vmw_balloon.c

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/*
 * 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 <[email protected]>
 */

/*
 * 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("1.2.1.3-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

/* Maximum number of refused pages we accumulate during inflation cycle */
#define VMW_BALLOON_MAX_REFUSED     16

/*
 * 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 statistics */
    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;
    unsigned int n_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;

/*
 * 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 int vmballoon_send_lock_page(struct vmballoon *b, unsigned long pfn,
                     unsigned int *hv_status)
{
    unsigned long status, dummy;
    u32 pfn32;

    pfn32 = (u32)pfn;
    if (pfn32 != pfn)
        return -1;

    STATS_INC(b->stats.lock);

    *hv_status = status = VMWARE_BALLOON_CMD(LOCK, pfn, dummy);
    if (vmballoon_check_status(b, status))
        return 0;

    pr_debug("%s - ppn %lx, hv returns %ld\n", __func__, pfn, status);
    STATS_INC(b->stats.lock_fail);
    return 1;
}

/*
 * 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;
    unsigned int hv_status;
    int locked;
    flags = can_sleep ? VMW_PAGE_ALLOC_CANSLEEP : VMW_PAGE_ALLOC_NOSLEEP;

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

        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), &hv_status);
        if (locked > 0) {
            STATS_INC(b->stats.refused_alloc);

            if (hv_status == VMW_BALLOON_ERROR_RESET ||
                hv_status == VMW_BALLOON_ERROR_PPN_NOTNEEDED) {
                __free_page(page);
                return -EIO;
            }

            /*
             * Place page on the list of non-balloonable pages
             * and retry allocation, unless we already accumulated
             * too many of them, in which case take a breather.
             */
            list_add(&page->lru, &b->refused_pages);
            if (++b->n_refused_pages >= VMW_BALLOON_MAX_REFUSED)
                return -EIO;
        }
    } while (locked != 0);

    /* 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);
    }

    b->n_refused_pages = 0;
}

/*
 * 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);
    }

    /*
     * We are using a freezable workqueue so that balloon operations are
     * stopped while the system transitions to/from sleep/hibernation.
     */
    queue_delayed_work(system_freezable_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;

    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");
        return -EIO;
    }

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

    error = vmballoon_debugfs_init(&balloon);
    if (error)
        return error;

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

    return 0;
}
module_init(vmballoon_init);

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

    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);