aboutsummaryrefslogtreecommitdiff
path: root/mm/ksm.c
blob: 2849422448a3f5314bb05f9430c42358f2884b06 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
/*
 * Memory merging support.
 *
 * This code enables dynamic sharing of identical pages found in different
 * memory areas, even if they are not shared by fork()
 *
 * Copyright (C) 2008-2009 Red Hat, Inc.
 * Authors:
 *	Izik Eidus
 *	Andrea Arcangeli
 *	Chris Wright
 *	Hugh Dickins
 *
 * This work is licensed under the terms of the GNU GPL, version 2.
 */

#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/mman.h>
#include <linux/sched.h>
#include <linux/rwsem.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/spinlock.h>
#include <linux/jhash.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/wait.h>
#include <linux/slab.h>
#include <linux/rbtree.h>
#include <linux/mmu_notifier.h>
#include <linux/ksm.h>

#include <asm/tlbflush.h>

/*
 * A few notes about the KSM scanning process,
 * to make it easier to understand the data structures below:
 *
 * In order to reduce excessive scanning, KSM sorts the memory pages by their
 * contents into a data structure that holds pointers to the pages' locations.
 *
 * Since the contents of the pages may change at any moment, KSM cannot just
 * insert the pages into a normal sorted tree and expect it to find anything.
 * Therefore KSM uses two data structures - the stable and the unstable tree.
 *
 * The stable tree holds pointers to all the merged pages (ksm pages), sorted
 * by their contents.  Because each such page is write-protected, searching on
 * this tree is fully assured to be working (except when pages are unmapped),
 * and therefore this tree is called the stable tree.
 *
 * In addition to the stable tree, KSM uses a second data structure called the
 * unstable tree: this tree holds pointers to pages which have been found to
 * be "unchanged for a period of time".  The unstable tree sorts these pages
 * by their contents, but since they are not write-protected, KSM cannot rely
 * upon the unstable tree to work correctly - the unstable tree is liable to
 * be corrupted as its contents are modified, and so it is called unstable.
 *
 * KSM solves this problem by several techniques:
 *
 * 1) The unstable tree is flushed every time KSM completes scanning all
 *    memory areas, and then the tree is rebuilt again from the beginning.
 * 2) KSM will only insert into the unstable tree, pages whose hash value
 *    has not changed since the previous scan of all memory areas.
 * 3) The unstable tree is a RedBlack Tree - so its balancing is based on the
 *    colors of the nodes and not on their contents, assuring that even when
 *    the tree gets "corrupted" it won't get out of balance, so scanning time
 *    remains the same (also, searching and inserting nodes in an rbtree uses
 *    the same algorithm, so we have no overhead when we flush and rebuild).
 * 4) KSM never flushes the stable tree, which means that even if it were to
 *    take 10 attempts to find a page in the unstable tree, once it is found,
 *    it is secured in the stable tree.  (When we scan a new page, we first
 *    compare it against the stable tree, and then against the unstable tree.)
 */

/**
 * struct mm_slot - ksm information per mm that is being scanned
 * @link: link to the mm_slots hash list
 * @mm_list: link into the mm_slots list, rooted in ksm_mm_head
 * @rmap_list: head for this mm_slot's list of rmap_items
 * @mm: the mm that this information is valid for
 */
struct mm_slot {
	struct hlist_node link;
	struct list_head mm_list;
	struct list_head rmap_list;
	struct mm_struct *mm;
};

/**
 * struct ksm_scan - cursor for scanning
 * @mm_slot: the current mm_slot we are scanning
 * @address: the next address inside that to be scanned
 * @rmap_item: the current rmap that we are scanning inside the rmap_list
 * @seqnr: count of completed full scans (needed when removing unstable node)
 *
 * There is only the one ksm_scan instance of this cursor structure.
 */
struct ksm_scan {
	struct mm_slot *mm_slot;
	unsigned long address;
	struct rmap_item *rmap_item;
	unsigned long seqnr;
};

/**
 * struct rmap_item - reverse mapping item for virtual addresses
 * @link: link into mm_slot's rmap_list (rmap_list is per mm)
 * @mm: the memory structure this rmap_item is pointing into
 * @address: the virtual address this rmap_item tracks (+ flags in low bits)
 * @oldchecksum: previous checksum of the page at that virtual address
 * @node: rb_node of this rmap_item in either unstable or stable tree
 * @next: next rmap_item hanging off the same node of the stable tree
 * @prev: previous rmap_item hanging off the same node of the stable tree
 */
struct rmap_item {
	struct list_head link;
	struct mm_struct *mm;
	unsigned long address;		/* + low bits used for flags below */
	union {
		unsigned int oldchecksum;		/* when unstable */
		struct rmap_item *next;			/* when stable */
	};
	union {
		struct rb_node node;			/* when tree node */
		struct rmap_item *prev;			/* in stable list */
	};
};

#define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
#define NODE_FLAG	0x100	/* is a node of unstable or stable tree */
#define STABLE_FLAG	0x200	/* is a node or list item of stable tree */

/* The stable and unstable tree heads */
static struct rb_root root_stable_tree = RB_ROOT;
static struct rb_root root_unstable_tree = RB_ROOT;

#define MM_SLOTS_HASH_HEADS 1024
static struct hlist_head *mm_slots_hash;

static struct mm_slot ksm_mm_head = {
	.mm_list = LIST_HEAD_INIT(ksm_mm_head.mm_list),
};
static struct ksm_scan ksm_scan = {
	.mm_slot = &ksm_mm_head,
};

static struct kmem_cache *rmap_item_cache;
static struct kmem_cache *mm_slot_cache;

/* The number of nodes in the stable tree */
static unsigned long ksm_pages_shared;

/* The number of page slots additionally sharing those nodes */
static unsigned long ksm_pages_sharing;

/* The number of nodes in the unstable tree */
static unsigned long ksm_pages_unshared;

/* The number of rmap_items in use: to calculate pages_volatile */
static unsigned long ksm_rmap_items;

/* Limit on the number of unswappable pages used */
static unsigned long ksm_max_kernel_pages;

/* Number of pages ksmd should scan in one batch */
static unsigned int ksm_thread_pages_to_scan;

