xref: /openbmc/linux/mm/ksm.c (revision 6774def6)
1 /*
2  * Memory merging support.
3  *
4  * This code enables dynamic sharing of identical pages found in different
5  * memory areas, even if they are not shared by fork()
6  *
7  * Copyright (C) 2008-2009 Red Hat, Inc.
8  * Authors:
9  *	Izik Eidus
10  *	Andrea Arcangeli
11  *	Chris Wright
12  *	Hugh Dickins
13  *
14  * This work is licensed under the terms of the GNU GPL, version 2.
15  */
16 
17 #include <linux/errno.h>
18 #include <linux/mm.h>
19 #include <linux/fs.h>
20 #include <linux/mman.h>
21 #include <linux/sched.h>
22 #include <linux/rwsem.h>
23 #include <linux/pagemap.h>
24 #include <linux/rmap.h>
25 #include <linux/spinlock.h>
26 #include <linux/jhash.h>
27 #include <linux/delay.h>
28 #include <linux/kthread.h>
29 #include <linux/wait.h>
30 #include <linux/slab.h>
31 #include <linux/rbtree.h>
32 #include <linux/memory.h>
33 #include <linux/mmu_notifier.h>
34 #include <linux/swap.h>
35 #include <linux/ksm.h>
36 #include <linux/hashtable.h>
37 #include <linux/freezer.h>
38 #include <linux/oom.h>
39 #include <linux/numa.h>
40 
41 #include <asm/tlbflush.h>
42 #include "internal.h"
43 
44 #ifdef CONFIG_NUMA
45 #define NUMA(x)		(x)
46 #define DO_NUMA(x)	do { (x); } while (0)
47 #else
48 #define NUMA(x)		(0)
49 #define DO_NUMA(x)	do { } while (0)
50 #endif
51 
52 /*
53  * A few notes about the KSM scanning process,
54  * to make it easier to understand the data structures below:
55  *
56  * In order to reduce excessive scanning, KSM sorts the memory pages by their
57  * contents into a data structure that holds pointers to the pages' locations.
58  *
59  * Since the contents of the pages may change at any moment, KSM cannot just
60  * insert the pages into a normal sorted tree and expect it to find anything.
61  * Therefore KSM uses two data structures - the stable and the unstable tree.
62  *
63  * The stable tree holds pointers to all the merged pages (ksm pages), sorted
64  * by their contents.  Because each such page is write-protected, searching on
65  * this tree is fully assured to be working (except when pages are unmapped),
66  * and therefore this tree is called the stable tree.
67  *
68  * In addition to the stable tree, KSM uses a second data structure called the
69  * unstable tree: this tree holds pointers to pages which have been found to
70  * be "unchanged for a period of time".  The unstable tree sorts these pages
71  * by their contents, but since they are not write-protected, KSM cannot rely
72  * upon the unstable tree to work correctly - the unstable tree is liable to
73  * be corrupted as its contents are modified, and so it is called unstable.
74  *
75  * KSM solves this problem by several techniques:
76  *
77  * 1) The unstable tree is flushed every time KSM completes scanning all
78  *    memory areas, and then the tree is rebuilt again from the beginning.
79  * 2) KSM will only insert into the unstable tree, pages whose hash value
80  *    has not changed since the previous scan of all memory areas.
81  * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the
82  *    colors of the nodes and not on their contents, assuring that even when
83  *    the tree gets "corrupted" it won't get out of balance, so scanning time
84  *    remains the same (also, searching and inserting nodes in an rbtree uses
85  *    the same algorithm, so we have no overhead when we flush and rebuild).
86  * 4) KSM never flushes the stable tree, which means that even if it were to
87  *    take 10 attempts to find a page in the unstable tree, once it is found,
88  *    it is secured in the stable tree.  (When we scan a new page, we first
89  *    compare it against the stable tree, and then against the unstable tree.)
90  *
91  * If the merge_across_nodes tunable is unset, then KSM maintains multiple
92  * stable trees and multiple unstable trees: one of each for each NUMA node.
93  */
94 
95 /**
96  * struct mm_slot - ksm information per mm that is being scanned
97  * @link: link to the mm_slots hash list
98  * @mm_list: link into the mm_slots list, rooted in ksm_mm_head
99  * @rmap_list: head for this mm_slot's singly-linked list of rmap_items
100  * @mm: the mm that this information is valid for
101  */
102 struct mm_slot {
103 	struct hlist_node link;
104 	struct list_head mm_list;
105 	struct rmap_item *rmap_list;
106 	struct mm_struct *mm;
107 };
108 
109 /**
110  * struct ksm_scan - cursor for scanning
111  * @mm_slot: the current mm_slot we are scanning
112  * @address: the next address inside that to be scanned
113  * @rmap_list: link to the next rmap to be scanned in the rmap_list
114  * @seqnr: count of completed full scans (needed when removing unstable node)
115  *
116  * There is only the one ksm_scan instance of this cursor structure.
117  */
118 struct ksm_scan {
119 	struct mm_slot *mm_slot;
120 	unsigned long address;
121 	struct rmap_item **rmap_list;
122 	unsigned long seqnr;
123 };
124 
125 /**
126  * struct stable_node - node of the stable rbtree
127  * @node: rb node of this ksm page in the stable tree
128  * @head: (overlaying parent) &migrate_nodes indicates temporarily on that list
129  * @list: linked into migrate_nodes, pending placement in the proper node tree
130  * @hlist: hlist head of rmap_items using this ksm page
131  * @kpfn: page frame number of this ksm page (perhaps temporarily on wrong nid)
132  * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
133  */
134 struct stable_node {
135 	union {
136 		struct rb_node node;	/* when node of stable tree */
137 		struct {		/* when listed for migration */
138 			struct list_head *head;
139 			struct list_head list;
140 		};
141 	};
142 	struct hlist_head hlist;
143 	unsigned long kpfn;
144 #ifdef CONFIG_NUMA
145 	int nid;
146 #endif
147 };
148 
149 /**
150  * struct rmap_item - reverse mapping item for virtual addresses
151  * @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
152  * @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
153  * @nid: NUMA node id of unstable tree in which linked (may not match page)
154  * @mm: the memory structure this rmap_item is pointing into
155  * @address: the virtual address this rmap_item tracks (+ flags in low bits)
156  * @oldchecksum: previous checksum of the page at that virtual address
157  * @node: rb node of this rmap_item in the unstable tree
158  * @head: pointer to stable_node heading this list in the stable tree
159  * @hlist: link into hlist of rmap_items hanging off that stable_node
160  */
161 struct rmap_item {
162 	struct rmap_item *rmap_list;
163 	union {
164 		struct anon_vma *anon_vma;	/* when stable */
165 #ifdef CONFIG_NUMA
166 		int nid;		/* when node of unstable tree */
167 #endif
168 	};
169 	struct mm_struct *mm;
170 	unsigned long address;		/* + low bits used for flags below */
171 	unsigned int oldchecksum;	/* when unstable */
172 	union {
173 		struct rb_node node;	/* when node of unstable tree */
174 		struct {		/* when listed from stable tree */
175 			struct stable_node *head;
176 			struct hlist_node hlist;
177 		};
178 	};
179 };
180 
181 #define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
182 #define UNSTABLE_FLAG	0x100	/* is a node of the unstable tree */
183 #define STABLE_FLAG	0x200	/* is listed from the stable tree */
184 
185 /* The stable and unstable tree heads */
186 static struct rb_root one_stable_tree[1] = { RB_ROOT };
187 static struct rb_root one_unstable_tree[1] = { RB_ROOT };
188 static struct rb_root *root_stable_tree = one_stable_tree;
189 static struct rb_root *root_unstable_tree = one_unstable_tree;
190 
191 /* Recently migrated nodes of stable tree, pending proper placement */
192 static LIST_HEAD(migrate_nodes);
193 
194 #define MM_SLOTS_HASH_BITS 10
195 static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
196 
197 static struct mm_slot ksm_mm_head = {
198 	.mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list),
199 };
200 static struct ksm_scan ksm_scan = {
201 	.mm_slot = &ksm_mm_head,
202 };
203 
204 static struct kmem_cache *rmap_item_cache;
205 static struct kmem_cache *stable_node_cache;
206 static struct kmem_cache *mm_slot_cache;
207 
208 /* The number of nodes in the stable tree */
209 static unsigned long ksm_pages_shared;
210 
211 /* The number of page slots additionally sharing those nodes */
212 static unsigned long ksm_pages_sharing;
213 
214 /* The number of nodes in the unstable tree */
215 static unsigned long ksm_pages_unshared;
216 
217 /* The number of rmap_items in use: to calculate pages_volatile */
218 static unsigned long ksm_rmap_items;
219 
220 /* Number of pages ksmd should scan in one batch */
221 static unsigned int ksm_thread_pages_to_scan = 100;
222 
223 /* Milliseconds ksmd should sleep between batches */
224 static unsigned int ksm_thread_sleep_millisecs = 20;
225 
226 #ifdef CONFIG_NUMA
227 /* Zeroed when merging across nodes is not allowed */
228 static unsigned int ksm_merge_across_nodes = 1;
229 static int ksm_nr_node_ids = 1;
230 #else
231 #define ksm_merge_across_nodes	1U
232 #define ksm_nr_node_ids		1
233 #endif
234 
235 #define KSM_RUN_STOP	0
236 #define KSM_RUN_MERGE	1
237 #define KSM_RUN_UNMERGE	2
238 #define KSM_RUN_OFFLINE	4
239 static unsigned long ksm_run = KSM_RUN_STOP;
240 static void wait_while_offlining(void);
241 
242 static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
243 static DEFINE_MUTEX(ksm_thread_mutex);
244 static DEFINE_SPINLOCK(ksm_mmlist_lock);
245 
246 #define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
247 		sizeof(struct __struct), __alignof__(struct __struct),\
248 		(__flags), NULL)
249 
250 static int __init ksm_slab_init(void)
251 {
252 	rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
253 	if (!rmap_item_cache)
254 		goto out;
255 
256 	stable_node_cache = KSM_KMEM_CACHE(stable_node, 0);
257 	if (!stable_node_cache)
258 		goto out_free1;
259 
260 	mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
261 	if (!mm_slot_cache)
262 		goto out_free2;
263 
264 	return 0;
265 
266 out_free2:
267 	kmem_cache_destroy(stable_node_cache);
268 out_free1:
269 	kmem_cache_destroy(rmap_item_cache);
270 out:
271 	return -ENOMEM;
272 }
273 
274 static void __init ksm_slab_free(void)
275 {
276 	kmem_cache_destroy(mm_slot_cache);
277 	kmem_cache_destroy(stable_node_cache);
278 	kmem_cache_destroy(rmap_item_cache);
279 	mm_slot_cache = NULL;
280 }
281 
282 static inline struct rmap_item *alloc_rmap_item(void)
283 {
284 	struct rmap_item *rmap_item;
285 
286 	rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
287 	if (rmap_item)
288 		ksm_rmap_items++;
289 	return rmap_item;
290 }
291 
292 static inline void free_rmap_item(struct rmap_item *rmap_item)
293 {
294 	ksm_rmap_items--;
295 	rmap_item->mm = NULL;	/* debug safety */
296 	kmem_cache_free(rmap_item_cache, rmap_item);
297 }
298 
299 static inline struct stable_node *alloc_stable_node(void)
300 {
301 	return kmem_cache_alloc(stable_node_cache, GFP_KERNEL);
302 }
303 
304 static inline void free_stable_node(struct stable_node *stable_node)
305 {
306 	kmem_cache_free(stable_node_cache, stable_node);
307 }
308 
309 static inline struct mm_slot *alloc_mm_slot(void)
310 {
311 	if (!mm_slot_cache)	/* initialization failed */
312 		return NULL;
313 	return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
314 }
315 
316 static inline void free_mm_slot(struct mm_slot *mm_slot)
317 {
318 	kmem_cache_free(mm_slot_cache, mm_slot);
319 }
320 
321 static struct mm_slot *get_mm_slot(struct mm_struct *mm)
322 {
323 	struct mm_slot *slot;
324 
325 	hash_for_each_possible(mm_slots_hash, slot, link, (unsigned long)mm)
326 		if (slot->mm == mm)
327 			return slot;
328 
329 	return NULL;
330 }
331 
332 static void insert_to_mm_slots_hash(struct mm_struct *mm,
333 				    struct mm_slot *mm_slot)
334 {
335 	mm_slot->mm = mm;
336 	hash_add(mm_slots_hash, &mm_slot->link, (unsigned long)mm);
337 }
338 
339 /*
340  * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
341  * page tables after it has passed through ksm_exit() - which, if necessary,
342  * takes mmap_sem briefly to serialize against them.  ksm_exit() does not set
343  * a special flag: they can just back out as soon as mm_users goes to zero.
