/*
 * 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:       Xavier Deguillard <xdeguillard@vmware.com>
 *                      Philip Moltmann <moltmann@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("1.2.1.3-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);