summaryrefslogtreecommitdiff
path: root/mm/ksm.c
blob: 4b25f642f4eca16c0f991b48329e477329d600b2 (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
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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
 */

#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/mman.h>
#include <linux/sched.h>
#include <linux/sched/mm.h>
#include <linux/sched/coredump.h>
#include <linux/rwsem.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
#include <linux/spinlock.h>
#include <linux/xxhash.h>
#include <linux/delay.h>
#include <linux/kthread.h>
#include <linux/wait.h>
#include <linux/slab.h>
#include <linux/rbtree.h>
#include <linux/memory.h>
#include <linux/mmu_notifier.h>
#include <linux/swap.h>
#include <linux/ksm.h>
#include <linux/hashtable.h>
#include <linux/freezer.h>
#include <linux/oom.h>
#include <linux/numa.h>

#include <asm/tlbflush.h>
#include "internal.h"

#ifdef CONFIG_NUMA
#define NUMA(x)		(x)
#define DO_NUMA(x)	do { (x); } while (0)
#else
#define NUMA(x)		(0)
#define DO_NUMA(x)	do { } while (0)
#endif

/**
 * DOC: Overview
 *
 * 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.
 *
 * The stable tree node includes information required for reverse
 * mapping from a KSM page to virtual addresses that map this page.
 *
 * In order to avoid large latencies of the rmap walks on KSM pages,
 * KSM maintains two types of nodes in the stable tree:
 *
 * * the regular nodes that keep the reverse mapping structures in a
 *   linked list
 * * the "chains" that link nodes ("dups") that represent the same
 *   write protected memory content, but each "dup" corresponds to a
 *   different KSM page copy of that content
 *
 * Internally, the regular nodes, "dups" and "chains" are represented
 * using the same struct stable_node structure.
 *
 * 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.)
 *
 * If the merge_across_nodes tunable is unset, then KSM maintains multiple
 * stable trees and multiple unstable trees: one of each for each NUMA node.
 */

/**
 * 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 singly-linked 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 rmap_item *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_list: link to the next rmap to be scanned in 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_list;
	unsigned long seqnr;
};

/**
 * struct stable_node - node of the stable rbtree
 * @node: rb node of this ksm page in the stable tree
 * @head: (overlaying parent) &migrate_nodes indicates temporarily on that list
 * @hlist_dup: linked into the stable_node->hlist with a stable_node chain
 * @list: linked into migrate_nodes, pending placement in the proper node tree
 * @hlist: hlist head of rmap_items using this ksm page
 * @kpfn: page frame number of this ksm page (perhaps temporarily on wrong nid)
 * @chain_prune_time: time of the last full garbage collection
 * @rmap_hlist_len: number of rmap_item entries in hlist or STABLE_NODE_CHAIN
 * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
 */
struct stable_node {
	union {
		struct rb_node node;	/* when node of stable tree */
		struct {		/* when listed for migration */
			struct list_head *head;
			struct {
				struct hlist_node hlist_dup;
				struct list_head list;
			};
		};
	};
	struct hlist_head hlist;
	union {
		unsigned long kpfn;
		unsigned long chain_prune_time;
	};
	/*
	 * STABLE_NODE_CHAIN can be any negative number in
	 * rmap_hlist_len negative range, but better not -1 to be able
	 * to reliably detect underflows.
	 */
#define STABLE_NODE_CHAIN -1024
	int rmap_hlist_len;
#ifdef CONFIG_NUMA
	int nid;
#endif
};

/**
 * struct rmap_item - reverse mapping item for virtual addresses
 * @rmap_list: next rmap_item in mm_slot's singly-linked rmap_list
 * @anon_vma: pointer to anon_vma for this mm,address, when in stable tree
 * @nid: NUMA node id of unstable tree in which linked (may not match page)
 * @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 the unstable tree
 * @head: pointer to stable_node heading this list in the stable tree
 * @hlist: link into hlist of rmap_items hanging off that stable_node
 */
struct rmap_item {
	struct rmap_item *rmap_list;
	union {
		struct anon_vma *anon_vma;	/* when stable */
#ifdef CONFIG_NUMA
		int nid;		/* when node of unstable tree */
#endif
	};
	struct mm_struct *mm;
	unsigned long address;		/* + low bits used for flags below */
	unsigned int oldchecksum;	/* when unstable */
	union {
		struct rb_node node;	/* when node of unstable tree */
		struct {		/* when listed from stable tree */
			struct stable_node *head;
			struct hlist_node hlist;
		};
	};
};

#define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
#define UNSTABLE_FLAG	0x100	/* is a node of the unstable tree */
#define STABLE_FLAG	0x200	/* is listed from the stable tree */

/* The stable and unstable tree heads */
static struct rb_root one_stable_tree[1] = { RB_ROOT };
static struct rb_root one_unstable_tree[1] = { RB_ROOT };
static struct rb_root *root_stable_tree = one_stable_tree;
static struct rb_root *root_unstable_tree = one_unstable_tree;

/* Recently migrated nodes of stable tree, pending proper placement */
static LIST_HEAD(migrate_nodes);
#define STABLE_NODE_DUP_HEAD ((struct list_head *)&migrate_nodes.prev)

#define MM_SLOTS_HASH_BITS 10
static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);

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 *stable_node_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;

/* The number of stable_node chains */
static unsigned long ksm_stable_node_chains;

/* The number of stable_node dups linked to the stable_node chains */
static unsigned long ksm_stable_node_dups;

/* Delay in pruning stale stable_node_dups in the stable_node_chains */
static int ksm_stable_node_chains_prune_millisecs = 2000;

/* Maximum number of page slots sharing a stable node */
static int ksm_max_page_sharing = 256;

/* Number of pages ksmd should scan in one batch */
static unsigned int ksm_thread_pages_to_scan = 100;

/* Milliseconds ksmd should sleep between batches */
static unsigned int ksm_thread_sleep_millisecs = 20;

/* Checksum of an empty (zeroed) page */
static unsigned int zero_checksum __read_mostly;

/* Whether to merge empty (zeroed) pages with actual zero pages */
static bool ksm_use_zero_pages __read_mostly;

#ifdef CONFIG_NUMA
/* Zeroed when merging across nodes is not allowed */
static unsigned int ksm_merge_across_nodes = 1;
static int ksm_nr_node_ids = 1;
#else
#define ksm_merge_across_nodes	1U
#define ksm_nr_node_ids		1
#endif

#define KSM_RUN_STOP	0
#define KSM_RUN_MERGE	1
#define KSM_RUN_UNMERGE	2
#define KSM_RUN_OFFLINE	4
static unsigned long ksm_run = KSM_RUN_STOP;
static void wait_while_offlining(void);

static DECLARE_WAIT_QUEUE_HEAD(ksm_thread_wait);
static DECLARE_WAIT_QUEUE_HEAD(ksm_iter_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;

	stable_node_cache = KSM_KMEM_CACHE(stable_node, 0);
	if (!stable_node_cache)
		goto out_free1;

	mm_slot_cache = KSM_KMEM_CACHE(mm_slot, 0);
	if (!mm_slot_cache)
		goto out_free2;

	return 0;

out_free2:
	kmem_cache_destroy(stable_node_cache);
out_free1:
	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(stable_node_cache);
	kmem_cache_destroy(rmap_item_cache);
	mm_slot_cache = NULL;
}

static __always_inline bool is_stable_node_chain(struct stable_node *chain)
{
	return chain->rmap_hlist_len == STABLE_NODE_CHAIN;
}

static __always_inline bool is_stable_node_dup(struct stable_node *dup)
{
	return dup->head == STABLE_NODE_DUP_HEAD;
}

static inline void stable_node_chain_add_dup(struct stable_node *dup,
					     struct stable_node *chain)
{
	VM_BUG_ON(is_stable_node_dup(dup));
	dup->head = STABLE_NODE_DUP_HEAD;
	VM_BUG_ON(!is_stable_node_chain(chain));
	hlist_add_head(&dup->hlist_dup, &chain->hlist);
	ksm_stable_node_dups++;
}

static inline void __stable_node_dup_del(struct stable_node *dup)
{
	VM_BUG_ON(!is_stable_node_dup(dup));
	hlist_del(&dup->hlist_dup);
	ksm_stable_node_dups--;
}

static inline void stable_node_dup_del(struct stable_node *dup)
{
	VM_BUG_ON(is_stable_node_chain(dup));
	if (is_stable_node_dup(dup))
		__stable_node_dup_del(dup);
	else
		rb_erase(&dup->node, root_stable_tree + NUMA(dup->nid));
#ifdef CONFIG_DEBUG_VM
	dup->head = NULL;
#endif
}

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

	rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL |
						__GFP_NORETRY | __GFP_NOWARN);
	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 stable_node *alloc_stable_node(void)
{
	/*
	 * The allocation can take too long with GFP_KERNEL when memory is under
	 * pressure, which may lead to hung task warnings.  Adding __GFP_HIGH
	 * grants access to memory reserves, helping to avoid this problem.
	 */
	return kmem_cache_alloc(stable_node_cache, GFP_KERNEL | __GFP_HIGH);
}

static inline void free_stable_node(struct stable_node *stable_node)
{
	VM_BUG_ON(stable_node->rmap_hlist_len &&
		  !is_stable_node_chain(stable_node));
	kmem_cache_free(stable_node_cache, stable_node);
}

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 struct mm_slot *get_mm_slot(struct mm_struct *mm)
{
	struct mm_slot *slot;

	hash_for_each_possible(mm_slots_hash, slot, link, (unsigned long)mm)
		if (slot->mm == mm)
			return slot;

	return NULL;
}

static void insert_to_mm_slots_hash(struct mm_struct *mm,
				    struct mm_slot *mm_slot)
{
	mm_slot->mm = mm;
	hash_add(mm_slots_hash, &mm_slot->link, (unsigned long)mm);
}

/*
 * ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
 * page tables after it has passed through ksm_exit() - which, if necessary,
 * takes mmap_lock briefly to serialize against them.  ksm_exit() does not set
 * a special flag: they can just back out as soon as mm_users goes to zero.
 * ksm_test_exit() is used throughout to make this test for exit: in some
 * places for correctness, in some places just to avoid unnecessary work.
 */
static inline bool ksm_test_exit(struct mm_struct *mm)
{
	return atomic_read(&mm->mm_users) == 0;
}