/* Milliseconds ksmd should sleep between batches */
static unsigned int ksm_thread_sleep_millisecs;

#define KSM_RUN_STOP	0
#define KSM_RUN_MERGE	1
#define KSM_RUN_UNMERGE	2
static unsigned int ksm_run;

static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
static DEFINE_MUTEX(ksm_thread_mutex);
static DEFINE_SPINLOCK(ksm_mmlist_lock);

#define KSM_KMEM_CACHE(__struct, __flags) kmem_cache_create("ksm_"#__struct,\
		sizeof(struct __struct), __alignof__(struct __struct),\
		(__flags), NULL)

static int __init ksm_slab_init(void)
{
	rmap_item_cache = KSM_KMEM_CACHE(rmap_item, 0);
	if (!rmap_item_cache)
		goto out;

	mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
	if (!mm_slot_cache)
		goto out_free;

	return 0;

out_free:
	kmem_cache_destroy(rmap_item_cache);
out:
	return -ENOMEM;
}

static void __init ksm_slab_free(void)
{
	kmem_cache_destroy(mm_slot_cache);
	kmem_cache_destroy(rmap_item_cache);
	mm_slot_cache = NULL;
}

static inline struct rmap_item *alloc_rmap_item(void)
{
	struct rmap_item *rmap_item;

	rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
	if (rmap_item)
		ksm_rmap_items++;
	return rmap_item;
}

static inline void free_rmap_item(struct rmap_item *rmap_item)
{
	ksm_rmap_items--;
	rmap_item->mm = NULL;	/* debug safety */
	kmem_cache_free(rmap_item_cache, rmap_item);
}

static inline struct mm_slot *alloc_mm_slot(void)
{
	if (!mm_slot_cache)	/* initialization failed */
		return NULL;
	return kmem_cache_zalloc(mm_slot_cache, GFP_KERNEL);
}

static inline void free_mm_slot(struct mm_slot *mm_slot)
{
	kmem_cache_free(mm_slot_cache, mm_slot);
}

static int __init mm_slots_hash_init(void)
{
	mm_slots_hash = kzalloc(MM_SLOTS_HASH_HEADS * sizeof(struct hlist_head),
				GFP_KERNEL);
	if (!mm_slots_hash)
		return -ENOMEM;
	return 0;
}

static void __init mm_slots_hash_free(void)
{
	kfree(mm_slots_hash);
}

static struct mm_slot *get_mm_slot(struct mm_struct *mm)
{
	struct mm_slot *mm_slot;
	struct hlist_head *bucket;
	struct hlist_node *node;

	bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
				% MM_SLOTS_HASH_HEADS];
	hlist_for_each_entry(mm_slot, node, bucket, link) {
		if (mm == mm_slot->mm)
			return mm_slot;
	}
	return NULL;
}

static void insert_to_mm_slots_hash(struct mm_struct *mm,
				    struct mm_slot *mm_slot)
{
	struct hlist_head *bucket;

	bucket = &mm_slots_hash[((unsigned long)mm / sizeof(struct mm_struct))
				% MM_SLOTS_HASH_HEADS];
	mm_slot->mm = mm;
	INIT_LIST_HEAD(&mm_slot->rmap_list);
	hlist_add_head(&mm_slot->link, bucket);
}

static inline int in_stable_tree(struct rmap_item *rmap_item)
{
	return rmap_item->address & STABLE_FLAG;
}

/*
 * We use break_ksm to break COW on a ksm page: it's a stripped down
 *
 *	if (get_user_pages(current, mm, addr, 1, 1, 1, &page, NULL) == 1)
 *		put_page(page);
 *
 * but taking great care only to touch a ksm page, in a VM_MERGEABLE vma,
 * in case the application has unmapped and remapped mm,addr meanwhile.
 * Could a ksm page appear anywhere else?  Actually yes, in a VM_PFNMAP
 * mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
 */
static void break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
	struct page *page;
	int ret;

	do {
		cond_resched();
		page = follow_page(vma, addr, FOLL_GET);
		if (!page)
			break;
		if (PageKsm(page))
			ret = handle_mm_fault(vma->vm_mm, vma, addr,
							FAULT_FLAG_WRITE);
		else
			ret = VM_FAULT_WRITE;
		put_page(page);
	} while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS)));

	/* Which leaves us looping there if VM_FAULT_OOM: hmmm... */
}

static void __break_cow(struct mm_struct *mm, unsigned long addr)
{
	struct vm_area_struct *vma;

	vma = find_vma(mm, addr);
	if (!vma || vma->vm_start > addr)
		return;
	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
		return;
	break_ksm(vma, addr);
}

static void break_cow(struct mm_struct *mm, unsigned long addr)
{
	down_read(&mm->mmap_sem);
	__break_cow(mm, addr);
	up_read(&mm->mmap_sem);
}

static struct page *get_mergeable_page(struct rmap_item *rmap_item)
{
	struct mm_struct *mm = rmap_item->mm;
	unsigned long addr = rmap_item->address;
	struct vm_area_struct *vma;
	struct page *page;

	down_read(&mm->mmap_sem);
	vma = find_vma(mm, addr);
	if (!vma || vma->vm_start > addr)
		goto out;
	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
		goto out;

	page = follow_page(vma, addr, FOLL_GET);
	if (!page)
		goto out;
	if (PageAnon(page)) {
		flush_anon_page(vma, page, addr);
		flush_dcache_page(page);
	} else {
		put_page(page);
out:		page = NULL;
	}
	up_read(&mm->mmap_sem);
	return page;
}

/*
 * get_ksm_page: checks if the page at the virtual address in rmap_item
 * is still PageKsm, in which case we can trust the content of the page,
 * and it returns the gotten page; but NULL if the page has been zapped.
 */
static struct page *get_ksm_page(struct rmap_item *rmap_item)
{
	struct page *page;

	page = get_mergeable_page(rmap_item);
	if (page && !PageKsm(page)) {
		put_page(page);
		page = NULL;
	}
	return page;
}

