xref: /openbmc/linux/drivers/xen/balloon.c (revision d9e32672)
1 /******************************************************************************
2  * Xen balloon driver - enables returning/claiming memory to/from Xen.
3  *
4  * Copyright (c) 2003, B Dragovic
5  * Copyright (c) 2003-2004, M Williamson, K Fraser
6  * Copyright (c) 2005 Dan M. Smith, IBM Corporation
7  * Copyright (c) 2010 Daniel Kiper
8  *
9  * Memory hotplug support was written by Daniel Kiper. Work on
10  * it was sponsored by Google under Google Summer of Code 2010
11  * program. Jeremy Fitzhardinge from Citrix was the mentor for
12  * this project.
13  *
14  * This program is free software; you can redistribute it and/or
15  * modify it under the terms of the GNU General Public License version 2
16  * as published by the Free Software Foundation; or, when distributed
17  * separately from the Linux kernel or incorporated into other
18  * software packages, subject to the following license:
19  *
20  * Permission is hereby granted, free of charge, to any person obtaining a copy
21  * of this source file (the "Software"), to deal in the Software without
22  * restriction, including without limitation the rights to use, copy, modify,
23  * merge, publish, distribute, sublicense, and/or sell copies of the Software,
24  * and to permit persons to whom the Software is furnished to do so, subject to
25  * the following conditions:
26  *
27  * The above copyright notice and this permission notice shall be included in
28  * all copies or substantial portions of the Software.
29  *
30  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
31  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
32  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
33  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
34  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
35  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
36  * IN THE SOFTWARE.
37  */
38 
39 #define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt
40 
41 #include <linux/cpu.h>
42 #include <linux/kernel.h>
43 #include <linux/sched.h>
44 #include <linux/cred.h>
45 #include <linux/errno.h>
46 #include <linux/mm.h>
47 #include <linux/memblock.h>
48 #include <linux/pagemap.h>
49 #include <linux/highmem.h>
50 #include <linux/mutex.h>
51 #include <linux/list.h>
52 #include <linux/gfp.h>
53 #include <linux/notifier.h>
54 #include <linux/memory.h>
55 #include <linux/memory_hotplug.h>
56 #include <linux/percpu-defs.h>
57 #include <linux/slab.h>
58 #include <linux/sysctl.h>
59 
60 #include <asm/page.h>
61 #include <asm/pgalloc.h>
62 #include <asm/pgtable.h>
63 #include <asm/tlb.h>
64 
65 #include <asm/xen/hypervisor.h>
66 #include <asm/xen/hypercall.h>
67 
68 #include <xen/xen.h>
69 #include <xen/interface/xen.h>
70 #include <xen/interface/memory.h>
71 #include <xen/balloon.h>
72 #include <xen/features.h>
73 #include <xen/page.h>
74 #include <xen/mem-reservation.h>
75 
76 static int xen_hotplug_unpopulated;
77 
78 #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
79 
80 static struct ctl_table balloon_table[] = {
81 	{
82 		.procname	= "hotplug_unpopulated",
83 		.data		= &xen_hotplug_unpopulated,
84 		.maxlen		= sizeof(int),
85 		.mode		= 0644,
86 		.proc_handler	= proc_dointvec_minmax,
87 		.extra1         = SYSCTL_ZERO,
88 		.extra2         = SYSCTL_ONE,
89 	},
90 	{ }
91 };
92 
93 static struct ctl_table balloon_root[] = {
94 	{
95 		.procname	= "balloon",
96 		.mode		= 0555,
97 		.child		= balloon_table,
98 	},
99 	{ }
100 };
101 
102 static struct ctl_table xen_root[] = {
103 	{
104 		.procname	= "xen",
105 		.mode		= 0555,
106 		.child		= balloon_root,
107 	},
108 	{ }
109 };
110 
111 #endif
112 
113 /*
114  * Use one extent per PAGE_SIZE to avoid to break down the page into
115  * multiple frame.
116  */
117 #define EXTENT_ORDER (fls(XEN_PFN_PER_PAGE) - 1)
118 
119 /*
120  * balloon_process() state:
121  *
122  * BP_DONE: done or nothing to do,
123  * BP_WAIT: wait to be rescheduled,
124  * BP_EAGAIN: error, go to sleep,
125  * BP_ECANCELED: error, balloon operation canceled.