/*
 * We use break_ksm to break COW on a ksm page: it's a stripped down
 *
 *	if (get_user_pages(addr, 1, FOLL_WRITE, &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.
 *
 * FAULT_FLAG/FOLL_REMOTE are because we do this outside the context
 * of the process that owns 'vma'.  We also do not want to enforce
 * protection keys here anyway.
 */
static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
	struct page *page;
	vm_fault_t ret = 0;

	do {
		cond_resched();
		page = follow_page(vma, addr,
				FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
		if (IS_ERR_OR_NULL(page))
			break;
		if (PageKsm(page))
			ret = handle_mm_fault(vma, addr,
					      FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
					      NULL);
		else
			ret = VM_FAULT_WRITE;
		put_page(page);
	} while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
	/*
	 * We must loop because handle_mm_fault() may back out if there's
	 * any difficulty e.g. if pte accessed bit gets updated concurrently.
	 *
	 * VM_FAULT_WRITE is what we have been hoping for: it indicates that
	 * COW has been broken, even if the vma does not permit VM_WRITE;
	 * but note that a concurrent fault might break PageKsm for us.
	 *
	 * VM_FAULT_SIGBUS could occur if we race with truncation of the
	 * backing file, which also invalidates anonymous pages: that's
	 * okay, that truncation will have unmapped the PageKsm for us.
	 *
	 * VM_FAULT_OOM: at the time of writing (late July 2009), setting
	 * aside mem_cgroup limits, VM_FAULT_OOM would only be set if the
	 * current task has TIF_MEMDIE set, and will be OOM killed on return
	 * to user; and ksmd, having no mm, would never be chosen for that.
	 *
	 * But if the mm is in a limited mem_cgroup, then the fault may fail
	 * with VM_FAULT_OOM even if the current task is not TIF_MEMDIE; and
	 * even ksmd can fail in this way - though it's usually breaking ksm
	 * just to undo a merge it made a moment before, so unlikely to oom.
	 *
	 * That's a pity: we might therefore have more kernel pages allocated
	 * than we're counting as nodes in the stable tree; but ksm_do_scan
	 * will retry to break_cow on each pass, so should recover the page
	 * in due course.  The important thing is to not let VM_MERGEABLE
	 * be cleared while any such pages might remain in the area.
	 */
	return (ret & VM_FAULT_OOM) ? -ENOMEM : 0;
}

static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
		unsigned long addr)
{
	struct vm_area_struct *vma;
	if (ksm_test_exit(mm))
		return NULL;
	vma = find_vma(mm, addr);
	if (!vma || vma->vm_start > addr)
		return NULL;
	if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
		return NULL;
	return vma;
}

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

	/*
	 * It is not an accident that whenever we want to break COW
	 * to undo, we also need to drop a reference to the anon_vma.
	 */
	put_anon_vma(rmap_item->anon_vma);

	mmap_read_lock(mm);
	vma = find_mergeable_vma(mm, addr);
	if (vma)
		break_ksm(vma, addr);
	mmap_read_unlock(mm);
}

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;

	mmap_read_lock(mm);
	vma = find_mergeable_vma(mm, addr);
	if (!vma)
		goto out;

	page = follow_page(vma, addr, FOLL_GET);
	if (IS_ERR_OR_NULL(page))
		goto out;
	if (PageAnon(page)) {
		flush_anon_page(vma, page, addr);
		flush_dcache_page(page);
	} else {
		put_page(page);
out:
		page = NULL;
	}
	mmap_read_unlock(mm);
	return page;
}

/*
 * This helper is used for getting right index into array of tree roots.
 * When merge_across_nodes knob is set to 1, there are only two rb-trees for
 * stable and unstable pages from all nodes with roots in index 0. Otherwise,
 * every node has its own stable and unstable tree.
 */
static inline int get_kpfn_nid(unsigned long kpfn)
{
	return ksm_merge_across_nodes ? 0 : NUMA(pfn_to_nid(kpfn));
}

static struct stable_node *alloc_stable_node_chain(struct stable_node *dup,
						   struct rb_root *root)
{
	struct stable_node *chain = alloc_stable_node();
	VM_BUG_ON(is_stable_node_chain(dup));
	if (likely(chain)) {
		INIT_HLIST_HEAD(&chain->hlist);
		chain->chain_prune_time = jiffies;
		chain->rmap_hlist_len = STABLE_NODE_CHAIN;
#if defined (CONFIG_DEBUG_VM) && defined(CONFIG_NUMA)
		chain->nid = NUMA_NO_NODE; /* debug */
#endif
		ksm_stable_node_chains++;

		/*
		 * Put the stable node chain in the first dimension of
		 * the stable tree and at the same time remove the old
		 * stable node.
		 */
		rb_replace_node(&dup->node, &chain->node, root);

		/*
		 * Move the old stable node to the second dimension
		 * queued in the hlist_dup. The invariant is that all
		 * dup stable_nodes in the chain->hlist point to pages
		 * that are write protected and have the exact same
		 * content.
		 */
		stable_node_chain_add_dup(dup, chain);
	}
	return chain;
}

static inline void free_stable_node_chain(struct stable_node *chain,
					  struct rb_root *root)
{
	rb_erase(&chain->node, root);
	free_stable_node(chain);
	ksm_stable_node_chains--;
}

static void remove_node_from_stable_tree(struct stable_node *stable_node)
{
	struct rmap_item *rmap_item;

	/* check it's not STABLE_NODE_CHAIN or negative */
	BUG_ON(stable_node->rmap_hlist_len < 0);

	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
		if (rmap_item->hlist.next)
			ksm_pages_sharing--;
		else
			ksm_pages_shared--;
		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
		stable_node->rmap_hlist_len--;
		put_anon_vma(rmap_item->anon_vma);
		rmap_item->address &= PAGE_MASK;
		cond_resched();
	}

	/*
	 * We need the second aligned pointer of the migrate_nodes
	 * list_head to stay clear from the rb_parent_color union
	 * (aligned and different than any node) and also different
	 * from &migrate_nodes. This will verify that future list.h changes
	 * don't break STABLE_NODE_DUP_HEAD. Only recent gcc can handle it.
	 */
#if defined(GCC_VERSION) && GCC_VERSION >= 40903
	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD <= &migrate_nodes);
	BUILD_BUG_ON(STABLE_NODE_DUP_HEAD >= &migrate_nodes + 1);
#endif

	if (stable_node->head == &migrate_nodes)
		list_del(&stable_node->list);
	else
		stable_node_dup_del(stable_node);
	free_stable_node(stable_node);
}

enum get_ksm_page_flags {
	GET_KSM_PAGE_NOLOCK,
	GET_KSM_PAGE_LOCK,
	GET_KSM_PAGE_TRYLOCK
};

/*
 * get_ksm_page: checks if the page indicated by the stable node
 * is still its ksm page, despite having held no reference to it.
 * In which case we can trust the content of the page, and it
 * returns the gotten page; but if the page has now been zapped,
 * remove the stale node from the stable tree and return NULL.
 * But beware, the stable node's page might be being migrated.
 *
 * You would expect the stable_node to hold a reference to the ksm page.
 * But if it increments the page's count, swapping out has to wait for
 * ksmd to come around again before it can free the page, which may take
 * seconds or even minutes: much too unresponsive.  So instead we use a
 * "keyhole reference": access to the ksm page from the stable node peeps
 * out through its keyhole to see if that page still holds the right key,
 * pointing back to this stable node.  This relies on freeing a PageAnon
 * page to reset its page->mapping to NULL, and relies on no other use of
 * a page to put something that might look like our key in page->mapping.
 * is on its way to being freed; but it is an anomaly to bear in mind.
 */
static struct page *get_ksm_page(struct stable_node *stable_node,
				 enum get_ksm_page_flags flags)
{
	struct page *page;
	void *expected_mapping;
	unsigned long kpfn;

	expected_mapping = (void *)((unsigned long)stable_node |
					PAGE_MAPPING_KSM);
again:
	kpfn = READ_ONCE(stable_node->kpfn); /* Address dependency. */
	page = pfn_to_page(kpfn);
	if (READ_ONCE(page->mapping) != expected_mapping)
		goto stale;

	/*
	 * We cannot do anything with the page while its refcount is 0.
	 * Usually 0 means free, or tail of a higher-order page: in which
	 * case this node is no longer referenced, and should be freed;
	 * however, it might mean that the page is under page_ref_freeze().
	 * The __remove_mapping() case is easy, again the node is now stale;
	 * the same is in reuse_ksm_page() case; but if page is swapcache
	 * in migrate_page_move_mapping(), it might still be our page,
	 * in which case it's essential to keep the node.
	 */
	while (!get_page_unless_zero(page)) {
		/*
		 * Another check for page->mapping != expected_mapping would
		 * work here too.  We have chosen the !PageSwapCache test to
		 * optimize the common case, when the page is or is about to
		 * be freed: PageSwapCache is cleared (under spin_lock_irq)
		 * in the ref_freeze section of __remove_mapping(); but Anon
		 * page->mapping reset to NULL later, in free_pages_prepare().
		 */
		if (!PageSwapCache(page))
			goto stale;
		cpu_relax();
	}

	if (READ_ONCE(page->mapping) != expected_mapping) {
		put_page(page);
		goto stale;
	}

	if (flags == GET_KSM_PAGE_TRYLOCK) {
		if (!trylock_page(page)) {
			put_page(page);
			return ERR_PTR(-EBUSY);
		}
	} else if (flags == GET_KSM_PAGE_LOCK)
		lock_page(page);

	if (flags != GET_KSM_PAGE_NOLOCK) {
		if (READ_ONCE(page->mapping) != expected_mapping) {
			unlock_page(page);
			put_page(page);
			goto stale;
		}
	}
	return page;

stale:
	/*
	 * We come here from above when page->mapping or !PageSwapCache
	 * suggests that the node is stale; but it might be under migration.
	 * We need smp_rmb(), matching the smp_wmb() in ksm_migrate_page(),
	 * before checking whether node->kpfn has been changed.
	 */
	smp_rmb();
	if (READ_ONCE(stable_node->kpfn) != kpfn)
		goto again;
	remove_node_from_stable_tree(stable_node);
	return NULL;
}