344  * ksm_test_exit() is used throughout to make this test for exit: in some
345  * places for correctness, in some places just to avoid unnecessary work.
346  */
347 static inline bool ksm_test_exit(struct mm_struct *mm)
348 {
349 	return atomic_read(&mm->mm_users) == 0;
350 }
351 
352 /*
353  * We use break_ksm to break COW on a ksm page: it's a stripped down
354  *
355  *	if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
356  *		put_page(page);
357  *
358  * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
359  * in case the application has unmapped and remapped mm,addr meanwhile.
360  * Could a ksm page appear anywhere else?  Actually yes, in a VM_PFNMAP
361  * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
362  */
363 static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
364 {
365 	struct page *page;
366 	int ret = 0;
367 
368 	do {
369 		cond_resched();
370 		page = follow_page(vma, addr, FOLL_GET | FOLL_MIGRATION);
371 		if (IS_ERR_OR_NULL(page))
372 			break;
373 		if (PageKsm(page))
374 			ret = handle_mm_fault(vma->vm_mm, vma, addr,
375 							FAULT_FLAG_WRITE);
376 		else
377 			ret = VM_FAULT_WRITE;
378 		put_page(page);
379 	} while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_OOM)));
380 	/*
381 	 * We must loop because handle_mm_fault() may back out if there's
382 	 * any difficulty e.g. if pte accessed bit gets updated concurrently.
383 	 *
384 	 * VM_FAULT_WRITE is what we have been hoping for: it indicates that
385 	 * COW has been broken, even if the vma does not permit VM_WRITE;
386 	 * but note that a concurrent fault might break PageKsm for us.
387 	 *
388 	 * VM_FAULT_SIGBUS could occur if we race with truncation of the
389 	 * backing file, which also invalidates anonymous pages: that's
390 	 * okay, that truncation will have unmapped the PageKsm for us.
391 	 *
392 	 * VM_FAULT_OOM: at the time of writing (late July 2009), setting
393 	 * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
394 	 * current task has TIF_MEMDIE set, and will be OOM killed on return
395 	 * to user; and ksmd, having no mm, would never be chosen for that.
396 	 *
397 	 * But if the mm is in a limited mem_cgroup, then the fault may fail
398 	 * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
399 	 * even ksmd can fail in this way - though it's usually breaking ksm
400 	 * just to undo a merge it made a moment before, so unlikely to oom.
401 	 *
402 	 * That's a pity: we might therefore have more kernel pages allocated
403 	 * than we're counting as nodes in the stable tree; but ksm_do_scan
404 	 * will retry to break_cow on each pass, so should recover the page
405 	 * in due course.  The important thing is to not let VM_MERGEABLE
406 	 * be cleared while any such pages might remain in the area.
407 	 */
408 	return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
409 }
410 
411 static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
412 		unsigned long addr)
413 {
414 	struct vm_area_struct *vma;
415 	if (ksm_test_exit(mm))
416 		return NULL;
417 	vma = find_vma(mm, addr);
418 	if (!vma || vma->vm_start > addr)
419 		return NULL;
420 	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
421 		return NULL;
422 	return vma;
423 }
424 
425 static void break_cow(struct rmap_item *rmap_item)
426 {
427 	struct mm_struct *mm = rmap_item->mm;
428 	unsigned long addr = rmap_item->address;
429 	struct vm_area_struct *vma;
430 
431 	/*
432 	 * It is not an accident that whenever we want to break COW
433 	 * to undo, we also need to drop a reference to the anon_vma.
434 	 */
435 	put_anon_vma(rmap_item->anon_vma);
436 
437 	down_read(&mm->mmap_sem);
438 	vma = find_mergeable_vma(mm, addr);
439 	if (vma)
440 		break_ksm(vma, addr);
441 	up_read(&mm->mmap_sem);
442 }
443 
444 static struct page *page_trans_compound_anon(struct page *page)
445 {
446 	if (PageTransCompound(page)) {
447 		struct page *head = compound_head(page);
448 		/*
449 		 * head may actually be splitted and freed from under
450 		 * us but it's ok here.
451 		 */
452 		if (PageAnon(head))
453 			return head;
454 	}
455 	return NULL;
456 }
457 
458 static struct page *get_mergeable_page(struct rmap_item *rmap_item)
459 {
460 	struct mm_struct *mm = rmap_item->mm;
461 	unsigned long addr = rmap_item->address;
462 	struct vm_area_struct *vma;
463 	struct page *page;
464 
465 	down_read(&mm->mmap_sem);
466 	vma = find_mergeable_vma(mm, addr);
467 	if (!vma)
468 		goto out;
469 
470 	page = follow_page(vma, addr, FOLL_GET);
471 	if (IS_ERR_OR_NULL(page))
472 		goto out;
473 	if (PageAnon(page) || page_trans_compound_anon(page)) {
474 		flush_anon_page(vma, page, addr);
475 		flush_dcache_page(page);
476 	} else {
477 		put_page(page);
478 out:		page = NULL;
479 	}
480 	up_read(&mm->mmap_sem);
481 	return page;
482 }
483 
484 /*
485  * This helper is used for getting right index into array of tree roots.
486  * When merge_across_nodes knob is set to 1, there are only two rb-trees for
487  * stable and unstable pages from all nodes with roots in index 0. Otherwise,
488  * every node has its own stable and unstable tree.
489  */
490 static inline int get_kpfn_nid(unsigned long kpfn)
491 {
492 	return ksm_merge_across_nodes ? 0 : NUMA(pfn_to_nid(kpfn));
493 }
494 
495 static void remove_node_from_stable_tree(struct stable_node *stable_node)
496 {
497 	struct rmap_item *rmap_item;
498 
499 	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
500 		if (rmap_item->hlist.next)
501 			ksm_pages_sharing--;
502 		else
503 			ksm_pages_shared--;
504 		put_anon_vma(rmap_item->anon_vma);
505 		rmap_item->address &= PAGE_MASK;
506 		cond_resched();
507 	}
508 
509 	if (stable_node->head == &migrate_nodes)
510 		list_del(&stable_node->list);
511 	else
512 		rb_erase(&stable_node->node,
513 			 root_stable_tree + NUMA(stable_node->nid));
514 	free_stable_node(stable_node);
515 }
516 
517 /*
518  * get_ksm_page: checks if the page indicated by the stable node
519  * is still its ksm page, despite having held no reference to it.
520  * In which case we can trust the content of the page, and it
521  * returns the gotten page; but if the page has now been zapped,
522  * remove the stale node from the stable tree and return NULL.
523  * But beware, the stable node's page might be being migrated.
524  *
525  * You would expect the stable_node to hold a reference to the ksm page.
526  * But if it increments the page's count, swapping out has to wait for
527  * ksmd to come around again before it can free the page, which may take
528  * seconds or even minutes: much too unresponsive.  So instead we use a
529  * "keyhole reference": access to the ksm page from the stable node peeps
530  * out through its keyhole to see if that page still holds the right key,
531  * pointing back to this stable node.  This relies on freeing a PageAnon
532  * page to reset its page->mapping to NULL, and relies on no other use of
533  * a page to put something that might look like our key in page->mapping.
534  * is on its way to being freed; but it is an anomaly to bear in mind.
535  */
536 static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
537 {
538 	struct page *page;
539 	void *expected_mapping;
540 	unsigned long kpfn;
541 
542 	expected_mapping = (void *)stable_node +
543 				(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
544 again:
545 	kpfn = ACCESS_ONCE(stable_node->kpfn);
546 	page = pfn_to_page(kpfn);
547 
548 	/*
549 	 * page is computed from kpfn, so on most architectures reading
550 	 * page->mapping is naturally ordered after reading node->kpfn,
551 	 * but on Alpha we need to be more careful.
552 	 */
553 	smp_read_barrier_depends();
554 	if (ACCESS_ONCE(page->mapping) != expected_mapping)
555 		goto stale;
556 
557 	/*
558 	 * We cannot do anything with the page while its refcount is 0.
559 	 * Usually 0 means free, or tail of a higher-order page: in which
560 	 * case this node is no longer referenced, and should be freed;
561 	 * however, it might mean that the page is under page_freeze_refs().
562 	 * The __remove_mapping() case is easy, again the node is now stale;
563 	 * but if page is swapcache in migrate_page_move_mapping(), it might
564 	 * still be our page, in which case it's essential to keep the node.
565 	 */
566 	while (!get_page_unless_zero(page)) {
567 		/*
568 		 * Another check for page->mapping != expected_mapping would
569 		 * work here too.  We have chosen the !PageSwapCache test to
570 		 * optimize the common case, when the page is or is about to
571 		 * be freed: PageSwapCache is cleared (under spin_lock_irq)
572 		 * in the freeze_refs section of __remove_mapping(); but Anon
573 		 * page->mapping reset to NULL later, in free_pages_prepare().