/*
 * Removing rmap_item from stable or unstable tree.
 * This function will clean the information from the stable/unstable tree.
 */
static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
{
	if (in_stable_tree(rmap_item)) {
		struct rmap_item *next_item = rmap_item->next;

		if (rmap_item->address & NODE_FLAG) {
			if (next_item) {
				rb_replace_node(&rmap_item->node,
						&next_item->node,
						&root_stable_tree);
				next_item->address |= NODE_FLAG;
				ksm_pages_sharing--;
			} else {
				rb_erase(&rmap_item->node, &root_stable_tree);
				ksm_pages_shared--;
			}
		} else {
			struct rmap_item *prev_item = rmap_item->prev;

			BUG_ON(prev_item->next != rmap_item);
			prev_item->next = next_item;
			if (next_item) {
				BUG_ON(next_item->prev != rmap_item);
				next_item->prev = rmap_item->prev;
			}
			ksm_pages_sharing--;
		}

		rmap_item->next = NULL;

	} else if (rmap_item->address & NODE_FLAG) {
		unsigned char age;
		/*
		 * ksm_thread can and must skip the rb_erase, because
		 * root_unstable_tree was already reset to RB_ROOT.
		 * But __ksm_exit has to be careful: do the rb_erase
		 * if it's interrupting a scan, and this rmap_item was
		 * inserted by this scan rather than left from before.
		 *
		 * Because of the case in which remove_mm_from_lists
		 * increments seqnr before removing rmaps, unstable_nr
		 * may even be 2 behind seqnr, but should never be
		 * further behind.  Yes, I did have trouble with this!
		 */
		age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
		BUG_ON(age > 2);
		if (!age)
			rb_erase(&rmap_item->node, &root_unstable_tree);
		ksm_pages_unshared--;
	}

	rmap_item->address &= PAGE_MASK;

	cond_resched();		/* we're called from many long loops */
}

static void remove_all_slot_rmap_items(struct mm_slot *mm_slot)
{
	struct rmap_item *rmap_item, *node;

	list_for_each_entry_safe(rmap_item, node, &mm_slot->rmap_list, link) {
		remove_rmap_item_from_tree(rmap_item);
		list_del(&rmap_item->link);
		free_rmap_item(rmap_item);
	}
}

static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
				       struct list_head *cur)
{
	struct rmap_item *rmap_item;

	while (cur != &mm_slot->rmap_list) {
		rmap_item = list_entry(cur, struct rmap_item, link);
		cur = cur->next;
		remove_rmap_item_from_tree(rmap_item);
		list_del(&rmap_item->link);
		free_rmap_item(rmap_item);
	}
}

/*
 * Though it's very tempting to unmerge in_stable_tree(rmap_item)s rather
 * than check every pte of a given vma, the locking doesn't quite work for
 * that - an rmap_item is assigned to the stable tree after inserting ksm
 * page and upping mmap_sem.  Nor does it fit with the way we skip dup'ing
 * rmap_items from parent to child at fork time (so as not to waste time
 * if exit comes before the next scan reaches it).
 */
static void unmerge_ksm_pages(struct vm_area_struct *vma,
			      unsigned long start, unsigned long end)
{
	unsigned long addr;

	for (addr = start; addr < end; addr += PAGE_SIZE)
		break_ksm(vma, addr);
}

static void unmerge_and_remove_all_rmap_items(void)
{
	struct mm_slot *mm_slot;
	struct mm_struct *mm;
	struct vm_area_struct *vma;

	list_for_each_entry(mm_slot, &ksm_mm_head.mm_list, mm_list) {
		mm = mm_slot->mm;
		down_read(&mm->mmap_sem);
		for (vma = mm->mmap; vma; vma = vma->vm_next) {
			if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
				continue;
			unmerge_ksm_pages(vma, vma->vm_start, vma->vm_end);
		}
		remove_all_slot_rmap_items(mm_slot);
		up_read(&mm->mmap_sem);
	}

	spin_lock(&ksm_mmlist_lock);
	if (ksm_scan.mm_slot != &ksm_mm_head) {
		ksm_scan.mm_slot = &ksm_mm_head;
		ksm_scan.seqnr++;
	}
	spin_unlock(&ksm_mmlist_lock);
}

static void remove_mm_from_lists(struct mm_struct *mm)
{
	struct mm_slot *mm_slot;

	spin_lock(&ksm_mmlist_lock);
	mm_slot = get_mm_slot(mm);

	/*
	 * This mm_slot is always at the scanning cursor when we're
	 * called from scan_get_next_rmap_item; but it's a special
	 * case when we're called from __ksm_exit.
	 */
	if (ksm_scan.mm_slot == mm_slot) {
		ksm_scan.mm_slot = list_entry(
			mm_slot->mm_list.next, struct mm_slot, mm_list);
		ksm_scan.address = 0;
		ksm_scan.rmap_item = list_entry(
			&ksm_scan.mm_slot->rmap_list, struct rmap_item, link);
		if (ksm_scan.mm_slot == &ksm_mm_head)
			ksm_scan.seqnr++;
	}

	hlist_del(&mm_slot->link);
	list_del(&mm_slot->mm_list);
	spin_unlock(&ksm_mmlist_lock);

	remove_all_slot_rmap_items(mm_slot);
	free_mm_slot(mm_slot);
	clear_bit(MMF_VM_MERGEABLE, &mm->flags);
}

static u32 calc_checksum(struct page *page)
{
	u32 checksum;
	void *addr = kmap_atomic(page, KM_USER0);
	checksum = jhash2(addr, PAGE_SIZE / 4, 17);
	kunmap_atomic(addr, KM_USER0);
	return checksum;
}

static int memcmp_pages(struct page *page1, struct page *page2)
{
	char *addr1, *addr2;
	int ret;

	addr1 = kmap_atomic(page1, KM_USER0);
	addr2 = kmap_atomic(page2, KM_USER1);
	ret = memcmp(addr1, addr2, PAGE_SIZE);
	kunmap_atomic(addr2, KM_USER1);
	kunmap_atomic(addr1, KM_USER0);
	return ret;
}

static inline int pages_identical(struct page *page1, struct page *page2)
{
	return !memcmp_pages(page1, page2);
}

static int write_protect_page(struct vm_area_struct *vma, struct page *page,
			      pte_t *orig_pte)
{
	struct mm_struct *mm = vma->vm_mm;
	unsigned long addr;
	pte_t *ptep;
	spinlock_t *ptl;
	int swapped;
	int err = -EFAULT;