/*
 * 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 (rmap_item->address & STABLE_FLAG) {
		struct stable_node *stable_node;
		struct page *page;

		stable_node = rmap_item->head;
		page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
		if (!page)
			goto out;

		hlist_del(&rmap_item->hlist);
		put_page(page);

		if (!hlist_empty(&stable_node->hlist))
			ksm_pages_sharing--;
		else
			ksm_pages_shared--;
		VM_BUG_ON(stable_node->rmap_hlist_len <= 0);
		stable_node->rmap_hlist_len--;

		put_anon_vma(rmap_item->anon_vma);
		rmap_item->address &= PAGE_MASK;

	} else if (rmap_item->address & UNSTABLE_FLAG) {
		unsigned char age;
		/*
		 * Usually ksmd can and must skip the rb_erase, because
		 * root_unstable_tree was already reset to RB_ROOT.
		 * But be careful when an mm is exiting: do the rb_erase
		 * if this rmap_item was inserted by this scan, rather
		 * than left over from before.
		 */
		age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
		BUG_ON(age > 1);
		if (!age)
			rb_erase(&rmap_item->node,
				 root_unstable_tree + NUMA(rmap_item->nid));
		ksm_pages_unshared--;
		rmap_item->address &= PAGE_MASK;
	}
out:
	cond_resched();		/* we're called from many long loops */
}

static void remove_trailing_rmap_items(struct mm_slot *mm_slot,
				       struct rmap_item **rmap_list)
{
	while (*rmap_list) {
		struct rmap_item *rmap_item = *rmap_list;
		*rmap_list = rmap_item->rmap_list;
		remove_rmap_item_from_tree(rmap_item);
		free_rmap_item(rmap_item);
	}
}

/*
 * Though it's very tempting to unmerge rmap_items from stable tree 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_lock.  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).
 *
 * Similarly, although we'd like to remove rmap_items (so updating counts
 * and freeing memory) when unmerging an area, it's easier to leave that
 * to the next pass of ksmd - consider, for example, how ksmd might be
 * in cmp_and_merge_page on one of the rmap_items we would be removing.
 */
static int unmerge_ksm_pages(struct vm_area_struct *vma,
			     unsigned long start, unsigned long end)
{
	unsigned long addr;
	int err = 0;

	for (addr = start; addr < end && !err; addr += PAGE_SIZE) {
		if (ksm_test_exit(vma->vm_mm))
			break;
		if (signal_pending(current))
			err = -ERESTARTSYS;
		else
			err = break_ksm(vma, addr);
	}
	return err;
}

static inline struct stable_node *page_stable_node(struct page *page)
{
	return PageKsm(page) ? page_rmapping(page) : NULL;
}

static inline void set_page_stable_node(struct page *page,
					struct stable_node *stable_node)
{
	page->mapping = (void *)((unsigned long)stable_node | PAGE_MAPPING_KSM);
}

#ifdef CONFIG_SYSFS
/*
 * Only called through the sysfs control interface:
 */
static int remove_stable_node(struct stable_node *stable_node)
{
	struct page *page;
	int err;

	page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
	if (!page) {
		/*
		 * get_ksm_page did remove_node_from_stable_tree itself.
		 */
		return 0;
	}

	/*
	 * Page could be still mapped if this races with __mmput() running in
	 * between ksm_exit() and exit_mmap(). Just refuse to let
	 * merge_across_nodes/max_page_sharing be switched.
	 */
	err = -EBUSY;
	if (!page_mapped(page)) {
		/*
		 * The stable node did not yet appear stale to get_ksm_page(),
		 * since that allows for an unmapped ksm page to be recognized
		 * right up until it is freed; but the node is safe to remove.
		 * This page might be in a pagevec waiting to be freed,
		 * or it might be PageSwapCache (perhaps under writeback),
		 * or it might have been removed from swapcache a moment ago.
		 */
		set_page_stable_node(page, NULL);
		remove_node_from_stable_tree(stable_node);
		err = 0;
	}

	unlock_page(page);
	put_page(page);
	return err;
}

static int remove_stable_node_chain(struct stable_node *stable_node,
				    struct rb_root *root)
{
	struct stable_node *dup;
	struct hlist_node *hlist_safe;

	if (!is_stable_node_chain(stable_node)) {
		VM_BUG_ON(is_stable_node_dup(stable_node));
		if (remove_stable_node(stable_node))
			return true;
		else
			return false;
	}

	hlist_for_each_entry_safe(dup, hlist_safe,
				  &stable_node->hlist, hlist_dup) {
		VM_BUG_ON(!is_stable_node_dup(dup));
		if (remove_stable_node(dup))
			return true;
	}
	BUG_ON(!hlist_empty(&stable_node->hlist));
	free_stable_node_chain(stable_node, root);
	return false;
}

static int remove_all_stable_nodes(void)
{
	struct stable_node *stable_node, *next;
	int nid;
	int err = 0;

	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
		while (root_stable_tree[nid].rb_node) {
			stable_node = rb_entry(root_stable_tree[nid].rb_node,
						struct stable_node, node);
			if (remove_stable_node_chain(stable_node,
						     root_stable_tree + nid)) {
				err = -EBUSY;
				break;	/* proceed to next nid */
			}
			cond_resched();
		}
	}
	list_for_each_entry_safe(stable_node, next, &migrate_nodes, list) {
		if (remove_stable_node(stable_node))
			err = -EBUSY;
		cond_resched();
	}
	return err;
}

static int unmerge_and_remove_all_rmap_items(void)
{
	struct mm_slot *mm_slot;
	struct mm_struct *mm;
	struct vm_area_struct *vma;
	int err = 0;

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

	for (mm_slot = ksm_scan.mm_slot;
			mm_slot != &ksm_mm_head; mm_slot = ksm_scan.mm_slot) {
		mm = mm_slot->mm;
		mmap_read_lock(mm);
		for (vma = mm->mmap; vma; vma = vma->vm_next) {
			if (ksm_test_exit(mm))
				break;
			if (!(vma->vm_flags & VM_MERGEABLE) || !vma->anon_vma)
				continue;
			err = unmerge_ksm_pages(vma,
						vma->vm_start, vma->vm_end);
			if (err)
				goto error;
		}

		remove_trailing_rmap_items(mm_slot, &mm_slot->rmap_list);
		mmap_read_unlock(mm);

		spin_lock(&ksm_mmlist_lock);
		ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
						struct mm_slot, mm_list);
		if (ksm_test_exit(mm)) {
			hash_del(&mm_slot->link);
			list_del(&mm_slot->mm_list);
			spin_unlock(&ksm_mmlist_lock);

			free_mm_slot(mm_slot);
			clear_bit(MMF_VM_MERGEABLE, &mm->flags);
			mmdrop(mm);
		} else
			spin_unlock(&ksm_mmlist_lock);
	}

	/* Clean up stable nodes, but don't worry if some are still busy */
	remove_all_stable_nodes();
	ksm_scan.seqnr = 0;
	return 0;

error:
	mmap_read_unlock(mm);
	spin_lock(&ksm_mmlist_lock);
	ksm_scan.mm_slot = &ksm_mm_head;
	spin_unlock(&ksm_mmlist_lock);
	return err;
}
#endif /* CONFIG_SYSFS */

static u32 calc_checksum(struct page *page)
{
	u32 checksum;
	void *addr = kmap_atomic(page);
	checksum = xxhash(addr, PAGE_SIZE, 0);
	kunmap_atomic(addr);
	return checksum;
}

static int write_protect_page(struct vm_area_struct *vma, struct page *page,
			      pte_t *orig_pte)
{
	struct mm_struct *mm = vma->vm_mm;
	struct page_vma_mapped_walk pvmw = {
		.page = page,
		.vma = vma,
	};
	int swapped;
	int err = -EFAULT;
	struct mmu_notifier_range range;

	pvmw.address = page_address_in_vma(page, vma);
	if (pvmw.address == -EFAULT)
		goto out;

	BUG_ON(PageTransCompound(page));

	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm,
				pvmw.address,
				pvmw.address + PAGE_SIZE);
	mmu_notifier_invalidate_range_start(&range);

	if (!page_vma_mapped_walk(&pvmw))
		goto out_mn;
	if (WARN_ONCE(!pvmw.pte, "Unexpected PMD mapping?"))
		goto out_unlock;

	if (pte_write(*pvmw.pte) || pte_dirty(*pvmw.pte) ||
	    (pte_protnone(*pvmw.pte) && pte_savedwrite(*pvmw.pte)) ||
						mm_tlb_flush_pending(mm)) {
		pte_t entry;

		swapped = PageSwapCache(page);
		flush_cache_page(vma, pvmw.address, page_to_pfn(page));
		/*
		 * Ok this is tricky, when get_user_pages_fast() run it doesn't
		 * 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.
		 *
		 * No need to notify as we are downgrading page table to read
		 * only not changing it to point to a new page.
		 *
		 * See Documentation/vm/mmu_notifier.rst
		 */
		entry = ptep_clear_flush(vma, pvmw.address, pvmw.pte);
		/*
		 * Check that no O_DIRECT or similar I/O is in progress on the
		 * page
		 */
		if (page_mapcount(page) + 1 + swapped != page_count(page)) {
			set_pte_at(mm, pvmw.address, pvmw.pte, entry);
			goto out_unlock;
		}
		if (pte_dirty(entry))
			set_page_dirty(page);

		if (pte_protnone(entry))
			entry = pte_mkclean(pte_clear_savedwrite(entry));
		else
			entry = pte_mkclean(pte_wrprotect(entry));
		set_pte_at_notify(mm, pvmw.address, pvmw.pte, entry);
	}
	*orig_pte = *pvmw.pte;
	err = 0;

out_unlock:
	page_vma_mapped_walk_done(&pvmw);
out_mn:
	mmu_notifier_invalidate_range_end(&range);
out:
	return err;
}

/**
 * replace_page - replace page in vma by new ksm page
 * @vma:      vma that holds the pte pointing to page
 * @page:     the page we are replacing by kpage
 * @kpage:    the ksm page we replace page 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 *page,
			struct page *kpage, pte_t orig_pte)
{
	struct mm_struct *mm = vma->vm_mm;
	pmd_t *pmd;
	pte_t *ptep;
	pte_t newpte;
	spinlock_t *ptl;
	unsigned long addr;
	int err = -EFAULT;
	struct mmu_notifier_range range;

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

	pmd = mm_find_pmd(mm, addr);
	if (!pmd)
		goto out;

