drivers/xen/balloon.c - kernel/msm - Gitiles

 /******************************************************************************
  * balloon.c
  *
  * Xen balloon driver - enables returning/claiming memory to/from Xen.
  *
  * Copyright (c) 2003, B Dragovic
  * Copyright (c) 2003-2004, M Williamson, K Fraser
  * Copyright (c) 2005 Dan M. Smith, IBM Corporation
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License version 2
  * as published by the Free Software Foundation; or, when distributed
  * separately from the Linux kernel or incorporated into other
  * software packages, subject to the following license:
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this source file (the "Software"), to deal in the Software without
  * restriction, including without limitation the rights to use, copy, modify,
  * merge, publish, distribute, sublicense, and/or sell copies of the Software,
  * and to permit persons to whom the Software is furnished to do so, subject to
  * the following conditions:
  *
  * The above copyright notice and this permission notice shall be included in
  * all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  * IN THE SOFTWARE.
  */

 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/errno.h>
 #include <linux/mm.h>
 #include <linux/bootmem.h>
 #include <linux/pagemap.h>
 #include <linux/highmem.h>
 #include <linux/mutex.h>
 #include <linux/list.h>
 #include <linux/sysdev.h>
 #include <linux/gfp.h>

 #include <asm/page.h>
 #include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/uaccess.h>
 #include <asm/tlb.h>

 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>

 #include <xen/xen.h>
 #include <xen/interface/xen.h>
 #include <xen/interface/memory.h>
 #include <xen/xenbus.h>
 #include <xen/features.h>
 #include <xen/page.h>

 #define PAGES2KB(_p) ((_p)<<(PAGE_SHIFT-10))

 #define BALLOON_CLASS_NAME "xen_memory"

 struct balloon_stats {
 	/* We aim for 'current allocation' == 'target allocation'. */
 	unsigned long current_pages;
 	unsigned long target_pages;
 	/*
 	 * Drivers may alter the memory reservation independently, but they
 	 * must inform the balloon driver so we avoid hitting the hard limit.
 	 */
 	unsigned long driver_pages;
 	/* Number of pages in high- and low-memory balloons. */
 	unsigned long balloon_low;
 	unsigned long balloon_high;
 };

 static DEFINE_MUTEX(balloon_mutex);

 static struct sys_device balloon_sysdev;

 static int register_balloon(struct sys_device *sysdev);

 static struct balloon_stats balloon_stats;

 /* We increase/decrease in batches which fit in a page */
 static unsigned long frame_list[PAGE_SIZE / sizeof(unsigned long)];

 #ifdef CONFIG_HIGHMEM
 #define inc_totalhigh_pages() (totalhigh_pages++)
 #define dec_totalhigh_pages() (totalhigh_pages--)
 #else
 #define inc_totalhigh_pages() do {} while(0)
 #define dec_totalhigh_pages() do {} while(0)
 #endif

 /* List of ballooned pages, threaded through the mem_map array. */
 static LIST_HEAD(ballooned_pages);

 /* Main work function, always executed in process context. */
 static void balloon_process(struct work_struct *work);
 static DECLARE_WORK(balloon_worker, balloon_process);
 static struct timer_list balloon_timer;

 /* When ballooning out (allocating memory to return to Xen) we don't really
    want the kernel to try too hard since that can trigger the oom killer. */
 #define GFP_BALLOON \
 	(GFP_HIGHUSER | __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC)

 static void scrub_page(struct page *page)
 {
 #ifdef CONFIG_XEN_SCRUB_PAGES
 	clear_highpage(page);
 #endif
 }

 /* balloon_append: add the given page to the balloon. */
 static void __balloon_append(struct page *page)
 {
 	/* Lowmem is re-populated first, so highmem pages go at list tail. */
 	if (PageHighMem(page)) {
 		list_add_tail(&page->lru, &ballooned_pages);
 		balloon_stats.balloon_high++;
 		dec_totalhigh_pages();
 	} else {
 		list_add(&page->lru, &ballooned_pages);
 		balloon_stats.balloon_low++;
 	}
 }

 static void balloon_append(struct page *page)
 {
 	__balloon_append(page);
 	totalram_pages--;
 }