	addr = page_address_in_vma(page, vma);
	if (addr == -EFAULT)
		goto out;

	ptep = page_check_address(page, mm, addr, &ptl, 0);
	if (!ptep)
		goto out;

	if (pte_write(*ptep)) {
		pte_t entry;

		swapped = PageSwapCache(page);
		flush_cache_page(vma, addr, page_to_pfn(page));
		/*
		 * Ok this is tricky, when get_user_pages_fast() run it doesnt
		 * take any lock, therefore the check that we are going to make
		 * with the pagecount against the mapcount is racey and
		 * O_DIRECT can happen right after the check.
		 * So we clear the pte and flush the tlb before the check
		 * this assure us that no O_DIRECT can happen after the check
		 * or in the middle of the check.
		 */
		entry = ptep_clear_flush(vma, addr, ptep);
		/*
		 * Check that no O_DIRECT or similar I/O is in progress on the
		 * page
		 */
		if ((page_mapcount(page) + 2 + swapped) != page_count(page)) {
			set_pte_at_notify(mm, addr, ptep, entry);
			goto out_unlock;
		}
		entry = pte_wrprotect(entry);
		set_pte_at_notify(mm, addr, ptep, entry);
	}
	*orig_pte = *ptep;
	err = 0;

out_unlock:
	pte_unmap_unlock(ptep, ptl);
out:
	return err;
}

/**
 * replace_page - replace page in vma by new ksm page
 * @vma:      vma that holds the pte pointing to oldpage
 * @oldpage:  the page we are replacing by newpage
 * @newpage:  the ksm page we replace oldpage by
 * @orig_pte: the original value of the pte
 *
 * Returns 0 on success, -EFAULT on failure.
 */
static int replace_page(struct vm_area_struct *vma, struct page *oldpage,
			struct page *newpage, pte_t orig_pte)
{
	struct mm_struct *mm = vma->vm_mm;
	pgd_t *pgd;
	pud_t *pud;
	pmd_t *pmd;
	pte_t *ptep;
	spinlock_t *ptl;
	unsigned long addr;
	pgprot_t prot;
	int err = -EFAULT;

	prot = vm_get_page_prot(vma->vm_flags & ~VM_WRITE);

	addr = page_address_in_vma(oldpage, vma);
	if (addr == -EFAULT)
		goto out;

	pgd = pgd_offset(mm, addr);
	if (!pgd_present(*pgd))
		goto out;

	pud = pud_offset(pgd, addr);
	if (!pud_present(*pud))
		goto out;

	pmd = pmd_offset(pud, addr);
	if (!pmd_present(*pmd))
		goto out;

	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
	if (!pte_same(*ptep, orig_pte)) {
		pte_unmap_unlock(ptep, ptl);
		goto out;
	}

	get_page(newpage);
	page_add_ksm_rmap(newpage);

	flush_cache_page(vma, addr, pte_pfn(*ptep));
	ptep_clear_flush(vma, addr, ptep);
	set_pte_at_notify(mm, addr, ptep, mk_pte(newpage, prot));

	page_remove_rmap(oldpage);
	put_page(oldpage);

	pte_unmap_unlock(ptep, ptl);
	err = 0;
out:
	return err;
}

/*
 * try_to_merge_one_page - take two pages and merge them into one
 * @vma: the vma that hold the pte pointing into oldpage
 * @oldpage: the page that we want to replace with newpage
 * @newpage: the page that we want to map instead of oldpage
 *
 * Note:
 * oldpage should be a PageAnon page, while newpage should be a PageKsm page,
 * or a newly allocated kernel page which page_add_ksm_rmap will make PageKsm.
 *
 * This function returns 0 if the pages were merged, -EFAULT otherwise.
 */
static int try_to_merge_one_page(struct vm_area_struct *vma,
				 struct page *oldpage,
				 struct page *newpage)
{
	pte_t orig_pte = __pte(0);
	int err = -EFAULT;

	if (!(vma->vm_flags & VM_MERGEABLE))
		goto out;

	if (!PageAnon(oldpage))
		goto out;

	get_page(newpage);
	get_page(oldpage);

	/*
	 * We need the page lock to read a stable PageSwapCache in
	 * write_protect_page().  We use trylock_page() instead of
	 * lock_page() because we don't want to wait here - we
	 * prefer to continue scanning and merging different pages,
	 * then come back to this page when it is unlocked.
	 */
	if (!trylock_page(oldpage))
		goto out_putpage;
	/*
	 * If this anonymous page is mapped only here, its pte may need
	 * to be write-protected.  If it's mapped elsewhere, all of its
	 * ptes are necessarily already write-protected.  But in either
	 * case, we need to lock and check page_count is not raised.
	 */
	if (write_protect_page(vma, oldpage, &orig_pte)) {
		unlock_page(oldpage);
		goto out_putpage;
	}
	unlock_page(oldpage);

	if (pages_identical(oldpage, newpage))
		err = replace_page(vma, oldpage, newpage, orig_pte);

out_putpage:
	put_page(oldpage);
	put_page(newpage);
out:
	return err;
}

/*
 * try_to_merge_two_pages - take two identical pages and prepare them
 * to be merged into one page.
 *
 * This function returns 0 if we successfully mapped two identical pages
 * into one page, -EFAULT otherwise.
 *
 * Note that this function allocates a new kernel page: if one of the pages
 * is already a ksm page, try_to_merge_with_ksm_page should be used.
 */
static int try_to_merge_two_pages(struct mm_struct *mm1, unsigned long addr1,
				  struct page *page1, struct mm_struct *mm2,
				  unsigned long addr2, struct page *page2)
{
	struct vm_area_struct *vma;
	struct page *kpage;
	int err = -EFAULT;