	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, addr,
				addr + PAGE_SIZE);
	mmu_notifier_invalidate_range_start(&range);

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

	/*
	 * No need to check ksm_use_zero_pages here: we can only have a
	 * zero_page here if ksm_use_zero_pages was enabled already.
	 */
	if (!is_zero_pfn(page_to_pfn(kpage))) {
		get_page(kpage);
		page_add_anon_rmap(kpage, vma, addr, false);
		newpte = mk_pte(kpage, vma->vm_page_prot);
	} else {
		newpte = pte_mkspecial(pfn_pte(page_to_pfn(kpage),
					       vma->vm_page_prot));
		/*
		 * We're replacing an anonymous page with a zero page, which is
		 * not anonymous. We need to do proper accounting otherwise we
		 * will get wrong values in /proc, and a BUG message in dmesg
		 * when tearing down the mm.
		 */
		dec_mm_counter(mm, MM_ANONPAGES);
	}

	flush_cache_page(vma, addr, pte_pfn(*ptep));
	/*
	 * No need to notify as we are replacing a read only page with another
	 * read only page with the same content.
	 *
	 * See Documentation/vm/mmu_notifier.rst
	 */
	ptep_clear_flush(vma, addr, ptep);
	set_pte_at_notify(mm, addr, ptep, newpte);

	page_remove_rmap(page, false);
	if (!page_mapped(page))
		try_to_free_swap(page);
	put_page(page);

	pte_unmap_unlock(ptep, ptl);
	err = 0;
out_mn:
	mmu_notifier_invalidate_range_end(&range);
out:
	return err;
}

/*
 * try_to_merge_one_page - take two pages and merge them into one
 * @vma: the vma that holds the pte pointing to page
 * @page: the PageAnon page that we want to replace with kpage
 * @kpage: the PageKsm page that we want to map instead of page,
 *         or NULL the first time when we want to use page as kpage.
 *
 * 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 *page, struct page *kpage)
{
	pte_t orig_pte = __pte(0);
	int err = -EFAULT;

	if (page == kpage)			/* ksm page forked */
		return 0;

	if (!PageAnon(page))
		goto out;

	/*
	 * 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(page))
		goto out;

	if (PageTransCompound(page)) {
		if (split_huge_page(page))
			goto out_unlock;
	}

	/*
	 * 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, page, &orig_pte) == 0) {
		if (!kpage) {
			/*
			 * While we hold page lock, upgrade page from
			 * PageAnon+anon_vma to PageKsm+NULL stable_node:
			 * stable_tree_insert() will update stable_node.
			 */
			set_page_stable_node(page, NULL);
			mark_page_accessed(page);
			/*
			 * Page reclaim just frees a clean page with no dirty
			 * ptes: make sure that the ksm page would be swapped.
			 */
			if (!PageDirty(page))
				SetPageDirty(page);
			err = 0;
		} else if (pages_identical(page, kpage))
			err = replace_page(vma, page, kpage, orig_pte);
	}

	if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
		munlock_vma_page(page);
		if (!PageMlocked(kpage)) {
			unlock_page(page);
			lock_page(kpage);
			mlock_vma_page(kpage);
			page = kpage;		/* for final unlock */
		}
	}

out_unlock:
	unlock_page(page);
out:
	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.
 *
 * This function returns 0 if the pages were merged, -EFAULT otherwise.
 */
static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
				      struct page *page, struct page *kpage)
{
	struct mm_struct *mm = rmap_item->mm;
	struct vm_area_struct *vma;
	int err = -EFAULT;

	mmap_read_lock(mm);
	vma = find_mergeable_vma(mm, rmap_item->address);
	if (!vma)
		goto out;

	err = try_to_merge_one_page(vma, page, kpage);
	if (err)
		goto out;

	/* Unstable nid is in union with stable anon_vma: remove first */
	remove_rmap_item_from_tree(rmap_item);

	/* Must get reference to anon_vma while still holding mmap_lock */
	rmap_item->anon_vma = vma->anon_vma;
	get_anon_vma(vma->anon_vma);
out:
	mmap_read_unlock(mm);
	return err;
}

/*
 * try_to_merge_two_pages - take two identical pages and prepare them
 * to be merged into one page.
 *
 * This function returns the kpage if we successfully merged two identical
 * pages into one ksm page, NULL otherwise.
 *
 * Note that this function upgrades page to ksm page: if one of the pages
 * is already a ksm page, try_to_merge_with_ksm_page should be used.
 */
static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
					   struct page *page,
					   struct rmap_item *tree_rmap_item,
					   struct page *tree_page)
{
	int err;

	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
	if (!err) {
		err = try_to_merge_with_ksm_page(tree_rmap_item,
							tree_page, page);
		/*
		 * If that fails, we have a ksm page with only one pte
		 * pointing to it: so break it.
		 */
		if (err)
			break_cow(rmap_item);
	}
	return err ? NULL : page;
}

static __always_inline
bool __is_page_sharing_candidate(struct stable_node *stable_node, int offset)
{
	VM_BUG_ON(stable_node->rmap_hlist_len < 0);
	/*
	 * Check that at least one mapping still exists, otherwise
	 * there's no much point to merge and share with this
	 * stable_node, as the underlying tree_page of the other
	 * sharer is going to be freed soon.
	 */
	return stable_node->rmap_hlist_len &&
		stable_node->rmap_hlist_len + offset < ksm_max_page_sharing;
}

static __always_inline
bool is_page_sharing_candidate(struct stable_node *stable_node)
{
	return __is_page_sharing_candidate(stable_node, 0);
}

static struct page *stable_node_dup(struct stable_node **_stable_node_dup,
				    struct stable_node **_stable_node,
				    struct rb_root *root,
				    bool prune_stale_stable_nodes)
{
	struct stable_node *dup, *found = NULL, *stable_node = *_stable_node;
	struct hlist_node *hlist_safe;
	struct page *_tree_page, *tree_page = NULL;
	int nr = 0;
	int found_rmap_hlist_len;

	if (!prune_stale_stable_nodes ||
	    time_before(jiffies, stable_node->chain_prune_time +
			msecs_to_jiffies(
				ksm_stable_node_chains_prune_millisecs)))
		prune_stale_stable_nodes = false;
	else
		stable_node->chain_prune_time = jiffies;

	hlist_for_each_entry_safe(dup, hlist_safe,
				  &stable_node->hlist, hlist_dup) {
		cond_resched();
		/*
		 * We must walk all stable_node_dup to prune the stale
		 * stable nodes during lookup.
		 *
		 * get_ksm_page can drop the nodes from the
		 * stable_node->hlist if they point to freed pages
		 * (that's why we do a _safe walk). The "dup"
		 * stable_node parameter itself will be freed from
		 * under us if it returns NULL.
		 */
		_tree_page = get_ksm_page(dup, GET_KSM_PAGE_NOLOCK);
		if (!_tree_page)
			continue;
		nr += 1;
		if (is_page_sharing_candidate(dup)) {
			if (!found ||
			    dup->rmap_hlist_len > found_rmap_hlist_len) {
				if (found)
					put_page(tree_page);
				found = dup;
				found_rmap_hlist_len = found->rmap_hlist_len;
				tree_page = _tree_page;

				/* skip put_page for found dup */
				if (!prune_stale_stable_nodes)
					break;
				continue;
			}
		}
		put_page(_tree_page);
	}

	if (found) {
		/*
		 * nr is counting all dups in the chain only if
		 * prune_stale_stable_nodes is true, otherwise we may
		 * break the loop at nr == 1 even if there are
		 * multiple entries.
		 */
		if (prune_stale_stable_nodes && nr == 1) {
			/*
			 * If there's not just one entry it would
			 * corrupt memory, better BUG_ON. In KSM
			 * context with no lock held it's not even
			 * fatal.
			 */
			BUG_ON(stable_node->hlist.first->next);

			/*
			 * There's just one entry and it is below the
			 * deduplication limit so drop the chain.
			 */
			rb_replace_node(&stable_node->node, &found->node,
					root);
			free_stable_node(stable_node);
			ksm_stable_node_chains--;
			ksm_stable_node_dups--;
			/*
			 * NOTE: the caller depends on the stable_node
			 * to be equal to stable_node_dup if the chain
			 * was collapsed.
			 */
			*_stable_node = found;
			/*
			 * Just for robustneess as stable_node is
			 * otherwise left as a stable pointer, the
			 * compiler shall optimize it away at build
			 * time.
			 */
			stable_node = NULL;
		} else if (stable_node->hlist.first != &found->hlist_dup &&
			   __is_page_sharing_candidate(found, 1)) {
			/*
			 * If the found stable_node dup can accept one
			 * more future merge (in addition to the one
			 * that is underway) and is not at the head of
			 * the chain, put it there so next search will
			 * be quicker in the !prune_stale_stable_nodes
			 * case.
			 *
			 * NOTE: it would be inaccurate to use nr > 1
			 * instead of checking the hlist.first pointer
			 * directly, because in the
			 * prune_stale_stable_nodes case "nr" isn't
			 * the position of the found dup in the chain,
			 * but the total number of dups in the chain.
			 */
			hlist_del(&found->hlist_dup);
			hlist_add_head(&found->hlist_dup,
				       &stable_node->hlist);
		}
	}

	*_stable_node_dup = found;
	return tree_page;
}

static struct stable_node *stable_node_dup_any(struct stable_node *stable_node,
					       struct rb_root *root)
{
	if (!is_stable_node_chain(stable_node))
		return stable_node;
	if (hlist_empty(&stable_node->hlist)) {
		free_stable_node_chain(stable_node, root);
		return NULL;
	}
	return hlist_entry(stable_node->hlist.first,
			   typeof(*stable_node), hlist_dup);
}