 /* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
 static struct page *balloon_retrieve(void)
 {
 	struct page *page;

 	if (list_empty(&ballooned_pages))
 		return NULL;

 	page = list_entry(ballooned_pages.next, struct page, lru);
 	list_del(&page->lru);

 	if (PageHighMem(page)) {
 		balloon_stats.balloon_high--;
 		inc_totalhigh_pages();
 	}
 	else
 		balloon_stats.balloon_low--;

 	totalram_pages++;

 	return page;
 }

 static struct page *balloon_first_page(void)
 {
 	if (list_empty(&ballooned_pages))
 		return NULL;
 	return list_entry(ballooned_pages.next, struct page, lru);
 }

 static struct page *balloon_next_page(struct page *page)
 {
 	struct list_head *next = page->lru.next;
 	if (next == &ballooned_pages)
 		return NULL;
 	return list_entry(next, struct page, lru);
 }

 static void balloon_alarm(unsigned long unused)
 {
 	schedule_work(&balloon_worker);
 }

 static unsigned long current_target(void)
 {
 	unsigned long target = balloon_stats.target_pages;

 	target = min(target,
 		     balloon_stats.current_pages +
 		     balloon_stats.balloon_low +
 		     balloon_stats.balloon_high);

 	return target;
 }

 static int increase_reservation(unsigned long nr_pages)
 {
 	unsigned long  pfn, i;
 	struct page   *page;
 	long           rc;
 	struct xen_memory_reservation reservation = {
 		.address_bits = 0,
 		.extent_order = 0,
 		.domid        = DOMID_SELF
 	};

 	if (nr_pages > ARRAY_SIZE(frame_list))
 		nr_pages = ARRAY_SIZE(frame_list);

 	page = balloon_first_page();
 	for (i = 0; i < nr_pages; i++) {
 		BUG_ON(page == NULL);
 		frame_list[i] = page_to_pfn(page);
 		page = balloon_next_page(page);
 	}

 	set_xen_guest_handle(reservation.extent_start, frame_list);
 	reservation.nr_extents = nr_pages;
 	rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);
 	if (rc < 0)
 		goto out;

 	for (i = 0; i < rc; i++) {
 		page = balloon_retrieve();
 		BUG_ON(page == NULL);

 		pfn = page_to_pfn(page);
 		BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
 		       phys_to_machine_mapping_valid(pfn));

 		set_phys_to_machine(pfn, frame_list[i]);

 		/* Link back into the page tables if not highmem. */
 		if (pfn < max_low_pfn) {
 			int ret;
 			ret = HYPERVISOR_update_va_mapping(
 				(unsigned long)__va(pfn << PAGE_SHIFT),
 				mfn_pte(frame_list[i], PAGE_KERNEL),
 				0);
 			BUG_ON(ret);
 		}

 		/* Relinquish the page back to the allocator. */
 		ClearPageReserved(page);
 		init_page_count(page);
 		__free_page(page);
 	}

 	balloon_stats.current_pages += rc;

  out:
 	return rc < 0 ? rc : rc != nr_pages;
 }

 static int decrease_reservation(unsigned long nr_pages)
 {
 	unsigned long  pfn, i;
 	struct page   *page;
 	int            need_sleep = 0;
 	int ret;
 	struct xen_memory_reservation reservation = {
 		.address_bits = 0,
 		.extent_order = 0,
 		.domid        = DOMID_SELF
 	};

 	if (nr_pages > ARRAY_SIZE(frame_list))
 		nr_pages = ARRAY_SIZE(frame_list);

 	for (i = 0; i < nr_pages; i++) {
 		if ((page = alloc_page(GFP_BALLOON)) == NULL) {
 			nr_pages = i;
 			need_sleep = 1;
 			break;
 		}

 		pfn = page_to_pfn(page);
 		frame_list[i] = pfn_to_mfn(pfn);

 		scrub_page(page);

 		if (!PageHighMem(page)) {
 			ret = HYPERVISOR_update_va_mapping(
 				(unsigned long)__va(pfn << PAGE_SHIFT),
 				__pte_ma(0), 0);
 			BUG_ON(ret);
                 }

