linux/drivers/misc/vmware_balloon.c

/*
 * VMware Balloon driver.
 *
 * Copyright (C) 2000-2010, VMware, Inc. All Rights Reserved.
 *
 * 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; version 2 of the License and no 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, GOOD TITLE or
 * NON INFRINGEMENT.  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.
 *
 * Maintained by: Dmitry Torokhov <dtor@vmware.com>
 */

/*
 * This is VMware physical memory management driver for Linux. The driver
 * acts like a "balloon" that can be inflated to reclaim physical pages by
 * reserving them in the guest and invalidating them in the monitor,
 * freeing up the underlying machine pages so they can be allocated to
 * other guests.  The balloon can also be deflated to allow the guest to
 * use more physical memory. Higher level policies can control the sizes
 * of balloons in VMs in order to manage physical memory resources.
 */

//#define DEBUG
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/workqueue.h>
#include <linux/debugfs.h>
#include <linux/seq_file.h>
#include <asm/hypervisor.h>

MODULE_AUTHOR("VMware, Inc.");
MODULE_DESCRIPTION("VMware Memory Control (Balloon) Driver");
MODULE_VERSION("1.2.1.0-K");
MODULE_ALIAS("dmi:*:svnVMware*:*");
MODULE_ALIAS("vmware_vmmemctl");
MODULE_LICENSE("GPL");

/*
 * Various constants controlling rate of inflaint/deflating balloon,
 * measured in pages.
 */

/*
 * Rate of allocating memory when there is no memory pressure
 * (driver performs non-sleeping allocations).
 */
#define VMW_BALLOON_NOSLEEP_ALLOC_MAX	16384U

/*
 * Rates of memory allocaton when guest experiences memory pressure
 * (driver performs sleeping allocations).
 */
#define VMW_BALLOON_RATE_ALLOC_MIN	512U
#define VMW_BALLOON_RATE_ALLOC_MAX	2048U
#define VMW_BALLOON_RATE_ALLOC_INC	16U

/*
 * Rates for releasing pages while deflating balloon.
 */
#define VMW_BALLOON_RATE_FREE_MIN	512U
#define VMW_BALLOON_RATE_FREE_MAX	16384U
#define VMW_BALLOON_RATE_FREE_INC	16U

/*
 * When guest is under memory pressure, use a reduced page allocation
 * rate for next several cycles.
 */
#define VMW_BALLOON_SLOW_CYCLES		4

/*
 * Use __GFP_HIGHMEM to allow pages from HIGHMEM zone. We don't
 * allow wait (__GFP_WAIT) for NOSLEEP page allocations. Use
 * __GFP_NOWARN, to suppress page allocation failure warnings.
 */
#define VMW_PAGE_ALLOC_NOSLEEP		(__GFP_HIGHMEM|__GFP_NOWARN)

/*
 * Use GFP_HIGHUSER when executing in a separate kernel thread
 * context and allocation can sleep.  This is less stressful to
 * the guest memory system, since it allows the thread to block
 * while memory is reclaimed, and won't take pages from emergency
 * low-memory pools.
 */
#define VMW_PAGE_ALLOC_CANSLEEP		(GFP_HIGHUSER)

/* Maximum number of page allocations without yielding processor */
#define VMW_BALLOON_YIELD_THRESHOLD	1024


/*
 * Hypervisor communication port definitions.
 */
#define VMW_BALLOON_HV_PORT		0x5670
#define VMW_BALLOON_HV_MAGIC		0x456c6d6f
#define VMW_BALLOON_PROTOCOL_VERSION	2
#define VMW_BALLOON_GUEST_ID		1	/* Linux */

#define VMW_BALLOON_CMD_START		0
#define VMW_BALLOON_CMD_GET_TARGET	1
#define VMW_BALLOON_CMD_LOCK		2
#define VMW_BALLOON_CMD_UNLOCK		3
#define VMW_BALLOON_CMD_GUEST_ID	4