/*
 * Like for get_ksm_page, this function can free the *_stable_node and
 * *_stable_node_dup if the returned tree_page is NULL.
 *
 * It can also free and overwrite *_stable_node with the found
 * stable_node_dup if the chain is collapsed (in which case
 * *_stable_node will be equal to *_stable_node_dup like if the chain
 * never existed). It's up to the caller to verify tree_page is not
 * NULL before dereferencing *_stable_node or *_stable_node_dup.
 *
 * *_stable_node_dup is really a second output parameter of this
 * function and will be overwritten in all cases, the caller doesn't
 * need to initialize it.
 */
static struct page *__stable_node_chain(struct stable_node **_stable_node_dup,
					struct stable_node **_stable_node,
					struct rb_root *root,
					bool prune_stale_stable_nodes)
{
	struct stable_node *stable_node = *_stable_node;
	if (!is_stable_node_chain(stable_node)) {
		if (is_page_sharing_candidate(stable_node)) {
			*_stable_node_dup = stable_node;
			return get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
		}
		/*
		 * _stable_node_dup set to NULL means the stable_node
		 * reached the ksm_max_page_sharing limit.
		 */
		*_stable_node_dup = NULL;
		return NULL;
	}
	return stable_node_dup(_stable_node_dup, _stable_node, root,
			       prune_stale_stable_nodes);
}

static __always_inline struct page *chain_prune(struct stable_node **s_n_d,
						struct stable_node **s_n,
						struct rb_root *root)
{
	return __stable_node_chain(s_n_d, s_n, root, true);
}

static __always_inline struct page *chain(struct stable_node **s_n_d,
					  struct stable_node *s_n,
					  struct rb_root *root)
{
	struct stable_node *old_stable_node = s_n;
	struct page *tree_page;

	tree_page = __stable_node_chain(s_n_d, &s_n, root, false);
	/* not pruning dups so s_n cannot have changed */
	VM_BUG_ON(s_n != old_stable_node);
	return tree_page;
}

/*
 * stable_tree_search - search for page inside the stable tree
 *
 * 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 returns the stable tree node of identical content if found,
 * NULL otherwise.
 */
static struct page *stable_tree_search(struct page *page)
{
	int nid;
	struct rb_root *root;
	struct rb_node **new;
	struct rb_node *parent;
	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
	struct stable_node *page_node;

	page_node = page_stable_node(page);
	if (page_node && page_node->head != &migrate_nodes) {
		/* ksm page forked */
		get_page(page);
		return page;
	}

	nid = get_kpfn_nid(page_to_pfn(page));
	root = root_stable_tree + nid;
again:
	new = &root->rb_node;
	parent = NULL;

	while (*new) {
		struct page *tree_page;
		int ret;

		cond_resched();
		stable_node = rb_entry(*new, struct stable_node, node);
		stable_node_any = NULL;
		tree_page = chain_prune(&stable_node_dup, &stable_node,	root);
		/*
		 * NOTE: stable_node may have been freed by
		 * chain_prune() if the returned stable_node_dup is
		 * not NULL. stable_node_dup may have been inserted in
		 * the rbtree instead as a regular stable_node (in
		 * order to collapse the stable_node chain if a single
		 * stable_node dup was found in it). In such case the
		 * stable_node is overwritten by the calleee to point
		 * to the stable_node_dup that was collapsed in the
		 * stable rbtree and stable_node will be equal to
		 * stable_node_dup like if the chain never existed.
		 */
		if (!stable_node_dup) {
			/*
			 * Either all stable_node dups were full in
			 * this stable_node chain, or this chain was
			 * empty and should be rb_erased.
			 */
			stable_node_any = stable_node_dup_any(stable_node,
							      root);
			if (!stable_node_any) {
				/* rb_erase just run */
				goto again;
			}
			/*
			 * Take any of the stable_node dups page of
			 * this stable_node chain to let the tree walk
			 * continue. All KSM pages belonging to the
			 * stable_node dups in a stable_node chain
			 * have the same content and they're
			 * write protected at all times. Any will work
			 * fine to continue the walk.
			 */
			tree_page = get_ksm_page(stable_node_any,
						 GET_KSM_PAGE_NOLOCK);
		}
		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
		if (!tree_page) {
			/*
			 * If we walked over a stale stable_node,
			 * get_ksm_page() will call rb_erase() and it
			 * may rebalance the tree from under us. So
			 * restart the search from scratch. Returning
			 * NULL would be safe too, but we'd generate
			 * false negative insertions just because some
			 * stable_node was stale.
			 */
			goto again;
		}

		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 {
			if (page_node) {
				VM_BUG_ON(page_node->head != &migrate_nodes);
				/*
				 * Test if the migrated page should be merged
				 * into a stable node dup. If the mapcount is
				 * 1 we can migrate it with another KSM page
				 * without adding it to the chain.
				 */
				if (page_mapcount(page) > 1)
					goto chain_append;
			}

			if (!stable_node_dup) {
				/*
				 * If the stable_node is a chain and
				 * we got a payload match in memcmp
				 * but we cannot merge the scanned
				 * page in any of the existing
				 * stable_node dups because they're
				 * all full, we need to wait the
				 * scanned page to find itself a match
				 * in the unstable tree to create a
				 * brand new KSM page to add later to
				 * the dups of this stable_node.
				 */
				return NULL;
			}

			/*
			 * Lock and unlock the stable_node's page (which
			 * might already have been migrated) so that page
			 * migration is sure to notice its raised count.
			 * It would be more elegant to return stable_node
			 * than kpage, but that involves more changes.
			 */
			tree_page = get_ksm_page(stable_node_dup,
						 GET_KSM_PAGE_TRYLOCK);

			if (PTR_ERR(tree_page) == -EBUSY)
				return ERR_PTR(-EBUSY);

			if (unlikely(!tree_page))
				/*
				 * The tree may have been rebalanced,
				 * so re-evaluate parent and new.
				 */
				goto again;
			unlock_page(tree_page);

			if (get_kpfn_nid(stable_node_dup->kpfn) !=
			    NUMA(stable_node_dup->nid)) {
				put_page(tree_page);
				goto replace;
			}
			return tree_page;
		}
	}

	if (!page_node)
		return NULL;

	list_del(&page_node->list);
	DO_NUMA(page_node->nid = nid);
	rb_link_node(&page_node->node, parent, new);
	rb_insert_color(&page_node->node, root);
out:
	if (is_page_sharing_candidate(page_node)) {
		get_page(page);
		return page;
	} else
		return NULL;

replace:
	/*
	 * If stable_node was a chain and chain_prune collapsed it,
	 * stable_node has been updated to be the new regular
	 * stable_node. A collapse of the chain is indistinguishable
	 * from the case there was no chain in the stable
	 * rbtree. Otherwise stable_node is the chain and
	 * stable_node_dup is the dup to replace.
	 */
	if (stable_node_dup == stable_node) {
		VM_BUG_ON(is_stable_node_chain(stable_node_dup));
		VM_BUG_ON(is_stable_node_dup(stable_node_dup));
		/* there is no chain */
		if (page_node) {
			VM_BUG_ON(page_node->head != &migrate_nodes);
			list_del(&page_node->list);
			DO_NUMA(page_node->nid = nid);
			rb_replace_node(&stable_node_dup->node,
					&page_node->node,
					root);
			if (is_page_sharing_candidate(page_node))
				get_page(page);
			else
				page = NULL;
		} else {
			rb_erase(&stable_node_dup->node, root);
			page = NULL;
		}
	} else {
		VM_BUG_ON(!is_stable_node_chain(stable_node));
		__stable_node_dup_del(stable_node_dup);
		if (page_node) {
			VM_BUG_ON(page_node->head != &migrate_nodes);
			list_del(&page_node->list);
			DO_NUMA(page_node->nid = nid);
			stable_node_chain_add_dup(page_node, stable_node);
			if (is_page_sharing_candidate(page_node))
				get_page(page);
			else
				page = NULL;
		} else {
			page = NULL;
		}
	}
	stable_node_dup->head = &migrate_nodes;
	list_add(&stable_node_dup->list, stable_node_dup->head);
	return page;

chain_append:
	/* stable_node_dup could be null if it reached the limit */
	if (!stable_node_dup)
		stable_node_dup = stable_node_any;
	/*
	 * If stable_node was a chain and chain_prune collapsed it,
	 * stable_node has been updated to be the new regular
	 * stable_node. A collapse of the chain is indistinguishable
	 * from the case there was no chain in the stable
	 * rbtree. Otherwise stable_node is the chain and
	 * stable_node_dup is the dup to replace.
	 */
	if (stable_node_dup == stable_node) {
		VM_BUG_ON(is_stable_node_dup(stable_node_dup));
		/* chain is missing so create it */
		stable_node = alloc_stable_node_chain(stable_node_dup,
						      root);
		if (!stable_node)
			return NULL;
	}
	/*
	 * Add this stable_node dup that was
	 * migrated to the stable_node chain
	 * of the current nid for this page
	 * content.
	 */
	VM_BUG_ON(!is_stable_node_dup(stable_node_dup));
	VM_BUG_ON(page_node->head != &migrate_nodes);
	list_del(&page_node->list);
	DO_NUMA(page_node->nid = nid);
	stable_node_chain_add_dup(page_node, stable_node);
	goto out;
}

/*
 * stable_tree_insert - insert stable tree node pointing to new ksm page
 * into the stable tree.
 *
 * This function returns the stable tree node just allocated on success,
 * NULL otherwise.
 */
static struct stable_node *stable_tree_insert(struct page *kpage)
{
	int nid;
	unsigned long kpfn;
	struct rb_root *root;
	struct rb_node **new;
	struct rb_node *parent;
	struct stable_node *stable_node, *stable_node_dup, *stable_node_any;
	bool need_chain = false;

	kpfn = page_to_pfn(kpage);
	nid = get_kpfn_nid(kpfn);
	root = root_stable_tree + nid;
again:
	parent = NULL;
	new = &root->rb_node;

	while (*new) {
		struct page *tree_page;
		int ret;

		cond_resched();
		stable_node = rb_entry(*new, struct stable_node, node);
		stable_node_any = NULL;
		tree_page = chain(&stable_node_dup, stable_node, root);
		if (!stable_node_dup) {
			/*
			 * Either all stable_node dups were full in
			 * this stable_node chain, or this chain was
			 * empty and should be rb_erased.
			 */
			stable_node_any = stable_node_dup_any(stable_node,
							      root);
			if (!stable_node_any) {
				/* rb_erase just run */
				goto again;
			}
			/*
			 * Take any of the stable_node dups page of
			 * this stable_node chain to let the tree walk
			 * continue. All KSM pages belonging to the
			 * stable_node dups in a stable_node chain
			 * have the same content and they're
			 * write protected at all times. Any will work
			 * fine to continue the walk.
			 */
			tree_page = get_ksm_page(stable_node_any,
						 GET_KSM_PAGE_NOLOCK);
		}
		VM_BUG_ON(!stable_node_dup ^ !!stable_node_any);
		if (!tree_page) {
			/*
			 * If we walked over a stale stable_node,
			 * get_ksm_page() will call rb_erase() and it
			 * may rebalance the tree from under us. So
			 * restart the search from scratch. Returning
			 * NULL would be safe too, but we'd generate
			 * false negative insertions just because some
			 * stable_node was stale.
			 */
			goto again;
		}