 	}

 	/* Ensure that ballooned highmem pages don't have kmaps. */
 	kmap_flush_unused();
 	flush_tlb_all();

 	/* No more mappings: invalidate P2M and add to balloon. */
 	for (i = 0; i < nr_pages; i++) {
 		pfn = mfn_to_pfn(frame_list[i]);
 		set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
 		balloon_append(pfn_to_page(pfn));
 	}

 	set_xen_guest_handle(reservation.extent_start, frame_list);
 	reservation.nr_extents   = nr_pages;
 	ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
 	BUG_ON(ret != nr_pages);

 	balloon_stats.current_pages -= nr_pages;

 	return need_sleep;
 }

 /*
  * We avoid multiple worker processes conflicting via the balloon mutex.
  * We may of course race updates of the target counts (which are protected
  * by the balloon lock), or with changes to the Xen hard limit, but we will
  * recover from these in time.
  */
 static void balloon_process(struct work_struct *work)
 {
 	int need_sleep = 0;
 	long credit;

 	mutex_lock(&balloon_mutex);

 	do {
 		credit = current_target() - balloon_stats.current_pages;
 		if (credit > 0)
 			need_sleep = (increase_reservation(credit) != 0);
 		if (credit < 0)
 			need_sleep = (decrease_reservation(-credit) != 0);

 #ifndef CONFIG_PREEMPT
 		if (need_resched())
 			schedule();
 #endif
 	} while ((credit != 0) && !need_sleep);

 	/* Schedule more work if there is some still to be done. */
 	if (current_target() != balloon_stats.current_pages)
 		mod_timer(&balloon_timer, jiffies + HZ);

 	mutex_unlock(&balloon_mutex);
 }

 /* Resets the Xen limit, sets new target, and kicks off processing. */
 static void balloon_set_new_target(unsigned long target)
 {
 	/* No need for lock. Not read-modify-write updates. */
 	balloon_stats.target_pages = target;
 	schedule_work(&balloon_worker);
 }

 static struct xenbus_watch target_watch =
 {
 	.node = "memory/target"
 };

 /* React to a change in the target key */
 static void watch_target(struct xenbus_watch *watch,
 			 const char **vec, unsigned int len)
 {
 	unsigned long long new_target;
 	int err;

 	err = xenbus_scanf(XBT_NIL, "memory", "target", "%llu", &new_target);
 	if (err != 1) {
 		/* This is ok (for domain0 at least) - so just return */
 		return;
 	}

 	/* The given memory/target value is in KiB, so it needs converting to
 	 * pages. PAGE_SHIFT converts bytes to pages, hence PAGE_SHIFT - 10.
 	 */
 	balloon_set_new_target(new_target >> (PAGE_SHIFT - 10));
 }

 static int balloon_init_watcher(struct notifier_block *notifier,
 				unsigned long event,
 				void *data)
 {
 	int err;

 	err = register_xenbus_watch(&target_watch);
 	if (err)
 		printk(KERN_ERR "Failed to set balloon watcher\n");

 	return NOTIFY_DONE;
 }

 static struct notifier_block xenstore_notifier;

 static int __init balloon_init(void)
 {
 	unsigned long pfn;
 	struct page *page;

 	if (!xen_pv_domain())
 		return -ENODEV;

 	pr_info("xen_balloon: Initialising balloon driver.\n");

 	balloon_stats.current_pages = min(xen_start_info->nr_pages, max_pfn);
 	balloon_stats.target_pages  = balloon_stats.current_pages;
 	balloon_stats.balloon_low   = 0;
 	balloon_stats.balloon_high  = 0;
 	balloon_stats.driver_pages  = 0UL;

 	init_timer(&balloon_timer);
 	balloon_timer.data = 0;
 	balloon_timer.function = balloon_alarm;

 	register_balloon(&balloon_sysdev);

 	/* Initialise the balloon with excess memory space. */
 	for (pfn = PFN_UP(xen_extra_mem_start);
 	     pfn < PFN_DOWN(xen_extra_mem_start + xen_extra_mem_size);
 	     pfn++) {
 		page = pfn_to_page(pfn);
 		/* totalram_pages doesn't include the boot-time
 		   balloon extension, so don't subtract from it. */
 		__balloon_append(page);
 	}

 	target_watch.callback = watch_target;
 	xenstore_notifier.notifier_call = balloon_init_watcher;

 	register_xenstore_notifier(&xenstore_notifier);

 	return 0;
 }

 subsys_initcall(balloon_init);

 static void balloon_exit(void)
 {
     /* XXX - release balloon here */
     return;
 }

 module_exit(balloon_exit);