	/*
	 * The number of nodes in the stable tree
	 * is the number of kernel pages that we hold.
	 */
	if (ksm_max_kernel_pages &&
	    ksm_max_kernel_pages <= ksm_pages_shared)
		return err;

	kpage = alloc_page(GFP_HIGHUSER);
	if (!kpage)
		return err;

	down_read(&mm1->mmap_sem);
	vma = find_vma(mm1, addr1);
	if (!vma || vma->vm_start > addr1) {
		put_page(kpage);
		up_read(&mm1->mmap_sem);
		return err;
	}

	copy_user_highpage(kpage, page1, addr1, vma);
	err = try_to_merge_one_page(vma, page1, kpage);
	up_read(&mm1->mmap_sem);

	if (!err) {
		down_read(&mm2->mmap_sem);
		vma = find_vma(mm2, addr2);
		if (!vma || vma->vm_start > addr2) {
			put_page(kpage);
			up_read(&mm2->mmap_sem);
			break_cow(mm1, addr1);
			return -EFAULT;
		}

		err = try_to_merge_one_page(vma, page2, kpage);
		up_read(&mm2->mmap_sem);

		/*
		 * If the second try_to_merge_one_page failed, we have a
		 * ksm page with just one pte pointing to it, so break it.
		 */
		if (err)
			break_cow(mm1, addr1);
	}

	put_page(kpage);
	return err;
}

/*
 * try_to_merge_with_ksm_page - like try_to_merge_two_pages,
 * but no new kernel page is allocated: kpage must already be a ksm page.
 */
static int try_to_merge_with_ksm_page(struct mm_struct *mm1,
				      unsigned long addr1,
				      struct page *page1,
				      struct page *kpage)
{
	struct vm_area_struct *vma;
	int err = -EFAULT;

	down_read(&mm1->mmap_sem);
	vma = find_vma(mm1, addr1);
	if (!vma || vma->vm_start > addr1) {
		up_read(&mm1->mmap_sem);
		return err;
	}

	err = try_to_merge_one_page(vma, page1, kpage);
	up_read(&mm1->mmap_sem);

	return err;
}

/*
 * stable_tree_search - search page inside the stable tree
 * @page: the page that we are searching identical pages to.
 * @page2: pointer into identical page that we are holding inside the stable
 *	   tree that we have found.
 * @rmap_item: the reverse mapping item
 *
 * This function checks if there is a page inside the stable tree
 * with identical content to the page that we are scanning right now.
 *
 * This function return rmap_item pointer to the identical item if found,
 * NULL otherwise.
 */
static struct rmap_item *stable_tree_search(struct page *page,
					    struct page **page2,
					    struct rmap_item *rmap_item)
{
	struct rb_node *node = root_stable_tree.rb_node;

	while (node) {
		struct rmap_item *tree_rmap_item, *next_rmap_item;
		int ret;

		tree_rmap_item = rb_entry(node, struct rmap_item, node);
		while (tree_rmap_item) {
			BUG_ON(!in_stable_tree(tree_rmap_item));
			cond_resched();
			page2[0] = get_ksm_page(tree_rmap_item);
			if (page2[0])
				break;
			next_rmap_item = tree_rmap_item->next;
			remove_rmap_item_from_tree(tree_rmap_item);
			tree_rmap_item = next_rmap_item;
		}
		if (!tree_rmap_item)
			return NULL;

		ret = memcmp_pages(page, page2[0]);

		if (ret < 0) {
			put_page(page2[0]);
			node = node->rb_left;
		} else if (ret > 0) {
			put_page(page2[0]);
			node = node->rb_right;
		} else {
			return tree_rmap_item;
		}
	}

	return NULL;
}

/*
 * stable_tree_insert - insert rmap_item pointing to new ksm page
 * into the stable tree.
 *
 * @page: the page that we are searching identical page to inside the stable
 *	  tree.
 * @rmap_item: pointer to the reverse mapping item.
 *
 * This function returns rmap_item if success, NULL otherwise.
 */
static struct rmap_item *stable_tree_insert(struct page *page,
					    struct rmap_item *rmap_item)
{
	struct rb_node **new = &root_stable_tree.rb_node;
	struct rb_node *parent = NULL;

	while (*new) {
		struct rmap_item *tree_rmap_item, *next_rmap_item;
		struct page *tree_page;
		int ret;

		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
		while (tree_rmap_item) {
			BUG_ON(!in_stable_tree(tree_rmap_item));
			cond_resched();
			tree_page = get_ksm_page(tree_rmap_item);
			if (tree_page)
				break;
			next_rmap_item = tree_rmap_item->next;
			remove_rmap_item_from_tree(tree_rmap_item);
			tree_rmap_item = next_rmap_item;
		}
		if (!tree_rmap_item)
			return NULL;

		ret = memcmp_pages(page, tree_page);
		put_page(tree_page);

		parent = *new;
		if (ret < 0)
			new = &parent->rb_left;
		else if (ret > 0)
			new = &parent->rb_right;
		else {
			/*
			 * It is not a bug that stable_tree_search() didn't
			 * find this node: because at that time our page was
			 * not yet write-protected, so may have changed since.
			 */
			return NULL;
		}
	}

	rmap_item->address |= NODE_FLAG | STABLE_FLAG;
	rmap_item->next = NULL;
	rb_link_node(&rmap_item->node, parent, new);
	rb_insert_color(&rmap_item->node, &root_stable_tree);

	ksm_pages_shared++;
	return rmap_item;
}

/*
 * unstable_tree_search_insert - search and insert items into the unstable tree.
 *
 * @page: the page that we are going to search for identical page or to insert
 *	  into the unstable tree
 * @page2: pointer into identical page that was found inside the unstable tree
 * @rmap_item: the reverse mapping item of page
 *
 * This function searches for a page in the unstable tree identical to the
 * page currently being scanned; and if no identical page is found in the
 * tree, we insert rmap_item as a new object into the unstable tree.
 *
 * This function returns pointer to rmap_item found to be identical
 * to the currently scanned page, NULL otherwise.
 *
 * This function does both searching and inserting, because they share
 * the same walking algorithm in an rbtree.
 */
static struct rmap_item *unstable_tree_search_insert(struct page *page,
						struct page **page2,
						struct rmap_item *rmap_item)
{
	struct rb_node **new = &root_unstable_tree.rb_node;
	struct rb_node *parent = NULL;

	while (*new) {
		struct rmap_item *tree_rmap_item;
		int ret;

		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
		page2[0] = get_mergeable_page(tree_rmap_item);
		if (!page2[0])
			return NULL;

