/******************************************************************************
 * Xen selfballoon driver (and optional frontswap self-shrinking driver)
 *
 * Copyright (c) 2009-2011, Dan Magenheimer, Oracle Corp.
 *
 * This code complements the cleancache and frontswap patchsets to optimize
 * support for Xen Transcendent Memory ("tmem").  The policy it implements
 * is rudimentary and will likely improve over time, but it does work well
 * enough today.
 *
 * Two functionalities are implemented here which both use "control theory"
 * (feedback) to optimize memory utilization. In a virtualized environment
 * such as Xen, RAM is often a scarce resource and we would like to ensure
 * that each of a possibly large number of virtual machines is using RAM
 * efficiently, i.e. using as little as possible when under light load
 * and obtaining as much as possible when memory demands are high.
 * Since RAM needs vary highly dynamically and sometimes dramatically,
 * "hysteresis" is used, that is, memory target is determined not just
 * on current data but also on past data stored in the system.
 *
 * "Selfballooning" creates memory pressure by managing the Xen balloon
 * driver to decrease and increase available kernel memory, driven
 * largely by the target value of "Committed_AS" (see /proc/meminfo).
 * Since Committed_AS does not account for clean mapped pages (i.e. pages
 * in RAM that are identical to pages on disk), selfballooning has the
 * affect of pushing less frequently used clean pagecache pages out of
 * kernel RAM and, presumably using cleancache, into Xen tmem where
 * Xen can more efficiently optimize RAM utilization for such pages.
 *
 * When kernel memory demand unexpectedly increases faster than Xen, via
 * the selfballoon driver, is able to (or chooses to) provide usable RAM,
 * the kernel may invoke swapping.  In most cases, frontswap is able
 * to absorb this swapping into Xen tmem.  However, due to the fact
 * that the kernel swap subsystem assumes swapping occurs to a disk,
 * swapped pages may sit on the disk for a very long time; even if
 * the kernel knows the page will never be used again.  This is because
 * the disk space costs very little and can be overwritten when
 * necessary.  When such stale pages are in frontswap, however, they
 * are taking up valuable real estate.  "Frontswap selfshrinking" works
 * to resolve this:  When frontswap activity is otherwise stable
 * and the guest kernel is not under memory pressure, the "frontswap
 * selfshrinking" accounts for this by providing pressure to remove some
 * pages from frontswap and return them to kernel memory.
 *
 * For both "selfballooning" and "frontswap-selfshrinking", a worker
 * thread is used and sysfs tunables are provided to adjust the frequency
 * and rate of adjustments to achieve the goal, as well as to disable one
 * or both functions independently.
 *
 * While some argue that this functionality can and should be implemented
 * in userspace, it has been observed that bad things happen (e.g. OOMs).
 *
 * System configuration note: Selfballooning should not be enabled on
 * systems without a sufficiently large swap device configured; for best
 * results, it is recommended that total swap be increased by the size
 * of the guest memory. Note, that selfballooning should be disabled by default
 * if frontswap is not configured.  Similarly selfballooning should be enabled
 * by default if frontswap is configured and can be disabled with the
 * "tmem.selfballooning=0" kernel boot option.  Finally, when frontswap is
 * configured, frontswap-selfshrinking can be disabled  with the
 * "tmem.selfshrink=0" kernel boot option.
 *
 * Selfballooning is disallowed in domain0 and force-disabled.
 *
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/kernel.h>
#include <linux/bootmem.h>
#include <linux/swap.h>
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/workqueue.h>
#include <linux/device.h>
#include <xen/balloon.h>
#include <xen/tmem.h>
#include <xen/xen.h>

/* Enable/disable with sysfs. */
static int xen_selfballooning_enabled __read_mostly;

/*
 * Controls rate at which memory target (this iteration) approaches
 * ultimate goal when memory need is increasing (up-hysteresis) or
 * decreasing (down-hysteresis). Higher values of hysteresis cause
 * slower increases/decreases. The default values for the various
 * parameters were deemed reasonable by experimentation, may be
 * workload-dependent, and can all be adjusted via sysfs.
 */
static unsigned int selfballoon_downhysteresis __read_mostly = 8;
static unsigned int selfballoon_uphysteresis __read_mostly = 1;

/* In HZ, controls frequency of worker invocation. */
static unsigned int selfballoon_interval __read_mostly = 5;

/*
 * Minimum usable RAM in MB for selfballooning target for balloon.
 * If non-zero, it is added to totalreserve_pages and self-ballooning
 * will not balloon below the sum.  If zero, a piecewise linear function
 * is calculated as a minimum and added to totalreserve_pages.  Note that
 * setting this value indiscriminately may cause OOMs and crashes.
 */
static unsigned int selfballoon_min_usable_mb;