 #define BALLOON_SHOW(name, format, args...)				\
 	static ssize_t show_##name(struct sys_device *dev,		\
 				   struct sysdev_attribute *attr,	\
 				   char *buf)				\
 	{								\
 		return sprintf(buf, format, ##args);			\
 	}								\
 	static SYSDEV_ATTR(name, S_IRUGO, show_##name, NULL)

 BALLOON_SHOW(current_kb, "%lu\n", PAGES2KB(balloon_stats.current_pages));
 BALLOON_SHOW(low_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_low));
 BALLOON_SHOW(high_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_high));
 BALLOON_SHOW(driver_kb, "%lu\n", PAGES2KB(balloon_stats.driver_pages));

 static ssize_t show_target_kb(struct sys_device *dev, struct sysdev_attribute *attr,
 			      char *buf)
 {
 	return sprintf(buf, "%lu\n", PAGES2KB(balloon_stats.target_pages));
 }

 static ssize_t store_target_kb(struct sys_device *dev,
 			       struct sysdev_attribute *attr,
 			       const char *buf,
 			       size_t count)
 {
 	char *endchar;
 	unsigned long long target_bytes;

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

 	target_bytes = simple_strtoull(buf, &endchar, 0) * 1024;

 	balloon_set_new_target(target_bytes >> PAGE_SHIFT);

 	return count;
 }

 static SYSDEV_ATTR(target_kb, S_IRUGO | S_IWUSR,
 		   show_target_kb, store_target_kb);


 static ssize_t show_target(struct sys_device *dev, struct sysdev_attribute *attr,
 			      char *buf)
 {
 	return sprintf(buf, "%llu\n",
 		       (unsigned long long)balloon_stats.target_pages
 		       << PAGE_SHIFT);
 }

 static ssize_t store_target(struct sys_device *dev,
 			    struct sysdev_attribute *attr,
 			    const char *buf,
 			    size_t count)
 {
 	char *endchar;
 	unsigned long long target_bytes;

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

 	target_bytes = memparse(buf, &endchar);

 	balloon_set_new_target(target_bytes >> PAGE_SHIFT);

 	return count;
 }

 static SYSDEV_ATTR(target, S_IRUGO | S_IWUSR,
 		   show_target, store_target);


 static struct sysdev_attribute *balloon_attrs[] = {
 	&attr_target_kb,
 	&attr_target,
 };

 static struct attribute *balloon_info_attrs[] = {
 	&attr_current_kb.attr,
 	&attr_low_kb.attr,
 	&attr_high_kb.attr,
 	&attr_driver_kb.attr,
 	NULL
 };

 static struct attribute_group balloon_info_group = {
 	.name = "info",
 	.attrs = balloon_info_attrs,
 };

 static struct sysdev_class balloon_sysdev_class = {
 	.name = BALLOON_CLASS_NAME,
 };

 static int register_balloon(struct sys_device *sysdev)
 {
 	int i, error;

 	error = sysdev_class_register(&balloon_sysdev_class);
 	if (error)
 		return error;

 	sysdev->id = 0;
 	sysdev->cls = &balloon_sysdev_class;

 	error = sysdev_register(sysdev);
 	if (error) {
 		sysdev_class_unregister(&balloon_sysdev_class);
 		return error;
 	}

 	for (i = 0; i < ARRAY_SIZE(balloon_attrs); i++) {
 		error = sysdev_create_file(sysdev, balloon_attrs[i]);
 		if (error)
 			goto fail;
 	}

 	error = sysfs_create_group(&sysdev->kobj, &balloon_info_group);
 	if (error)
 		goto fail;

 	return 0;

  fail:
 	while (--i >= 0)
 		sysdev_remove_file(sysdev, balloon_attrs[i]);
 	sysdev_unregister(sysdev);
 	sysdev_class_unregister(&balloon_sysdev_class);
 	return error;
 }

