xref: /openbmc/linux/mm/khugepaged.c (revision 8ee90c5c)
1 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
2 
3 #include <linux/mm.h>
4 #include <linux/sched.h>
5 #include <linux/sched/mm.h>
6 #include <linux/sched/coredump.h>
7 #include <linux/mmu_notifier.h>
8 #include <linux/rmap.h>
9 #include <linux/swap.h>
10 #include <linux/mm_inline.h>
11 #include <linux/kthread.h>
12 #include <linux/khugepaged.h>
13 #include <linux/freezer.h>
14 #include <linux/mman.h>
15 #include <linux/hashtable.h>
16 #include <linux/userfaultfd_k.h>
17 #include <linux/page_idle.h>
18 #include <linux/swapops.h>
19 #include <linux/shmem_fs.h>
20 
21 #include <asm/tlb.h>
22 #include <asm/pgalloc.h>
23 #include "internal.h"
24 
25 enum scan_result {
26 	SCAN_FAIL,
27 	SCAN_SUCCEED,
28 	SCAN_PMD_NULL,
29 	SCAN_EXCEED_NONE_PTE,
30 	SCAN_PTE_NON_PRESENT,
31 	SCAN_PAGE_RO,
32 	SCAN_LACK_REFERENCED_PAGE,
33 	SCAN_PAGE_NULL,
34 	SCAN_SCAN_ABORT,
35 	SCAN_PAGE_COUNT,
36 	SCAN_PAGE_LRU,
37 	SCAN_PAGE_LOCK,
38 	SCAN_PAGE_ANON,
39 	SCAN_PAGE_COMPOUND,
40 	SCAN_ANY_PROCESS,
41 	SCAN_VMA_NULL,
42 	SCAN_VMA_CHECK,
43 	SCAN_ADDRESS_RANGE,
44 	SCAN_SWAP_CACHE_PAGE,
45 	SCAN_DEL_PAGE_LRU,
46 	SCAN_ALLOC_HUGE_PAGE_FAIL,
47 	SCAN_CGROUP_CHARGE_FAIL,
48 	SCAN_EXCEED_SWAP_PTE,
49 	SCAN_TRUNCATED,
50 };
51 
52 #define CREATE_TRACE_POINTS
53 #include <trace/events/huge_memory.h>
54 
55 /* default scan 8*512 pte (or vmas) every 30 second */
56 static unsigned int khugepaged_pages_to_scan __read_mostly;
57 static unsigned int khugepaged_pages_collapsed;
58 static unsigned int khugepaged_full_scans;
59 static unsigned int khugepaged_scan_sleep_millisecs __read_mostly = 10000;
60 /* during fragmentation poll the hugepage allocator once every minute */
61 static unsigned int khugepaged_alloc_sleep_millisecs __read_mostly = 60000;
62 static unsigned long khugepaged_sleep_expire;
63 static DEFINE_SPINLOCK(khugepaged_mm_lock);
64 static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
65 /*
66  * default collapse hugepages if there is at least one pte mapped like
67  * it would have happened if the vma was large enough during page
68  * fault.
69  */
70 static unsigned int khugepaged_max_ptes_none __read_mostly;
71 static unsigned int khugepaged_max_ptes_swap __read_mostly;
72 
73 #define MM_SLOTS_HASH_BITS 10
74 static __read_mostly DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
75 
76 static struct kmem_cache *mm_slot_cache __read_mostly;
77 
78 /**
79  * struct mm_slot - hash lookup from mm to mm_slot
80  * @hash: hash collision list
81  * @mm_node: khugepaged scan list headed in khugepaged_scan.mm_head
82  * @mm: the mm that this information is valid for
83  */
84 struct mm_slot {
85 	struct hlist_node hash;
86 	struct list_head mm_node;
87 	struct mm_struct *mm;
88 };
89 
90 /**
91  * struct khugepaged_scan - cursor for scanning
92  * @mm_head: the head of the mm list to scan
93  * @mm_slot: the current mm_slot we are scanning
94  * @address: the next address inside that to be scanned
95  *
96  * There is only the one khugepaged_scan instance of this cursor structure.
97  */
98 struct khugepaged_scan {
99 	struct list_head mm_head;
100 	struct mm_slot *mm_slot;
101 	unsigned long address;
102 };
103 
104 static struct khugepaged_scan khugepaged_scan = {
105 	.mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
106 };
107 
108 #ifdef CONFIG_SYSFS
109 static ssize_t scan_sleep_millisecs_show(struct kobject *kobj,
110 					 struct kobj_attribute *attr,
111 					 char *buf)
112 {
113 	return sprintf(buf, "%u\n", khugepaged_scan_sleep_millisecs);
114 }
115 
116 static ssize_t scan_sleep_millisecs_store(struct kobject *kobj,
117 					  struct kobj_attribute *attr,
118 					  const char *buf, size_t count)
119 {
120 	unsigned long msecs;
121 	int err;
122 
123 	err = kstrtoul(buf, 10, &msecs);
124 	if (err || msecs > UINT_MAX)
125 		return -EINVAL;
126 
127 	khugepaged_scan_sleep_millisecs = msecs;
128 	khugepaged_sleep_expire = 0;
129 	wake_up_interruptible(&khugepaged_wait);
130 
131 	return count;
132 }
133 static struct kobj_attribute scan_sleep_millisecs_attr =
134 	__ATTR(scan_sleep_millisecs, 0644, scan_sleep_millisecs_show,
135 	       scan_sleep_millisecs_store);
136 
137 static ssize_t alloc_sleep_millisecs_show(struct kobject *kobj,
138 					  struct kobj_attribute *attr,
139 					  char *buf)
140 {
141 	return sprintf(buf, "%u\n", khugepaged_alloc_sleep_millisecs);
142 }
143 
144 static ssize_t alloc_sleep_millisecs_store(struct kobject *kobj,
145 					   struct kobj_attribute *attr,
146 					   const char *buf, size_t count)
147 {
148 	unsigned long msecs;
149 	int err;
150 
151 	err = kstrtoul(buf, 10, &msecs);
152 	if (err || msecs > UINT_MAX)
153 		return -EINVAL;
154 
155 	khugepaged_alloc_sleep_millisecs = msecs;
156 	khugepaged_sleep_expire = 0;
157 	wake_up_interruptible(&khugepaged_wait);
158 
159 	return count;
160 }
161 static struct kobj_attribute alloc_sleep_millisecs_attr =
162 	__ATTR(alloc_sleep_millisecs, 0644, alloc_sleep_millisecs_show,
163 	       alloc_sleep_millisecs_store);
164 
165 static ssize_t pages_to_scan_show(struct kobject *kobj,
166 				  struct kobj_attribute *attr,
167 				  char *buf)
168 {
169 	return sprintf(buf, "%u\n", khugepaged_pages_to_scan);
170 }
171 static ssize_t pages_to_scan_store(struct kobject *kobj,
172 				   struct kobj_attribute *attr,
173 				   const char *buf, size_t count)
174 {
175 	int err;
176 	unsigned long pages;
177 
178 	err = kstrtoul(buf, 10, &pages);
179 	if (err || !pages || pages > UINT_MAX)
180 		return -EINVAL;
181 
182 	khugepaged_pages_to_scan = pages;
183 
184 	return count;
185 }
186 static struct kobj_attribute pages_to_scan_attr =
187 	__ATTR(pages_to_scan, 0644, pages_to_scan_show,
188 	       pages_to_scan_store);
189 
190 static ssize_t pages_collapsed_show(struct kobject *kobj,
191 				    struct kobj_attribute *attr,
192 				    char *buf)
193 {
194 	return sprintf(buf, "%u\n", khugepaged_pages_collapsed);
195 }
196 static struct kobj_attribute pages_collapsed_attr =
197 	__ATTR_RO(pages_collapsed);
198 
199 static ssize_t full_scans_show(struct kobject *kobj,
200 			       struct kobj_attribute *attr,
201 			       char *buf)
202 {
203 	return sprintf(buf, "%u\n", khugepaged_full_scans);
204 }
205 static struct kobj_attribute full_scans_attr =
206 	__ATTR_RO(full_scans);
207 
208 static ssize_t khugepaged_defrag_show(struct kobject *kobj,
209 				      struct kobj_attribute *attr, char *buf)
210 {
211 	return single_hugepage_flag_show(kobj, attr, buf,
212 				TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
213 }
214 static ssize_t khugepaged_defrag_store(struct kobject *kobj,
215 				       struct kobj_attribute *attr,
216 				       const char *buf, size_t count)
217 {
218 	return single_hugepage_flag_store(kobj, attr, buf, count,
219 				 TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
220 }
221 static struct kobj_attribute khugepaged_defrag_attr =
222 	__ATTR(defrag, 0644, khugepaged_defrag_show,
223 	       khugepaged_defrag_store);
224 
225 /*
226  * max_ptes_none controls if khugepaged should collapse hugepages over
227  * any unmapped ptes in turn potentially increasing the memory
228  * footprint of the vmas. When max_ptes_none is 0 khugepaged will not
229  * reduce the available free memory in the system as it
230  * runs. Increasing max_ptes_none will instead potentially reduce the
231  * free memory in the system during the khugepaged scan.