/*
 * Amount of RAM in MB to add to the target number of pages.
 * Can be used to reserve some more room for caches and the like.
 */
static unsigned int selfballoon_reserved_mb;

static void selfballoon_process(struct work_struct *work);
static DECLARE_DELAYED_WORK(selfballoon_worker, selfballoon_process);

#ifdef CONFIG_FRONTSWAP
#include <linux/frontswap.h>

/* Enable/disable with sysfs. */
static bool frontswap_selfshrinking __read_mostly;

/*
 * The default values for the following parameters were deemed reasonable
 * by experimentation, may be workload-dependent, and can all be
 * adjusted via sysfs.
 */

/* Control rate for frontswap shrinking. Higher hysteresis is slower. */
static unsigned int frontswap_hysteresis __read_mostly = 20;

/*
 * Number of selfballoon worker invocations to wait before observing that
 * frontswap selfshrinking should commence. Note that selfshrinking does
 * not use a separate worker thread.
 */
static unsigned int frontswap_inertia __read_mostly = 3;

/* Countdown to next invocation of frontswap_shrink() */
static unsigned long frontswap_inertia_counter;

/*
 * Invoked by the selfballoon worker thread, uses current number of pages
 * in frontswap (frontswap_curr_pages()), previous status, and control
 * values (hysteresis and inertia) to determine if frontswap should be
 * shrunk and what the new frontswap size should be.  Note that
 * frontswap_shrink is essentially a partial swapoff that immediately
 * transfers pages from the "swap device" (frontswap) back into kernel
 * RAM; despite the name, frontswap "shrinking" is very different from
 * the "shrinker" interface used by the kernel MM subsystem to reclaim
 * memory.
 */
static void frontswap_selfshrink(void)
{
        static unsigned long cur_frontswap_pages;
        static unsigned long last_frontswap_pages;
        static unsigned long tgt_frontswap_pages;

        last_frontswap_pages = cur_frontswap_pages;
        cur_frontswap_pages = frontswap_curr_pages();
        if (!cur_frontswap_pages ||
                        (cur_frontswap_pages > last_frontswap_pages)) {
                frontswap_inertia_counter = frontswap_inertia;
                return;
        }
        if (frontswap_inertia_counter && --frontswap_inertia_counter)
                return;
        if (cur_frontswap_pages <= frontswap_hysteresis)
                tgt_frontswap_pages = 0;
        else
                tgt_frontswap_pages = cur_frontswap_pages -
                        (cur_frontswap_pages / frontswap_hysteresis);
        frontswap_shrink(tgt_frontswap_pages);
        frontswap_inertia_counter = frontswap_inertia;
}

#endif /* CONFIG_FRONTSWAP */

#define MB2PAGES(mb)    ((mb) << (20 - PAGE_SHIFT))
#define PAGES2MB(pages) ((pages) >> (20 - PAGE_SHIFT))

/*
 * Use current balloon size, the goal (vm_committed_as), and hysteresis
 * parameters to set a new target balloon size
 */
static void selfballoon_process(struct work_struct *work)
{
        unsigned long cur_pages, goal_pages, tgt_pages, floor_pages;
        unsigned long useful_pages;
        bool reset_timer = false;

        if (xen_selfballooning_enabled) {
                cur_pages = totalram_pages;
                tgt_pages = cur_pages; /* default is no change */
                goal_pages = vm_memory_committed() +
                                totalreserve_pages +
                                MB2PAGES(selfballoon_reserved_mb);
#ifdef CONFIG_FRONTSWAP
                /* allow space for frontswap pages to be repatriated */
                if (frontswap_selfshrinking)
                        goal_pages += frontswap_curr_pages();
#endif
                if (cur_pages > goal_pages)
                        tgt_pages = cur_pages -
                                ((cur_pages - goal_pages) /
                                  selfballoon_downhysteresis);
                else if (cur_pages < goal_pages)
                        tgt_pages = cur_pages +
                                ((goal_pages - cur_pages) /
                                  selfballoon_uphysteresis);
                /* else if cur_pages == goal_pages, no change */
                useful_pages = max_pfn - totalreserve_pages;
                if (selfballoon_min_usable_mb != 0)
                        floor_pages = totalreserve_pages +
                                        MB2PAGES(selfballoon_min_usable_mb);
                /* piecewise linear function ending in ~3% slope */
                else if (useful_pages < MB2PAGES(16))
                        floor_pages = max_pfn; /* not worth ballooning */
                else if (useful_pages < MB2PAGES(64))
                        floor_pages = totalreserve_pages + MB2PAGES(16) +
                                        ((useful_pages - MB2PAGES(16)) >> 1);
                else if (useful_pages < MB2PAGES(512))
                        floor_pages = totalreserve_pages + MB2PAGES(40) +
                                        ((useful_pages - MB2PAGES(40)) >> 3);
                else /* useful_pages >= MB2PAGES(512) */
                        floor_pages = totalreserve_pages + MB2PAGES(99) +
                                        ((useful_pages - MB2PAGES(99)) >> 5);
                if (tgt_pages < floor_pages)
                        tgt_pages = floor_pages;
                balloon_set_new_target(tgt_pages +
                        balloon_stats.current_pages - totalram_pages);
                reset_timer = true;
        }
#ifdef CONFIG_FRONTSWAP
        if (frontswap_selfshrinking) {
                frontswap_selfshrink();
                reset_timer = true;
        }
#endif
        if (reset_timer)
                schedule_delayed_work(&selfballoon_worker,
                        selfballoon_interval * HZ);
}

