_table = id_table,
	.probe = virtballoon_probe,
	.remove = virtballoon_remove,
	.config_changed = virtballoon_changed,
	#ifdef CONFIG_PM_SLEEP
	.freeze = virtballoon_freeze,
	.restore = virtballoon_restore,
	#endif
	};

	module_virtio_driver(virtio_balloon_driver);
	MODULE_DEVICE_TABLE(virtio, id_table);
	MODULE_DESCRIPTION("Virtio balloon driver");
	MODULE_LICENSE("GPL");