232  */
233 static ssize_t khugepaged_max_ptes_none_show(struct kobject *kobj,
234 					     struct kobj_attribute *attr,
235 					     char *buf)
236 {
237 	return sprintf(buf, "%u\n", khugepaged_max_ptes_none);
238 }
239 static ssize_t khugepaged_max_ptes_none_store(struct kobject *kobj,
240 					      struct kobj_attribute *attr,
241 					      const char *buf, size_t count)
242 {
243 	int err;
244 	unsigned long max_ptes_none;
245 
246 	err = kstrtoul(buf, 10, &max_ptes_none);
247 	if (err || max_ptes_none > HPAGE_PMD_NR-1)
248 		return -EINVAL;
249 
250 	khugepaged_max_ptes_none = max_ptes_none;
251 
252 	return count;
253 }
254 static struct kobj_attribute khugepaged_max_ptes_none_attr =
255 	__ATTR(max_ptes_none, 0644, khugepaged_max_ptes_none_show,
256 	       khugepaged_max_ptes_none_store);
257 
258 static ssize_t khugepaged_max_ptes_swap_show(struct kobject *kobj,
259 					     struct kobj_attribute *attr,
260 					     char *buf)
261 {
262 	return sprintf(buf, "%u\n", khugepaged_max_ptes_swap);
263 }
264 
265 static ssize_t khugepaged_max_ptes_swap_store(struct kobject *kobj,
266 					      struct kobj_attribute *attr,
267 					      const char *buf, size_t count)
268 {
269 	int err;
270 	unsigned long max_ptes_swap;
271 
272 	err  = kstrtoul(buf, 10, &max_ptes_swap);
273 	if (err || max_ptes_swap > HPAGE_PMD_NR-1)
274 		return -EINVAL;
275 
276 	khugepaged_max_ptes_swap = max_ptes_swap;
277 
278 	return count;
279 }
280 
281 static struct kobj_attribute khugepaged_max_ptes_swap_attr =
282 	__ATTR(max_ptes_swap, 0644, khugepaged_max_ptes_swap_show,
283 	       khugepaged_max_ptes_swap_store);
284 
285 static struct attribute *khugepaged_attr[] = {
286 	&khugepaged_defrag_attr.attr,
287 	&khugepaged_max_ptes_none_attr.attr,
288 	&pages_to_scan_attr.attr,
289 	&pages_collapsed_attr.attr,
290 	&full_scans_attr.attr,
291 	&scan_sleep_millisecs_attr.attr,
292 	&alloc_sleep_millisecs_attr.attr,
293 	&khugepaged_max_ptes_swap_attr.attr,
294 	NULL,
295 };
296 
297 struct attribute_group khugepaged_attr_group = {
298 	.attrs = khugepaged_attr,
299 	.name = "khugepaged",
300 };
301 #endif /* CONFIG_SYSFS */
302 
303 #define VM_NO_KHUGEPAGED (VM_SPECIAL | VM_HUGETLB)
304 
305 int hugepage_madvise(struct vm_area_struct *vma,
306 		     unsigned long *vm_flags, int advice)
307 {
308 	switch (advice) {
309 	case MADV_HUGEPAGE:
310 #ifdef CONFIG_S390
311 		/*
312 		 * qemu blindly sets MADV_HUGEPAGE on all allocations, but s390
313 		 * can't handle this properly after s390_enable_sie, so we simply
314 		 * ignore the madvise to prevent qemu from causing a SIGSEGV.
315 		 */
316 		if (mm_has_pgste(vma->vm_mm))
317 			return 0;
318 #endif
319 		*vm_flags &= ~VM_NOHUGEPAGE;
320 		*vm_flags |= VM_HUGEPAGE;
321 		/*
322 		 * If the vma become good for khugepaged to scan,
323 		 * register it here without waiting a page fault that
324 		 * may not happen any time soon.
325 		 */
326 		if (!(*vm_flags & VM_NO_KHUGEPAGED) &&
327 				khugepaged_enter_vma_merge(vma, *vm_flags))
328 			return -ENOMEM;
329 		break;
330 	case MADV_NOHUGEPAGE:
331 		*vm_flags &= ~VM_HUGEPAGE;
332 		*vm_flags |= VM_NOHUGEPAGE;
333 		/*
334 		 * Setting VM_NOHUGEPAGE will prevent khugepaged from scanning
335 		 * this vma even if we leave the mm registered in khugepaged if
336 		 * it got registered before VM_NOHUGEPAGE was set.
337 		 */
338 		break;
339 	}
340 
341 	return 0;
342 }
343 
344 int __init khugepaged_init(void)
345 {
346 	mm_slot_cache = kmem_cache_create("khugepaged_mm_slot",
347 					  sizeof(struct mm_slot),
348 					  __alignof__(struct mm_slot), 0, NULL);
349 	if (!mm_slot_cache)
350 		return -ENOMEM;
351 
352 	khugepaged_pages_to_scan = HPAGE_PMD_NR * 8;
353 	khugepaged_max_ptes_none = HPAGE_PMD_NR - 1;
354 	khugepaged_max_ptes_swap = HPAGE_PMD_NR / 8;
355 
356 	return 0;
357 }
358 
359 void __init khugepaged_destroy(void)
360 {
361 	kmem_cache_destroy(mm_slot_cache);
362 }
363 
364 static inline struct mm_slot *alloc_mm_slot(void)
365 {
366 	if (!mm_slot_cache)	/* initialization failed */
367 		return NULL;
368 	return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
369 }
370 
371 static inline void free_mm_slot(struct mm_slot *mm_slot)
372 {
373 	kmem_cache_free(mm_slot_cache, mm_slot);
374 }
375 
376 static struct mm_slot *get_mm_slot(struct mm_struct *mm)
377 {
378 	struct mm_slot *mm_slot;
379 
380 	hash_for_each_possible(mm_slots_hash, mm_slot, hash, (unsigned long)mm)
381 		if (mm == mm_slot->mm)
382 			return mm_slot;
383 
384 	return NULL;
385 }
386 
387 static void insert_to_mm_slots_hash(struct mm_struct *mm,
388 				    struct mm_slot *mm_slot)
389 {
390 	mm_slot->mm = mm;
391 	hash_add(mm_slots_hash, &mm_slot->hash, (long)mm);
392 }
393 
394 static inline int khugepaged_test_exit(struct mm_struct *mm)
395 {
396 	return atomic_read(&mm->mm_users) == 0;
397 }
398 
399 int __khugepaged_enter(struct mm_struct *mm)
400 {
401 	struct mm_slot *mm_slot;
402 	int wakeup;
403 
404 	mm_slot = alloc_mm_slot();
405 	if (!mm_slot)
406 		return -ENOMEM;
407 
408 	/* __khugepaged_exit() must not run from under us */
409 	VM_BUG_ON_MM(khugepaged_test_exit(mm), mm);
410 	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
411 		free_mm_slot(mm_slot);
412 		return 0;
413 	}
414 
415 	spin_lock(&khugepaged_mm_lock);
416 	insert_to_mm_slots_hash(mm, mm_slot);
417 	/*
418 	 * Insert just behind the scanning cursor, to let the area settle
419 	 * down a little.
420 	 */
421 	wakeup = list_empty(&khugepaged_scan.mm_head);
422 	list_add_tail(&mm_slot->mm_node, &khugepaged_scan.mm_head);
423 	spin_unlock(&khugepaged_mm_lock);
424 
425 	mmgrab(mm);
426 	if (wakeup)
427 		wake_up_interruptible(&khugepaged_wait);
428 
429 	return 0;
430 }
431 
432 int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
433 			       unsigned long vm_flags)
434 {
435 	unsigned long hstart, hend;
436 	if (!vma->anon_vma)
437 		/*
438 		 * Not yet faulted in so we will register later in the
439 		 * page fault if needed.
440 		 */
441 		return 0;
442 	if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
443 		/* khugepaged not yet working on file or special mappings */
444 		return 0;
445 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
446 	hend = vma->vm_end & HPAGE_PMD_MASK;
447 	if (hstart < hend)
448 		return khugepaged_enter(vma, vm_flags);
449 	return 0;
450 }
451 
452 void __khugepaged_exit(struct mm_struct *mm)
453 {
454 	struct mm_slot *mm_slot;
455 	int free = 0;
456 
457 	spin_lock(&khugepaged_mm_lock);
458 	mm_slot = get_mm_slot(mm);
459 	if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
460 		hash_del(&mm_slot->hash);
461 		list_del(&mm_slot->mm_node);
462 		free = 1;
463 	}
464 	spin_unlock(&khugepaged_mm_lock);
465 
466 	if (free) {
467 		clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
468 		free_mm_slot(mm_slot);
469 		mmdrop(mm);
470 	} else if (mm_slot) {
471 		/*
472 		 * This is required to serialize against
473 		 * khugepaged_test_exit() (which is guaranteed to run
474 		 * under mmap sem read mode). Stop here (after we
475 		 * return all pagetables will be destroyed) until
476 		 * khugepaged has finished working on the pagetables
477 		 * under the mmap_sem.