574 		 */
575 		if (!PageSwapCache(page))
576 			goto stale;
577 		cpu_relax();
578 	}
579 
580 	if (ACCESS_ONCE(page->mapping) != expected_mapping) {
581 		put_page(page);
582 		goto stale;
583 	}
584 
585 	if (lock_it) {
586 		lock_page(page);
587 		if (ACCESS_ONCE(page->mapping) != expected_mapping) {
588 			unlock_page(page);
589 			put_page(page);
590 			goto stale;
591 		}
592 	}
593 	return page;
594 
595 stale:
596 	/*
597 	 * We come here from above when page->mapping or !PageSwapCache
598 	 * suggests that the node is stale; but it might be under migration.
599 	 * We need smp_rmb(), matching the smp_wmb() in ksm_migrate_page(),
600 	 * before checking whether node->kpfn has been changed.
601 	 */
602 	smp_rmb();
603 	if (ACCESS_ONCE(stable_node->kpfn) != kpfn)
604 		goto again;
605 	remove_node_from_stable_tree(stable_node);
606 	return NULL;
607 }
608 
609 /*
610  * Removing rmap_item from stable or unstable tree.
611  * This function will clean the information from the stable/unstable tree.
612  */
613 static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
614 {
615 	if (rmap_item->address & STABLE_FLAG) {
616 		struct stable_node *stable_node;
617 		struct page *page;
618 
619 		stable_node = rmap_item->head;
620 		page = get_ksm_page(stable_node, true);
621 		if (!page)
622 			goto out;
623 
624 		hlist_del(&rmap_item->hlist);
625 		unlock_page(page);
626 		put_page(page);
627 
628 		if (stable_node->hlist.first)
629 			ksm_pages_sharing--;
630 		else
631 			ksm_pages_shared--;
632 
633 		put_anon_vma(rmap_item->anon_vma);
634 		rmap_item->address &= PAGE_MASK;
635 
636 	} else if (rmap_item->address & UNSTABLE_FLAG) {
637 		unsigned char age;
638 		/*
639 		 * Usually ksmd can and must skip the rb_erase, because
640 		 * root_unstable_tree was already reset to RB_ROOT.
641 		 * But be careful when an mm is exiting: do the rb_erase
642 		 * if this rmap_item was inserted by this scan, rather
643 		 * than left over from before.
644 		 */
645 		age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
646 		BUG_ON(age > 1);
647 		if (!age)
648 			rb_erase(&rmap_item->node,
649 				 root_unstable_tree + NUMA(rmap_item->nid));
650 		ksm_pages_unshared--;
651 		rmap_item->address &= PAGE_MASK;
652 	}
653 out:
654 	cond_resched();		/* we're called from many long loops */
655 }
656 
657 static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
658 				       struct rmap_item **rmap_list)
659 {
660 	while (*rmap_list) {
661 		struct rmap_item *rmap_item = *rmap_list;
662 		*rmap_list = rmap_item->rmap_list;
663 		remove_rmap_item_from_tree(rmap_item);
664 		free_rmap_item(rmap_item);
665 	}
666 }
667 
668 /*
669  * Though it's very tempting to unmerge rmap_items from stable tree rather
670  * than check every pte of a given vma, the locking doesn't quite work for
671  * that - an rmap_item is assigned to the stable tree after inserting ksm
672  * page and upping mmap_sem.  Nor does it fit with the way we skip dup'ing
673  * rmap_items from parent to child at fork time (so as not to waste time
674  * if exit comes before the next scan reaches it).
675  *
676  * Similarly, although we'd like to remove rmap_items (so updating counts
677  * and freeing memory) when unmerging an area, it's easier to leave that
678  * to the next pass of ksmd - consider, for example, how ksmd might be
679  * in cmp_and_merge_page on one of the rmap_items we would be removing.
680  */
681 static int unmerge_ksm_pages(struct vm_area_struct *vma,
682 			     unsigned long start, unsigned long end)
683 {
684 	unsigned long addr;
685 	int err = 0;
686 
687 	for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
688 		if (ksm_test_exit(vma->vm_mm))
689 			break;
690 		if (signal_pending(current))
691 			err = -ERESTARTSYS;
692 		else
693 			err = break_ksm(vma, addr);
694 	}
695 	return err;
696 }
697 
698 #ifdef CONFIG_SYSFS
699 /*
700  * Only called through the sysfs control interface:
701  */
702 static int remove_stable_node(struct stable_node *stable_node)
703 {
704 	struct page *page;
705 	int err;
706 
707 	page = get_ksm_page(stable_node, true);
708 	if (!page) {
709 		/*
710 		 * get_ksm_page did remove_node_from_stable_tree itself.
711 		 */
712 		return 0;
713 	}
714 
715 	if (WARN_ON_ONCE(page_mapped(page))) {
716 		/*
717 		 * This should not happen: but if it does, just refuse to let
718 		 * merge_across_nodes be switched - there is no need to panic.
719 		 */
720 		err = -EBUSY;
721 	} else {
722 		/*
723 		 * The stable node did not yet appear stale to get_ksm_page(),
724 		 * since that allows for an unmapped ksm page to be recognized
725 		 * right up until it is freed; but the node is safe to remove.
726 		 * This page might be in a pagevec waiting to be freed,
727 		 * or it might be PageSwapCache (perhaps under writeback),
728 		 * or it might have been removed from swapcache a moment ago.
729 		 */
730 		set_page_stable_node(page, NULL);
731 		remove_node_from_stable_tree(stable_node);
732 		err = 0;
733 	}
734 
735 	unlock_page(page);
736 	put_page(page);
737 	return err;
738 }
739 
740 static int remove_all_stable_nodes(void)
741 {
742 	struct stable_node *stable_node;
743 	struct list_head *this, *next;
744 	int nid;
745 	int err = 0;
746 
747 	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
748 		while (root_stable_tree[nid].rb_node) {
749 			stable_node = rb_entry(root_stable_tree[nid].rb_node,
750 						struct stable_node, node);
751 			if (remove_stable_node(stable_node)) {
752 				err = -EBUSY;
753 				break;	/* proceed to next nid */
754 			}
755 			cond_resched();
756 		}
757 	}
758 	list_for_each_safe(this, next, &migrate_nodes) {
759 		stable_node = list_entry(this, struct stable_node, list);
760 		if (remove_stable_node(stable_node))
761 			err = -EBUSY;
762 		cond_resched();
763 	}
764 	return err;
765 }
766 
767 static int unmerge_and_remove_all_rmap_items(void)
768 {
769 	struct mm_slot *mm_slot;
770 	struct mm_struct *mm;
771 	struct vm_area_struct *vma;
772 	int err = 0;
773 
774 	spin_lock(&ksm_mmlist_lock);
775 	ksm_scan.mm_slot = list_entry(ksm_mm_head.mm_list.next,
776 						struct mm_slot, mm_list);
777 	spin_unlock(&ksm_mmlist_lock);
778 
779 	for (mm_slot = ksm_scan.mm_slot;
780 			mm_slot != &ksm_mm_head; mm_slot = ksm_scan.mm_slot) {
781 		mm = mm_slot->mm;
782 		down_read(&mm->mmap_sem);
783 		for (vma = mm->mmap; vma; vma = vma->vm_next) {
784 			if (ksm_test_exit(mm))
785 				break;
786 			if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
787 				continue;
788 			err = unmerge_ksm_pages(vma,
789 						vma->vm_start, vma->vm_end);
790 			if (err)
791 				goto error;
792 		}
793 
794 		remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list);
795 
796 		spin_lock(&ksm_mmlist_lock);
797 		ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
798 						struct mm_slot, mm_list);
799 		if (ksm_test_exit(mm)) {
800 			hash_del(&mm_slot->link);
801 			list_del(&mm_slot->mm_list);
802 			spin_unlock(&ksm_mmlist_lock);
803 
804 			free_mm_slot(mm_slot);
805 			clear_bit(MMF_VM_MERGEABLE, &mm->flags);
806 			up_read(&mm->mmap_sem);
807 			mmdrop(mm);
808 		} else {
809 			spin_unlock(&ksm_mmlist_lock);
810 			up_read(&mm->mmap_sem);
811 		}
812 	}
813 
814 	/* Clean up stable nodes, but don't worry if some are still busy */
815 	remove_all_stable_nodes();
816 	ksm_scan.seqnr = 0;
817 	return 0;
818 
819 error:
820 	up_read(&mm->mmap_sem);
821 	spin_lock(&ksm_mmlist_lock);
822 	ksm_scan.mm_slot = &ksm_mm_head;
823 	spin_unlock(&ksm_mmlist_lock);
824 	return err;
825 }
826 #endif /* CONFIG_SYSFS */
827 
828 static u32 calc_checksum(struct page *page)
829 {
830 	u32 checksum;
831 	void *addr = kmap_atomic(page);
832 	checksum = jhash2(addr, PAGE_SIZE / 4, 17);
833 	kunmap_atomic(addr);
834 	return checksum;
835 }
836 
837 static int memcmp_pages(struct page *page1, struct page *page2)
838 {
839 	char *addr1, *addr2;
840 	int ret;
841 
842 	addr1 = kmap_atomic(page1);
843 	addr2 = kmap_atomic(page2);
844 	ret = memcmp(addr1, addr2, PAGE_SIZE);
845 	kunmap_atomic(addr2);
846 	kunmap_atomic(addr1);
847 	return ret;
848 }
849 
850 static inline int pages_identical(struct page *page1, struct page *page2)
851 {
852 	return !memcmp_pages(page1, page2);
853 }
854 
855 static int write_protect_page(struct vm_area_struct *vma, struct page *page,
856 			      pte_t *orig_pte)
857 {
858 	struct mm_struct *mm = vma->vm_mm;
859 	unsigned long addr;
860 	pte_t *ptep;
861 	spinlock_t *ptl;
862 	int swapped;
863 	int err = -EFAULT;
864 	unsigned long mmun_start;	/* For mmu_notifiers */
865 	unsigned long mmun_end;		/* For mmu_notifiers */
866 
867 	addr = page_address_in_vma(page, vma);
868 	if (addr == -EFAULT)
869 		goto out;
870 
871 	BUG_ON(PageTransCompound(page));
872 
873 	mmun_start = addr;
874 	mmun_end   = addr + PAGE_SIZE;
875 	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
876 
877 	ptep = page_check_address(page, mm, addr, &ptl, 0);
878 	if (!ptep)
879 		goto out_mn;
880 
881 	if (pte_write(*ptep) || pte_dirty(*ptep)) {
882 		pte_t entry;
883 
884 		swapped = PageSwapCache(page);
885 		flush_cache_page(vma, addr, page_to_pfn(page));
886 		/*
887 		 * Ok this is tricky, when get_user_pages_fast() run it doesn't
888 		 * take any lock, therefore the check that we are going to make
889 		 * with the pagecount against the mapcount is racey and
890 		 * O_DIRECT can happen right after the check.