/* error codes */
#define VMW_BALLOON_SUCCESS		0
#define VMW_BALLOON_FAILURE		-1
#define VMW_BALLOON_ERROR_CMD_INVALID	1
#define VMW_BALLOON_ERROR_PPN_INVALID	2
#define VMW_BALLOON_ERROR_PPN_LOCKED	3
#define VMW_BALLOON_ERROR_PPN_UNLOCKED	4
#define VMW_BALLOON_ERROR_PPN_PINNED	5
#define VMW_BALLOON_ERROR_PPN_NOTNEEDED	6
#define VMW_BALLOON_ERROR_RESET		7
#define VMW_BALLOON_ERROR_BUSY		8

#define VMWARE_BALLOON_CMD(cmd, data, result)		\
({							\
	unsigned long __stat, __dummy1, __dummy2;	\
	__asm__ __volatile__ ("inl (%%dx)" :		\
		"=a"(__stat),				\
		"=c"(__dummy1),				\
		"=d"(__dummy2),				\
		"=b"(result) :				\
		"0"(VMW_BALLOON_HV_MAGIC),		\
		"1"(VMW_BALLOON_CMD_##cmd),		\
		"2"(VMW_BALLOON_HV_PORT),		\
		"3"(data) :				\
		"memory");				\
	result &= -1UL;					\
	__stat & -1UL;					\
})

#ifdef CONFIG_DEBUG_FS
struct vmballoon_stats {
	unsigned int timer;

	/* allocation statustics */
	unsigned int alloc;
	unsigned int alloc_fail;
	unsigned int sleep_alloc;
	unsigned int sleep_alloc_fail;
	unsigned int refused_alloc;
	unsigned int refused_free;
	unsigned int free;

	/* monitor operations */
	unsigned int lock;
	unsigned int lock_fail;
	unsigned int unlock;
	unsigned int unlock_fail;
	unsigned int target;
	unsigned int target_fail;
	unsigned int start;
	unsigned int start_fail;
	unsigned int guest_type;
	unsigned int guest_type_fail;
};

#define STATS_INC(stat) (stat)++
#else
#define STATS_INC(stat)
#endif

struct vmballoon {

	/* list of reserved physical pages */
	struct list_head pages;

	/* transient list of non-balloonable pages */
	struct list_head refused_pages;

	/* balloon size in pages */
	unsigned int size;
	unsigned int target;

	/* reset flag */
	bool reset_required;

	/* adjustment rates (pages per second) */
	unsigned int rate_alloc;
	unsigned int rate_free;

	/* slowdown page allocations for next few cycles */
	unsigned int slow_allocation_cycles;

#ifdef CONFIG_DEBUG_FS
	/* statistics */
	struct vmballoon_stats stats;

	/* debugfs file exporting statistics */
	struct dentry *dbg_entry;
#endif

	struct sysinfo sysinfo;

	struct delayed_work dwork;
};

static struct vmballoon balloon;
static struct workqueue_struct *vmballoon_wq;

/*
 * Send "start" command to the host, communicating supported version
 * of the protocol.
 */
static bool vmballoon_send_start(struct vmballoon *b)
{
	unsigned long status, dummy;

	STATS_INC(b->stats.start);

	status = VMWARE_BALLOON_CMD(START, VMW_BALLOON_PROTOCOL_VERSION, dummy);
	if (status == VMW_BALLOON_SUCCESS)
		return true;

	pr_debug("%s - failed, hv returns %ld\n", __func__, status);
	STATS_INC(b->stats.start_fail);
	return false;
}

static bool vmballoon_check_status(struct vmballoon *b, unsigned long status)
{
	switch (status) {
	case VMW_BALLOON_SUCCESS:
		return true;

	case VMW_BALLOON_ERROR_RESET:
		b->reset_required = true;
		/* fall through */

	default:
		return false;
	}
}

/*
 * Communicate guest type to the host so that it can adjust ballooning
 * algorithm to the one most appropriate for the guest. This command
 * is normally issued after sending "start" command and is part of
 * standard reset sequence.
 */
static bool vmballoon_send_guest_id(struct vmballoon *b)
{
	unsigned long status, dummy;

	status = VMWARE_BALLOON_CMD(GUEST_ID, VMW_BALLOON_GUEST_ID, dummy);

	STATS_INC(b->stats.guest_type);

	if (vmballoon_check_status(b, status))
		return true;

	pr_debug("%s - failed, hv returns %ld\n", __func__, status);
	STATS_INC(b->stats.guest_type_fail);
	return false;
}