126  */
127 
128 enum bp_state {
129 	BP_DONE,
130 	BP_WAIT,
131 	BP_EAGAIN,
132 	BP_ECANCELED
133 };
134 
135 
136 static DEFINE_MUTEX(balloon_mutex);
137 
138 struct balloon_stats balloon_stats;
139 EXPORT_SYMBOL_GPL(balloon_stats);
140 
141 /* We increase/decrease in batches which fit in a page */
142 static xen_pfn_t frame_list[PAGE_SIZE / sizeof(xen_pfn_t)];
143 
144 
145 /* List of ballooned pages, threaded through the mem_map array. */
146 static LIST_HEAD(ballooned_pages);
147 static DECLARE_WAIT_QUEUE_HEAD(balloon_wq);
148 
149 /* Main work function, always executed in process context. */
150 static void balloon_process(struct work_struct *work);
151 static DECLARE_DELAYED_WORK(balloon_worker, balloon_process);
152 
153 /* When ballooning out (allocating memory to return to Xen) we don't really
154    want the kernel to try too hard since that can trigger the oom killer. */
155 #define GFP_BALLOON \
156 	(GFP_HIGHUSER | __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC)
157 
158 /* balloon_append: add the given page to the balloon. */
159 static void balloon_append(struct page *page)
160 {
161 	__SetPageOffline(page);
162 
163 	/* Lowmem is re-populated first, so highmem pages go at list tail. */
164 	if (PageHighMem(page)) {
165 		list_add_tail(&page->lru, &ballooned_pages);
166 		balloon_stats.balloon_high++;
167 	} else {
168 		list_add(&page->lru, &ballooned_pages);
169 		balloon_stats.balloon_low++;
170 	}
171 	wake_up(&balloon_wq);
172 }
173 
174 /* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
175 static struct page *balloon_retrieve(bool require_lowmem)
176 {
177 	struct page *page;
178 
179 	if (list_empty(&ballooned_pages))
180 		return NULL;
181 
182 	page = list_entry(ballooned_pages.next, struct page, lru);
183 	if (require_lowmem && PageHighMem(page))
184 		return NULL;
185 	list_del(&page->lru);
186 
187 	if (PageHighMem(page))
188 		balloon_stats.balloon_high--;
189 	else
190 		balloon_stats.balloon_low--;
191 
192 	__ClearPageOffline(page);
193 	return page;
194 }
195 
196 static struct page *balloon_next_page(struct page *page)
197 {
198 	struct list_head *next = page->lru.next;
199 	if (next == &ballooned_pages)
200 		return NULL;
201 	return list_entry(next, struct page, lru);
202 }
203 
204 static enum bp_state update_schedule(enum bp_state state)
205 {
206 	if (state == BP_WAIT)
207 		return BP_WAIT;
208 
209 	if (state == BP_ECANCELED)
210 		return BP_ECANCELED;
211 
212 	if (state == BP_DONE) {
213 		balloon_stats.schedule_delay = 1;
214 		balloon_stats.retry_count = 1;
215 		return BP_DONE;
216 	}
217 
218 	++balloon_stats.retry_count;
219 
220 	if (balloon_stats.max_retry_count != RETRY_UNLIMITED &&
221 			balloon_stats.retry_count > balloon_stats.max_retry_count) {
222 		balloon_stats.schedule_delay = 1;
223 		balloon_stats.retry_count = 1;
224 		return BP_ECANCELED;
225 	}
226 
227 	balloon_stats.schedule_delay <<= 1;
228 
229 	if (balloon_stats.schedule_delay > balloon_stats.max_schedule_delay)
230 		balloon_stats.schedule_delay = balloon_stats.max_schedule_delay;
231 
232 	return BP_EAGAIN;
233 }
234 
235 #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
236 static void release_memory_resource(struct resource *resource)