891 		 * So we clear the pte and flush the tlb before the check
892 		 * this assure us that no O_DIRECT can happen after the check
893 		 * or in the middle of the check.
894 		 */
895 		entry = ptep_clear_flush(vma, addr, ptep);
896 		/*
897 		 * Check that no O_DIRECT or similar I/O is in progress on the
898 		 * page
899 		 */
900 		if (page_mapcount(page) + 1 + swapped != page_count(page)) {
901 			set_pte_at(mm, addr, ptep, entry);
902 			goto out_unlock;
903 		}
904 		if (pte_dirty(entry))
905 			set_page_dirty(page);
906 		entry = pte_mkclean(pte_wrprotect(entry));
907 		set_pte_at_notify(mm, addr, ptep, entry);
908 	}
909 	*orig_pte = *ptep;
910 	err = 0;
911 
912 out_unlock:
913 	pte_unmap_unlock(ptep, ptl);
914 out_mn:
915 	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
916 out:
917 	return err;
918 }
919 
920 /**
921  * replace_page - replace page in vma by new ksm page
922  * @vma:      vma that holds the pte pointing to page
923  * @page:     the page we are replacing by kpage
924  * @kpage:    the ksm page we replace page by
925  * @orig_pte: the original value of the pte
926  *
927  * Returns 0 on success, -EFAULT on failure.
928  */
929 static int replace_page(struct vm_area_struct *vma, struct page *page,
930 			struct page *kpage, pte_t orig_pte)
931 {
932 	struct mm_struct *mm = vma->vm_mm;
933 	pmd_t *pmd;
934 	pte_t *ptep;
935 	spinlock_t *ptl;
936 	unsigned long addr;
937 	int err = -EFAULT;
938 	unsigned long mmun_start;	/* For mmu_notifiers */
939 	unsigned long mmun_end;		/* For mmu_notifiers */
940 
941 	addr = page_address_in_vma(page, vma);
942 	if (addr == -EFAULT)
943 		goto out;
944 
945 	pmd = mm_find_pmd(mm, addr);
946 	if (!pmd)
947 		goto out;
948 
949 	mmun_start = addr;
950 	mmun_end   = addr + PAGE_SIZE;
951 	mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end);
952 
953 	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
954 	if (!pte_same(*ptep, orig_pte)) {
955 		pte_unmap_unlock(ptep, ptl);
956 		goto out_mn;
957 	}
958 
959 	get_page(kpage);
960 	page_add_anon_rmap(kpage, vma, addr);
961 
962 	flush_cache_page(vma, addr, pte_pfn(*ptep));
963 	ptep_clear_flush(vma, addr, ptep);
964 	set_pte_at_notify(mm, addr, ptep, mk_pte(kpage, vma->vm_page_prot));
965 
966 	page_remove_rmap(page);
967 	if (!page_mapped(page))
968 		try_to_free_swap(page);
969 	put_page(page);
970 
971 	pte_unmap_unlock(ptep, ptl);
972 	err = 0;
973 out_mn:
974 	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
975 out:
976 	return err;
977 }
978 
979 static int page_trans_compound_anon_split(struct page *page)
980 {
981 	int ret = 0;
982 	struct page *transhuge_head = page_trans_compound_anon(page);
983 	if (transhuge_head) {
984 		/* Get the reference on the head to split it. */
985 		if (get_page_unless_zero(transhuge_head)) {
986 			/*
987 			 * Recheck we got the reference while the head
988 			 * was still anonymous.
989 			 */
990 			if (PageAnon(transhuge_head))
991 				ret = split_huge_page(transhuge_head);
992 			else
993 				/*
994 				 * Retry later if split_huge_page run
995 				 * from under us.
996 				 */
997 				ret = 1;
998 			put_page(transhuge_head);
999 		} else
1000 			/* Retry later if split_huge_page run from under us. */
1001 			ret = 1;
1002 	}
1003 	return ret;
1004 }
1005 
1006 /*
1007  * try_to_merge_one_page - take two pages and merge them into one
1008  * @vma: the vma that holds the pte pointing to page
1009  * @page: the PageAnon page that we want to replace with kpage
1010  * @kpage: the PageKsm page that we want to map instead of page,
1011  *         or NULL the first time when we want to use page as kpage.
1012  *
1013  * This function returns 0 if the pages were merged, -EFAULT otherwise.
1014  */
1015 static int try_to_merge_one_page(struct vm_area_struct *vma,
1016 				 struct page *page, struct page *kpage)
1017 {
1018 	pte_t orig_pte = __pte(0);
1019 	int err = -EFAULT;
1020 
1021 	if (page == kpage)			/* ksm page forked */
1022 		return 0;
1023 
1024 	if (!(vma->vm_flags & VM_MERGEABLE))
1025 		goto out;
1026 	if (PageTransCompound(page) && page_trans_compound_anon_split(page))
1027 		goto out;
1028 	BUG_ON(PageTransCompound(page));
1029 	if (!PageAnon(page))
1030 		goto out;
1031 
1032 	/*
1033 	 * We need the page lock to read a stable PageSwapCache in
1034 	 * write_protect_page().  We use trylock_page() instead of
1035 	 * lock_page() because we don't want to wait here - we
1036 	 * prefer to continue scanning and merging different pages,
1037 	 * then come back to this page when it is unlocked.
1038 	 */
1039 	if (!trylock_page(page))
1040 		goto out;
1041 	/*
1042 	 * If this anonymous page is mapped only here, its pte may need
1043 	 * to be write-protected.  If it's mapped elsewhere, all of its
1044 	 * ptes are necessarily already write-protected.  But in either
1045 	 * case, we need to lock and check page_count is not raised.
1046 	 */
1047 	if (write_protect_page(vma, page, &orig_pte) == 0) {
1048 		if (!kpage) {
1049 			/*
1050 			 * While we hold page lock, upgrade page from
1051 			 * PageAnon+anon_vma to PageKsm+NULL stable_node:
1052 			 * stable_tree_insert() will update stable_node.
1053 			 */
1054 			set_page_stable_node(page, NULL);
1055 			mark_page_accessed(page);
1056 			err = 0;
1057 		} else if (pages_identical(page, kpage))
1058 			err = replace_page(vma, page, kpage, orig_pte);
1059 	}
1060 
1061 	if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
1062 		munlock_vma_page(page);
1063 		if (!PageMlocked(kpage)) {
1064 			unlock_page(page);
1065 			lock_page(kpage);
1066 			mlock_vma_page(kpage);
1067 			page = kpage;		/* for final unlock */
1068 		}
1069 	}
1070 
1071 	unlock_page(page);
1072 out:
1073 	return err;
1074 }
1075 
1076 /*
1077  * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
1078  * but no new kernel page is allocated: kpage must already be a ksm page.
1079  *
1080  * This function returns 0 if the pages were merged, -EFAULT otherwise.
1081  */
1082 static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
1083 				      struct page *page, struct page *kpage)
1084 {
1085 	struct mm_struct *mm = rmap_item->mm;
1086 	struct vm_area_struct *vma;
1087 	int err = -EFAULT;
1088 
1089 	down_read(&mm->mmap_sem);
1090 	if (ksm_test_exit(mm))
1091 		goto out;
1092 	vma = find_vma(mm, rmap_item->address);
1093 	if (!vma || vma->vm_start > rmap_item->address)
1094 		goto out;
1095 
1096 	err = try_to_merge_one_page(vma, page, kpage);
1097 	if (err)
1098 		goto out;
1099 
1100 	/* Unstable nid is in union with stable anon_vma: remove first */
1101 	remove_rmap_item_from_tree(rmap_item);
1102 
1103 	/* Must get reference to anon_vma while still holding mmap_sem */
1104 	rmap_item->anon_vma = vma->anon_vma;
1105 	get_anon_vma(vma->anon_vma);
1106 out:
1107 	up_read(&mm->mmap_sem);
1108 	return err;
1109 }
1110 
1111 /*
1112  * try_to_merge_two_pages - take two identical pages and prepare them
1113  * to be merged into one page.
1114  *
1115  * This function returns the kpage if we successfully merged two identical
1116  * pages into one ksm page, NULL otherwise.
1117  *
1118  * Note that this function upgrades page to ksm page: if one of the pages
1119  * is already a ksm page, try_to_merge_with_ksm_page should be used.
1120  */
1121 static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
1122 					   struct page *page,
1123 					   struct rmap_item *tree_rmap_item,
1124 					   struct page *tree_page)
1125 {
1126 	int err;
1127 
1128 	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
1129 	if (!err) {
1130 		err = try_to_merge_with_ksm_page(tree_rmap_item,
1131 							tree_page, page);
1132 		/*
1133 		 * If that fails, we have a ksm page with only one pte
1134 		 * pointing to it: so break it.
1135 		 */
1136 		if (err)
1137 			break_cow(rmap_item);
1138 	}
1139 	return err ? NULL : page;
1140 }
1141 
1142 /*
1143  * stable_tree_search - search for page inside the stable tree
1144  *
1145  * This function checks if there is a page inside the stable tree
1146  * with identical content to the page that we are scanning right now.
1147  *
1148  * This function returns the stable tree node of identical content if found,
1149  * NULL otherwise.
1150  */
1151 static struct page *stable_tree_search(struct page *page)
1152 {
1153 	int nid;
1154 	struct rb_root *root;
1155 	struct rb_node **new;
1156 	struct rb_node *parent;
1157 	struct stable_node *stable_node;
1158 	struct stable_node *page_node;
1159 
1160 	page_node = page_stable_node(page);
1161 	if (page_node && page_node->head != &migrate_nodes) {
1162 		/* ksm page forked */
1163 		get_page(page);
1164 		return page;
1165 	}
1166 
1167 	nid = get_kpfn_nid(page_to_pfn(page));
1168 	root = root_stable_tree + nid;
1169 again:
1170 	new = &root->rb_node;
1171 	parent = NULL;
1172 
1173 	while (*new) {
1174 		struct page *tree_page;
1175 		int ret;
1176 
1177 		cond_resched();
1178 		stable_node = rb_entry(*new, struct stable_node, node);
1179 		tree_page = get_ksm_page(stable_node, false);
1180 		if (!tree_page)
1181 			return NULL;
1182 
1183 		ret = memcmp_pages(page, tree_page);
1184 		put_page(tree_page);
1185 
1186 		parent = *new;
1187 		if (ret < 0)
1188 			new = &parent->rb_left;
1189 		else if (ret > 0)
1190 			new = &parent->rb_right;
1191 		else {
1192 			/*
1193 			 * Lock and unlock the stable_node's page (which
1194 			 * might already have been migrated) so that page
1195 			 * migration is sure to notice its raised count.