478 		 */
479 		down_write(&mm->mmap_sem);
480 		up_write(&mm->mmap_sem);
481 	}
482 }
483 
484 static void release_pte_page(struct page *page)
485 {
486 	dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
487 	unlock_page(page);
488 	putback_lru_page(page);
489 }
490 
491 static void release_pte_pages(pte_t *pte, pte_t *_pte)
492 {
493 	while (--_pte >= pte) {
494 		pte_t pteval = *_pte;
495 		if (!pte_none(pteval) && !is_zero_pfn(pte_pfn(pteval)))
496 			release_pte_page(pte_page(pteval));
497 	}
498 }
499 
500 static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
501 					unsigned long address,
502 					pte_t *pte)
503 {
504 	struct page *page = NULL;
505 	pte_t *_pte;
506 	int none_or_zero = 0, result = 0, referenced = 0;
507 	bool writable = false;
508 
509 	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
510 	     _pte++, address += PAGE_SIZE) {
511 		pte_t pteval = *_pte;
512 		if (pte_none(pteval) || (pte_present(pteval) &&
513 				is_zero_pfn(pte_pfn(pteval)))) {
514 			if (!userfaultfd_armed(vma) &&
515 			    ++none_or_zero <= khugepaged_max_ptes_none) {
516 				continue;
517 			} else {
518 				result = SCAN_EXCEED_NONE_PTE;
519 				goto out;
520 			}
521 		}
522 		if (!pte_present(pteval)) {
523 			result = SCAN_PTE_NON_PRESENT;
524 			goto out;
525 		}
526 		page = vm_normal_page(vma, address, pteval);
527 		if (unlikely(!page)) {
528 			result = SCAN_PAGE_NULL;
529 			goto out;
530 		}
531 
532 		VM_BUG_ON_PAGE(PageCompound(page), page);
533 		VM_BUG_ON_PAGE(!PageAnon(page), page);
534 
535 		/*
536 		 * We can do it before isolate_lru_page because the
537 		 * page can't be freed from under us. NOTE: PG_lock
538 		 * is needed to serialize against split_huge_page
539 		 * when invoked from the VM.
540 		 */
541 		if (!trylock_page(page)) {
542 			result = SCAN_PAGE_LOCK;
543 			goto out;
544 		}
545 
546 		/*
547 		 * cannot use mapcount: can't collapse if there's a gup pin.
548 		 * The page must only be referenced by the scanned process
549 		 * and page swap cache.
550 		 */
551 		if (page_count(page) != 1 + PageSwapCache(page)) {
552 			unlock_page(page);
553 			result = SCAN_PAGE_COUNT;
554 			goto out;
555 		}
556 		if (pte_write(pteval)) {
557 			writable = true;
558 		} else {
559 			if (PageSwapCache(page) &&
560 			    !reuse_swap_page(page, NULL)) {
561 				unlock_page(page);
562 				result = SCAN_SWAP_CACHE_PAGE;
563 				goto out;
564 			}
565 			/*
566 			 * Page is not in the swap cache. It can be collapsed
567 			 * into a THP.
568 			 */
569 		}
570 
571 		/*
572 		 * Isolate the page to avoid collapsing an hugepage
573 		 * currently in use by the VM.
574 		 */
575 		if (isolate_lru_page(page)) {
576 			unlock_page(page);
577 			result = SCAN_DEL_PAGE_LRU;
578 			goto out;
579 		}
580 		inc_node_page_state(page,
581 				NR_ISOLATED_ANON + page_is_file_cache(page));
582 		VM_BUG_ON_PAGE(!PageLocked(page), page);
583 		VM_BUG_ON_PAGE(PageLRU(page), page);
584 
585 		/* There should be enough young pte to collapse the page */
586 		if (pte_young(pteval) ||
587 		    page_is_young(page) || PageReferenced(page) ||
588 		    mmu_notifier_test_young(vma->vm_mm, address))
589 			referenced++;
590 	}
591 	if (likely(writable)) {
592 		if (likely(referenced)) {
593 			result = SCAN_SUCCEED;
594 			trace_mm_collapse_huge_page_isolate(page, none_or_zero,
595 							    referenced, writable, result);
596 			return 1;
597 		}
598 	} else {
599 		result = SCAN_PAGE_RO;
600 	}
601 
602 out:
603 	release_pte_pages(pte, _pte);
604 	trace_mm_collapse_huge_page_isolate(page, none_or_zero,
605 					    referenced, writable, result);
606 	return 0;
607 }
608 
609 static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
610 				      struct vm_area_struct *vma,
611 				      unsigned long address,
612 				      spinlock_t *ptl)
613 {
614 	pte_t *_pte;
615 	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
616 				_pte++, page++, address += PAGE_SIZE) {
617 		pte_t pteval = *_pte;
618 		struct page *src_page;
619 
620 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
621 			clear_user_highpage(page, address);
622 			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
623 			if (is_zero_pfn(pte_pfn(pteval))) {
624 				/*
625 				 * ptl mostly unnecessary.
626 				 */
627 				spin_lock(ptl);
628 				/*
629 				 * paravirt calls inside pte_clear here are
630 				 * superfluous.
631 				 */
632 				pte_clear(vma->vm_mm, address, _pte);
633 				spin_unlock(ptl);
634 			}
635 		} else {
636 			src_page = pte_page(pteval);
637 			copy_user_highpage(page, src_page, address, vma);
638 			VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
639 			release_pte_page(src_page);
640 			/*
641 			 * ptl mostly unnecessary, but preempt has to
642 			 * be disabled to update the per-cpu stats
643 			 * inside page_remove_rmap().
644 			 */
645 			spin_lock(ptl);
646 			/*
647 			 * paravirt calls inside pte_clear here are
648 			 * superfluous.
649 			 */
650 			pte_clear(vma->vm_mm, address, _pte);
651 			page_remove_rmap(src_page, false);
652 			spin_unlock(ptl);
653 			free_page_and_swap_cache(src_page);
654 		}
655 	}
656 }
657 
658 static void khugepaged_alloc_sleep(void)
659 {
660 	DEFINE_WAIT(wait);
661 
662 	add_wait_queue(&khugepaged_wait, &wait);
663 	freezable_schedule_timeout_interruptible(
664 		msecs_to_jiffies(khugepaged_alloc_sleep_millisecs));
665 	remove_wait_queue(&khugepaged_wait, &wait);
666 }
667 
668 static int khugepaged_node_load[MAX_NUMNODES];
669 
670 static bool khugepaged_scan_abort(int nid)
671 {
672 	int i;
673 
674 	/*
675 	 * If node_reclaim_mode is disabled, then no extra effort is made to
676 	 * allocate memory locally.