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

		parent = *new;
		if (ret < 0)
			new = &parent->rb_left;
		else if (ret > 0)
			new = &parent->rb_right;
		else {
			need_chain = true;
			break;
		}
	}

	stable_node_dup = alloc_stable_node();
	if (!stable_node_dup)
		return NULL;

	INIT_HLIST_HEAD(&stable_node_dup->hlist);
	stable_node_dup->kpfn = kpfn;
	set_page_stable_node(kpage, stable_node_dup);
	stable_node_dup->rmap_hlist_len = 0;
	DO_NUMA(stable_node_dup->nid = nid);
	if (!need_chain) {
		rb_link_node(&stable_node_dup->node, parent, new);
		rb_insert_color(&stable_node_dup->node, root);
	} else {
		if (!is_stable_node_chain(stable_node)) {
			struct stable_node *orig = stable_node;
			/* chain is missing so create it */
			stable_node = alloc_stable_node_chain(orig, root);
			if (!stable_node) {
				free_stable_node(stable_node_dup);
				return NULL;
			}
		}
		stable_node_chain_add_dup(stable_node_dup, stable_node);
	}

	return stable_node_dup;
}

/*
 * unstable_tree_search_insert - search for identical page,
 * else insert rmap_item into the unstable tree.
 *
 * 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 rmap_item *rmap_item,
					      struct page *page,
					      struct page **tree_pagep)
{
	struct rb_node **new;
	struct rb_root *root;
	struct rb_node *parent = NULL;
	int nid;

	nid = get_kpfn_nid(page_to_pfn(page));
	root = root_unstable_tree + nid;
	new = &root->rb_node;

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

		cond_resched();
		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
		tree_page = get_mergeable_page(tree_rmap_item);
		if (!tree_page)
			return NULL;

		/*
		 * Don't substitute a ksm page for a forked page.
		 */
		if (page == tree_page) {
			put_page(tree_page);
			return NULL;
		}

		ret = memcmp_pages(page, tree_page);

		parent = *new;
		if (ret < 0) {
			put_page(tree_page);
			new = &parent->rb_left;
		} else if (ret > 0) {
			put_page(tree_page);
			new = &parent->rb_right;
		} else if (!ksm_merge_across_nodes &&
			   page_to_nid(tree_page) != nid) {
			/*
			 * If tree_page has been migrated to another NUMA node,
			 * it will be flushed out and put in the right unstable
			 * tree next time: only merge with it when across_nodes.
			 */
			put_page(tree_page);
			return NULL;
		} else {
			*tree_pagep = tree_page;
			return tree_rmap_item;
		}
	}

	rmap_item->address |= UNSTABLE_FLAG;
	rmap_item->address |= (ksm_scan.seqnr & SEQNR_MASK);
	DO_NUMA(rmap_item->nid = nid);
	rb_link_node(&rmap_item->node, parent, new);
	rb_insert_color(&rmap_item->node, root);

	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 stable_node *stable_node,
			       bool max_page_sharing_bypass)
{
	/*
	 * rmap won't find this mapping if we don't insert the
	 * rmap_item in the right stable_node
	 * duplicate. page_migration could break later if rmap breaks,
	 * so we can as well crash here. We really need to check for
	 * rmap_hlist_len == STABLE_NODE_CHAIN, but we can as well check
	 * for other negative values as an underflow if detected here
	 * for the first time (and not when decreasing rmap_hlist_len)
	 * would be sign of memory corruption in the stable_node.
	 */
	BUG_ON(stable_node->rmap_hlist_len < 0);

	stable_node->rmap_hlist_len++;
	if (!max_page_sharing_bypass)
		/* possibly non fatal but unexpected overflow, only warn */
		WARN_ON_ONCE(stable_node->rmap_hlist_len >
			     ksm_max_page_sharing);

	rmap_item->head = stable_node;
	rmap_item->address |= STABLE_FLAG;
	hlist_add_head(&rmap_item->hlist, &stable_node->hlist);

	if (rmap_item->hlist.next)
		ksm_pages_sharing++;
	else
		ksm_pages_shared++;
}

/*
 * cmp_and_merge_page - first see if page can be merged into the stable tree;
 * if not, compare checksum to previous and if it's the same, see if page can
 * be inserted into the unstable tree, or merged with a page already there and
 * both transferred to the stable tree.
 *
 * @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 mm_struct *mm = rmap_item->mm;
	struct rmap_item *tree_rmap_item;
	struct page *tree_page = NULL;
	struct stable_node *stable_node;
	struct page *kpage;
	unsigned int checksum;
	int err;
	bool max_page_sharing_bypass = false;

	stable_node = page_stable_node(page);
	if (stable_node) {
		if (stable_node->head != &migrate_nodes &&
		    get_kpfn_nid(READ_ONCE(stable_node->kpfn)) !=
		    NUMA(stable_node->nid)) {
			stable_node_dup_del(stable_node);
			stable_node->head = &migrate_nodes;
			list_add(&stable_node->list, stable_node->head);
		}
		if (stable_node->head != &migrate_nodes &&
		    rmap_item->head == stable_node)
			return;
		/*
		 * If it's a KSM fork, allow it to go over the sharing limit
		 * without warnings.
		 */
		if (!is_page_sharing_candidate(stable_node))
			max_page_sharing_bypass = true;
	}

	/* We first start with searching the page inside the stable tree */
	kpage = stable_tree_search(page);
	if (kpage == page && rmap_item->head == stable_node) {
		put_page(kpage);
		return;
	}

	remove_rmap_item_from_tree(rmap_item);

	if (kpage) {
		if (PTR_ERR(kpage) == -EBUSY)
			return;

		err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
		if (!err) {
			/*
			 * The page was successfully merged:
			 * add its rmap_item to the stable tree.
			 */
			lock_page(kpage);
			stable_tree_append(rmap_item, page_stable_node(kpage),
					   max_page_sharing_bypass);
			unlock_page(kpage);
		}
		put_page(kpage);
		return;
	}

	/*
	 * If the hash value of the page has changed from the last time
	 * we calculated it, this page is changing frequently: therefore we
	 * don't want to insert it in the unstable tree, and we don't want
	 * to waste our time searching for something identical to it there.
	 */
	checksum = calc_checksum(page);
	if (rmap_item->oldchecksum != checksum) {
		rmap_item->oldchecksum = checksum;
		return;
	}

	/*
	 * Same checksum as an empty page. We attempt to merge it with the
	 * appropriate zero page if the user enabled this via sysfs.
	 */
	if (ksm_use_zero_pages && (checksum == zero_checksum)) {
		struct vm_area_struct *vma;

		mmap_read_lock(mm);
		vma = find_mergeable_vma(mm, rmap_item->address);
		if (vma) {
			err = try_to_merge_one_page(vma, page,
					ZERO_PAGE(rmap_item->address));
		} else {
			/*
			 * If the vma is out of date, we do not need to
			 * continue.
			 */
			err = 0;
		}
		mmap_read_unlock(mm);
		/*
		 * In case of failure, the page was not really empty, so we
		 * need to continue. Otherwise we're done.
		 */
		if (!err)
			return;
	}
	tree_rmap_item =
		unstable_tree_search_insert(rmap_item, page, &tree_page);
	if (tree_rmap_item) {
		bool split;

		kpage = try_to_merge_two_pages(rmap_item, page,
						tree_rmap_item, tree_page);
		/*
		 * If both pages we tried to merge belong to the same compound
		 * page, then we actually ended up increasing the reference
		 * count of the same compound page twice, and split_huge_page
		 * failed.
		 * Here we set a flag if that happened, and we use it later to
		 * try split_huge_page again. Since we call put_page right
		 * afterwards, the reference count will be correct and
		 * split_huge_page should succeed.
		 */
		split = PageTransCompound(page)
			&& compound_head(page) == compound_head(tree_page);
		put_page(tree_page);
		if (kpage) {
			/*
			 * The pages were successfully merged: insert new
			 * node in the stable tree and add both rmap_items.
			 */
			lock_page(kpage);
			stable_node = stable_tree_insert(kpage);
			if (stable_node) {
				stable_tree_append(tree_rmap_item, stable_node,
						   false);
				stable_tree_append(rmap_item, stable_node,
						   false);
			}
			unlock_page(kpage);

			/*
			 * 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_node) {
				break_cow(tree_rmap_item);
				break_cow(rmap_item);
			}
		} else if (split) {
			/*
			 * We are here if we tried to merge two pages and
			 * failed because they both belonged to the same
			 * compound page. We will split the page now, but no
			 * merging will take place.
			 * We do not want to add the cost of a full lock; if
			 * the page is locked, it is better to skip it and
			 * perhaps try again later.
			 */
			if (!trylock_page(page))
				return;
			split_huge_page(page);
			unlock_page(page);
		}
	}
}

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

	while (*rmap_list) {
		rmap_item = *rmap_list;
		if ((rmap_item->address & PAGE_MASK) == addr)
			return rmap_item;
		if (rmap_item->address > addr)
			break;
		*rmap_list = rmap_item->rmap_list;
		remove_rmap_item_from_tree(rmap_item);
		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;
		rmap_item->rmap_list = *rmap_list;
		*rmap_list = rmap_item;
	}
	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;
	int nid;

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

	slot = ksm_scan.mm_slot;
	if (slot == &ksm_mm_head) {
		/*
		 * A number of pages can hang around indefinitely on per-cpu
		 * pagevecs, raised page count preventing write_protect_page
		 * from merging them.  Though it doesn't really matter much,
		 * it is puzzling to see some stuck in pages_volatile until
		 * other activity jostles them out, and they also prevented
		 * LTP's KSM test from succeeding deterministically; so drain
		 * them here (here rather than on entry to ksm_do_scan(),
		 * so we don't IPI too often when pages_to_scan is set low).
		 */
		lru_add_drain_all();