1196 			 * It would be more elegant to return stable_node
1197 			 * than kpage, but that involves more changes.
1198 			 */
1199 			tree_page = get_ksm_page(stable_node, true);
1200 			if (tree_page) {
1201 				unlock_page(tree_page);
1202 				if (get_kpfn_nid(stable_node->kpfn) !=
1203 						NUMA(stable_node->nid)) {
1204 					put_page(tree_page);
1205 					goto replace;
1206 				}
1207 				return tree_page;
1208 			}
1209 			/*
1210 			 * There is now a place for page_node, but the tree may
1211 			 * have been rebalanced, so re-evaluate parent and new.
1212 			 */
1213 			if (page_node)
1214 				goto again;
1215 			return NULL;
1216 		}
1217 	}
1218 
1219 	if (!page_node)
1220 		return NULL;
1221 
1222 	list_del(&page_node->list);
1223 	DO_NUMA(page_node->nid = nid);
1224 	rb_link_node(&page_node->node, parent, new);
1225 	rb_insert_color(&page_node->node, root);
1226 	get_page(page);
1227 	return page;
1228 
1229 replace:
1230 	if (page_node) {
1231 		list_del(&page_node->list);
1232 		DO_NUMA(page_node->nid = nid);
1233 		rb_replace_node(&stable_node->node, &page_node->node, root);
1234 		get_page(page);
1235 	} else {
1236 		rb_erase(&stable_node->node, root);
1237 		page = NULL;
1238 	}
1239 	stable_node->head = &migrate_nodes;
1240 	list_add(&stable_node->list, stable_node->head);
1241 	return page;
1242 }
1243 
1244 /*
1245  * stable_tree_insert - insert stable tree node pointing to new ksm page
1246  * into the stable tree.
1247  *
1248  * This function returns the stable tree node just allocated on success,
1249  * NULL otherwise.
1250  */
1251 static struct stable_node *stable_tree_insert(struct page *kpage)
1252 {
1253 	int nid;
1254 	unsigned long kpfn;
1255 	struct rb_root *root;
1256 	struct rb_node **new;
1257 	struct rb_node *parent = NULL;
1258 	struct stable_node *stable_node;
1259 
1260 	kpfn = page_to_pfn(kpage);
1261 	nid = get_kpfn_nid(kpfn);
1262 	root = root_stable_tree + nid;
1263 	new = &root->rb_node;
1264 
1265 	while (*new) {
1266 		struct page *tree_page;
1267 		int ret;
1268 
1269 		cond_resched();
1270 		stable_node = rb_entry(*new, struct stable_node, node);
1271 		tree_page = get_ksm_page(stable_node, false);
1272 		if (!tree_page)
1273 			return NULL;
1274 
1275 		ret = memcmp_pages(kpage, tree_page);
1276 		put_page(tree_page);
1277 
1278 		parent = *new;
1279 		if (ret < 0)
1280 			new = &parent->rb_left;
1281 		else if (ret > 0)
1282 			new = &parent->rb_right;
1283 		else {
1284 			/*
1285 			 * It is not a bug that stable_tree_search() didn't
1286 			 * find this node: because at that time our page was
1287 			 * not yet write-protected, so may have changed since.
1288 			 */
1289 			return NULL;
1290 		}
1291 	}
1292 
1293 	stable_node = alloc_stable_node();
1294 	if (!stable_node)
1295 		return NULL;
1296 
1297 	INIT_HLIST_HEAD(&stable_node->hlist);
1298 	stable_node->kpfn = kpfn;
1299 	set_page_stable_node(kpage, stable_node);
1300 	DO_NUMA(stable_node->nid = nid);
1301 	rb_link_node(&stable_node->node, parent, new);
1302 	rb_insert_color(&stable_node->node, root);
1303 
1304 	return stable_node;
1305 }
1306 
1307 /*
1308  * unstable_tree_search_insert - search for identical page,
1309  * else insert rmap_item into the unstable tree.
1310  *
1311  * This function searches for a page in the unstable tree identical to the
1312  * page currently being scanned; and if no identical page is found in the
1313  * tree, we insert rmap_item as a new object into the unstable tree.
1314  *
1315  * This function returns pointer to rmap_item found to be identical
1316  * to the currently scanned page, NULL otherwise.
1317  *
1318  * This function does both searching and inserting, because they share
1319  * the same walking algorithm in an rbtree.
1320  */
1321 static
1322 struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
1323 					      struct page *page,
1324 					      struct page **tree_pagep)
1325 {
1326 	struct rb_node **new;
1327 	struct rb_root *root;
1328 	struct rb_node *parent = NULL;
1329 	int nid;
1330 
1331 	nid = get_kpfn_nid(page_to_pfn(page));
1332 	root = root_unstable_tree + nid;
1333 	new = &root->rb_node;
1334 
1335 	while (*new) {
1336 		struct rmap_item *tree_rmap_item;
1337 		struct page *tree_page;
1338 		int ret;
1339 
1340 		cond_resched();
1341 		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
1342 		tree_page = get_mergeable_page(tree_rmap_item);
1343 		if (IS_ERR_OR_NULL(tree_page))
1344 			return NULL;
1345 
1346 		/*
1347 		 * Don't substitute a ksm page for a forked page.
1348 		 */
1349 		if (page == tree_page) {
1350 			put_page(tree_page);
1351 			return NULL;
1352 		}
1353 
1354 		ret = memcmp_pages(page, tree_page);
1355 
1356 		parent = *new;
1357 		if (ret < 0) {
1358 			put_page(tree_page);
1359 			new = &parent->rb_left;
1360 		} else if (ret > 0) {
1361 			put_page(tree_page);
1362 			new = &parent->rb_right;
1363 		} else if (!ksm_merge_across_nodes &&
1364 			   page_to_nid(tree_page) != nid) {
1365 			/*
1366 			 * If tree_page has been migrated to another NUMA node,
1367 			 * it will be flushed out and put in the right unstable
1368 			 * tree next time: only merge with it when across_nodes.
1369 			 */
1370 			put_page(tree_page);
1371 			return NULL;
1372 		} else {
1373 			*tree_pagep = tree_page;
1374 			return tree_rmap_item;
1375 		}
1376 	}
1377 
1378 	rmap_item->address |= UNSTABLE_FLAG;
1379 	rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
1380 	DO_NUMA(rmap_item->nid = nid);
1381 	rb_link_node(&rmap_item->node, parent, new);
1382 	rb_insert_color(&rmap_item->node, root);
1383 
1384 	ksm_pages_unshared++;
1385 	return NULL;
1386 }
1387 
1388 /*
1389  * stable_tree_append - add another rmap_item to the linked list of
1390  * rmap_items hanging off a given node of the stable tree, all sharing
1391  * the same ksm page.
1392  */
1393 static void stable_tree_append(struct rmap_item *rmap_item,
1394 			       struct stable_node *stable_node)
1395 {
1396 	rmap_item->head = stable_node;
1397 	rmap_item->address |= STABLE_FLAG;
1398 	hlist_add_head(&rmap_item->hlist, &stable_node->hlist);
1399 
1400 	if (rmap_item->hlist.next)
1401 		ksm_pages_sharing++;
1402 	else
1403 		ksm_pages_shared++;
1404 }
1405 
1406 /*
1407  * cmp_and_merge_page - first see if page can be merged into the stable tree;
1408  * if not, compare checksum to previous and if it's the same, see if page can
1409  * be inserted into the unstable tree, or merged with a page already there and
1410  * both transferred to the stable tree.
1411  *
1412  * @page: the page that we are searching identical page to.
1413  * @rmap_item: the reverse mapping into the virtual address of this page
1414  */
1415 static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
1416 {
1417 	struct rmap_item *tree_rmap_item;
1418 	struct page *tree_page = NULL;
1419 	struct stable_node *stable_node;
1420 	struct page *kpage;
1421 	unsigned int checksum;
1422 	int err;
1423 
1424 	stable_node = page_stable_node(page);
1425 	if (stable_node) {
1426 		if (stable_node->head != &migrate_nodes &&
1427 		    get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid)) {
1428 			rb_erase(&stable_node->node,
1429 				 root_stable_tree + NUMA(stable_node->nid));
1430 			stable_node->head = &migrate_nodes;
1431 			list_add(&stable_node->list, stable_node->head);
1432 		}
1433 		if (stable_node->head != &migrate_nodes &&
1434 		    rmap_item->head == stable_node)
1435 			return;
1436 	}
1437 
1438 	/* We first start with searching the page inside the stable tree */
1439 	kpage = stable_tree_search(page);
1440 	if (kpage == page && rmap_item->head == stable_node) {
1441 		put_page(kpage);
1442 		return;
1443 	}
1444 
1445 	remove_rmap_item_from_tree(rmap_item);
1446 
1447 	if (kpage) {
1448 		err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
1449 		if (!err) {
1450 			/*
1451 			 * The page was successfully merged:
1452 			 * add its rmap_item to the stable tree.
1453 			 */
1454 			lock_page(kpage);
1455 			stable_tree_append(rmap_item, page_stable_node(kpage));
1456 			unlock_page(kpage);
1457 		}
1458 		put_page(kpage);
1459 		return;
1460 	}
1461 
1462 	/*
1463 	 * If the hash value of the page has changed from the last time
1464 	 * we calculated it, this page is changing frequently: therefore we
1465 	 * don't want to insert it in the unstable tree, and we don't want
1466 	 * to waste our time searching for something identical to it there.
1467 	 */
1468 	checksum = calc_checksum(page);
1469 	if (rmap_item->oldchecksum != checksum) {
1470 		rmap_item->oldchecksum = checksum;
1471 		return;
1472 	}
1473 
1474 	tree_rmap_item =
1475 		unstable_tree_search_insert(rmap_item, page, &tree_page);
1476 	if (tree_rmap_item) {
1477 		kpage = try_to_merge_two_pages(rmap_item, page,
1478 						tree_rmap_item, tree_page);
1479 		put_page(tree_page);
1480 		if (kpage) {
1481 			/*
1482 			 * The pages were successfully merged: insert new
1483 			 * node in the stable tree and add both rmap_items.