677 	 */
678 	if (!node_reclaim_mode)
679 		return false;
680 
681 	/* If there is a count for this node already, it must be acceptable */
682 	if (khugepaged_node_load[nid])
683 		return false;
684 
685 	for (i = 0; i < MAX_NUMNODES; i++) {
686 		if (!khugepaged_node_load[i])
687 			continue;
688 		if (node_distance(nid, i) > RECLAIM_DISTANCE)
689 			return true;
690 	}
691 	return false;
692 }
693 
694 /* Defrag for khugepaged will enter direct reclaim/compaction if necessary */
695 static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)
696 {
697 	return khugepaged_defrag() ? GFP_TRANSHUGE : GFP_TRANSHUGE_LIGHT;
698 }
699 
700 #ifdef CONFIG_NUMA
701 static int khugepaged_find_target_node(void)
702 {
703 	static int last_khugepaged_target_node = NUMA_NO_NODE;
704 	int nid, target_node = 0, max_value = 0;
705 
706 	/* find first node with max normal pages hit */
707 	for (nid = 0; nid < MAX_NUMNODES; nid++)
708 		if (khugepaged_node_load[nid] > max_value) {
709 			max_value = khugepaged_node_load[nid];
710 			target_node = nid;
711 		}
712 
713 	/* do some balance if several nodes have the same hit record */
714 	if (target_node <= last_khugepaged_target_node)
715 		for (nid = last_khugepaged_target_node + 1; nid < MAX_NUMNODES;
716 				nid++)
717 			if (max_value == khugepaged_node_load[nid]) {
718 				target_node = nid;
719 				break;
720 			}
721 
722 	last_khugepaged_target_node = target_node;
723 	return target_node;
724 }
725 
726 static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
727 {
728 	if (IS_ERR(*hpage)) {
729 		if (!*wait)
730 			return false;
731 
732 		*wait = false;
733 		*hpage = NULL;
734 		khugepaged_alloc_sleep();
735 	} else if (*hpage) {
736 		put_page(*hpage);
737 		*hpage = NULL;
738 	}
739 
740 	return true;
741 }
742 
743 static struct page *
744 khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
745 {
746 	VM_BUG_ON_PAGE(*hpage, *hpage);
747 
748 	*hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
749 	if (unlikely(!*hpage)) {
750 		count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
751 		*hpage = ERR_PTR(-ENOMEM);
752 		return NULL;
753 	}
754 
755 	prep_transhuge_page(*hpage);
756 	count_vm_event(THP_COLLAPSE_ALLOC);
757 	return *hpage;
758 }
759 #else
760 static int khugepaged_find_target_node(void)
761 {
762 	return 0;
763 }
764 
765 static inline struct page *alloc_khugepaged_hugepage(void)
766 {
767 	struct page *page;
768 
769 	page = alloc_pages(alloc_hugepage_khugepaged_gfpmask(),
770 			   HPAGE_PMD_ORDER);
771 	if (page)
772 		prep_transhuge_page(page);
773 	return page;
774 }
775 
776 static struct page *khugepaged_alloc_hugepage(bool *wait)
777 {
778 	struct page *hpage;
779 
780 	do {
781 		hpage = alloc_khugepaged_hugepage();
782 		if (!hpage) {
783 			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
784 			if (!*wait)
785 				return NULL;
786 
787 			*wait = false;
788 			khugepaged_alloc_sleep();
789 		} else
790 			count_vm_event(THP_COLLAPSE_ALLOC);
791 	} while (unlikely(!hpage) && likely(khugepaged_enabled()));
792 
793 	return hpage;
794 }
795 
796 static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
797 {
798 	if (!*hpage)
799 		*hpage = khugepaged_alloc_hugepage(wait);
800 
801 	if (unlikely(!*hpage))
802 		return false;
803 
804 	return true;
805 }
806 
807 static struct page *
808 khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
809 {
810 	VM_BUG_ON(!*hpage);
811 
812 	return  *hpage;
813 }
814 #endif
815 
816 static bool hugepage_vma_check(struct vm_area_struct *vma)
817 {
818 	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
819 	    (vma->vm_flags & VM_NOHUGEPAGE) ||
820 	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
821 		return false;
822 	if (shmem_file(vma->vm_file)) {
823 		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
824 			return false;
825 		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
826 				HPAGE_PMD_NR);
827 	}
828 	if (!vma->anon_vma || vma->vm_ops)
829 		return false;
830 	if (is_vma_temporary_stack(vma))
831 		return false;
832 	return !(vma->vm_flags & VM_NO_KHUGEPAGED);
833 }
834 
835 /*
836  * If mmap_sem temporarily dropped, revalidate vma
837  * before taking mmap_sem.
838  * Return 0 if succeeds, otherwise return none-zero
839  * value (scan code).
840  */
841 
842 static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
843 		struct vm_area_struct **vmap)
844 {
845 	struct vm_area_struct *vma;
846 	unsigned long hstart, hend;
847 
848 	if (unlikely(khugepaged_test_exit(mm)))
849 		return SCAN_ANY_PROCESS;
850 
851 	*vmap = vma = find_vma(mm, address);
852 	if (!vma)
853 		return SCAN_VMA_NULL;
854 
855 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
856 	hend = vma->vm_end & HPAGE_PMD_MASK;
857 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
858 		return SCAN_ADDRESS_RANGE;
859 	if (!hugepage_vma_check(vma))
860 		return SCAN_VMA_CHECK;
861 	return 0;
862 }
863 
864 /*
865  * Bring missing pages in from swap, to complete THP collapse.
866  * Only done if khugepaged_scan_pmd believes it is worthwhile.
867  *
868  * Called and returns without pte mapped or spinlocks held,
869  * but with mmap_sem held to protect against vma changes.
870  */
871 
872 static bool __collapse_huge_page_swapin(struct mm_struct *mm,
873 					struct vm_area_struct *vma,
874 					unsigned long address, pmd_t *pmd,
875 					int referenced)
876 {
877 	int swapped_in = 0, ret = 0;
878 	struct vm_fault vmf = {
879 		.vma = vma,
880 		.address = address,
881 		.flags = FAULT_FLAG_ALLOW_RETRY,
882 		.pmd = pmd,
883 		.pgoff = linear_page_index(vma, address),
884 	};
885 
886 	/* we only decide to swapin, if there is enough young ptes */
887 	if (referenced < HPAGE_PMD_NR/2) {
888 		trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
889 		return false;
890 	}
891 	vmf.pte = pte_offset_map(pmd, address);
892 	for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
893 			vmf.pte++, vmf.address += PAGE_SIZE) {
894 		vmf.orig_pte = *vmf.pte;
895 		if (!is_swap_pte(vmf.orig_pte))
896 			continue;
897 		swapped_in++;
898 		ret = do_swap_page(&vmf);
899 
900 		/* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
901 		if (ret & VM_FAULT_RETRY) {
902 			down_read(&mm->mmap_sem);
903 			if (hugepage_vma_revalidate(mm, address, &vmf.vma)) {
904 				/* vma is no longer available, don't continue to swapin */
905 				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
906 				return false;
907 			}
908 			/* check if the pmd is still valid */
909 			if (mm_find_pmd(mm, address) != pmd) {
910 				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
911 				return false;
912 			}
913 		}
914 		if (ret & VM_FAULT_ERROR) {
915 			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
916 			return false;
917 		}
918 		/* pte is unmapped now, we need to map it */
919 		vmf.pte = pte_offset_map(pmd, vmf.address);
920 	}
921 	vmf.pte--;
922 	pte_unmap(vmf.pte);
923 	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);
924 	return true;
925 }
926 
927 static void collapse_huge_page(struct mm_struct *mm,
928 				   unsigned long address,
929 				   struct page **hpage,
930 				   int node, int referenced)
931 {
932 	pmd_t *pmd, _pmd;
933 	pte_t *pte;
934 	pgtable_t pgtable;
935 	struct page *new_page;
936 	spinlock_t *pmd_ptl, *pte_ptl;
937 	int isolated = 0, result = 0;
938 	struct mem_cgroup *memcg;
939 	struct vm_area_struct *vma;
940 	unsigned long mmun_start;	/* For mmu_notifiers */
941 	unsigned long mmun_end;		/* For mmu_notifiers */
942 	gfp_t gfp;
943 
944 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
945 
946 	/* Only allocate from the target node */
947 	gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
948 
949 	/*
950 	 * Before allocating the hugepage, release the mmap_sem read lock.
951 	 * The allocation can take potentially a long time if it involves
952 	 * sync compaction, and we do not need to hold the mmap_sem during
953 	 * that. We will recheck the vma after taking it again in write mode.
954 	 */
955 	up_read(&mm->mmap_sem);
956 	new_page = khugepaged_alloc_page(hpage, gfp, node);
957 	if (!new_page) {
958 		result = SCAN_ALLOC_HUGE_PAGE_FAIL;
959 		goto out_nolock;
960 	}
961 
962 	if (unlikely(mem_cgroup_try_charge(new_page, mm, gfp, &memcg, true))) {
963 		result = SCAN_CGROUP_CHARGE_FAIL;
964 		goto out_nolock;
965 	}
966 
967 	down_read(&mm->mmap_sem);
968 	result = hugepage_vma_revalidate(mm, address, &vma);
969 	if (result) {
970 		mem_cgroup_cancel_charge(new_page, memcg, true);
971 		up_read(&mm->mmap_sem);
972 		goto out_nolock;
973 	}
974 
975 	pmd = mm_find_pmd(mm, address);
976 	if (!pmd) {
977 		result = SCAN_PMD_NULL;
978 		mem_cgroup_cancel_charge(new_page, memcg, true);
979 		up_read(&mm->mmap_sem);
980 		goto out_nolock;
981 	}
982 
983 	/*
984 	 * __collapse_huge_page_swapin always returns with mmap_sem locked.
985 	 * If it fails, we release mmap_sem and jump out_nolock.
986 	 * Continuing to collapse causes inconsistency.
987 	 */
988 	if (!__collapse_huge_page_swapin(mm, vma, address, pmd, referenced)) {
989 		mem_cgroup_cancel_charge(new_page, memcg, true);
990 		up_read(&mm->mmap_sem);
991 		goto out_nolock;
992 	}
993 
994 	up_read(&mm->mmap_sem);
995 	/*
996 	 * Prevent all access to pagetables with the exception of
997 	 * gup_fast later handled by the ptep_clear_flush and the VM
998 	 * handled by the anon_vma lock + PG_lock.