 MODULE_LICENSE("GPL");
	/******************************************************************************
	* balloon.c
	*
	* Xen balloon driver - enables returning/claiming memory to/from Xen.
	*
	* Copyright (c) 2003, B Dragovic
	* Copyright (c) 2003-2004, M Williamson, K Fraser
	* Copyright (c) 2005 Dan M. Smith, IBM Corporation
	*
	* This program is free software; you can redistribute it and/or
	* modify it under the terms of the GNU General Public License version 2
	* as published by the Free Software Foundation; or, when distributed
	* separately from the Linux kernel or incorporated into other
	* software packages, subject to the following license:
	*
	* Permission is hereby granted, free of charge, to any person obtaining a copy
	* of this source file (the "Software"), to deal in the Software without
	* restriction, including without limitation the rights to use, copy, modify,
	* merge, publish, distribute, sublicense, and/or sell copies of the Software,
	* and to permit persons to whom the Software is furnished to do so, subject to
	* the following conditions:
	*
	* The above copyright notice and this permission notice shall be included in
	* all copies or substantial portions of the Software.
	*
	* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
	* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
	* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
	* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
	* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
	* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
	* IN THE SOFTWARE.
	*/

	#include <linux/kernel.h>
	#include <linux/module.h>
	#include <linux/sched.h>
	#include <linux/errno.h>
	#include <linux/mm.h>
	#include <linux/bootmem.h>
	#include <linux/pagemap.h>
	#include <linux/highmem.h>
	#include <linux/mutex.h>
	#include <linux/list.h>
	#include <linux/sysdev.h>
	#include <linux/gfp.h>

	#include <asm/page.h>
	#include <asm/pgalloc.h>
	#include <asm/pgtable.h>
	#include <asm/uaccess.h>
	#include <asm/tlb.h>

	#include <asm/xen/hypervisor.h>
	#include <asm/xen/hypercall.h>

	#include <xen/xen.h>
	#include <xen/interface/xen.h>
	#include <xen/interface/memory.h>
	#include <xen/xenbus.h>
	#include <xen/features.h>
	#include <xen/page.h>

	#define PAGES2KB(_p) ((_p)<<(PAGE_SHIFT-10))

	#define BALLOON_CLASS_NAME "xen_memory"

	struct balloon_stats {
	/* We aim for 'current allocation' == 'target allocation'. */
	unsigned long current_pages;
	unsigned long target_pages;
	/*
	* Drivers may alter the memory reservation independently, but they
	* must inform the balloon driver so we avoid hitting the hard limit.
	*/
	unsigned long driver_pages;
	/* Number of pages in high- and low-memory balloons. */
	unsigned long balloon_low;
	unsigned long balloon_high;
	};

	static DEFINE_MUTEX(balloon_mutex);

	static struct sys_device balloon_sysdev;

	static int register_balloon(struct sys_device *sysdev);

	static struct balloon_stats balloon_stats;

	/* We increase/decrease in batches which fit in a page */
	static unsigned long frame_list[PAGE_SIZE / sizeof(unsigned long)];

	#ifdef CONFIG_HIGHMEM
	#define inc_totalhigh_pages() (totalhigh_pages++)
	#define dec_totalhigh_pages() (totalhigh_pages--)
	#else
	#define inc_totalhigh_pages() do {} while(0)
	#define dec_totalhigh_pages() do {} while(0)
	#endif

	/* List of ballooned pages, threaded through the mem_map array. */
	static LIST_HEAD(ballooned_pages);

	/* Main work function, always executed in process context. */
	static void balloon_process(struct work_struct *work);
	static DECLARE_WORK(balloon_worker, balloon_process);
	static struct timer_list balloon_timer;

	/* When ballooning out (allocating memory to return to Xen) we don't really
	want the kernel to try too hard since that can trigger the oom killer. */
	#define GFP_BALLOON \
	(GFP_HIGHUSER \| __GFP_NOWARN \| __GFP_NORETRY \| __GFP_NOMEMALLOC)

	static void scrub_page(struct page *page)
	{
	#ifdef CONFIG_XEN_SCRUB_PAGES
	clear_highpage(page);
	#endif
	}