/*
 * Retrieve desired balloon size from the host.
 */
static bool vmballoon_send_get_target(struct vmballoon *b, u32 *new_target)
{
	unsigned long status;
	unsigned long target;
	unsigned long limit;
	u32 limit32;

	/*
	 * si_meminfo() is cheap. Moreover, we want to provide dynamic
	 * max balloon size later. So let us call si_meminfo() every
	 * iteration.
	 */
	si_meminfo(&b->sysinfo);
	limit = b->sysinfo.totalram;

	/* Ensure limit fits in 32-bits */
	limit32 = (u32)limit;
	if (limit != limit32)
		return false;

	/* update stats */
	STATS_INC(b->stats.target);

	status = VMWARE_BALLOON_CMD(GET_TARGET, limit, target);
	if (vmballoon_check_status(b, status)) {
		*new_target = target;
		return true;
	}

	pr_debug("%s - failed, hv returns %ld\n", __func__, status);
	STATS_INC(b->stats.target_fail);
	return false;
}

/*
 * Notify the host about allocated page so that host can use it without
 * fear that guest will need it. Host may reject some pages, we need to
 * check the return value and maybe submit a different page.
 */
static bool vmballoon_send_lock_page(struct vmballoon *b, unsigned long pfn)
{
	unsigned long status, dummy;
	u32 pfn32;

	pfn32 = (u32)pfn;
	if (pfn32 != pfn)
		return false;

	STATS_INC(b->stats.lock);

	status = VMWARE_BALLOON_CMD(LOCK, pfn, dummy);
	if (vmballoon_check_status(b, status))
		return true;

	pr_debug("%s - ppn %lx, hv returns %ld\n", __func__, pfn, status);
	STATS_INC(b->stats.lock_fail);
	return false;
}

/*
 * Notify the host that guest intends to release given page back into
 * the pool of available (to the guest) pages.
 */
static bool vmballoon_send_unlock_page(struct vmballoon *b, unsigned long pfn)
{
	unsigned long status, dummy;
	u32 pfn32;

	pfn32 = (u32)pfn;
	if (pfn32 != pfn)
		return false;

	STATS_INC(b->stats.unlock);

	status = VMWARE_BALLOON_CMD(UNLOCK, pfn, dummy);
	if (vmballoon_check_status(b, status))
		return true;

	pr_debug("%s - ppn %lx, hv returns %ld\n", __func__, pfn, status);
	STATS_INC(b->stats.unlock_fail);
	return false;
}

/*
 * Quickly release all pages allocated for the balloon. This function is
 * called when host decides to "reset" balloon for one reason or another.
 * Unlike normal "deflate" we do not (shall not) notify host of the pages
 * being released.
 */
static void vmballoon_pop(struct vmballoon *b)
{
	struct page *page, *next;
	unsigned int count = 0;

	list_for_each_entry_safe(page, next, &b->pages, lru) {
		list_del(&page->lru);
		__free_page(page);
		STATS_INC(b->stats.free);
		b->size--;

		if (++count >= b->rate_free) {
			count = 0;
			cond_resched();
		}
	}
}

/*
 * Perform standard reset sequence by popping the balloon (in case it
 * is not  empty) and then restarting protocol. This operation normally
 * happens when host responds with VMW_BALLOON_ERROR_RESET to a command.
 */
static void vmballoon_reset(struct vmballoon *b)
{
	/* free all pages, skipping monitor unlock */
	vmballoon_pop(b);

	if (vmballoon_send_start(b)) {
		b->reset_required = false;
		if (!vmballoon_send_guest_id(b))
			pr_err("failed to send guest ID to the host\n");
	}
}