999 	 */
1000 	down_write(&mm->mmap_sem);
1001 	result = hugepage_vma_revalidate(mm, address, &vma);
1002 	if (result)
1003 		goto out;
1004 	/* check if the pmd is still valid */
1005 	if (mm_find_pmd(mm, address) != pmd)
1006 		goto out;
1007 
1008 	anon_vma_lock_write(vma->anon_vma);
1009 
1010 	pte = pte_offset_map(pmd, address);
1011 	pte_ptl = pte_lockptr(mm, pmd);
1012 
1013 	mmun_start = address;
1014 	mmun_end   = address + HPAGE_PMD_SIZE;
1015 	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
1016 	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
1017 	/*
1018 	 * After this gup_fast can't run anymore. This also removes
1019 	 * any huge TLB entry from the CPU so we won't allow
1020 	 * huge and small TLB entries for the same virtual address
1021 	 * to avoid the risk of CPU bugs in that area.
1022 	 */
1023 	_pmd = pmdp_collapse_flush(vma, address, pmd);
1024 	spin_unlock(pmd_ptl);
1025 	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
1026 
1027 	spin_lock(pte_ptl);
1028 	isolated = __collapse_huge_page_isolate(vma, address, pte);
1029 	spin_unlock(pte_ptl);
1030 
1031 	if (unlikely(!isolated)) {
1032 		pte_unmap(pte);
1033 		spin_lock(pmd_ptl);
1034 		BUG_ON(!pmd_none(*pmd));
1035 		/*
1036 		 * We can only use set_pmd_at when establishing
1037 		 * hugepmds and never for establishing regular pmds that
1038 		 * points to regular pagetables. Use pmd_populate for that
1039 		 */
1040 		pmd_populate(mm, pmd, pmd_pgtable(_pmd));
1041 		spin_unlock(pmd_ptl);
1042 		anon_vma_unlock_write(vma->anon_vma);
1043 		result = SCAN_FAIL;
1044 		goto out;
1045 	}
1046 
1047 	/*
1048 	 * All pages are isolated and locked so anon_vma rmap
1049 	 * can't run anymore.
1050 	 */
1051 	anon_vma_unlock_write(vma->anon_vma);
1052 
1053 	__collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl);
1054 	pte_unmap(pte);
1055 	__SetPageUptodate(new_page);
1056 	pgtable = pmd_pgtable(_pmd);
1057 
1058 	_pmd = mk_huge_pmd(new_page, vma->vm_page_prot);
1059 	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
1060 
1061 	/*
1062 	 * spin_lock() below is not the equivalent of smp_wmb(), so
1063 	 * this is needed to avoid the copy_huge_page writes to become
1064 	 * visible after the set_pmd_at() write.
1065 	 */
1066 	smp_wmb();
1067 
1068 	spin_lock(pmd_ptl);
1069 	BUG_ON(!pmd_none(*pmd));
1070 	page_add_new_anon_rmap(new_page, vma, address, true);
1071 	mem_cgroup_commit_charge(new_page, memcg, false, true);
1072 	lru_cache_add_active_or_unevictable(new_page, vma);
1073 	pgtable_trans_huge_deposit(mm, pmd, pgtable);
1074 	set_pmd_at(mm, address, pmd, _pmd);
1075 	update_mmu_cache_pmd(vma, address, pmd);
1076 	spin_unlock(pmd_ptl);
1077 
1078 	*hpage = NULL;
1079 
1080 	khugepaged_pages_collapsed++;
1081 	result = SCAN_SUCCEED;
1082 out_up_write:
1083 	up_write(&mm->mmap_sem);
1084 out_nolock:
1085 	trace_mm_collapse_huge_page(mm, isolated, result);
1086 	return;
1087 out:
1088 	mem_cgroup_cancel_charge(new_page, memcg, true);
1089 	goto out_up_write;
1090 }
1091 
1092 static int khugepaged_scan_pmd(struct mm_struct *mm,
1093 			       struct vm_area_struct *vma,
1094 			       unsigned long address,
1095 			       struct page **hpage)
1096 {
1097 	pmd_t *pmd;
1098 	pte_t *pte, *_pte;
1099 	int ret = 0, none_or_zero = 0, result = 0, referenced = 0;
1100 	struct page *page = NULL;
1101 	unsigned long _address;
1102 	spinlock_t *ptl;
1103 	int node = NUMA_NO_NODE, unmapped = 0;
1104 	bool writable = false;
1105 
1106 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
1107 
1108 	pmd = mm_find_pmd(mm, address);
1109 	if (!pmd) {
1110 		result = SCAN_PMD_NULL;
1111 		goto out;
1112 	}
1113 
1114 	memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
1115 	pte = pte_offset_map_lock(mm, pmd, address, &ptl);
1116 	for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
1117 	     _pte++, _address += PAGE_SIZE) {
1118 		pte_t pteval = *_pte;
1119 		if (is_swap_pte(pteval)) {
1120 			if (++unmapped <= khugepaged_max_ptes_swap) {
1121 				continue;
1122 			} else {
1123 				result = SCAN_EXCEED_SWAP_PTE;
1124 				goto out_unmap;
1125 			}
1126 		}
1127 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
1128 			if (!userfaultfd_armed(vma) &&
1129 			    ++none_or_zero <= khugepaged_max_ptes_none) {
1130 				continue;
1131 			} else {
1132 				result = SCAN_EXCEED_NONE_PTE;
1133 				goto out_unmap;
1134 			}
1135 		}
1136 		if (!pte_present(pteval)) {
1137 			result = SCAN_PTE_NON_PRESENT;
1138 			goto out_unmap;
1139 		}
1140 		if (pte_write(pteval))
1141 			writable = true;
1142 
1143 		page = vm_normal_page(vma, _address, pteval);
1144 		if (unlikely(!page)) {
1145 			result = SCAN_PAGE_NULL;
1146 			goto out_unmap;
1147 		}
1148 
1149 		/* TODO: teach khugepaged to collapse THP mapped with pte */
1150 		if (PageCompound(page)) {
1151 			result = SCAN_PAGE_COMPOUND;
1152 			goto out_unmap;
1153 		}
1154 
1155 		/*
1156 		 * Record which node the original page is from and save this
1157 		 * information to khugepaged_node_load[].
1158 		 * Khupaged will allocate hugepage from the node has the max
1159 		 * hit record.
1160 		 */
1161 		node = page_to_nid(page);
1162 		if (khugepaged_scan_abort(node)) {
1163 			result = SCAN_SCAN_ABORT;
1164 			goto out_unmap;
1165 		}
1166 		khugepaged_node_load[node]++;
1167 		if (!PageLRU(page)) {
1168 			result = SCAN_PAGE_LRU;
1169 			goto out_unmap;
1170 		}
1171 		if (PageLocked(page)) {
1172 			result = SCAN_PAGE_LOCK;
1173 			goto out_unmap;
1174 		}
1175 		if (!PageAnon(page)) {
1176 			result = SCAN_PAGE_ANON;
1177 			goto out_unmap;
1178 		}
1179 
1180 		/*
1181 		 * cannot use mapcount: can't collapse if there's a gup pin.
1182 		 * The page must only be referenced by the scanned process
1183 		 * and page swap cache.
1184 		 */
1185 		if (page_count(page) != 1 + PageSwapCache(page)) {
1186 			result = SCAN_PAGE_COUNT;
1187 			goto out_unmap;
1188 		}
1189 		if (pte_young(pteval) ||
1190 		    page_is_young(page) || PageReferenced(page) ||
1191 		    mmu_notifier_test_young(vma->vm_mm, address))
1192 			referenced++;
1193 	}
1194 	if (writable) {
1195 		if (referenced) {
1196 			result = SCAN_SUCCEED;
1197 			ret = 1;
1198 		} else {
1199 			result = SCAN_LACK_REFERENCED_PAGE;
1200 		}
1201 	} else {
1202 		result = SCAN_PAGE_RO;
1203 	}
1204 out_unmap:
1205 	pte_unmap_unlock(pte, ptl);
1206 	if (ret) {
1207 		node = khugepaged_find_target_node();
1208 		/* collapse_huge_page will return with the mmap_sem released */
1209 		collapse_huge_page(mm, address, hpage, node, referenced);
1210 	}
1211 out:
1212 	trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
1213 				     none_or_zero, result, unmapped);
1214 	return ret;
1215 }
1216 
1217 static void collect_mm_slot(struct mm_slot *mm_slot)
1218 {
1219 	struct mm_struct *mm = mm_slot->mm;
1220 
1221 	VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));
1222 
1223 	if (khugepaged_test_exit(mm)) {
1224 		/* free mm_slot */
1225 		hash_del(&mm_slot->hash);
1226 		list_del(&mm_slot->mm_node);
1227 
1228 		/*
1229 		 * Not strictly needed because the mm exited already.