	/* balloon_append: add the given page to the balloon. */
	static void __balloon_append(struct page *page)
	{
	/* Lowmem is re-populated first, so highmem pages go at list tail. */
	if (PageHighMem(page)) {
	list_add_tail(&page->lru, &ballooned_pages);
	balloon_stats.balloon_high++;
	dec_totalhigh_pages();
	} else {
	list_add(&page->lru, &ballooned_pages);
	balloon_stats.balloon_low++;
	}
	}

	static void balloon_append(struct page *page)
	{
	__balloon_append(page);
	totalram_pages--;
	}

	/* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
	static struct page *balloon_retrieve(void)
	{
	struct page *page;

	if (list_empty(&ballooned_pages))
	return NULL;

	page = list_entry(ballooned_pages.next, struct page, lru);
	list_del(&page->lru);

	if (PageHighMem(page)) {
	balloon_stats.balloon_high--;
	inc_totalhigh_pages();
	}
	else
	balloon_stats.balloon_low--;

	totalram_pages++;

	return page;
	}

	static struct page *balloon_first_page(void)
	{
	if (list_empty(&ballooned_pages))
	return NULL;
	return list_entry(ballooned_pages.next, struct page, lru);
	}

	static struct page balloon_next_page(struct page page)
	{
	struct list_head *next = page->lru.next;
	if (next == &ballooned_pages)
	return NULL;
	return list_entry(next, struct page, lru);
	}

	static void balloon_alarm(unsigned long unused)
	{
	schedule_work(&balloon_worker);
	}

	static unsigned long current_target(void)
	{
	unsigned long target = balloon_stats.target_pages;

	target = min(target,
	balloon_stats.current_pages +
	balloon_stats.balloon_low +
	balloon_stats.balloon_high);

	return target;
	}

	static int increase_reservation(unsigned long nr_pages)
	{
	unsigned long pfn, i;
	struct page *page;
	long rc;
	struct xen_memory_reservation reservation = {
	.address_bits = 0,
	.extent_order = 0,
	.domid = DOMID_SELF
	};

	if (nr_pages > ARRAY_SIZE(frame_list))
	nr_pages = ARRAY_SIZE(frame_list);

	page = balloon_first_page();
	for (i = 0; i < nr_pages; i++) {
	BUG_ON(page == NULL);
	frame_list[i] = page_to_pfn(page);
	page = balloon_next_page(page);
	}

	set_xen_guest_handle(reservation.extent_start, frame_list);
	reservation.nr_extents = nr_pages;
	rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);
	if (rc < 0)
	goto out;

	for (i = 0; i < rc; i++) {
	page = balloon_retrieve();
	BUG_ON(page == NULL);

	pfn = page_to_pfn(page);
	BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
	phys_to_machine_mapping_valid(pfn));

	set_phys_to_machine(pfn, frame_list[i]);

	/* Link back into the page tables if not highmem. */
	if (pfn < max_low_pfn) {
	int ret;
	ret = HYPERVISOR_update_va_mapping(
	(unsigned long)__va(pfn << PAGE_SHIFT),
	mfn_pte(frame_list[i], PAGE_KERNEL),
	0);
	BUG_ON(ret);
	}

	/* Relinquish the page back to the allocator. */
	ClearPageReserved(page);
	init_page_count(page);
	__free_page(page);
	}

	balloon_stats.current_pages += rc;

	out:
	return rc < 0 ? rc : rc != nr_pages;
	}

	static int decrease_reservation(unsigned long nr_pages)
	{
	unsigned long pfn, i;
	struct page *page;
	int need_sleep = 0;
	int ret;
	struct xen_memory_reservation reservation = {
	.address_bits = 0,
	.extent_order = 0,
	.domid = DOMID_SELF
	};

	if (nr_pages > ARRAY_SIZE(frame_list))
	nr_pages = ARRAY_SIZE(frame_list);

	for (i = 0; i < nr_pages; i++) {
	if ((page = alloc_page(GFP_BALLOON)) == NULL) {
	nr_pages = i;
	need_sleep = 1;
	break;
	}

	pfn = page_to_pfn(page);
	frame_list[i] = pfn_to_mfn(pfn);

	scrub_page(page);

	if (!PageHighMem(page)) {
	ret = HYPERVISOR_update_va_mapping(
	(unsigned long)__va(pfn << PAGE_SHIFT),
	__pte_ma(0), 0);
	BUG_ON(ret);
	}