/*
 * Allocate (or reserve) a page for the balloon and notify the host.  If host
 * refuses the page put it on "refuse" list and allocate another one until host
 * is satisfied. "Refused" pages are released at the end of inflation cycle
 * (when we allocate b->rate_alloc pages).
 */
static int vmballoon_reserve_page(struct vmballoon *b, bool can_sleep)
{
	struct page *page;
	gfp_t flags;
	bool locked = false;

	do {
		if (!can_sleep)
			STATS_INC(b->stats.alloc);
		else
			STATS_INC(b->stats.sleep_alloc);

		flags = can_sleep ? VMW_PAGE_ALLOC_CANSLEEP : VMW_PAGE_ALLOC_NOSLEEP;
		page = alloc_page(flags);
		if (!page) {
			if (!can_sleep)
				STATS_INC(b->stats.alloc_fail);
			else
				STATS_INC(b->stats.sleep_alloc_fail);
			return -ENOMEM;
		}

		/* inform monitor */
		locked = vmballoon_send_lock_page(b, page_to_pfn(page));
		if (!locked) {
			if (b->reset_required) {
				__free_page(page);
				return -EIO;
			}

			/* place on list of non-balloonable pages, retry allocation */
			list_add(&page->lru, &b->refused_pages);
			STATS_INC(b->stats.refused_alloc);
		}
	} while (!locked);

	/* track allocated page */
	list_add(&page->lru, &b->pages);

	/* update balloon size */
	b->size++;

	return 0;
}

/*
 * Release the page allocated for the balloon. Note that we first notify
 * the host so it can make sure the page will be available for the guest
 * to use, if needed.
 */
static int vmballoon_release_page(struct vmballoon *b, struct page *page)
{
	if (!vmballoon_send_unlock_page(b, page_to_pfn(page)))
		return -EIO;

	list_del(&page->lru);

	/* deallocate page */
	__free_page(page);
	STATS_INC(b->stats.free);

	/* update balloon size */
	b->size--;

	return 0;
}

/*
 * Release pages that were allocated while attempting to inflate the
 * balloon but were refused by the host for one reason or another.
 */
static void vmballoon_release_refused_pages(struct vmballoon *b)
{
	struct page *page, *next;

	list_for_each_entry_safe(page, next, &b->refused_pages, lru) {
		list_del(&page->lru);
		__free_page(page);
		STATS_INC(b->stats.refused_free);
	}
}

/*
 * Inflate the balloon towards its target size. Note that we try to limit
 * the rate of allocation to make sure we are not choking the rest of the
 * system.
 */
static void vmballoon_inflate(struct vmballoon *b)
{
	unsigned int goal;
	unsigned int rate;
	unsigned int i;
	unsigned int allocations = 0;
	int error = 0;
	bool alloc_can_sleep = false;

	pr_debug("%s - size: %d, target %d\n", __func__, b->size, b->target);

	/*
	 * First try NOSLEEP page allocations to inflate balloon.
	 *
	 * If we do not throttle nosleep allocations, we can drain all
	 * free pages in the guest quickly (if the balloon target is high).
	 * As a side-effect, draining free pages helps to inform (force)
	 * the guest to start swapping if balloon target is not met yet,
	 * which is a desired behavior. However, balloon driver can consume
	 * all available CPU cycles if too many pages are allocated in a
	 * second. Therefore, we throttle nosleep allocations even when
	 * the guest is not under memory pressure. OTOH, if we have already
	 * predicted that the guest is under memory pressure, then we
	 * slowdown page allocations considerably.
	 */

	goal = b->target - b->size;
	/*
	 * Start with no sleep allocation rate which may be higher
	 * than sleeping allocation rate.
	 */
	rate = b->slow_allocation_cycles ?
			b->rate_alloc : VMW_BALLOON_NOSLEEP_ALLOC_MAX;

	pr_debug("%s - goal: %d, no-sleep rate: %d, sleep rate: %d\n",
		 __func__, goal, rate, b->rate_alloc);

	for (i = 0; i < goal; i++) {

		error = vmballoon_reserve_page(b, alloc_can_sleep);
		if (error) {
			if (error != -ENOMEM) {
				/*
				 * Not a page allocation failure, stop this
				 * cycle. Maybe we'll get new target from
				 * the host soon.
				 */
				break;
			}

			if (alloc_can_sleep) {
				/*
				 * CANSLEEP page allocation failed, so guest
				 * is under severe memory pressure. Quickly
				 * decrease allocation rate.
				 */
				b->rate_alloc = max(b->rate_alloc / 2,
						    VMW_BALLOON_RATE_ALLOC_MIN);
				break;
			}