1230 		 *
1231 		 * clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
1232 		 */
1233 
1234 		/* khugepaged_mm_lock actually not necessary for the below */
1235 		free_mm_slot(mm_slot);
1236 		mmdrop(mm);
1237 	}
1238 }
1239 
1240 #if defined(CONFIG_SHMEM) && defined(CONFIG_TRANSPARENT_HUGE_PAGECACHE)
1241 static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
1242 {
1243 	struct vm_area_struct *vma;
1244 	unsigned long addr;
1245 	pmd_t *pmd, _pmd;
1246 
1247 	i_mmap_lock_write(mapping);
1248 	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
1249 		/* probably overkill */
1250 		if (vma->anon_vma)
1251 			continue;
1252 		addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
1253 		if (addr & ~HPAGE_PMD_MASK)
1254 			continue;
1255 		if (vma->vm_end < addr + HPAGE_PMD_SIZE)
1256 			continue;
1257 		pmd = mm_find_pmd(vma->vm_mm, addr);
1258 		if (!pmd)
1259 			continue;
1260 		/*
1261 		 * We need exclusive mmap_sem to retract page table.
1262 		 * If trylock fails we would end up with pte-mapped THP after
1263 		 * re-fault. Not ideal, but it's more important to not disturb
1264 		 * the system too much.
1265 		 */
1266 		if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
1267 			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
1268 			/* assume page table is clear */
1269 			_pmd = pmdp_collapse_flush(vma, addr, pmd);
1270 			spin_unlock(ptl);
1271 			up_write(&vma->vm_mm->mmap_sem);
1272 			atomic_long_dec(&vma->vm_mm->nr_ptes);
1273 			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
1274 		}
1275 	}
1276 	i_mmap_unlock_write(mapping);
1277 }
1278 
1279 /**
1280  * collapse_shmem - collapse small tmpfs/shmem pages into huge one.
1281  *
1282  * Basic scheme is simple, details are more complex:
1283  *  - allocate and freeze a new huge page;
1284  *  - scan over radix tree replacing old pages the new one
1285  *    + swap in pages if necessary;
1286  *    + fill in gaps;
1287  *    + keep old pages around in case if rollback is required;
1288  *  - if replacing succeed:
1289  *    + copy data over;
1290  *    + free old pages;
1291  *    + unfreeze huge page;
1292  *  - if replacing failed;
1293  *    + put all pages back and unfreeze them;
1294  *    + restore gaps in the radix-tree;
1295  *    + free huge page;
1296  */
1297 static void collapse_shmem(struct mm_struct *mm,
1298 		struct address_space *mapping, pgoff_t start,
1299 		struct page **hpage, int node)
1300 {
1301 	gfp_t gfp;
1302 	struct page *page, *new_page, *tmp;
1303 	struct mem_cgroup *memcg;
1304 	pgoff_t index, end = start + HPAGE_PMD_NR;
1305 	LIST_HEAD(pagelist);
1306 	struct radix_tree_iter iter;
1307 	void **slot;
1308 	int nr_none = 0, result = SCAN_SUCCEED;
1309 
1310 	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
1311 
1312 	/* Only allocate from the target node */
1313 	gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
1314 
1315 	new_page = khugepaged_alloc_page(hpage, gfp, node);
1316 	if (!new_page) {
1317 		result = SCAN_ALLOC_HUGE_PAGE_FAIL;
1318 		goto out;
1319 	}
1320 
1321 	if (unlikely(mem_cgroup_try_charge(new_page, mm, gfp, &memcg, true))) {
1322 		result = SCAN_CGROUP_CHARGE_FAIL;
1323 		goto out;
1324 	}
1325 
1326 	new_page->index = start;
1327 	new_page->mapping = mapping;
1328 	__SetPageSwapBacked(new_page);
1329 	__SetPageLocked(new_page);
1330 	BUG_ON(!page_ref_freeze(new_page, 1));
1331 
1332 
1333 	/*
1334 	 * At this point the new_page is 'frozen' (page_count() is zero), locked
1335 	 * and not up-to-date. It's safe to insert it into radix tree, because
1336 	 * nobody would be able to map it or use it in other way until we
1337 	 * unfreeze it.
1338 	 */
1339 
1340 	index = start;
1341 	spin_lock_irq(&mapping->tree_lock);
1342 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
1343 		int n = min(iter.index, end) - index;
1344 
1345 		/*
1346 		 * Handle holes in the radix tree: charge it from shmem and
1347 		 * insert relevant subpage of new_page into the radix-tree.
1348 		 */
1349 		if (n && !shmem_charge(mapping->host, n)) {
1350 			result = SCAN_FAIL;
1351 			break;
1352 		}
1353 		nr_none += n;
1354 		for (; index < min(iter.index, end); index++) {
1355 			radix_tree_insert(&mapping->page_tree, index,
1356 					new_page + (index % HPAGE_PMD_NR));
1357 		}
1358 
1359 		/* We are done. */
1360 		if (index >= end)
1361 			break;
1362 
1363 		page = radix_tree_deref_slot_protected(slot,
1364 				&mapping->tree_lock);
1365 		if (radix_tree_exceptional_entry(page) || !PageUptodate(page)) {
1366 			spin_unlock_irq(&mapping->tree_lock);
1367 			/* swap in or instantiate fallocated page */
1368 			if (shmem_getpage(mapping->host, index, &page,
1369 						SGP_NOHUGE)) {
1370 				result = SCAN_FAIL;
1371 				goto tree_unlocked;
1372 			}
1373 			spin_lock_irq(&mapping->tree_lock);
1374 		} else if (trylock_page(page)) {
1375 			get_page(page);
1376 		} else {
1377 			result = SCAN_PAGE_LOCK;
1378 			break;
1379 		}
1380 
1381 		/*
1382 		 * The page must be locked, so we can drop the tree_lock
1383 		 * without racing with truncate.
1384 		 */
1385 		VM_BUG_ON_PAGE(!PageLocked(page), page);
1386 		VM_BUG_ON_PAGE(!PageUptodate(page), page);
1387 		VM_BUG_ON_PAGE(PageTransCompound(page), page);
1388 
1389 		if (page_mapping(page) != mapping) {
1390 			result = SCAN_TRUNCATED;
1391 			goto out_unlock;
1392 		}
1393 		spin_unlock_irq(&mapping->tree_lock);
1394 
1395 		if (isolate_lru_page(page)) {
1396 			result = SCAN_DEL_PAGE_LRU;
1397 			goto out_isolate_failed;
1398 		}
1399 
1400 		if (page_mapped(page))
1401 			unmap_mapping_range(mapping, index << PAGE_SHIFT,
1402 					PAGE_SIZE, 0);
1403 
1404 		spin_lock_irq(&mapping->tree_lock);
1405 
1406 		slot = radix_tree_lookup_slot(&mapping->page_tree, index);
1407 		VM_BUG_ON_PAGE(page != radix_tree_deref_slot_protected(slot,
1408 					&mapping->tree_lock), page);
1409 		VM_BUG_ON_PAGE(page_mapped(page), page);
1410 
1411 		/*
1412 		 * The page is expected to have page_count() == 3:
1413 		 *  - we hold a pin on it;
1414 		 *  - one reference from radix tree;
1415 		 *  - one from isolate_lru_page;
1416 		 */
1417 		if (!page_ref_freeze(page, 3)) {
1418 			result = SCAN_PAGE_COUNT;
1419 			goto out_lru;
1420 		}
1421 
1422 		/*
1423 		 * Add the page to the list to be able to undo the collapse if
1424 		 * something go wrong.
1425 		 */
1426 		list_add_tail(&page->lru, &pagelist);
1427 
1428 		/* Finally, replace with the new page. */
1429 		radix_tree_replace_slot(&mapping->page_tree, slot,
1430 				new_page + (index % HPAGE_PMD_NR));
1431 
1432 		slot = radix_tree_iter_resume(slot, &iter);
1433 		index++;
1434 		continue;
1435 out_lru:
1436 		spin_unlock_irq(&mapping->tree_lock);
1437 		putback_lru_page(page);
1438 out_isolate_failed:
1439 		unlock_page(page);
1440 		put_page(page);
1441 		goto tree_unlocked;
1442 out_unlock:
1443 		unlock_page(page);
1444 		put_page(page);
1445 		break;
1446 	}
1447 
1448 	/*
1449 	 * Handle hole in radix tree at the end of the range.
1450 	 * This code only triggers if there's nothing in radix tree
1451 	 * beyond 'end'.