	}

	/* Ensure that ballooned highmem pages don't have kmaps. */
	kmap_flush_unused();
	flush_tlb_all();

	/* No more mappings: invalidate P2M and add to balloon. */
	for (i = 0; i < nr_pages; i++) {
	pfn = mfn_to_pfn(frame_list[i]);
	set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
	balloon_append(pfn_to_page(pfn));
	}

	set_xen_guest_handle(reservation.extent_start, frame_list);
	reservation.nr_extents = nr_pages;
	ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
	BUG_ON(ret != nr_pages);

	balloon_stats.current_pages -= nr_pages;

	return need_sleep;
	}

	/*
	* We avoid multiple worker processes conflicting via the balloon mutex.
	* We may of course race updates of the target counts (which are protected
	* by the balloon lock), or with changes to the Xen hard limit, but we will
	* recover from these in time.
	*/
	static void balloon_process(struct work_struct *work)
	{
	int need_sleep = 0;
	long credit;

	mutex_lock(&balloon_mutex);

	do {
	credit = current_target() - balloon_stats.current_pages;
	if (credit > 0)
	need_sleep = (increase_reservation(credit) != 0);
	if (credit < 0)
	need_sleep = (decrease_reservation(-credit) != 0);

	#ifndef CONFIG_PREEMPT
	if (need_resched())
	schedule();
	#endif
	} while ((credit != 0) && !need_sleep);

	/* Schedule more work if there is some still to be done. */
	if (current_target() != balloon_stats.current_pages)
	mod_timer(&balloon_timer, jiffies + HZ);

	mutex_unlock(&balloon_mutex);
	}

	/* Resets the Xen limit, sets new target, and kicks off processing. */
	static void balloon_set_new_target(unsigned long target)
	{
	/* No need for lock. Not read-modify-write updates. */
	balloon_stats.target_pages = target;
	schedule_work(&balloon_worker);
	}

	static struct xenbus_watch target_watch =
	{
	.node = "memory/target"
	};

	/* React to a change in the target key */
	static void watch_target(struct xenbus_watch *watch,
	const char **vec, unsigned int len)
	{
	unsigned long long new_target;
	int err;

	err = xenbus_scanf(XBT_NIL, "memory", "target", "%llu", &new_target);
	if (err != 1) {
	/* This is ok (for domain0 at least) - so just return */
	return;
	}

	/* The given memory/target value is in KiB, so it needs converting to
	* pages. PAGE_SHIFT converts bytes to pages, hence PAGE_SHIFT - 10.
	*/
	balloon_set_new_target(new_target >> (PAGE_SHIFT - 10));
	}

	static int balloon_init_watcher(struct notifier_block *notifier,
	unsigned long event,
	void *data)
	{
	int err;

	err = register_xenbus_watch(&target_watch);
	if (err)
	printk(KERN_ERR "Failed to set balloon watcher\n");

	return NOTIFY_DONE;
	}

	static struct notifier_block xenstore_notifier;

	static int __init balloon_init(void)
	{
	unsigned long pfn;
	struct page *page;

	if (!xen_pv_domain())
	return -ENODEV;

	pr_info("xen_balloon: Initialising balloon driver.\n");

	balloon_stats.current_pages = min(xen_start_info->nr_pages, max_pfn);
	balloon_stats.target_pages = balloon_stats.current_pages;
	balloon_stats.balloon_low = 0;
	balloon_stats.balloon_high = 0;
	balloon_stats.driver_pages = 0UL;

	init_timer(&balloon_timer);
	balloon_timer.data = 0;
	balloon_timer.function = balloon_alarm;

	register_balloon(&balloon_sysdev);