237 {
238 	if (!resource)
239 		return;
240 
241 	/*
242 	 * No need to reset region to identity mapped since we now
243 	 * know that no I/O can be in this region
244 	 */
245 	release_resource(resource);
246 	kfree(resource);
247 }
248 
249 static struct resource *additional_memory_resource(phys_addr_t size)
250 {
251 	struct resource *res;
252 	int ret;
253 
254 	res = kzalloc(sizeof(*res), GFP_KERNEL);
255 	if (!res)
256 		return NULL;
257 
258 	res->name = "System RAM";
259 	res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
260 
261 	ret = allocate_resource(&iomem_resource, res,
262 				size, 0, -1,
263 				PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
264 	if (ret < 0) {
265 		pr_err("Cannot allocate new System RAM resource\n");
266 		kfree(res);
267 		return NULL;
268 	}
269 
270 #ifdef CONFIG_SPARSEMEM
271 	{
272 		unsigned long limit = 1UL << (MAX_PHYSMEM_BITS - PAGE_SHIFT);
273 		unsigned long pfn = res->start >> PAGE_SHIFT;
274 
275 		if (pfn > limit) {
276 			pr_err("New System RAM resource outside addressable RAM (%lu > %lu)\n",
277 			       pfn, limit);
278 			release_memory_resource(res);
279 			return NULL;
280 		}
281 	}
282 #endif
283 
284 	return res;
285 }
286 
287 static enum bp_state reserve_additional_memory(void)
288 {
289 	long credit;
290 	struct resource *resource;
291 	int nid, rc;
292 	unsigned long balloon_hotplug;
293 
294 	credit = balloon_stats.target_pages + balloon_stats.target_unpopulated
295 		- balloon_stats.total_pages;
296 
297 	/*
298 	 * Already hotplugged enough pages?  Wait for them to be
299 	 * onlined.
300 	 */
301 	if (credit <= 0)
302 		return BP_WAIT;
303 
304 	balloon_hotplug = round_up(credit, PAGES_PER_SECTION);
305 
306 	resource = additional_memory_resource(balloon_hotplug * PAGE_SIZE);
307 	if (!resource)
308 		goto err;
309 
310 	nid = memory_add_physaddr_to_nid(resource->start);
311 
312 #ifdef CONFIG_XEN_HAVE_PVMMU
313 	/*
314 	 * We don't support PV MMU when Linux and Xen is using
315 	 * different page granularity.
316 	 */
317 	BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
318 
319         /*
320          * add_memory() will build page tables for the new memory so
321          * the p2m must contain invalid entries so the correct
322          * non-present PTEs will be written.
323          *
324          * If a failure occurs, the original (identity) p2m entries
325          * are not restored since this region is now known not to
326          * conflict with any devices.
327          */
328 	if (!xen_feature(XENFEAT_auto_translated_physmap)) {
329 		unsigned long pfn, i;
330 
331 		pfn = PFN_DOWN(resource->start);
332 		for (i = 0; i < balloon_hotplug; i++) {
333 			if (!set_phys_to_machine(pfn + i, INVALID_P2M_ENTRY)) {
334 				pr_warn("set_phys_to_machine() failed, no memory added\n");
335 				goto err;
336 			}
337                 }
338 	}
339 #endif
340 
341 	/*
342 	 * add_memory_resource() will call online_pages() which in its turn
343 	 * will call xen_online_page() callback causing deadlock if we don't
344 	 * release balloon_mutex here. Unlocking here is safe because the
345 	 * callers drop the mutex before trying again.