		/*
		 * Don't substitute an unswappable ksm page
		 * just for one good swappable forked page.
		 */
		if (page == page2[0]) {
			put_page(page2[0]);
			return NULL;
		}

		ret = memcmp_pages(page, page2[0]);

		parent = *new;
		if (ret < 0) {
			put_page(page2[0]);
			new = &parent->rb_left;
		} else if (ret > 0) {
			put_page(page2[0]);
			new = &parent->rb_right;
		} else {
			return tree_rmap_item;
		}
	}

	rmap_item->address |= NODE_FLAG;
	rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
	rb_link_node(&rmap_item->node, parent, new);
	rb_insert_color(&rmap_item->node, &root_unstable_tree);

	ksm_pages_unshared++;
	return NULL;
}

/*
 * stable_tree_append - add another rmap_item to the linked list of
 * rmap_items hanging off a given node of the stable tree, all sharing
 * the same ksm page.
 */
static void stable_tree_append(struct rmap_item *rmap_item,
			       struct rmap_item *tree_rmap_item)
{
	rmap_item->next = tree_rmap_item->next;
	rmap_item->prev = tree_rmap_item;

	if (tree_rmap_item->next)
		tree_rmap_item->next->prev = rmap_item;

	tree_rmap_item->next = rmap_item;
	rmap_item->address |= STABLE_FLAG;

	ksm_pages_sharing++;
}

/*
 * cmp_and_merge_page - take a page computes its hash value and check if there
 * is similar hash value to different page,
 * in case we find that there is similar hash to different page we call to
 * try_to_merge_two_pages().
 *
 * @page: the page that we are searching identical page to.
 * @rmap_item: the reverse mapping into the virtual address of this page
 */
static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
{
	struct page *page2[1];
	struct rmap_item *tree_rmap_item;
	unsigned int checksum;
	int err;

	if (in_stable_tree(rmap_item))
		remove_rmap_item_from_tree(rmap_item);

	/* We first start with searching the page inside the stable tree */
	tree_rmap_item = stable_tree_search(page, page2, rmap_item);
	if (tree_rmap_item) {
		if (page == page2[0])			/* forked */
			err = 0;
		else
			err = try_to_merge_with_ksm_page(rmap_item->mm,
							 rmap_item->address,
							 page, page2[0]);
		put_page(page2[0]);

		if (!err) {
			/*
			 * The page was successfully merged:
			 * add its rmap_item to the stable tree.
			 */
			stable_tree_append(rmap_item, tree_rmap_item);
		}
		return;
	}

	/*
	 * A ksm page might have got here by fork, but its other
	 * references have already been removed from the stable tree.
	 */
	if (PageKsm(page))
		break_cow(rmap_item->mm, rmap_item->address);

	/*
	 * In case the hash value of the page was changed from the last time we
	 * have calculated it, this page to be changed frequely, therefore we
	 * don't want to insert it to the unstable tree, and we don't want to
	 * waste our time to search if there is something identical to it there.
	 */
	checksum = calc_checksum(page);
	if (rmap_item->oldchecksum != checksum) {
		rmap_item->oldchecksum = checksum;
		return;
	}

	tree_rmap_item = unstable_tree_search_insert(page, page2, rmap_item);
	if (tree_rmap_item) {
		err = try_to_merge_two_pages(rmap_item->mm,
					     rmap_item->address, page,
					     tree_rmap_item->mm,
					     tree_rmap_item->address, page2[0]);
		/*
		 * As soon as we merge this page, we want to remove the
		 * rmap_item of the page we have merged with from the unstable
		 * tree, and insert it instead as new node in the stable tree.
		 */
		if (!err) {
			rb_erase(&tree_rmap_item->node, &root_unstable_tree);
			tree_rmap_item->address &= ~NODE_FLAG;
			ksm_pages_unshared--;

			/*
			 * If we fail to insert the page into the stable tree,
			 * we will have 2 virtual addresses that are pointing
			 * to a ksm page left outside the stable tree,
			 * in which case we need to break_cow on both.
			 */
			if (stable_tree_insert(page2[0], tree_rmap_item))
				stable_tree_append(rmap_item, tree_rmap_item);
			else {
				break_cow(tree_rmap_item->mm,
						tree_rmap_item->address);
				break_cow(rmap_item->mm, rmap_item->address);
			}
		}

		put_page(page2[0]);
	}
}

static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
					    struct list_head *cur,
					    unsigned long addr)
{
	struct rmap_item *rmap_item;

	while (cur != &mm_slot->rmap_list) {
		rmap_item = list_entry(cur, struct rmap_item, link);
		if ((rmap_item->address & PAGE_MASK) == addr) {
			if (!in_stable_tree(rmap_item))
				remove_rmap_item_from_tree(rmap_item);
			return rmap_item;
		}
		if (rmap_item->address > addr)
			break;
		cur = cur->next;
		remove_rmap_item_from_tree(rmap_item);
		list_del(&rmap_item->link);
		free_rmap_item(rmap_item);
	}

	rmap_item = alloc_rmap_item();
	if (rmap_item) {
		/* It has already been zeroed */
		rmap_item->mm = mm_slot->mm;
		rmap_item->address = addr;
		list_add_tail(&rmap_item->link, cur);
	}
	return rmap_item;
}

static struct rmap_item *scan_get_next_rmap_item(struct page **page)
{
	struct mm_struct *mm;
	struct mm_slot *slot;
	struct vm_area_struct *vma;
	struct rmap_item *rmap_item;

	if (list_empty(&ksm_mm_head.mm_list))
		return NULL;

	slot = ksm_scan.mm_slot;
	if (slot == &ksm_mm_head) {
		root_unstable_tree = RB_ROOT;

		spin_lock(&ksm_mmlist_lock);
		slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
		ksm_scan.mm_slot = slot;
		spin_unlock(&ksm_mmlist_lock);
next_mm:
		ksm_scan.address = 0;
		ksm_scan.rmap_item = list_entry(&slot->rmap_list,
						struct rmap_item, link);
	}