1452 	 */
1453 	if (result == SCAN_SUCCEED && index < end) {
1454 		int n = end - index;
1455 
1456 		if (!shmem_charge(mapping->host, n)) {
1457 			result = SCAN_FAIL;
1458 			goto tree_locked;
1459 		}
1460 
1461 		for (; index < end; index++) {
1462 			radix_tree_insert(&mapping->page_tree, index,
1463 					new_page + (index % HPAGE_PMD_NR));
1464 		}
1465 		nr_none += n;
1466 	}
1467 
1468 tree_locked:
1469 	spin_unlock_irq(&mapping->tree_lock);
1470 tree_unlocked:
1471 
1472 	if (result == SCAN_SUCCEED) {
1473 		unsigned long flags;
1474 		struct zone *zone = page_zone(new_page);
1475 
1476 		/*
1477 		 * Replacing old pages with new one has succeed, now we need to
1478 		 * copy the content and free old pages.
1479 		 */
1480 		list_for_each_entry_safe(page, tmp, &pagelist, lru) {
1481 			copy_highpage(new_page + (page->index % HPAGE_PMD_NR),
1482 					page);
1483 			list_del(&page->lru);
1484 			unlock_page(page);
1485 			page_ref_unfreeze(page, 1);
1486 			page->mapping = NULL;
1487 			ClearPageActive(page);
1488 			ClearPageUnevictable(page);
1489 			put_page(page);
1490 		}
1491 
1492 		local_irq_save(flags);
1493 		__inc_node_page_state(new_page, NR_SHMEM_THPS);
1494 		if (nr_none) {
1495 			__mod_node_page_state(zone->zone_pgdat, NR_FILE_PAGES, nr_none);
1496 			__mod_node_page_state(zone->zone_pgdat, NR_SHMEM, nr_none);
1497 		}
1498 		local_irq_restore(flags);
1499 
1500 		/*
1501 		 * Remove pte page tables, so we can re-faulti
1502 		 * the page as huge.
1503 		 */
1504 		retract_page_tables(mapping, start);
1505 
1506 		/* Everything is ready, let's unfreeze the new_page */
1507 		set_page_dirty(new_page);
1508 		SetPageUptodate(new_page);
1509 		page_ref_unfreeze(new_page, HPAGE_PMD_NR);
1510 		mem_cgroup_commit_charge(new_page, memcg, false, true);
1511 		lru_cache_add_anon(new_page);
1512 		unlock_page(new_page);
1513 
1514 		*hpage = NULL;
1515 	} else {
1516 		/* Something went wrong: rollback changes to the radix-tree */
1517 		shmem_uncharge(mapping->host, nr_none);
1518 		spin_lock_irq(&mapping->tree_lock);
1519 		radix_tree_for_each_slot(slot, &mapping->page_tree, &iter,
1520 				start) {
1521 			if (iter.index >= end)
1522 				break;
1523 			page = list_first_entry_or_null(&pagelist,
1524 					struct page, lru);
1525 			if (!page || iter.index < page->index) {
1526 				if (!nr_none)
1527 					break;
1528 				nr_none--;
1529 				/* Put holes back where they were */
1530 				radix_tree_delete(&mapping->page_tree,
1531 						  iter.index);
1532 				continue;
1533 			}
1534 
1535 			VM_BUG_ON_PAGE(page->index != iter.index, page);
1536 
1537 			/* Unfreeze the page. */
1538 			list_del(&page->lru);
1539 			page_ref_unfreeze(page, 2);
1540 			radix_tree_replace_slot(&mapping->page_tree,
1541 						slot, page);
1542 			slot = radix_tree_iter_resume(slot, &iter);
1543 			spin_unlock_irq(&mapping->tree_lock);
1544 			putback_lru_page(page);
1545 			unlock_page(page);
1546 			spin_lock_irq(&mapping->tree_lock);
1547 		}
1548 		VM_BUG_ON(nr_none);
1549 		spin_unlock_irq(&mapping->tree_lock);
1550 
1551 		/* Unfreeze new_page, caller would take care about freeing it */
1552 		page_ref_unfreeze(new_page, 1);
1553 		mem_cgroup_cancel_charge(new_page, memcg, true);
1554 		unlock_page(new_page);
1555 		new_page->mapping = NULL;
1556 	}
1557 out:
1558 	VM_BUG_ON(!list_empty(&pagelist));
1559 	/* TODO: tracepoints */
1560 }
1561 
1562 static void khugepaged_scan_shmem(struct mm_struct *mm,
1563 		struct address_space *mapping,
1564 		pgoff_t start, struct page **hpage)
1565 {
1566 	struct page *page = NULL;
1567 	struct radix_tree_iter iter;
1568 	void **slot;
1569 	int present, swap;
1570 	int node = NUMA_NO_NODE;
1571 	int result = SCAN_SUCCEED;
1572 
1573 	present = 0;
1574 	swap = 0;
1575 	memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
1576 	rcu_read_lock();
1577 	radix_tree_for_each_slot(slot, &mapping->page_tree, &iter, start) {
1578 		if (iter.index >= start + HPAGE_PMD_NR)
1579 			break;
1580 
1581 		page = radix_tree_deref_slot(slot);
1582 		if (radix_tree_deref_retry(page)) {
1583 			slot = radix_tree_iter_retry(&iter);
1584 			continue;
1585 		}
1586 
1587 		if (radix_tree_exception(page)) {
1588 			if (++swap > khugepaged_max_ptes_swap) {
1589 				result = SCAN_EXCEED_SWAP_PTE;
1590 				break;
1591 			}
1592 			continue;
1593 		}
1594 
1595 		if (PageTransCompound(page)) {
1596 			result = SCAN_PAGE_COMPOUND;
1597 			break;
1598 		}
1599 
1600 		node = page_to_nid(page);
1601 		if (khugepaged_scan_abort(node)) {
1602 			result = SCAN_SCAN_ABORT;
1603 			break;
1604 		}
1605 		khugepaged_node_load[node]++;
1606 
1607 		if (!PageLRU(page)) {
1608 			result = SCAN_PAGE_LRU;
1609 			break;
1610 		}
1611 
1612 		if (page_count(page) != 1 + page_mapcount(page)) {
1613 			result = SCAN_PAGE_COUNT;
1614 			break;
1615 		}
1616 
1617 		/*
1618 		 * We probably should check if the page is referenced here, but
1619 		 * nobody would transfer pte_young() to PageReferenced() for us.
1620 		 * And rmap walk here is just too costly...
1621 		 */
1622 
1623 		present++;
1624 
1625 		if (need_resched()) {
1626 			slot = radix_tree_iter_resume(slot, &iter);
1627 			cond_resched_rcu();
1628 		}
1629 	}
1630 	rcu_read_unlock();
1631 
1632 	if (result == SCAN_SUCCEED) {
1633 		if (present < HPAGE_PMD_NR - khugepaged_max_ptes_none) {
1634 			result = SCAN_EXCEED_NONE_PTE;
1635 		} else {
1636 			node = khugepaged_find_target_node();
1637 			collapse_shmem(mm, mapping, start, hpage, node);
1638 		}
1639 	}
1640 
1641 	/* TODO: tracepoints */
1642 }
1643 #else
1644 static void khugepaged_scan_shmem(struct mm_struct *mm,
1645 		struct address_space *mapping,
1646 		pgoff_t start, struct page **hpage)
1647 {
1648 	BUILD_BUG();
1649 }
1650 #endif
1651 
1652 static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
1653 					    struct page **hpage)
1654 	__releases(&khugepaged_mm_lock)
1655 	__acquires(&khugepaged_mm_lock)
1656 {
1657 	struct mm_slot *mm_slot;
1658 	struct mm_struct *mm;
1659 	struct vm_area_struct *vma;
1660 	int progress = 0;
1661 
1662 	VM_BUG_ON(!pages);
1663 	VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));
1664 
1665 	if (khugepaged_scan.mm_slot)
1666 		mm_slot = khugepaged_scan.mm_slot;
1667 	else {
1668 		mm_slot = list_entry(khugepaged_scan.mm_head.next,
1669 				     struct mm_slot, mm_node);
1670 		khugepaged_scan.address = 0;
1671 		khugepaged_scan.mm_slot = mm_slot;
1672 	}
1673 	spin_unlock(&khugepaged_mm_lock);
1674 
1675 	mm = mm_slot->mm;
1676 	down_read(&mm->mmap_sem);
1677 	if (unlikely(khugepaged_test_exit(mm)))
1678 		vma = NULL;
1679 	else
1680 		vma = find_vma(mm, khugepaged_scan.address);
1681 
1682 	progress++;
1683 	for (; vma; vma = vma->vm_next) {
1684 		unsigned long hstart, hend;
1685 
1686 		cond_resched();
1687 		if (unlikely(khugepaged_test_exit(mm))) {
1688 			progress++;
1689 			break;
1690 		}
1691 		if (!hugepage_vma_check(vma)) {
1692 skip:
1693 			progress++;
1694 			continue;
1695 		}
1696 		hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
1697 		hend = vma->vm_end & HPAGE_PMD_MASK;
1698 		if (hstart >= hend)
1699 			goto skip;
1700 		if (khugepaged_scan.address > hend)
1701 			goto skip;
1702 		if (khugepaged_scan.address < hstart)
1703 			khugepaged_scan.address = hstart;
1704 		VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);
1705 
1706 		while (khugepaged_scan.address < hend) {
1707 			int ret;
1708 			cond_resched();
1709 			if (unlikely(khugepaged_test_exit(mm)))
1710 				goto breakouterloop;
1711 
1712 			VM_BUG_ON(khugepaged_scan.address < hstart ||
1713 				  khugepaged_scan.address + HPAGE_PMD_SIZE >
1714 				  hend);
1715 			if (shmem_file(vma->vm_file)) {
1716 				struct file *file;
1717 				pgoff_t pgoff = linear_page_index(vma,
1718 						khugepaged_scan.address);
1719 				if (!shmem_huge_enabled(vma))
1720 					goto skip;
1721 				file = get_file(vma->vm_file);
1722 				up_read(&mm->mmap_sem);
1723 				ret = 1;
1724 				khugepaged_scan_shmem(mm, file->f_mapping,
1725 						pgoff, hpage);
1726 				fput(file);
1727 			} else {
1728 				ret = khugepaged_scan_pmd(mm, vma,
1729 						khugepaged_scan.address,
1730 						hpage);
1731 			}
1732 			/* move to next address */
1733 			khugepaged_scan.address += HPAGE_PMD_SIZE;
1734 			progress += HPAGE_PMD_NR;
1735 			if (ret)
1736 				/* we released mmap_sem so break loop */
1737 				goto breakouterloop_mmap_sem;
1738 			if (progress >= pages)
1739 				goto breakouterloop;
1740 		}
1741 	}
1742 breakouterloop:
1743 	up_read(&mm->mmap_sem); /* exit_mmap will destroy ptes after this */
1744 breakouterloop_mmap_sem:
1745 
1746 	spin_lock(&khugepaged_mm_lock);
1747 	VM_BUG_ON(khugepaged_scan.mm_slot != mm_slot);
1748 	/*
1749 	 * Release the current mm_slot if this mm is about to die, or
1750 	 * if we scanned all vmas of this mm.