1484 			 */
1485 			lock_page(kpage);
1486 			stable_node = stable_tree_insert(kpage);
1487 			if (stable_node) {
1488 				stable_tree_append(tree_rmap_item, stable_node);
1489 				stable_tree_append(rmap_item, stable_node);
1490 			}
1491 			unlock_page(kpage);
1492 
1493 			/*
1494 			 * If we fail to insert the page into the stable tree,
1495 			 * we will have 2 virtual addresses that are pointing
1496 			 * to a ksm page left outside the stable tree,
1497 			 * in which case we need to break_cow on both.
1498 			 */
1499 			if (!stable_node) {
1500 				break_cow(tree_rmap_item);
1501 				break_cow(rmap_item);
1502 			}
1503 		}
1504 	}
1505 }
1506 
1507 static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
1508 					    struct rmap_item **rmap_list,
1509 					    unsigned long addr)
1510 {
1511 	struct rmap_item *rmap_item;
1512 
1513 	while (*rmap_list) {
1514 		rmap_item = *rmap_list;
1515 		if ((rmap_item->address & PAGE_MASK) == addr)
1516 			return rmap_item;
1517 		if (rmap_item->address > addr)
1518 			break;
1519 		*rmap_list = rmap_item->rmap_list;
1520 		remove_rmap_item_from_tree(rmap_item);
1521 		free_rmap_item(rmap_item);
1522 	}
1523 
1524 	rmap_item = alloc_rmap_item();
1525 	if (rmap_item) {
1526 		/* It has already been zeroed */
1527 		rmap_item->mm = mm_slot->mm;
1528 		rmap_item->address = addr;
1529 		rmap_item->rmap_list = *rmap_list;
1530 		*rmap_list = rmap_item;
1531 	}
1532 	return rmap_item;
1533 }
1534 
1535 static struct rmap_item *scan_get_next_rmap_item(struct page **page)
1536 {
1537 	struct mm_struct *mm;
1538 	struct mm_slot *slot;
1539 	struct vm_area_struct *vma;
1540 	struct rmap_item *rmap_item;
1541 	int nid;
1542 
1543 	if (list_empty(&ksm_mm_head.mm_list))
1544 		return NULL;
1545 
1546 	slot = ksm_scan.mm_slot;
1547 	if (slot == &ksm_mm_head) {
1548 		/*
1549 		 * A number of pages can hang around indefinitely on per-cpu
1550 		 * pagevecs, raised page count preventing write_protect_page
1551 		 * from merging them.  Though it doesn't really matter much,
1552 		 * it is puzzling to see some stuck in pages_volatile until
1553 		 * other activity jostles them out, and they also prevented
1554 		 * LTP's KSM test from succeeding deterministically; so drain
1555 		 * them here (here rather than on entry to ksm_do_scan(),
1556 		 * so we don't IPI too often when pages_to_scan is set low).
1557 		 */
1558 		lru_add_drain_all();
1559 
1560 		/*
1561 		 * Whereas stale stable_nodes on the stable_tree itself
1562 		 * get pruned in the regular course of stable_tree_search(),
1563 		 * those moved out to the migrate_nodes list can accumulate:
1564 		 * so prune them once before each full scan.
1565 		 */
1566 		if (!ksm_merge_across_nodes) {
1567 			struct stable_node *stable_node;
1568 			struct list_head *this, *next;
1569 			struct page *page;
1570 
1571 			list_for_each_safe(this, next, &migrate_nodes) {
1572 				stable_node = list_entry(this,
1573 						struct stable_node, list);
1574 				page = get_ksm_page(stable_node, false);
1575 				if (page)
1576 					put_page(page);
1577 				cond_resched();
1578 			}
1579 		}
1580 
1581 		for (nid = 0; nid < ksm_nr_node_ids; nid++)
1582 			root_unstable_tree[nid] = RB_ROOT;
1583 
1584 		spin_lock(&ksm_mmlist_lock);
1585 		slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
1586 		ksm_scan.mm_slot = slot;
1587 		spin_unlock(&ksm_mmlist_lock);
1588 		/*
1589 		 * Although we tested list_empty() above, a racing __ksm_exit
1590 		 * of the last mm on the list may have removed it since then.
1591 		 */
1592 		if (slot == &ksm_mm_head)
1593 			return NULL;
1594 next_mm:
1595 		ksm_scan.address = 0;
1596 		ksm_scan.rmap_list = &slot->rmap_list;
1597 	}
1598 
1599 	mm = slot->mm;
1600 	down_read(&mm->mmap_sem);
1601 	if (ksm_test_exit(mm))
1602 		vma = NULL;
1603 	else
1604 		vma = find_vma(mm, ksm_scan.address);
1605 
1606 	for (; vma; vma = vma->vm_next) {
1607 		if (!(vma->vm_flags & VM_MERGEABLE))
1608 			continue;
1609 		if (ksm_scan.address < vma->vm_start)
1610 			ksm_scan.address = vma->vm_start;
1611 		if (!vma->anon_vma)
1612 			ksm_scan.address = vma->vm_end;
1613 
1614 		while (ksm_scan.address < vma->vm_end) {
1615 			if (ksm_test_exit(mm))
1616 				break;
1617 			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
1618 			if (IS_ERR_OR_NULL(*page)) {
1619 				ksm_scan.address += PAGE_SIZE;
1620 				cond_resched();
1621 				continue;
1622 			}
1623 			if (PageAnon(*page) ||
1624 			    page_trans_compound_anon(*page)) {
1625 				flush_anon_page(vma, *page, ksm_scan.address);
1626 				flush_dcache_page(*page);
1627 				rmap_item = get_next_rmap_item(slot,
1628 					ksm_scan.rmap_list, ksm_scan.address);
1629 				if (rmap_item) {
1630 					ksm_scan.rmap_list =
1631 							&rmap_item->rmap_list;
1632 					ksm_scan.address += PAGE_SIZE;
1633 				} else
1634 					put_page(*page);
1635 				up_read(&mm->mmap_sem);
1636 				return rmap_item;
1637 			}
1638 			put_page(*page);
1639 			ksm_scan.address += PAGE_SIZE;
1640 			cond_resched();
1641 		}
1642 	}
1643 
1644 	if (ksm_test_exit(mm)) {
1645 		ksm_scan.address = 0;
1646 		ksm_scan.rmap_list = &slot->rmap_list;
1647 	}
1648 	/*
1649 	 * Nuke all the rmap_items that are above this current rmap:
1650 	 * because there were no VM_MERGEABLE vmas with such addresses.
1651 	 */
1652 	remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
1653 
1654 	spin_lock(&ksm_mmlist_lock);
1655 	ksm_scan.mm_slot = list_entry(slot->mm_list.next,
1656 						struct mm_slot, mm_list);
1657 	if (ksm_scan.address == 0) {
1658 		/*
1659 		 * We've completed a full scan of all vmas, holding mmap_sem
1660 		 * throughout, and found no VM_MERGEABLE: so do the same as
1661 		 * __ksm_exit does to remove this mm from all our lists now.
1662 		 * This applies either when cleaning up after __ksm_exit
1663 		 * (but beware: we can reach here even before __ksm_exit),
1664 		 * or when all VM_MERGEABLE areas have been unmapped (and
1665 		 * mmap_sem then protects against race with MADV_MERGEABLE).
1666 		 */
1667 		hash_del(&slot->link);
1668 		list_del(&slot->mm_list);
1669 		spin_unlock(&ksm_mmlist_lock);
1670 
1671 		free_mm_slot(slot);
1672 		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
1673 		up_read(&mm->mmap_sem);
1674 		mmdrop(mm);
1675 	} else {
1676 		spin_unlock(&ksm_mmlist_lock);
1677 		up_read(&mm->mmap_sem);
1678 	}
1679 
1680 	/* Repeat until we've completed scanning the whole list */
1681 	slot = ksm_scan.mm_slot;
1682 	if (slot != &ksm_mm_head)
1683 		goto next_mm;
1684 
1685 	ksm_scan.seqnr++;
1686 	return NULL;
1687 }
1688 
1689 /**
1690  * ksm_do_scan  - the ksm scanner main worker function.
1691  * @scan_npages - number of pages we want to scan before we return.
1692  */
1693 static void ksm_do_scan(unsigned int scan_npages)
1694 {
1695 	struct rmap_item *rmap_item;
1696 	struct page *uninitialized_var(page);
1697 
1698 	while (scan_npages-- && likely(!freezing(current))) {
1699 		cond_resched();
1700 		rmap_item = scan_get_next_rmap_item(&page);
1701 		if (!rmap_item)
1702 			return;
1703 		cmp_and_merge_page(page, rmap_item);
1704 		put_page(page);
1705 	}
1706 }
1707 
1708 static int ksmd_should_run(void)
1709 {
1710 	return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
1711 }
1712 
1713 static int ksm_scan_thread(void *nothing)
1714 {
1715 	set_freezable();
1716 	set_user_nice(current, 5);
1717 
1718 	while (!kthread_should_stop()) {
1719 		mutex_lock(&ksm_thread_mutex);
1720 		wait_while_offlining();
1721 		if (ksmd_should_run())
1722 			ksm_do_scan(ksm_thread_pages_to_scan);
1723 		mutex_unlock(&ksm_thread_mutex);
1724 
1725 		try_to_freeze();
1726 
1727 		if (ksmd_should_run()) {
1728 			schedule_timeout_interruptible(
1729 				msecs_to_jiffies(ksm_thread_sleep_millisecs));
1730 		} else {
1731 			wait_event_freezable(ksm_thread_wait,
1732 				ksmd_should_run() || kthread_should_stop());
1733 		}
1734 	}
1735 	return 0;
1736 }
1737 
1738 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
1739 		unsigned long end, int advice, unsigned long *vm_flags)
1740 {
1741 	struct mm_struct *mm = vma->vm_mm;
1742 	int err;
1743 
1744 	switch (advice) {
1745 	case MADV_MERGEABLE:
1746 		/*
1747 		 * Be somewhat over-protective for now!