346 	 */
347 	mutex_unlock(&balloon_mutex);
348 	/* add_memory_resource() requires the device_hotplug lock */
349 	lock_device_hotplug();
350 	rc = add_memory_resource(nid, resource);
351 	unlock_device_hotplug();
352 	mutex_lock(&balloon_mutex);
353 
354 	if (rc) {
355 		pr_warn("Cannot add additional memory (%i)\n", rc);
356 		goto err;
357 	}
358 
359 	balloon_stats.total_pages += balloon_hotplug;
360 
361 	return BP_WAIT;
362   err:
363 	release_memory_resource(resource);
364 	return BP_ECANCELED;
365 }
366 
367 static void xen_online_page(struct page *page, unsigned int order)
368 {
369 	unsigned long i, size = (1 << order);
370 	unsigned long start_pfn = page_to_pfn(page);
371 	struct page *p;
372 
373 	pr_debug("Online %lu pages starting at pfn 0x%lx\n", size, start_pfn);
374 	mutex_lock(&balloon_mutex);
375 	for (i = 0; i < size; i++) {
376 		p = pfn_to_page(start_pfn + i);
377 		__online_page_set_limits(p);
378 		balloon_append(p);
379 	}
380 	mutex_unlock(&balloon_mutex);
381 }
382 
383 static int xen_memory_notifier(struct notifier_block *nb, unsigned long val, void *v)
384 {
385 	if (val == MEM_ONLINE)
386 		schedule_delayed_work(&balloon_worker, 0);
387 
388 	return NOTIFY_OK;
389 }
390 
391 static struct notifier_block xen_memory_nb = {
392 	.notifier_call = xen_memory_notifier,
393 	.priority = 0
394 };
395 #else
396 static enum bp_state reserve_additional_memory(void)
397 {
398 	balloon_stats.target_pages = balloon_stats.current_pages;
399 	return BP_ECANCELED;
400 }
401 #endif /* CONFIG_XEN_BALLOON_MEMORY_HOTPLUG */
402 
403 static long current_credit(void)
404 {
405 	return balloon_stats.target_pages - balloon_stats.current_pages;
406 }
407 
408 static bool balloon_is_inflated(void)
409 {
410 	return balloon_stats.balloon_low || balloon_stats.balloon_high;
411 }
412 
413 static enum bp_state increase_reservation(unsigned long nr_pages)
414 {
415 	int rc;
416 	unsigned long i;
417 	struct page   *page;
418 
419 	if (nr_pages > ARRAY_SIZE(frame_list))
420 		nr_pages = ARRAY_SIZE(frame_list);
421 
422 	page = list_first_entry_or_null(&ballooned_pages, struct page, lru);
423 	for (i = 0; i < nr_pages; i++) {
424 		if (!page) {
425 			nr_pages = i;
426 			break;
427 		}
428 
429 		frame_list[i] = page_to_xen_pfn(page);
430 		page = balloon_next_page(page);
431 	}
432 
433 	rc = xenmem_reservation_increase(nr_pages, frame_list);
434 	if (rc <= 0)
435 		return BP_EAGAIN;
436 
437 	for (i = 0; i < rc; i++) {
438 		page = balloon_retrieve(false);
439 		BUG_ON(page == NULL);
440 
441 		xenmem_reservation_va_mapping_update(1, &page, &frame_list[i]);
442 
443 		/* Relinquish the page back to the allocator. */
444 		free_reserved_page(page);
445 	}
446 
447 	balloon_stats.current_pages += rc;
448 
449 	return BP_DONE;
450 }
451 
452 static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
453 {
454 	enum bp_state state = BP_DONE;
455 	unsigned long i;
456 	struct page *page, *tmp;
457 	int ret;
458 	LIST_HEAD(pages);
459 
460 	if (nr_pages > ARRAY_SIZE(frame_list))
461 		nr_pages = ARRAY_SIZE(frame_list);
462 
463 	for (i = 0; i < nr_pages; i++) {
464 		page = alloc_page(gfp);
465 		if (page == NULL) {
466 			nr_pages = i;
467 			state = BP_EAGAIN;
468 			break;
469 		}
470 		adjust_managed_page_count(page, -1);
471 		xenmem_reservation_scrub_page(page);
472 		list_add(&page->lru, &pages);
473 	}
474 
475 	/*
476 	 * Ensure that ballooned highmem pages don't have kmaps.
477 	 *
478 	 * Do this before changing the p2m as kmap_flush_unused()
479 	 * reads PTEs to obtain pages (and hence needs the original
480 	 * p2m entry).
481 	 */
482 	kmap_flush_unused();
483 
484 	/*
485 	 * Setup the frame, update direct mapping, invalidate P2M,
486 	 * and add to balloon.