#ifdef CONFIG_SYSFS

#include <linux/capability.h>

#define SELFBALLOON_SHOW(name, format, args...)                         \
        static ssize_t show_##name(struct device *dev,  \
                                          struct device_attribute *attr, \
                                          char *buf) \
        { \
                return sprintf(buf, format, ##args); \
        }

SELFBALLOON_SHOW(selfballooning, "%d\n", xen_selfballooning_enabled);

static ssize_t store_selfballooning(struct device *dev,
                            struct device_attribute *attr,
                            const char *buf,
                            size_t count)
{
        bool was_enabled = xen_selfballooning_enabled;
        unsigned long tmp;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        err = kstrtoul(buf, 10, &tmp);
        if (err)
                return err;
        if ((tmp != 0) && (tmp != 1))
                return -EINVAL;

        xen_selfballooning_enabled = !!tmp;
        if (!was_enabled && xen_selfballooning_enabled)
                schedule_delayed_work(&selfballoon_worker,
                        selfballoon_interval * HZ);

        return count;
}

static DEVICE_ATTR(selfballooning, S_IRUGO | S_IWUSR,
                   show_selfballooning, store_selfballooning);

SELFBALLOON_SHOW(selfballoon_interval, "%d\n", selfballoon_interval);

static ssize_t store_selfballoon_interval(struct device *dev,
                                          struct device_attribute *attr,
                                          const char *buf,
                                          size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        selfballoon_interval = val;
        return count;
}

static DEVICE_ATTR(selfballoon_interval, S_IRUGO | S_IWUSR,
                   show_selfballoon_interval, store_selfballoon_interval);

SELFBALLOON_SHOW(selfballoon_downhys, "%d\n", selfballoon_downhysteresis);

static ssize_t store_selfballoon_downhys(struct device *dev,
                                         struct device_attribute *attr,
                                         const char *buf,
                                         size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        selfballoon_downhysteresis = val;
        return count;
}

static DEVICE_ATTR(selfballoon_downhysteresis, S_IRUGO | S_IWUSR,
                   show_selfballoon_downhys, store_selfballoon_downhys);


SELFBALLOON_SHOW(selfballoon_uphys, "%d\n", selfballoon_uphysteresis);

static ssize_t store_selfballoon_uphys(struct device *dev,
                                       struct device_attribute *attr,
                                       const char *buf,
                                       size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        selfballoon_uphysteresis = val;
        return count;
}

static DEVICE_ATTR(selfballoon_uphysteresis, S_IRUGO | S_IWUSR,
                   show_selfballoon_uphys, store_selfballoon_uphys);

SELFBALLOON_SHOW(selfballoon_min_usable_mb, "%d\n",
                                selfballoon_min_usable_mb);

static ssize_t store_selfballoon_min_usable_mb(struct device *dev,
                                               struct device_attribute *attr,
                                               const char *buf,
                                               size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        selfballoon_min_usable_mb = val;
        return count;
}

static DEVICE_ATTR(selfballoon_min_usable_mb, S_IRUGO | S_IWUSR,
                   show_selfballoon_min_usable_mb,
                   store_selfballoon_min_usable_mb);

SELFBALLOON_SHOW(selfballoon_reserved_mb, "%d\n",
                                selfballoon_reserved_mb);

static ssize_t store_selfballoon_reserved_mb(struct device *dev,
                                             struct device_attribute *attr,
                                             const char *buf,
                                             size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        selfballoon_reserved_mb = val;
        return count;
}

static DEVICE_ATTR(selfballoon_reserved_mb, S_IRUGO | S_IWUSR,
                   show_selfballoon_reserved_mb,
                   store_selfballoon_reserved_mb);