1748 		 */
1749 		if (*vm_flags & (VM_MERGEABLE | VM_SHARED  | VM_MAYSHARE   |
1750 				 VM_PFNMAP    | VM_IO      | VM_DONTEXPAND |
1751 				 VM_HUGETLB | VM_NONLINEAR | VM_MIXEDMAP))
1752 			return 0;		/* just ignore the advice */
1753 
1754 #ifdef VM_SAO
1755 		if (*vm_flags & VM_SAO)
1756 			return 0;
1757 #endif
1758 
1759 		if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
1760 			err = __ksm_enter(mm);
1761 			if (err)
1762 				return err;
1763 		}
1764 
1765 		*vm_flags |= VM_MERGEABLE;
1766 		break;
1767 
1768 	case MADV_UNMERGEABLE:
1769 		if (!(*vm_flags & VM_MERGEABLE))
1770 			return 0;		/* just ignore the advice */
1771 
1772 		if (vma->anon_vma) {
1773 			err = unmerge_ksm_pages(vma, start, end);
1774 			if (err)
1775 				return err;
1776 		}
1777 
1778 		*vm_flags &= ~VM_MERGEABLE;
1779 		break;
1780 	}
1781 
1782 	return 0;
1783 }
1784 
1785 int __ksm_enter(struct mm_struct *mm)
1786 {
1787 	struct mm_slot *mm_slot;
1788 	int needs_wakeup;
1789 
1790 	mm_slot = alloc_mm_slot();
1791 	if (!mm_slot)
1792 		return -ENOMEM;
1793 
1794 	/* Check ksm_run too?  Would need tighter locking */
1795 	needs_wakeup = list_empty(&ksm_mm_head.mm_list);
1796 
1797 	spin_lock(&ksm_mmlist_lock);
1798 	insert_to_mm_slots_hash(mm, mm_slot);
1799 	/*
1800 	 * When KSM_RUN_MERGE (or KSM_RUN_STOP),
1801 	 * insert just behind the scanning cursor, to let the area settle
1802 	 * down a little; when fork is followed by immediate exec, we don't
1803 	 * want ksmd to waste time setting up and tearing down an rmap_list.
1804 	 *
1805 	 * But when KSM_RUN_UNMERGE, it's important to insert ahead of its
1806 	 * scanning cursor, otherwise KSM pages in newly forked mms will be
1807 	 * missed: then we might as well insert at the end of the list.
1808 	 */
1809 	if (ksm_run & KSM_RUN_UNMERGE)
1810 		list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
1811 	else
1812 		list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
1813 	spin_unlock(&ksm_mmlist_lock);
1814 
1815 	set_bit(MMF_VM_MERGEABLE, &mm->flags);
1816 	atomic_inc(&mm->mm_count);
1817 
1818 	if (needs_wakeup)
1819 		wake_up_interruptible(&ksm_thread_wait);
1820 
1821 	return 0;
1822 }
1823 
1824 void __ksm_exit(struct mm_struct *mm)
1825 {
1826 	struct mm_slot *mm_slot;
1827 	int easy_to_free = 0;
1828 
1829 	/*
1830 	 * This process is exiting: if it's straightforward (as is the
1831 	 * case when ksmd was never running), free mm_slot immediately.
1832 	 * But if it's at the cursor or has rmap_items linked to it, use
1833 	 * mmap_sem to synchronize with any break_cows before pagetables
1834 	 * are freed, and leave the mm_slot on the list for ksmd to free.
1835 	 * Beware: ksm may already have noticed it exiting and freed the slot.
1836 	 */
1837 
1838 	spin_lock(&ksm_mmlist_lock);
1839 	mm_slot = get_mm_slot(mm);
1840 	if (mm_slot && ksm_scan.mm_slot != mm_slot) {
1841 		if (!mm_slot->rmap_list) {
1842 			hash_del(&mm_slot->link);
1843 			list_del(&mm_slot->mm_list);
1844 			easy_to_free = 1;
1845 		} else {
1846 			list_move(&mm_slot->mm_list,
1847 				  &ksm_scan.mm_slot->mm_list);
1848 		}
1849 	}
1850 	spin_unlock(&ksm_mmlist_lock);
1851 
1852 	if (easy_to_free) {
1853 		free_mm_slot(mm_slot);
1854 		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
1855 		mmdrop(mm);
1856 	} else if (mm_slot) {
1857 		down_write(&mm->mmap_sem);
1858 		up_write(&mm->mmap_sem);
1859 	}
1860 }
1861 
1862 struct page *ksm_might_need_to_copy(struct page *page,
1863 			struct vm_area_struct *vma, unsigned long address)
1864 {
1865 	struct anon_vma *anon_vma = page_anon_vma(page);
1866 	struct page *new_page;
1867 
1868 	if (PageKsm(page)) {
1869 		if (page_stable_node(page) &&
1870 		    !(ksm_run & KSM_RUN_UNMERGE))
1871 			return page;	/* no need to copy it */
1872 	} else if (!anon_vma) {
1873 		return page;		/* no need to copy it */
1874 	} else if (anon_vma->root == vma->anon_vma->root &&
1875 		 page->index == linear_page_index(vma, address)) {
1876 		return page;		/* still no need to copy it */
1877 	}
1878 	if (!PageUptodate(page))
1879 		return page;		/* let do_swap_page report the error */
1880 
1881 	new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
1882 	if (new_page) {
1883 		copy_user_highpage(new_page, page, address, vma);
1884 
1885 		SetPageDirty(new_page);
1886 		__SetPageUptodate(new_page);
1887 		__set_page_locked(new_page);
1888 	}
1889 
1890 	return new_page;
1891 }
1892 
1893 int rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
1894 {
1895 	struct stable_node *stable_node;
1896 	struct rmap_item *rmap_item;
1897 	int ret = SWAP_AGAIN;
1898 	int search_new_forks = 0;
1899 
1900 	VM_BUG_ON_PAGE(!PageKsm(page), page);
1901 
1902 	/*
1903 	 * Rely on the page lock to protect against concurrent modifications
1904 	 * to that page's node of the stable tree.
1905 	 */
1906 	VM_BUG_ON_PAGE(!PageLocked(page), page);
1907 
1908 	stable_node = page_stable_node(page);
1909 	if (!stable_node)
1910 		return ret;
1911 again:
1912 	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
1913 		struct anon_vma *anon_vma = rmap_item->anon_vma;
1914 		struct anon_vma_chain *vmac;
1915 		struct vm_area_struct *vma;
1916 
1917 		anon_vma_lock_read(anon_vma);
1918 		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
1919 					       0, ULONG_MAX) {
1920 			vma = vmac->vma;
1921 			if (rmap_item->address < vma->vm_start ||
1922 			    rmap_item->address >= vma->vm_end)
1923 				continue;
1924 			/*
1925 			 * Initially we examine only the vma which covers this
1926 			 * rmap_item; but later, if there is still work to do,
1927 			 * we examine covering vmas in other mms: in case they
1928 			 * were forked from the original since ksmd passed.
1929 			 */
1930 			if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
1931 				continue;
1932 
1933 			if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
1934 				continue;
1935 
1936 			ret = rwc->rmap_one(page, vma,
1937 					rmap_item->address, rwc->arg);
1938 			if (ret != SWAP_AGAIN) {
1939 				anon_vma_unlock_read(anon_vma);
1940 				goto out;
1941 			}
1942 			if (rwc->done && rwc->done(page)) {
1943 				anon_vma_unlock_read(anon_vma);
1944 				goto out;
1945 			}
1946 		}
1947 		anon_vma_unlock_read(anon_vma);
1948 	}
1949 	if (!search_new_forks++)
1950 		goto again;
1951 out:
1952 	return ret;
1953 }
1954 
1955 #ifdef CONFIG_MIGRATION
1956 void ksm_migrate_page(struct page *newpage, struct page *oldpage)
1957 {
1958 	struct stable_node *stable_node;
1959 
1960 	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
1961 	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
1962 	VM_BUG_ON_PAGE(newpage->mapping != oldpage->mapping, newpage);
1963 
1964 	stable_node = page_stable_node(newpage);
1965 	if (stable_node) {
1966 		VM_BUG_ON_PAGE(stable_node->kpfn != page_to_pfn(oldpage), oldpage);
1967 		stable_node->kpfn = page_to_pfn(newpage);
1968 		/*
1969 		 * newpage->mapping was set in advance; now we need smp_wmb()
1970 		 * to make sure that the new stable_node->kpfn is visible
1971 		 * to get_ksm_page() before it can see that oldpage->mapping
1972 		 * has gone stale (or that PageSwapCache has been cleared).
1973 		 */
1974 		smp_wmb();
1975 		set_page_stable_node(oldpage, NULL);
1976 	}
1977 }
1978 #endif /* CONFIG_MIGRATION */
1979 
1980 #ifdef CONFIG_MEMORY_HOTREMOVE
1981 static void wait_while_offlining(void)
1982 {
1983 	while (ksm_run & KSM_RUN_OFFLINE) {
1984 		mutex_unlock(&ksm_thread_mutex);
1985 		wait_on_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE),
1986 			    TASK_UNINTERRUPTIBLE);
1987 		mutex_lock(&ksm_thread_mutex);
1988 	}
1989 }
1990 
1991 static void ksm_check_stable_tree(unsigned long start_pfn,
1992 				  unsigned long end_pfn)
1993 {
1994 	struct stable_node *stable_node;
1995 	struct list_head *this, *next;
1996 	struct rb_node *node;
1997 	int nid;
1998 
1999 	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
2000 		node = rb_first(root_stable_tree + nid);
2001 		while (node) {
2002 			stable_node = rb_entry(node, struct stable_node, node);
2003 			if (stable_node->kpfn >= start_pfn &&
2004 			    stable_node->kpfn < end_pfn) {
2005 				/*
2006 				 * Don't get_ksm_page, page has already gone:
2007 				 * which is why we keep kpfn instead of page*
2008 				 */
2009 				remove_node_from_stable_tree(stable_node);
2010 				node = rb_first(root_stable_tree + nid);
2011 			} else
2012 				node = rb_next(node);
2013 			cond_resched();
2014 		}
2015 	}
2016 	list_for_each_safe(this, next, &migrate_nodes) {
2017 		stable_node = list_entry(this, struct stable_node, list);
2018 		if (stable_node->kpfn >= start_pfn &&
2019 		    stable_node->kpfn < end_pfn)
2020 			remove_node_from_stable_tree(stable_node);
2021 		cond_resched();
2022 	}
2023 }
2024 
2025 static int ksm_memory_callback(struct notifier_block *self,
2026 			       unsigned long action, void *arg)
2027 {
2028 	struct memory_notify *mn = arg;
2029 
2030 	switch (action) {
2031 	case MEM_GOING_OFFLINE:
2032 		/*
2033 		 * Prevent ksm_do_scan(), unmerge_and_remove_all_rmap_items()
2034 		 * and remove_all_stable_nodes() while memory is going offline:
2035 		 * it is unsafe for them to touch the stable tree at this time.
2036 		 * But unmerge_ksm_pages(), rmap lookups and other entry points
2037 		 * which do not need the ksm_thread_mutex are all safe.
2038 		 */
2039 		mutex_lock(&ksm_thread_mutex);
2040 		ksm_run |= KSM_RUN_OFFLINE;
2041 		mutex_unlock(&ksm_thread_mutex);
2042 		break;
2043 
2044 	case MEM_OFFLINE:
2045 		/*
2046 		 * Most of the work is done by page migration; but there might
2047 		 * be a few stable_nodes left over, still pointing to struct
2048 		 * pages which have been offlined: prune those from the tree,
2049 		 * otherwise get_ksm_page() might later try to access a
2050 		 * non-existent struct page.