			/*
			 * NOSLEEP page allocation failed, so the guest is
			 * under memory pressure. Let us slow down page
			 * allocations for next few cycles so that the guest
			 * gets out of memory pressure. Also, if we already
			 * allocated b->rate_alloc pages, let's pause,
			 * otherwise switch to sleeping allocations.
			 */
			b->slow_allocation_cycles = VMW_BALLOON_SLOW_CYCLES;

			if (i >= b->rate_alloc)
				break;

			alloc_can_sleep = true;
			/* Lower rate for sleeping allocations. */
			rate = b->rate_alloc;
		}

		if (++allocations > VMW_BALLOON_YIELD_THRESHOLD) {
			cond_resched();
			allocations = 0;
		}

		if (i >= rate) {
			/* We allocated enough pages, let's take a break. */
			break;
		}
	}

	/*
	 * We reached our goal without failures so try increasing
	 * allocation rate.
	 */
	if (error == 0 && i >= b->rate_alloc) {
		unsigned int mult = i / b->rate_alloc;

		b->rate_alloc =
			min(b->rate_alloc + mult * VMW_BALLOON_RATE_ALLOC_INC,
			    VMW_BALLOON_RATE_ALLOC_MAX);
	}

	vmballoon_release_refused_pages(b);
}

/*
 * Decrease the size of the balloon allowing guest to use more memory.
 */
static void vmballoon_deflate(struct vmballoon *b)
{
	struct page *page, *next;
	unsigned int i = 0;
	unsigned int goal;
	int error;

	pr_debug("%s - size: %d, target %d\n", __func__, b->size, b->target);

	/* limit deallocation rate */
	goal = min(b->size - b->target, b->rate_free);

	pr_debug("%s - goal: %d, rate: %d\n", __func__, goal, b->rate_free);

	/* free pages to reach target */
	list_for_each_entry_safe(page, next, &b->pages, lru) {
		error = vmballoon_release_page(b, page);
		if (error) {
			/* quickly decrease rate in case of error */
			b->rate_free = max(b->rate_free / 2,
					   VMW_BALLOON_RATE_FREE_MIN);
			return;
		}

		if (++i >= goal)
			break;
	}

	/* slowly increase rate if there were no errors */
	b->rate_free = min(b->rate_free + VMW_BALLOON_RATE_FREE_INC,
			   VMW_BALLOON_RATE_FREE_MAX);
}

/*
 * Balloon work function: reset protocol, if needed, get the new size and
 * adjust balloon as needed. Repeat in 1 sec.
 */
static void vmballoon_work(struct work_struct *work)
{
	struct delayed_work *dwork = to_delayed_work(work);
	struct vmballoon *b = container_of(dwork, struct vmballoon, dwork);
	unsigned int target;

	STATS_INC(b->stats.timer);

	if (b->reset_required)
		vmballoon_reset(b);

	if (b->slow_allocation_cycles > 0)
		b->slow_allocation_cycles--;

	if (vmballoon_send_get_target(b, &target)) {
		/* update target, adjust size */
		b->target = target;

		if (b->size < target)
			vmballoon_inflate(b);
		else if (b->size > target)
			vmballoon_deflate(b);
	}

	queue_delayed_work(vmballoon_wq, dwork, round_jiffies_relative(HZ));
}

/*
 * DEBUGFS Interface
 */
#ifdef CONFIG_DEBUG_FS

static int vmballoon_debug_show(struct seq_file *f, void *offset)
{
	struct vmballoon *b = f->private;
	struct vmballoon_stats *stats = &b->stats;

	/* format size info */
	seq_printf(f,
		   "target:             %8d pages\n"
		   "current:            %8d pages\n",
		   b->target, b->size);