		/*
		 * Whereas stale stable_nodes on the stable_tree itself
		 * get pruned in the regular course of stable_tree_search(),
		 * those moved out to the migrate_nodes list can accumulate:
		 * so prune them once before each full scan.
		 */
		if (!ksm_merge_across_nodes) {
			struct stable_node *stable_node, *next;
			struct page *page;

			list_for_each_entry_safe(stable_node, next,
						 &migrate_nodes, list) {
				page = get_ksm_page(stable_node,
						    GET_KSM_PAGE_NOLOCK);
				if (page)
					put_page(page);
				cond_resched();
			}
		}

		for (nid = 0; nid < ksm_nr_node_ids; nid++)
			root_unstable_tree[nid] = 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);
		/*
		 * Although we tested list_empty() above, a racing __ksm_exit
		 * of the last mm on the list may have removed it since then.
		 */
		if (slot == &ksm_mm_head)
			return NULL;
next_mm:
		ksm_scan.address = 0;
		ksm_scan.rmap_list = &slot->rmap_list;
	}

	mm = slot->mm;
	mmap_read_lock(mm);
	if (ksm_test_exit(mm))
		vma = NULL;
	else
		vma = find_vma(mm, ksm_scan.address);

	for (; 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) {
			if (ksm_test_exit(mm))
				break;
			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
			if (IS_ERR_OR_NULL(*page)) {
				ksm_scan.address += PAGE_SIZE;
				cond_resched();
				continue;
			}
			if (PageAnon(*page)) {
				flush_anon_page(vma, *page, ksm_scan.address);
				flush_dcache_page(*page);
				rmap_item = get_next_rmap_item(slot,
					ksm_scan.rmap_list, ksm_scan.address);
				if (rmap_item) {
					ksm_scan.rmap_list =
							&rmap_item->rmap_list;
					ksm_scan.address += PAGE_SIZE;
				} else
					put_page(*page);
				mmap_read_unlock(mm);
				return rmap_item;
			}
			put_page(*page);
			ksm_scan.address += PAGE_SIZE;
			cond_resched();
		}
	}

	if (ksm_test_exit(mm)) {
		ksm_scan.address = 0;
		ksm_scan.rmap_list = &slot->rmap_list;
	}
	/*
	 * 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_list);

	spin_lock(&ksm_mmlist_lock);
	ksm_scan.mm_slot = list_entry(slot->mm_list.next,
						struct mm_slot, mm_list);
	if (ksm_scan.address == 0) {
		/*
		 * We've completed a full scan of all vmas, holding mmap_lock
		 * throughout, and found no VM_MERGEABLE: so do the same as
		 * __ksm_exit does to remove this mm from all our lists now.
		 * This applies either when cleaning up after __ksm_exit
		 * (but beware: we can reach here even before __ksm_exit),
		 * or when all VM_MERGEABLE areas have been unmapped (and
		 * mmap_lock then protects against race with MADV_MERGEABLE).
		 */
		hash_del(&slot->link);
		list_del(&slot->mm_list);
		spin_unlock(&ksm_mmlist_lock);

		free_mm_slot(slot);
		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
		mmap_read_unlock(mm);
		mmdrop(mm);
	} else {
		mmap_read_unlock(mm);
		/*
		 * mmap_read_unlock(mm) first because after
		 * spin_unlock(&ksm_mmlist_lock) run, the "mm" may
		 * already have been freed under us by __ksm_exit()
		 * because the "mm_slot" is still hashed and
		 * ksm_scan.mm_slot doesn't point to it anymore.
		 */
		spin_unlock(&ksm_mmlist_lock);
	}

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

	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-- && likely(!freezing(current))) {
		cond_resched();
		rmap_item = scan_get_next_rmap_item(&page);
		if (!rmap_item)
			return;
		cmp_and_merge_page(page, rmap_item);
		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)
{
	unsigned int sleep_ms;

	set_freezable();
	set_user_nice(current, 5);

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

		try_to_freeze();

		if (ksmd_should_run()) {
			sleep_ms = READ_ONCE(ksm_thread_sleep_millisecs);
			wait_event_interruptible_timeout(ksm_iter_wait,
				sleep_ms != READ_ONCE(ksm_thread_sleep_millisecs),
				msecs_to_jiffies(sleep_ms));
		} else {
			wait_event_freezable(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;
	int err;

	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_HUGETLB | VM_MIXEDMAP))
			return 0;		/* just ignore the advice */

		if (vma_is_dax(vma))
			return 0;

#ifdef VM_SAO
		if (*vm_flags & VM_SAO)
			return 0;
#endif
#ifdef VM_SPARC_ADI
		if (*vm_flags & VM_SPARC_ADI)
			return 0;
#endif

		if (!test_bit(MMF_VM_MERGEABLE, &mm->flags)) {
			err = __ksm_enter(mm);
			if (err)
				return err;
		}

		*vm_flags |= VM_MERGEABLE;
		break;

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

		if (vma->anon_vma) {
			err = unmerge_ksm_pages(vma, start, end);
			if (err)
				return err;
		}

		*vm_flags &= ~VM_MERGEABLE;
		break;
	}

	return 0;
}
EXPORT_SYMBOL_GPL(ksm_madvise);

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);
	/*
	 * When KSM_RUN_MERGE (or KSM_RUN_STOP),
	 * 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.
	 *
	 * But when KSM_RUN_UNMERGE, it's important to insert ahead of its
	 * scanning cursor, otherwise KSM pages in newly forked mms will be
	 * missed: then we might as well insert at the end of the list.
	 */
	if (ksm_run & KSM_RUN_UNMERGE)
		list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list);
	else
		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);
	mmgrab(mm);

	if (needs_wakeup)
		wake_up_interruptible(&ksm_thread_wait);

	return 0;
}

void __ksm_exit(struct mm_struct *mm)
{
	struct mm_slot *mm_slot;
	int easy_to_free = 0;

	/*
	 * This process is exiting: if it's straightforward (as is the
	 * case when ksmd was never running), free mm_slot immediately.
	 * But if it's at the cursor or has rmap_items linked to it, use
	 * mmap_lock to synchronize with any break_cows before pagetables
	 * are freed, and leave the mm_slot on the list for ksmd to free.
	 * Beware: ksm may already have noticed it exiting and freed the slot.
	 */

	spin_lock(&ksm_mmlist_lock);
	mm_slot = get_mm_slot(mm);
	if (mm_slot && ksm_scan.mm_slot != mm_slot) {
		if (!mm_slot->rmap_list) {
			hash_del(&mm_slot->link);
			list_del(&mm_slot->mm_list);
			easy_to_free = 1;
		} else {
			list_move(&mm_slot->mm_list,
				  &ksm_scan.mm_slot->mm_list);
		}
	}
	spin_unlock(&ksm_mmlist_lock);

	if (easy_to_free) {
		free_mm_slot(mm_slot);
		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
		mmdrop(mm);
	} else if (mm_slot) {
		mmap_write_lock(mm);
		mmap_write_unlock(mm);
	}
}

struct page *ksm_might_need_to_copy(struct page *page,
			struct vm_area_struct *vma, unsigned long address)
{
	struct anon_vma *anon_vma = page_anon_vma(page);
	struct page *new_page;

	if (PageKsm(page)) {
		if (page_stable_node(page) &&
		    !(ksm_run & KSM_RUN_UNMERGE))
			return page;	/* no need to copy it */
	} else if (!anon_vma) {
		return page;		/* no need to copy it */
	} else if (anon_vma->root == vma->anon_vma->root &&
		 page->index == linear_page_index(vma, address)) {
		return page;		/* still no need to copy it */
	}
	if (!PageUptodate(page))
		return page;		/* let do_swap_page report the error */

	new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
	if (new_page && mem_cgroup_charge(new_page, vma->vm_mm, GFP_KERNEL)) {
		put_page(new_page);
		new_page = NULL;
	}
	if (new_page) {
		copy_user_highpage(new_page, page, address, vma);

		SetPageDirty(new_page);
		__SetPageUptodate(new_page);
		__SetPageLocked(new_page);
	}

	return new_page;
}

void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
{
	struct stable_node *stable_node;
	struct rmap_item *rmap_item;
	int search_new_forks = 0;

	VM_BUG_ON_PAGE(!PageKsm(page), page);

	/*
	 * Rely on the page lock to protect against concurrent modifications
	 * to that page's node of the stable tree.
	 */
	VM_BUG_ON_PAGE(!PageLocked(page), page);

	stable_node = page_stable_node(page);
	if (!stable_node)
		return;
again:
	hlist_for_each_entry(rmap_item, &stable_node->hlist, hlist) {
		struct anon_vma *anon_vma = rmap_item->anon_vma;
		struct anon_vma_chain *vmac;
		struct vm_area_struct *vma;

		cond_resched();
		anon_vma_lock_read(anon_vma);
		anon_vma_interval_tree_foreach(vmac, &anon_vma->rb_root,
					       0, ULONG_MAX) {
			unsigned long addr;

			cond_resched();
			vma = vmac->vma;

			/* Ignore the stable/unstable/sqnr flags */
			addr = rmap_item->address & PAGE_MASK;

			if (addr < vma->vm_start || addr >= vma->vm_end)
				continue;
			/*
			 * Initially we examine only the vma which covers this
			 * rmap_item; but later, if there is still work to do,
			 * we examine covering vmas in other mms: in case they
			 * were forked from the original since ksmd passed.
			 */
			if ((rmap_item->mm == vma->vm_mm) == search_new_forks)
				continue;

			if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
				continue;

			if (!rwc->rmap_one(page, vma, addr, rwc->arg)) {
				anon_vma_unlock_read(anon_vma);
				return;
			}
			if (rwc->done && rwc->done(page)) {
				anon_vma_unlock_read(anon_vma);
				return;
			}
		}
		anon_vma_unlock_read(anon_vma);
	}
	if (!search_new_forks++)
		goto again;
}

#ifdef CONFIG_MIGRATION
void ksm_migrate_page(struct page *newpage, struct page *oldpage)
{
	struct stable_node *stable_node;

	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
	VM_BUG_ON_PAGE(newpage->mapping != oldpage->mapping, newpage);

	stable_node = page_stable_node(newpage);
	if (stable_node) {
		VM_BUG_ON_PAGE(stable_node->kpfn != page_to_pfn(oldpage), oldpage);
		stable_node->kpfn = page_to_pfn(newpage);
		/*
		 * newpage->mapping was set in advance; now we need smp_wmb()
		 * to make sure that the new stable_node->kpfn is visible
		 * to get_ksm_page() before it can see that oldpage->mapping
		 * has gone stale (or that PageSwapCache has been cleared).
		 */
		smp_wmb();
		set_page_stable_node(oldpage, NULL);
	}
}
#endif /* CONFIG_MIGRATION */