1751 	 */
1752 	if (khugepaged_test_exit(mm) || !vma) {
1753 		/*
1754 		 * Make sure that if mm_users is reaching zero while
1755 		 * khugepaged runs here, khugepaged_exit will find
1756 		 * mm_slot not pointing to the exiting mm.
1757 		 */
1758 		if (mm_slot->mm_node.next != &khugepaged_scan.mm_head) {
1759 			khugepaged_scan.mm_slot = list_entry(
1760 				mm_slot->mm_node.next,
1761 				struct mm_slot, mm_node);
1762 			khugepaged_scan.address = 0;
1763 		} else {
1764 			khugepaged_scan.mm_slot = NULL;
1765 			khugepaged_full_scans++;
1766 		}
1767 
1768 		collect_mm_slot(mm_slot);
1769 	}
1770 
1771 	return progress;
1772 }
1773 
1774 static int khugepaged_has_work(void)
1775 {
1776 	return !list_empty(&khugepaged_scan.mm_head) &&
1777 		khugepaged_enabled();
1778 }
1779 
1780 static int khugepaged_wait_event(void)
1781 {
1782 	return !list_empty(&khugepaged_scan.mm_head) ||
1783 		kthread_should_stop();
1784 }
1785 
1786 static void khugepaged_do_scan(void)
1787 {
1788 	struct page *hpage = NULL;
1789 	unsigned int progress = 0, pass_through_head = 0;
1790 	unsigned int pages = khugepaged_pages_to_scan;
1791 	bool wait = true;
1792 
1793 	barrier(); /* write khugepaged_pages_to_scan to local stack */
1794 
1795 	while (progress < pages) {
1796 		if (!khugepaged_prealloc_page(&hpage, &wait))
1797 			break;
1798 
1799 		cond_resched();
1800 
1801 		if (unlikely(kthread_should_stop() || try_to_freeze()))
1802 			break;
1803 
1804 		spin_lock(&khugepaged_mm_lock);
1805 		if (!khugepaged_scan.mm_slot)
1806 			pass_through_head++;
1807 		if (khugepaged_has_work() &&
1808 		    pass_through_head < 2)
1809 			progress += khugepaged_scan_mm_slot(pages - progress,
1810 							    &hpage);
1811 		else
1812 			progress = pages;
1813 		spin_unlock(&khugepaged_mm_lock);
1814 	}
1815 
1816 	if (!IS_ERR_OR_NULL(hpage))
1817 		put_page(hpage);
1818 }
1819 
1820 static bool khugepaged_should_wakeup(void)
1821 {
1822 	return kthread_should_stop() ||
1823 	       time_after_eq(jiffies, khugepaged_sleep_expire);
1824 }
1825 
1826 static void khugepaged_wait_work(void)
1827 {
1828 	if (khugepaged_has_work()) {
1829 		const unsigned long scan_sleep_jiffies =
1830 			msecs_to_jiffies(khugepaged_scan_sleep_millisecs);
1831 
1832 		if (!scan_sleep_jiffies)
1833 			return;
1834 
1835 		khugepaged_sleep_expire = jiffies + scan_sleep_jiffies;
1836 		wait_event_freezable_timeout(khugepaged_wait,
1837 					     khugepaged_should_wakeup(),
1838 					     scan_sleep_jiffies);
1839 		return;
1840 	}
1841 
1842 	if (khugepaged_enabled())
1843 		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
1844 }
1845 
1846 static int khugepaged(void *none)
1847 {
1848 	struct mm_slot *mm_slot;
1849 
1850 	set_freezable();
1851 	set_user_nice(current, MAX_NICE);
1852 
1853 	while (!kthread_should_stop()) {
1854 		khugepaged_do_scan();
1855 		khugepaged_wait_work();
1856 	}
1857 
1858 	spin_lock(&khugepaged_mm_lock);
1859 	mm_slot = khugepaged_scan.mm_slot;
1860 	khugepaged_scan.mm_slot = NULL;
1861 	if (mm_slot)
1862 		collect_mm_slot(mm_slot);
1863 	spin_unlock(&khugepaged_mm_lock);
1864 	return 0;
1865 }
1866 
1867 static void set_recommended_min_free_kbytes(void)
1868 {
1869 	struct zone *zone;
1870 	int nr_zones = 0;
1871 	unsigned long recommended_min;
1872 
1873 	for_each_populated_zone(zone)
1874 		nr_zones++;
1875 
1876 	/* Ensure 2 pageblocks are free to assist fragmentation avoidance */
1877 	recommended_min = pageblock_nr_pages * nr_zones * 2;
1878 
1879 	/*
1880 	 * Make sure that on average at least two pageblocks are almost free
1881 	 * of another type, one for a migratetype to fall back to and a
1882 	 * second to avoid subsequent fallbacks of other types There are 3
1883 	 * MIGRATE_TYPES we care about.
1884 	 */
1885 	recommended_min += pageblock_nr_pages * nr_zones *
1886 			   MIGRATE_PCPTYPES * MIGRATE_PCPTYPES;
1887 
1888 	/* don't ever allow to reserve more than 5% of the lowmem */
1889 	recommended_min = min(recommended_min,
1890 			      (unsigned long) nr_free_buffer_pages() / 20);
1891 	recommended_min <<= (PAGE_SHIFT-10);
1892 
1893 	if (recommended_min > min_free_kbytes) {
1894 		if (user_min_free_kbytes >= 0)
1895 			pr_info("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n",
1896 				min_free_kbytes, recommended_min);
1897 
1898 		min_free_kbytes = recommended_min;
1899 	}
1900 	setup_per_zone_wmarks();
1901 }
1902 
1903 int start_stop_khugepaged(void)
1904 {
1905 	static struct task_struct *khugepaged_thread __read_mostly;
1906 	static DEFINE_MUTEX(khugepaged_mutex);
1907 	int err = 0;
1908 
1909 	mutex_lock(&khugepaged_mutex);
1910 	if (khugepaged_enabled()) {
1911 		if (!khugepaged_thread)
1912 			khugepaged_thread = kthread_run(khugepaged, NULL,
1913 							"khugepaged");
1914 		if (IS_ERR(khugepaged_thread)) {
1915 			pr_err("khugepaged: kthread_run(khugepaged) failed\n");
1916 			err = PTR_ERR(khugepaged_thread);
1917 			khugepaged_thread = NULL;
1918 			goto fail;
1919 		}
1920 
1921 		if (!list_empty(&khugepaged_scan.mm_head))
1922 			wake_up_interruptible(&khugepaged_wait);
1923 
1924 		set_recommended_min_free_kbytes();
1925 	} else if (khugepaged_thread) {
1926 		kthread_stop(khugepaged_thread);
1927 		khugepaged_thread = NULL;
1928 	}
1929 fail:
1930 	mutex_unlock(&khugepaged_mutex);
1931 	return err;
1932 }
1933