	mm = slot->mm;
	down_read(&mm->mmap_sem);
	for (vma = find_vma(mm, ksm_scan.address); vma; vma = vma->vm_next) {
		if (!(vma->vm_flags & VM_MERGEABLE))
			continue;
		if (ksm_scan.address < vma->vm_start)
			ksm_scan.address = vma->vm_start;
		if (!vma->anon_vma)
			ksm_scan.address = vma->vm_end;

		while (ksm_scan.address < vma->vm_end) {
			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
			if (*page && PageAnon(*page)) {
				flush_anon_page(vma, *page, ksm_scan.address);
				flush_dcache_page(*page);
				rmap_item = get_next_rmap_item(slot,
					ksm_scan.rmap_item->link.next,
					ksm_scan.address);
				if (rmap_item) {
					ksm_scan.rmap_item = rmap_item;
					ksm_scan.address += PAGE_SIZE;
				} else
					put_page(*page);
				up_read(&mm->mmap_sem);
				return rmap_item;
			}
			if (*page)
				put_page(*page);
			ksm_scan.address += PAGE_SIZE;
			cond_resched();
		}
	}

	if (!ksm_scan.address) {
		/*
		 * We've completed a full scan of all vmas, holding mmap_sem
		 * throughout, and found no VM_MERGEABLE: so do the same as
		 * __ksm_exit does to remove this mm from all our lists now.
		 */
		remove_mm_from_lists(mm);
		up_read(&mm->mmap_sem);
		slot = ksm_scan.mm_slot;
		if (slot != &ksm_mm_head)
			goto next_mm;
		return NULL;
	}

	/*
	 * Nuke all the rmap_items that are above this current rmap:
	 * because there were no VM_MERGEABLE vmas with such addresses.
	 */
	remove_trailing_rmap_items(slot, ksm_scan.rmap_item->link.next);
	up_read(&mm->mmap_sem);

	spin_lock(&ksm_mmlist_lock);
	slot = list_entry(slot->mm_list.next, struct mm_slot, mm_list);
	ksm_scan.mm_slot = slot;
	spin_unlock(&ksm_mmlist_lock);

	/* Repeat until we've completed scanning the whole list */
	if (slot != &ksm_mm_head)
		goto next_mm;

	/*
	 * Bump seqnr here rather than at top, so that __ksm_exit
	 * can skip rb_erase on unstable tree until we run again.
	 */
	ksm_scan.seqnr++;
	return NULL;
}

/**
 * ksm_do_scan  - the ksm scanner main worker function.
 * @scan_npages - number of pages we want to scan before we return.
 */
static void ksm_do_scan(unsigned int scan_npages)
{
	struct rmap_item *rmap_item;
	struct page *page;

	while (scan_npages--) {
		cond_resched();
		rmap_item = scan_get_next_rmap_item(&page);
		if (!rmap_item)
			return;
		if (!PageKsm(page) || !in_stable_tree(rmap_item))
			cmp_and_merge_page(page, rmap_item);
		else if (page_mapcount(page) == 1) {
			/*
			 * Replace now-unshared ksm page by ordinary page.
			 */
			break_cow(rmap_item->mm, rmap_item->address);
			remove_rmap_item_from_tree(rmap_item);
			rmap_item->oldchecksum = calc_checksum(page);
		}
		put_page(page);
	}
}

static int ksmd_should_run(void)
{
	return (ksm_run & KSM_RUN_MERGE) && !list_empty(&ksm_mm_head.mm_list);
}

static int ksm_scan_thread(void *nothing)
{
	set_user_nice(current, 5);

	while (!kthread_should_stop()) {
		mutex_lock(&ksm_thread_mutex);
		if (ksmd_should_run())
			ksm_do_scan(ksm_thread_pages_to_scan);
		mutex_unlock(&ksm_thread_mutex);

		if (ksmd_should_run()) {
			schedule_timeout_interruptible(
				msecs_to_jiffies(ksm_thread_sleep_millisecs));
		} else {
			wait_event_interruptible(ksm_thread_wait,
				ksmd_should_run() || kthread_should_stop());
		}
	}
	return 0;
}

int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
		unsigned long end, int advice, unsigned long *vm_flags)
{
	struct mm_struct *mm = vma->vm_mm;

	switch (advice) {
	case MADV_MERGEABLE:
		/*
		 * Be somewhat over-protective for now!
		 */
		if (*vm_flags & (VM_MERGEABLE | VM_SHARED  | VM_MAYSHARE   |
				 VM_PFNMAP    | VM_IO      | VM_DONTEXPAND |
				 VM_RESERVED  | VM_HUGETLB | VM_INSERTPAGE |
				 VM_MIXEDMAP  | VM_SAO))
			return 0;		/* just ignore the advice */

		if (!test_bit(MMF_VM_MERGEABLE, &mm->flags))
			if (__ksm_enter(mm) < 0)
				return -EAGAIN;

		*vm_flags |= VM_MERGEABLE;
		break;

	case MADV_UNMERGEABLE:
		if (!(*vm_flags & VM_MERGEABLE))
			return 0;		/* just ignore the advice */

		if (vma->anon_vma)
			unmerge_ksm_pages(vma, start, end);

		*vm_flags &= ~VM_MERGEABLE;
		break;
	}

	return 0;
}

int __ksm_enter(struct mm_struct *mm)
{
	struct mm_slot *mm_slot;
	int needs_wakeup;

	mm_slot = alloc_mm_slot();
	if (!mm_slot)
		return -ENOMEM;

	/* Check ksm_run too?  Would need tighter locking */
	needs_wakeup = list_empty(&ksm_mm_head.mm_list);

	spin_lock(&ksm_mmlist_lock);
	insert_to_mm_slots_hash(mm, mm_slot);
	/*
	 * Insert just behind the scanning cursor, to let the area settle
	 * down a little; when fork is followed by immediate exec, we don't
	 * want ksmd to waste time setting up and tearing down an rmap_list.
	 */
	list_add_tail(&mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);
	spin_unlock(&ksm_mmlist_lock);

	set_bit(MMF_VM_MERGEABLE, &mm->flags);

	if (needs_wakeup)
		wake_up_interruptible(&ksm_thread_wait);

	return 0;
}

void __ksm_exit(struct mm_struct *mm)
{
	/*
	 * This process is exiting: doesn't hold and doesn't need mmap_sem;
	 * but we do need to exclude ksmd and other exiters while we modify
	 * the various lists and trees.
	 */
	mutex_lock(&ksm_thread_mutex);
	remove_mm_from_lists(mm);
	mutex_unlock(&ksm_thread_mutex);
}