487 	 */
488 	i = 0;
489 	list_for_each_entry_safe(page, tmp, &pages, lru) {
490 		frame_list[i++] = xen_page_to_gfn(page);
491 
492 		xenmem_reservation_va_mapping_reset(1, &page);
493 
494 		list_del(&page->lru);
495 
496 		balloon_append(page);
497 	}
498 
499 	flush_tlb_all();
500 
501 	ret = xenmem_reservation_decrease(nr_pages, frame_list);
502 	BUG_ON(ret != nr_pages);
503 
504 	balloon_stats.current_pages -= nr_pages;
505 
506 	return state;
507 }
508 
509 /*
510  * As this is a work item it is guaranteed to run as a single instance only.
511  * We may of course race updates of the target counts (which are protected
512  * by the balloon lock), or with changes to the Xen hard limit, but we will
513  * recover from these in time.
514  */
515 static void balloon_process(struct work_struct *work)
516 {
517 	enum bp_state state = BP_DONE;
518 	long credit;
519 
520 
521 	do {
522 		mutex_lock(&balloon_mutex);
523 
524 		credit = current_credit();
525 
526 		if (credit > 0) {
527 			if (balloon_is_inflated())
528 				state = increase_reservation(credit);
529 			else
530 				state = reserve_additional_memory();
531 		}
532 
533 		if (credit < 0) {
534 			long n_pages;
535 
536 			n_pages = min(-credit, si_mem_available());
537 			state = decrease_reservation(n_pages, GFP_BALLOON);
538 			if (state == BP_DONE && n_pages != -credit &&
539 			    n_pages < totalreserve_pages)
540 				state = BP_EAGAIN;
541 		}
542 
543 		state = update_schedule(state);
544 
545 		mutex_unlock(&balloon_mutex);
546 
547 		cond_resched();
548 
549 	} while (credit && state == BP_DONE);
550 
551 	/* Schedule more work if there is some still to be done. */
552 	if (state == BP_EAGAIN)
553 		schedule_delayed_work(&balloon_worker, balloon_stats.schedule_delay * HZ);
554 }
555 
556 /* Resets the Xen limit, sets new target, and kicks off processing. */
557 void balloon_set_new_target(unsigned long target)
558 {
559 	/* No need for lock. Not read-modify-write updates. */
560 	balloon_stats.target_pages = target;
561 	schedule_delayed_work(&balloon_worker, 0);
562 }
563 EXPORT_SYMBOL_GPL(balloon_set_new_target);
564 
565 static int add_ballooned_pages(int nr_pages)
566 {
567 	enum bp_state st;
568 
569 	if (xen_hotplug_unpopulated) {
570 		st = reserve_additional_memory();
571 		if (st != BP_ECANCELED) {
572 			mutex_unlock(&balloon_mutex);
573 			wait_event(balloon_wq,
574 				   !list_empty(&ballooned_pages));
575 			mutex_lock(&balloon_mutex);
576 			return 0;
577 		}
578 	}
579 
580 	if (si_mem_available() < nr_pages)
581 		return -ENOMEM;
582 
583 	st = decrease_reservation(nr_pages, GFP_USER);
584 	if (st != BP_DONE)
585 		return -ENOMEM;
586 
587 	return 0;
588 }
589 
590 /**
591  * alloc_xenballooned_pages - get pages that have been ballooned out
592  * @nr_pages: Number of pages to get
593  * @pages: pages returned
594  * @return 0 on success, error otherwise
595  */
596 int alloc_xenballooned_pages(int nr_pages, struct page **pages)
597 {
598 	int pgno = 0;
599 	struct page *page;
600 	int ret;
601 
602 	mutex_lock(&balloon_mutex);
603 
604 	balloon_stats.target_unpopulated += nr_pages;
605 
606 	while (pgno < nr_pages) {
607 		page = balloon_retrieve(true);
608 		if (page) {
609 			pages[pgno++] = page;
610 #ifdef CONFIG_XEN_HAVE_PVMMU
611 			/*
612 			 * We don't support PV MMU when Linux and Xen is using
613 			 * different page granularity.