#ifdef CONFIG_FRONTSWAP
SELFBALLOON_SHOW(frontswap_selfshrinking, "%d\n", frontswap_selfshrinking);

static ssize_t store_frontswap_selfshrinking(struct device *dev,
                                             struct device_attribute *attr,
                                             const char *buf,
                                             size_t count)
{
        bool was_enabled = frontswap_selfshrinking;
        unsigned long tmp;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &tmp);
        if (err)
                return err;
        if ((tmp != 0) && (tmp != 1))
                return -EINVAL;
        frontswap_selfshrinking = !!tmp;
        if (!was_enabled && !xen_selfballooning_enabled &&
             frontswap_selfshrinking)
                schedule_delayed_work(&selfballoon_worker,
                        selfballoon_interval * HZ);

        return count;
}

static DEVICE_ATTR(frontswap_selfshrinking, S_IRUGO | S_IWUSR,
                   show_frontswap_selfshrinking, store_frontswap_selfshrinking);

SELFBALLOON_SHOW(frontswap_inertia, "%d\n", frontswap_inertia);

static ssize_t store_frontswap_inertia(struct device *dev,
                                       struct device_attribute *attr,
                                       const char *buf,
                                       size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        frontswap_inertia = val;
        frontswap_inertia_counter = val;
        return count;
}

static DEVICE_ATTR(frontswap_inertia, S_IRUGO | S_IWUSR,
                   show_frontswap_inertia, store_frontswap_inertia);

SELFBALLOON_SHOW(frontswap_hysteresis, "%d\n", frontswap_hysteresis);

static ssize_t store_frontswap_hysteresis(struct device *dev,
                                          struct device_attribute *attr,
                                          const char *buf,
                                          size_t count)
{
        unsigned long val;
        int err;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
        err = kstrtoul(buf, 10, &val);
        if (err)
                return err;
        if (val == 0)
                return -EINVAL;
        frontswap_hysteresis = val;
        return count;
}

static DEVICE_ATTR(frontswap_hysteresis, S_IRUGO | S_IWUSR,
                   show_frontswap_hysteresis, store_frontswap_hysteresis);

#endif /* CONFIG_FRONTSWAP */

static struct attribute *selfballoon_attrs[] = {
        &dev_attr_selfballooning.attr,
        &dev_attr_selfballoon_interval.attr,
        &dev_attr_selfballoon_downhysteresis.attr,
        &dev_attr_selfballoon_uphysteresis.attr,
        &dev_attr_selfballoon_min_usable_mb.attr,
        &dev_attr_selfballoon_reserved_mb.attr,
#ifdef CONFIG_FRONTSWAP
        &dev_attr_frontswap_selfshrinking.attr,
        &dev_attr_frontswap_hysteresis.attr,
        &dev_attr_frontswap_inertia.attr,
#endif
        NULL
};

static const struct attribute_group selfballoon_group = {
        .name = "selfballoon",
        .attrs = selfballoon_attrs
};
#endif

int register_xen_selfballooning(struct device *dev)
{
        int error = -1;

#ifdef CONFIG_SYSFS
        error = sysfs_create_group(&dev->kobj, &selfballoon_group);
#endif
        return error;
}
EXPORT_SYMBOL(register_xen_selfballooning);

int xen_selfballoon_init(bool use_selfballooning, bool use_frontswap_selfshrink)
{
        bool enable = false;
        unsigned long reserve_pages;

        if (!xen_domain())
                return -ENODEV;

        if (xen_initial_domain()) {
                pr_info("Xen selfballooning driver disabled for domain0\n");
                return -ENODEV;
        }

        xen_selfballooning_enabled = tmem_enabled && use_selfballooning;
        if (xen_selfballooning_enabled) {
                pr_info("Initializing Xen selfballooning driver\n");
                enable = true;
        }
#ifdef CONFIG_FRONTSWAP
        frontswap_selfshrinking = tmem_enabled && use_frontswap_selfshrink;
        if (frontswap_selfshrinking) {
                pr_info("Initializing frontswap selfshrinking driver\n");
                enable = true;
        }
#endif
        if (!enable)
                return -ENODEV;

        /*
         * Give selfballoon_reserved_mb a default value(10% of total ram pages)
         * to make selfballoon not so aggressive.
         *
         * There are mainly two reasons:
         * 1) The original goal_page didn't consider some pages used by kernel
         *    space, like slab pages and memory used by device drivers.
         *
         * 2) The balloon driver may not give back memory to guest OS fast
         *    enough when the workload suddenly aquries a lot of physical memory.
         *
         * In both cases, the guest OS will suffer from memory pressure and
         * OOM killer may be triggered.
         * By reserving extra 10% of total ram pages, we can keep the system
         * much more reliably and response faster in some cases.
         */
        if (!selfballoon_reserved_mb) {
                reserve_pages = totalram_pages / 10;
                selfballoon_reserved_mb = PAGES2MB(reserve_pages);
        }
        schedule_delayed_work(&selfballoon_worker, selfballoon_interval * HZ);

        return 0;
}
EXPORT_SYMBOL(xen_selfballoon_init);