#define KSM_ATTR_RO(_name) \
	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
#define KSM_ATTR(_name) \
	static struct kobj_attribute _name##_attr = \
		__ATTR(_name, 0644, _name##_show, _name##_store)

static ssize_t sleep_millisecs_show(struct kobject *kobj,
				    struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%u\n", ksm_thread_sleep_millisecs);
}

static ssize_t sleep_millisecs_store(struct kobject *kobj,
				     struct kobj_attribute *attr,
				     const char *buf, size_t count)
{
	unsigned long msecs;
	int err;

	err = strict_strtoul(buf, 10, &msecs);
	if (err || msecs > UINT_MAX)
		return -EINVAL;

	ksm_thread_sleep_millisecs = msecs;

	return count;
}
KSM_ATTR(sleep_millisecs);

static ssize_t pages_to_scan_show(struct kobject *kobj,
				  struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%u\n", ksm_thread_pages_to_scan);
}

static ssize_t pages_to_scan_store(struct kobject *kobj,
				   struct kobj_attribute *attr,
				   const char *buf, size_t count)
{
	int err;
	unsigned long nr_pages;

	err = strict_strtoul(buf, 10, &nr_pages);
	if (err || nr_pages > UINT_MAX)
		return -EINVAL;

	ksm_thread_pages_to_scan = nr_pages;

	return count;
}
KSM_ATTR(pages_to_scan);

static ssize_t run_show(struct kobject *kobj, struct kobj_attribute *attr,
			char *buf)
{
	return sprintf(buf, "%u\n", ksm_run);
}

static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
			 const char *buf, size_t count)
{
	int err;
	unsigned long flags;

	err = strict_strtoul(buf, 10, &flags);
	if (err || flags > UINT_MAX)
		return -EINVAL;
	if (flags > KSM_RUN_UNMERGE)
		return -EINVAL;

	/*
	 * KSM_RUN_MERGE sets ksmd running, and 0 stops it running.
	 * KSM_RUN_UNMERGE stops it running and unmerges all rmap_items,
	 * breaking COW to free the unswappable pages_shared (but leaves
	 * mm_slots on the list for when ksmd may be set running again).
	 */

	mutex_lock(&ksm_thread_mutex);
	if (ksm_run != flags) {
		ksm_run = flags;
		if (flags & KSM_RUN_UNMERGE)
			unmerge_and_remove_all_rmap_items();
	}
	mutex_unlock(&ksm_thread_mutex);

	if (flags & KSM_RUN_MERGE)
		wake_up_interruptible(&ksm_thread_wait);

	return count;
}
KSM_ATTR(run);

static ssize_t max_kernel_pages_store(struct kobject *kobj,
				      struct kobj_attribute *attr,
				      const char *buf, size_t count)
{
	int err;
	unsigned long nr_pages;

	err = strict_strtoul(buf, 10, &nr_pages);
	if (err)
		return -EINVAL;

	ksm_max_kernel_pages = nr_pages;

	return count;
}

static ssize_t max_kernel_pages_show(struct kobject *kobj,
				     struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%lu\n", ksm_max_kernel_pages);
}
KSM_ATTR(max_kernel_pages);

static ssize_t pages_shared_show(struct kobject *kobj,
				 struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%lu\n", ksm_pages_shared);
}
KSM_ATTR_RO(pages_shared);

static ssize_t pages_sharing_show(struct kobject *kobj,
				  struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%lu\n", ksm_pages_sharing);
}
KSM_ATTR_RO(pages_sharing);

static ssize_t pages_unshared_show(struct kobject *kobj,
				   struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%lu\n", ksm_pages_unshared);
}
KSM_ATTR_RO(pages_unshared);

static ssize_t pages_volatile_show(struct kobject *kobj,
				   struct kobj_attribute *attr, char *buf)
{
	long ksm_pages_volatile;

	ksm_pages_volatile = ksm_rmap_items - ksm_pages_shared
				- ksm_pages_sharing - ksm_pages_unshared;
	/*
	 * It was not worth any locking to calculate that statistic,
	 * but it might therefore sometimes be negative: conceal that.
	 */
	if (ksm_pages_volatile < 0)
		ksm_pages_volatile = 0;
	return sprintf(buf, "%ld\n", ksm_pages_volatile);
}
KSM_ATTR_RO(pages_volatile);

static ssize_t full_scans_show(struct kobject *kobj,
			       struct kobj_attribute *attr, char *buf)
{
	return sprintf(buf, "%lu\n", ksm_scan.seqnr);
}
KSM_ATTR_RO(full_scans);

static struct attribute *ksm_attrs[] = {
	&sleep_millisecs_attr.attr,
	&pages_to_scan_attr.attr,
	&run_attr.attr,
	&max_kernel_pages_attr.attr,
	&pages_shared_attr.attr,
	&pages_sharing_attr.attr,
	&pages_unshared_attr.attr,
	&pages_volatile_attr.attr,
	&full_scans_attr.attr,
	NULL,
};

static struct attribute_group ksm_attr_group = {
	.attrs = ksm_attrs,
	.name = "ksm",
};

static int __init ksm_init(void)
{
	struct task_struct *ksm_thread;
	int err;

	err = ksm_slab_init();
	if (err)
		goto out;

	err = mm_slots_hash_init();
	if (err)
		goto out_free1;

	ksm_thread = kthread_run(ksm_scan_thread, NULL, "ksmd");
	if (IS_ERR(ksm_thread)) {
		printk(KERN_ERR "ksm: creating kthread failed\n");
		err = PTR_ERR(ksm_thread);
		goto out_free2;
	}

	err = sysfs_create_group(mm_kobj, &ksm_attr_group);
	if (err) {
		printk(KERN_ERR "ksm: register sysfs failed\n");
		goto out_free3;
	}

	return 0;

out_free3:
	kthread_stop(ksm_thread);
out_free2:
	mm_slots_hash_free();
out_free1:
	ksm_slab_free();
out:
	return err;
}
module_init(ksm_init)