614 			 */
615 			BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
616 
617 			if (!xen_feature(XENFEAT_auto_translated_physmap)) {
618 				ret = xen_alloc_p2m_entry(page_to_pfn(page));
619 				if (ret < 0)
620 					goto out_undo;
621 			}
622 #endif
623 		} else {
624 			ret = add_ballooned_pages(nr_pages - pgno);
625 			if (ret < 0)
626 				goto out_undo;
627 		}
628 	}
629 	mutex_unlock(&balloon_mutex);
630 	return 0;
631  out_undo:
632 	mutex_unlock(&balloon_mutex);
633 	free_xenballooned_pages(pgno, pages);
634 	return ret;
635 }
636 EXPORT_SYMBOL(alloc_xenballooned_pages);
637 
638 /**
639  * free_xenballooned_pages - return pages retrieved with get_ballooned_pages
640  * @nr_pages: Number of pages
641  * @pages: pages to return
642  */
643 void free_xenballooned_pages(int nr_pages, struct page **pages)
644 {
645 	int i;
646 
647 	mutex_lock(&balloon_mutex);
648 
649 	for (i = 0; i < nr_pages; i++) {
650 		if (pages[i])
651 			balloon_append(pages[i]);
652 	}
653 
654 	balloon_stats.target_unpopulated -= nr_pages;
655 
656 	/* The balloon may be too large now. Shrink it if needed. */
657 	if (current_credit())
658 		schedule_delayed_work(&balloon_worker, 0);
659 
660 	mutex_unlock(&balloon_mutex);
661 }
662 EXPORT_SYMBOL(free_xenballooned_pages);
663 
664 #ifdef CONFIG_XEN_PV
665 static void __init balloon_add_region(unsigned long start_pfn,
666 				      unsigned long pages)
667 {
668 	unsigned long pfn, extra_pfn_end;
669 
670 	/*
671 	 * If the amount of usable memory has been limited (e.g., with
672 	 * the 'mem' command line parameter), don't add pages beyond
673 	 * this limit.
674 	 */
675 	extra_pfn_end = min(max_pfn, start_pfn + pages);
676 
677 	for (pfn = start_pfn; pfn < extra_pfn_end; pfn++) {
678 		/* totalram_pages and totalhigh_pages do not
679 		   include the boot-time balloon extension, so
680 		   don't subtract from it. */
681 		balloon_append(pfn_to_page(pfn));
682 	}
683 
684 	balloon_stats.total_pages += extra_pfn_end - start_pfn;
685 }
686 #endif
687 
688 static int __init balloon_init(void)
689 {
690 	if (!xen_domain())
691 		return -ENODEV;
692 
693 	pr_info("Initialising balloon driver\n");
694 
695 #ifdef CONFIG_XEN_PV
696 	balloon_stats.current_pages = xen_pv_domain()
697 		? min(xen_start_info->nr_pages - xen_released_pages, max_pfn)
698 		: get_num_physpages();
699 #else
700 	balloon_stats.current_pages = get_num_physpages();
701 #endif
702 	balloon_stats.target_pages  = balloon_stats.current_pages;
703 	balloon_stats.balloon_low   = 0;
704 	balloon_stats.balloon_high  = 0;
705 	balloon_stats.total_pages   = balloon_stats.current_pages;
706 
707 	balloon_stats.schedule_delay = 1;
708 	balloon_stats.max_schedule_delay = 32;
709 	balloon_stats.retry_count = 1;
710 	balloon_stats.max_retry_count = 4;
711 
712 #ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
713 	set_online_page_callback(&xen_online_page);
714 	register_memory_notifier(&xen_memory_nb);
715 	register_sysctl_table(xen_root);
716 #endif
717 
718 #ifdef CONFIG_XEN_PV
719 	{
720 		int i;
721 
722 		/*
723 		 * Initialize the balloon with pages from the extra memory
724 		 * regions (see arch/x86/xen/setup.c).
725 		 */
726 		for (i = 0; i < XEN_EXTRA_MEM_MAX_REGIONS; i++)
727 			if (xen_extra_mem[i].n_pfns)
728 				balloon_add_region(xen_extra_mem[i].start_pfn,
729 						   xen_extra_mem[i].n_pfns);
730 	}
731 #endif
732 
733 	/* Init the xen-balloon driver. */
734 	xen_balloon_init();
735 
736 	return 0;
737 }
738 subsys_initcall(balloon_init);
739