	/* format rate info */
	seq_printf(f,
		   "rateNoSleepAlloc:   %8d pages/sec\n"
		   "rateSleepAlloc:     %8d pages/sec\n"
		   "rateFree:           %8d pages/sec\n",
		   VMW_BALLOON_NOSLEEP_ALLOC_MAX,
		   b->rate_alloc, b->rate_free);

	seq_printf(f,
		   "\n"
		   "timer:              %8u\n"
		   "start:              %8u (%4u failed)\n"
		   "guestType:          %8u (%4u failed)\n"
		   "lock:               %8u (%4u failed)\n"
		   "unlock:             %8u (%4u failed)\n"
		   "target:             %8u (%4u failed)\n"
		   "primNoSleepAlloc:   %8u (%4u failed)\n"
		   "primCanSleepAlloc:  %8u (%4u failed)\n"
		   "primFree:           %8u\n"
		   "errAlloc:           %8u\n"
		   "errFree:            %8u\n",
		   stats->timer,
		   stats->start, stats->start_fail,
		   stats->guest_type, stats->guest_type_fail,
		   stats->lock,  stats->lock_fail,
		   stats->unlock, stats->unlock_fail,
		   stats->target, stats->target_fail,
		   stats->alloc, stats->alloc_fail,
		   stats->sleep_alloc, stats->sleep_alloc_fail,
		   stats->free,
		   stats->refused_alloc, stats->refused_free);

	return 0;
}

static int vmballoon_debug_open(struct inode *inode, struct file *file)
{
	return single_open(file, vmballoon_debug_show, inode->i_private);
}

static const struct file_operations vmballoon_debug_fops = {
	.owner		= THIS_MODULE,
	.open		= vmballoon_debug_open,
	.read		= seq_read,
	.llseek		= seq_lseek,
	.release	= single_release,
};

static int __init vmballoon_debugfs_init(struct vmballoon *b)
{
	int error;

	b->dbg_entry = debugfs_create_file("vmmemctl", S_IRUGO, NULL, b,
					   &vmballoon_debug_fops);
	if (IS_ERR(b->dbg_entry)) {
		error = PTR_ERR(b->dbg_entry);
		pr_err("failed to create debugfs entry, error: %d\n", error);
		return error;
	}

	return 0;
}

static void __exit vmballoon_debugfs_exit(struct vmballoon *b)
{
	debugfs_remove(b->dbg_entry);
}

#else

static inline int vmballoon_debugfs_init(struct vmballoon *b)
{
	return 0;
}

static inline void vmballoon_debugfs_exit(struct vmballoon *b)
{
}

#endif	/* CONFIG_DEBUG_FS */

static int __init vmballoon_init(void)
{
	int error;

	/*
	 * Check if we are running on VMware's hypervisor and bail out
	 * if we are not.
	 */
	if (x86_hyper != &x86_hyper_vmware)
		return -ENODEV;

	vmballoon_wq = create_freezeable_workqueue("vmmemctl");
	if (!vmballoon_wq) {
		pr_err("failed to create workqueue\n");
		return -ENOMEM;
	}

	INIT_LIST_HEAD(&balloon.pages);
	INIT_LIST_HEAD(&balloon.refused_pages);

	/* initialize rates */
	balloon.rate_alloc = VMW_BALLOON_RATE_ALLOC_MAX;
	balloon.rate_free = VMW_BALLOON_RATE_FREE_MAX;

	INIT_DELAYED_WORK(&balloon.dwork, vmballoon_work);

	/*
	 * Start balloon.
	 */
	if (!vmballoon_send_start(&balloon)) {
		pr_err("failed to send start command to the host\n");
		error = -EIO;
		goto fail;
	}

	if (!vmballoon_send_guest_id(&balloon)) {
		pr_err("failed to send guest ID to the host\n");
		error = -EIO;
		goto fail;
	}

	error = vmballoon_debugfs_init(&balloon);
	if (error)
		goto fail;

	queue_delayed_work(vmballoon_wq, &balloon.dwork, 0);

	return 0;

fail:
	destroy_workqueue(vmballoon_wq);
	return error;
}
module_init(vmballoon_init);

static void __exit vmballoon_exit(void)
{
	cancel_delayed_work_sync(&balloon.dwork);
	destroy_workqueue(vmballoon_wq);

	vmballoon_debugfs_exit(&balloon);

	/*
	 * Deallocate all reserved memory, and reset connection with monitor.
	 * Reset connection before deallocating memory to avoid potential for
	 * additional spurious resets from guest touching deallocated pages.
	 */
	vmballoon_send_start(&balloon);
	vmballoon_pop(&balloon);
}
module_exit(vmballoon_exit);