	/* Initialise the balloon with excess memory space. */
	for (pfn = PFN_UP(xen_extra_mem_start);
	pfn < PFN_DOWN(xen_extra_mem_start + xen_extra_mem_size);
	pfn++) {
	page = pfn_to_page(pfn);
	/* totalram_pages doesn't include the boot-time
	balloon extension, so don't subtract from it. */
	__balloon_append(page);
	}

	target_watch.callback = watch_target;
	xenstore_notifier.notifier_call = balloon_init_watcher;

	register_xenstore_notifier(&xenstore_notifier);

	return 0;
	}

	subsys_initcall(balloon_init);

	static void balloon_exit(void)
	{
	/* XXX - release balloon here */
	return;
	}

	module_exit(balloon_exit);

	#define BALLOON_SHOW(name, format, args...) \
	static ssize_t show_##name(struct sys_device *dev, \
	struct sysdev_attribute *attr, \
	char *buf) \
	{ \
	return sprintf(buf, format, ##args); \
	} \
	static SYSDEV_ATTR(name, S_IRUGO, show_##name, NULL)

	BALLOON_SHOW(current_kb, "%lu\n", PAGES2KB(balloon_stats.current_pages));
	BALLOON_SHOW(low_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_low));
	BALLOON_SHOW(high_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_high));
	BALLOON_SHOW(driver_kb, "%lu\n", PAGES2KB(balloon_stats.driver_pages));

	static ssize_t show_target_kb(struct sys_device dev, struct sysdev_attribute attr,
	char *buf)
	{
	return sprintf(buf, "%lu\n", PAGES2KB(balloon_stats.target_pages));
	}

	static ssize_t store_target_kb(struct sys_device *dev,
	struct sysdev_attribute *attr,
	const char *buf,
	size_t count)
	{
	char *endchar;
	unsigned long long target_bytes;

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

	target_bytes = simple_strtoull(buf, &endchar, 0) * 1024;

	balloon_set_new_target(target_bytes >> PAGE_SHIFT);

	return count;
	}

	static SYSDEV_ATTR(target_kb, S_IRUGO \| S_IWUSR,
	show_target_kb, store_target_kb);


	static ssize_t show_target(struct sys_device dev, struct sysdev_attribute attr,
	char *buf)
	{
	return sprintf(buf, "%llu\n",
	(unsigned long long)balloon_stats.target_pages
	<< PAGE_SHIFT);
	}

	static ssize_t store_target(struct sys_device *dev,
	struct sysdev_attribute *attr,
	const char *buf,
	size_t count)
	{
	char *endchar;
	unsigned long long target_bytes;

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

	target_bytes = memparse(buf, &endchar);

	balloon_set_new_target(target_bytes >> PAGE_SHIFT);

	return count;
	}

	static SYSDEV_ATTR(target, S_IRUGO \| S_IWUSR,
	show_target, store_target);


	static struct sysdev_attribute *balloon_attrs[] = {
	&attr_target_kb,
	&attr_target,
	};

	static struct attribute *balloon_info_attrs[] = {
	&attr_current_kb.attr,
	&attr_low_kb.attr,
	&attr_high_kb.attr,
	&attr_driver_kb.attr,
	NULL
	};

	static struct attribute_group balloon_info_group = {
	.name = "info",
	.attrs = balloon_info_attrs,
	};

	static struct sysdev_class balloon_sysdev_class = {
	.name = BALLOON_CLASS_NAME,
	};

	static int register_balloon(struct sys_device *sysdev)
	{
	int i, error;

	error = sysdev_class_register(&balloon_sysdev_class);
	if (error)
	return error;

	sysdev->id = 0;
	sysdev->cls = &balloon_sysdev_class;

	error = sysdev_register(sysdev);
	if (error) {
	sysdev_class_unregister(&balloon_sysdev_class);
	return error;
	}

	for (i = 0; i < ARRAY_SIZE(balloon_attrs); i++) {
	error = sysdev_create_file(sysdev, balloon_attrs[i]);
	if (error)
	goto fail;
	}

	error = sysfs_create_group(&sysdev->kobj, &balloon_info_group);
	if (error)
	goto fail;

	return 0;

	fail:
	while (--i >= 0)
	sysdev_remove_file(sysdev, balloon_attrs[i]);
	sysdev_unregister(sysdev);
	sysdev_class_unregister(&balloon_sysdev_class);
	return error;
	}

	MODULE_LICENSE("GPL");