2051 		 */
2052 		ksm_check_stable_tree(mn->start_pfn,
2053 				      mn->start_pfn + mn->nr_pages);
2054 		/* fallthrough */
2055 
2056 	case MEM_CANCEL_OFFLINE:
2057 		mutex_lock(&ksm_thread_mutex);
2058 		ksm_run &= ~KSM_RUN_OFFLINE;
2059 		mutex_unlock(&ksm_thread_mutex);
2060 
2061 		smp_mb();	/* wake_up_bit advises this */
2062 		wake_up_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE));
2063 		break;
2064 	}
2065 	return NOTIFY_OK;
2066 }
2067 #else
2068 static void wait_while_offlining(void)
2069 {
2070 }
2071 #endif /* CONFIG_MEMORY_HOTREMOVE */
2072 
2073 #ifdef CONFIG_SYSFS
2074 /*
2075  * This all compiles without CONFIG_SYSFS, but is a waste of space.
2076  */
2077 
2078 #define KSM_ATTR_RO(_name) \
2079 	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
2080 #define KSM_ATTR(_name) \
2081 	static struct kobj_attribute _name##_attr = \
2082 		__ATTR(_name, 0644, _name##_show, _name##_store)
2083 
2084 static ssize_t sleep_millisecs_show(struct kobject *kobj,
2085 				    struct kobj_attribute *attr, char *buf)
2086 {
2087 	return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
2088 }
2089 
2090 static ssize_t sleep_millisecs_store(struct kobject *kobj,
2091 				     struct kobj_attribute *attr,
2092 				     const char *buf, size_t count)
2093 {
2094 	unsigned long msecs;
2095 	int err;
2096 
2097 	err = kstrtoul(buf, 10, &msecs);
2098 	if (err || msecs > UINT_MAX)
2099 		return -EINVAL;
2100 
2101 	ksm_thread_sleep_millisecs = msecs;
2102 
2103 	return count;
2104 }
2105 KSM_ATTR(sleep_millisecs);
2106 
2107 static ssize_t pages_to_scan_show(struct kobject *kobj,
2108 				  struct kobj_attribute *attr, char *buf)
2109 {
2110 	return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
2111 }
2112 
2113 static ssize_t pages_to_scan_store(struct kobject *kobj,
2114 				   struct kobj_attribute *attr,
2115 				   const char *buf, size_t count)
2116 {
2117 	int err;
2118 	unsigned long nr_pages;
2119 
2120 	err = kstrtoul(buf, 10, &nr_pages);
2121 	if (err || nr_pages > UINT_MAX)
2122 		return -EINVAL;
2123 
2124 	ksm_thread_pages_to_scan = nr_pages;
2125 
2126 	return count;
2127 }
2128 KSM_ATTR(pages_to_scan);
2129 
2130 static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
2131 			char *buf)
2132 {
2133 	return sprintf(buf, "%lu\n", ksm_run);
2134 }
2135 
2136 static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
2137 			 const char *buf, size_t count)
2138 {
2139 	int err;
2140 	unsigned long flags;
2141 
2142 	err = kstrtoul(buf, 10, &flags);
2143 	if (err || flags > UINT_MAX)
2144 		return -EINVAL;
2145 	if (flags > KSM_RUN_UNMERGE)
2146 		return -EINVAL;
2147 
2148 	/*
2149 	 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
2150 	 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
2151 	 * breaking COW to free the pages_shared (but leaves mm_slots
2152 	 * on the list for when ksmd may be set running again).
2153 	 */
2154 
2155 	mutex_lock(&ksm_thread_mutex);
2156 	wait_while_offlining();
2157 	if (ksm_run != flags) {
2158 		ksm_run = flags;
2159 		if (flags & KSM_RUN_UNMERGE) {
2160 			set_current_oom_origin();
2161 			err = unmerge_and_remove_all_rmap_items();
2162 			clear_current_oom_origin();
2163 			if (err) {
2164 				ksm_run = KSM_RUN_STOP;
2165 				count = err;
2166 			}
2167 		}
2168 	}
2169 	mutex_unlock(&ksm_thread_mutex);
2170 
2171 	if (flags & KSM_RUN_MERGE)
2172 		wake_up_interruptible(&ksm_thread_wait);
2173 
2174 	return count;
2175 }
2176 KSM_ATTR(run);
2177 
2178 #ifdef CONFIG_NUMA
2179 static ssize_t merge_across_nodes_show(struct kobject *kobj,
2180 				struct kobj_attribute *attr, char *buf)
2181 {
2182 	return sprintf(buf, "%u\n", ksm_merge_across_nodes);
2183 }
2184 
2185 static ssize_t merge_across_nodes_store(struct kobject *kobj,
2186 				   struct kobj_attribute *attr,
2187 				   const char *buf, size_t count)
2188 {
2189 	int err;
2190 	unsigned long knob;
2191 
2192 	err = kstrtoul(buf, 10, &knob);
2193 	if (err)
2194 		return err;
2195 	if (knob > 1)
2196 		return -EINVAL;
2197 
2198 	mutex_lock(&ksm_thread_mutex);
2199 	wait_while_offlining();
2200 	if (ksm_merge_across_nodes != knob) {
2201 		if (ksm_pages_shared || remove_all_stable_nodes())
2202 			err = -EBUSY;
2203 		else if (root_stable_tree == one_stable_tree) {
2204 			struct rb_root *buf;
2205 			/*
2206 			 * This is the first time that we switch away from the
2207 			 * default of merging across nodes: must now allocate
2208 			 * a buffer to hold as many roots as may be needed.
2209 			 * Allocate stable and unstable together:
2210 			 * MAXSMP NODES_SHIFT 10 will use 16kB.
2211 			 */
2212 			buf = kcalloc(nr_node_ids + nr_node_ids, sizeof(*buf),
2213 				      GFP_KERNEL);
2214 			/* Let us assume that RB_ROOT is NULL is zero */
2215 			if (!buf)
2216 				err = -ENOMEM;
2217 			else {
2218 				root_stable_tree = buf;
2219 				root_unstable_tree = buf + nr_node_ids;
2220 				/* Stable tree is empty but not the unstable */
2221 				root_unstable_tree[0] = one_unstable_tree[0];
2222 			}
2223 		}
2224 		if (!err) {
2225 			ksm_merge_across_nodes = knob;
2226 			ksm_nr_node_ids = knob ? 1 : nr_node_ids;
2227 		}
2228 	}
2229 	mutex_unlock(&ksm_thread_mutex);
2230 
2231 	return err ? err : count;
2232 }
2233 KSM_ATTR(merge_across_nodes);
2234 #endif
2235 
2236 static ssize_t pages_shared_show(struct kobject *kobj,
2237 				 struct kobj_attribute *attr, char *buf)
2238 {
2239 	return sprintf(buf, "%lu\n", ksm_pages_shared);
2240 }
2241 KSM_ATTR_RO(pages_shared);
2242 
2243 static ssize_t pages_sharing_show(struct kobject *kobj,
2244 				  struct kobj_attribute *attr, char *buf)
2245 {
2246 	return sprintf(buf, "%lu\n", ksm_pages_sharing);
2247 }
2248 KSM_ATTR_RO(pages_sharing);
2249 
2250 static ssize_t pages_unshared_show(struct kobject *kobj,
2251 				   struct kobj_attribute *attr, char *buf)
2252 {
2253 	return sprintf(buf, "%lu\n", ksm_pages_unshared);
2254 }
2255 KSM_ATTR_RO(pages_unshared);
2256 
2257 static ssize_t pages_volatile_show(struct kobject *kobj,
2258 				   struct kobj_attribute *attr, char *buf)
2259 {
2260 	long ksm_pages_volatile;
2261 
2262 	ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
2263 				- ksm_pages_sharing - ksm_pages_unshared;
2264 	/*
2265 	 * It was not worth any locking to calculate that statistic,
2266 	 * but it might therefore sometimes be negative: conceal that.
2267 	 */
2268 	if (ksm_pages_volatile < 0)
2269 		ksm_pages_volatile = 0;
2270 	return sprintf(buf, "%ld\n", ksm_pages_volatile);
2271 }
2272 KSM_ATTR_RO(pages_volatile);
2273 
2274 static ssize_t full_scans_show(struct kobject *kobj,
2275 			       struct kobj_attribute *attr, char *buf)
2276 {
2277 	return sprintf(buf, "%lu\n", ksm_scan.seqnr);
2278 }
2279 KSM_ATTR_RO(full_scans);
2280 
2281 static struct attribute *ksm_attrs[] = {
2282 	&sleep_millisecs_attr.attr,
2283 	&pages_to_scan_attr.attr,
2284 	&run_attr.attr,
2285 	&pages_shared_attr.attr,
2286 	&pages_sharing_attr.attr,
2287 	&pages_unshared_attr.attr,
2288 	&pages_volatile_attr.attr,
2289 	&full_scans_attr.attr,
2290 #ifdef CONFIG_NUMA
2291 	&merge_across_nodes_attr.attr,
2292 #endif
2293 	NULL,
2294 };
2295 
2296 static struct attribute_group ksm_attr_group = {
2297 	.attrs = ksm_attrs,
2298 	.name = "ksm",
2299 };
2300 #endif /* CONFIG_SYSFS */
2301 
2302 static int __init ksm_init(void)
2303 {
2304 	struct task_struct *ksm_thread;
2305 	int err;
2306 
2307 	err = ksm_slab_init();
2308 	if (err)
2309 		goto out;
2310 
2311 	ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
2312 	if (IS_ERR(ksm_thread)) {
2313 		pr_err("ksm: creating kthread failed\n");
2314 		err = PTR_ERR(ksm_thread);
2315 		goto out_free;
2316 	}
2317 
2318 #ifdef CONFIG_SYSFS
2319 	err = sysfs_create_group(mm_kobj, &ksm_attr_group);
2320 	if (err) {
2321 		pr_err("ksm: register sysfs failed\n");
2322 		kthread_stop(ksm_thread);
2323 		goto out_free;
2324 	}
2325 #else
2326 	ksm_run = KSM_RUN_MERGE;	/* no way for user to start it */
2327 
2328 #endif /* CONFIG_SYSFS */
2329 
2330 #ifdef CONFIG_MEMORY_HOTREMOVE
2331 	/* There is no significance to this priority 100 */
2332 	hotplug_memory_notifier(ksm_memory_callback, 100);
2333 #endif
2334 	return 0;
2335 
2336 out_free:
2337 	ksm_slab_free();
2338 out:
2339 	return err;
2340 }
2341 subsys_initcall(ksm_init);
2342