#ifdef CONFIG_MEMORY_HOTREMOVE
static void wait_while_offlining(void)
{
	while (ksm_run & KSM_RUN_OFFLINE) {
		mutex_unlock(&ksm_thread_mutex);
		wait_on_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE),
			    TASK_UNINTERRUPTIBLE);
		mutex_lock(&ksm_thread_mutex);
	}
}

static bool stable_node_dup_remove_range(struct stable_node *stable_node,
					 unsigned long start_pfn,
					 unsigned long end_pfn)
{
	if (stable_node->kpfn >= start_pfn &&
	    stable_node->kpfn < end_pfn) {
		/*
		 * Don't get_ksm_page, page has already gone:
		 * which is why we keep kpfn instead of page*
		 */
		remove_node_from_stable_tree(stable_node);
		return true;
	}
	return false;
}

static bool stable_node_chain_remove_range(struct stable_node *stable_node,
					   unsigned long start_pfn,
					   unsigned long end_pfn,
					   struct rb_root *root)
{
	struct stable_node *dup;
	struct hlist_node *hlist_safe;

	if (!is_stable_node_chain(stable_node)) {
		VM_BUG_ON(is_stable_node_dup(stable_node));
		return stable_node_dup_remove_range(stable_node, start_pfn,
						    end_pfn);
	}

	hlist_for_each_entry_safe(dup, hlist_safe,
				  &stable_node->hlist, hlist_dup) {
		VM_BUG_ON(!is_stable_node_dup(dup));
		stable_node_dup_remove_range(dup, start_pfn, end_pfn);
	}
	if (hlist_empty(&stable_node->hlist)) {
		free_stable_node_chain(stable_node, root);
		return true; /* notify caller that tree was rebalanced */
	} else
		return false;
}

static void ksm_check_stable_tree(unsigned long start_pfn,
				  unsigned long end_pfn)
{
	struct stable_node *stable_node, *next;
	struct rb_node *node;
	int nid;

	for (nid = 0; nid < ksm_nr_node_ids; nid++) {
		node = rb_first(root_stable_tree + nid);
		while (node) {
			stable_node = rb_entry(node, struct stable_node, node);
			if (stable_node_chain_remove_range(stable_node,
							   start_pfn, end_pfn,
							   root_stable_tree +
							   nid))
				node = rb_first(root_stable_tree + nid);
			else
				node = rb_next(node);
			cond_resched();
		}
	}
	list_for_each_entry_safe(stable_node, next, &migrate_nodes, list) {
		if (stable_node->kpfn >= start_pfn &&
		    stable_node->kpfn < end_pfn)
			remove_node_from_stable_tree(stable_node);
		cond_resched();
	}
}

static int ksm_memory_callback(struct notifier_block *self,
			       unsigned long action, void *arg)
{
	struct memory_notify *mn = arg;

	switch (action) {
	case MEM_GOING_OFFLINE:
		/*
		 * Prevent ksm_do_scan(), unmerge_and_remove_all_rmap_items()
		 * and remove_all_stable_nodes() while memory is going offline:
		 * it is unsafe for them to touch the stable tree at this time.
		 * But unmerge_ksm_pages(), rmap lookups and other entry points
		 * which do not need the ksm_thread_mutex are all safe.
		 */
		mutex_lock(&ksm_thread_mutex);
		ksm_run |= KSM_RUN_OFFLINE;
		mutex_unlock(&ksm_thread_mutex);
		break;

	case MEM_OFFLINE:
		/*
		 * Most of the work is done by page migration; but there might
		 * be a few stable_nodes left over, still pointing to struct
		 * pages which have been offlined: prune those from the tree,
		 * otherwise get_ksm_page() might later try to access a
		 * non-existent struct page.
		 */
		ksm_check_stable_tree(mn->start_pfn,
				      mn->start_pfn + mn->nr_pages);
		fallthrough;
	case MEM_CANCEL_OFFLINE:
		mutex_lock(&ksm_thread_mutex);
		ksm_run &= ~KSM_RUN_OFFLINE;
		mutex_unlock(&ksm_thread_mutex);

		smp_mb();	/* wake_up_bit advises this */
		wake_up_bit(&ksm_run, ilog2(KSM_RUN_OFFLINE));
		break;
	}
	return NOTIFY_OK;
}
#else
static void wait_while_offlining(void)
{
}
#endif /* CONFIG_MEMORY_HOTREMOVE */

#ifdef CONFIG_SYSFS
/*
 * This all compiles without CONFIG_SYSFS, but is a waste of space.
 */

#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 sysfs_emit(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 int msecs;
	int err;

	err = kstrtouint(buf, 10, &msecs);
	if (err)
		return -EINVAL;

	ksm_thread_sleep_millisecs = msecs;
	wake_up_interruptible(&ksm_iter_wait);

	return count;
}
KSM_ATTR(sleep_millisecs);

static ssize_t pages_to_scan_show(struct kobject *kobj,
				  struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(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)
{
	unsigned int nr_pages;
	int err;

	err = kstrtouint(buf, 10, &nr_pages);
	if (err)
		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 sysfs_emit(buf, "%lu\n", ksm_run);
}

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

	err = kstrtouint(buf, 10, &flags);
	if (err)
		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 pages_shared (but leaves mm_slots
	 * on the list for when ksmd may be set running again).
	 */

	mutex_lock(&ksm_thread_mutex);
	wait_while_offlining();
	if (ksm_run != flags) {
		ksm_run = flags;
		if (flags & KSM_RUN_UNMERGE) {
			set_current_oom_origin();
			err = unmerge_and_remove_all_rmap_items();
			clear_current_oom_origin();
			if (err) {
				ksm_run = KSM_RUN_STOP;
				count = err;
			}
		}
	}
	mutex_unlock(&ksm_thread_mutex);

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

	return count;
}
KSM_ATTR(run);

#ifdef CONFIG_NUMA
static ssize_t merge_across_nodes_show(struct kobject *kobj,
				       struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(buf, "%u\n", ksm_merge_across_nodes);
}

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

	err = kstrtoul(buf, 10, &knob);
	if (err)
		return err;
	if (knob > 1)
		return -EINVAL;

	mutex_lock(&ksm_thread_mutex);
	wait_while_offlining();
	if (ksm_merge_across_nodes != knob) {
		if (ksm_pages_shared || remove_all_stable_nodes())
			err = -EBUSY;
		else if (root_stable_tree == one_stable_tree) {
			struct rb_root *buf;
			/*
			 * This is the first time that we switch away from the
			 * default of merging across nodes: must now allocate
			 * a buffer to hold as many roots as may be needed.
			 * Allocate stable and unstable together:
			 * MAXSMP NODES_SHIFT 10 will use 16kB.
			 */
			buf = kcalloc(nr_node_ids + nr_node_ids, sizeof(*buf),
				      GFP_KERNEL);
			/* Let us assume that RB_ROOT is NULL is zero */
			if (!buf)
				err = -ENOMEM;
			else {
				root_stable_tree = buf;
				root_unstable_tree = buf + nr_node_ids;
				/* Stable tree is empty but not the unstable */
				root_unstable_tree[0] = one_unstable_tree[0];
			}
		}
		if (!err) {
			ksm_merge_across_nodes = knob;
			ksm_nr_node_ids = knob ? 1 : nr_node_ids;
		}
	}
	mutex_unlock(&ksm_thread_mutex);

	return err ? err : count;
}
KSM_ATTR(merge_across_nodes);
#endif

static ssize_t use_zero_pages_show(struct kobject *kobj,
				   struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(buf, "%u\n", ksm_use_zero_pages);
}
static ssize_t use_zero_pages_store(struct kobject *kobj,
				   struct kobj_attribute *attr,
				   const char *buf, size_t count)
{
	int err;
	bool value;

	err = kstrtobool(buf, &value);
	if (err)
		return -EINVAL;

	ksm_use_zero_pages = value;

	return count;
}
KSM_ATTR(use_zero_pages);

static ssize_t max_page_sharing_show(struct kobject *kobj,
				     struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(buf, "%u\n", ksm_max_page_sharing);
}

static ssize_t max_page_sharing_store(struct kobject *kobj,
				      struct kobj_attribute *attr,
				      const char *buf, size_t count)
{
	int err;
	int knob;

	err = kstrtoint(buf, 10, &knob);
	if (err)
		return err;
	/*
	 * When a KSM page is created it is shared by 2 mappings. This
	 * being a signed comparison, it implicitly verifies it's not
	 * negative.
	 */
	if (knob < 2)
		return -EINVAL;

	if (READ_ONCE(ksm_max_page_sharing) == knob)
		return count;

	mutex_lock(&ksm_thread_mutex);
	wait_while_offlining();
	if (ksm_max_page_sharing != knob) {
		if (ksm_pages_shared || remove_all_stable_nodes())
			err = -EBUSY;
		else
			ksm_max_page_sharing = knob;
	}
	mutex_unlock(&ksm_thread_mutex);

	return err ? err : count;
}
KSM_ATTR(max_page_sharing);

static ssize_t pages_shared_show(struct kobject *kobj,
				 struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(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 sysfs_emit(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 sysfs_emit(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 sysfs_emit(buf, "%ld\n", ksm_pages_volatile);
}
KSM_ATTR_RO(pages_volatile);

static ssize_t stable_node_dups_show(struct kobject *kobj,
				     struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(buf, "%lu\n", ksm_stable_node_dups);
}
KSM_ATTR_RO(stable_node_dups);

static ssize_t stable_node_chains_show(struct kobject *kobj,
				       struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(buf, "%lu\n", ksm_stable_node_chains);
}
KSM_ATTR_RO(stable_node_chains);

static ssize_t
stable_node_chains_prune_millisecs_show(struct kobject *kobj,
					struct kobj_attribute *attr,
					char *buf)
{
	return sysfs_emit(buf, "%u\n", ksm_stable_node_chains_prune_millisecs);
}

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

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

	ksm_stable_node_chains_prune_millisecs = msecs;

	return count;
}
KSM_ATTR(stable_node_chains_prune_millisecs);

static ssize_t full_scans_show(struct kobject *kobj,
			       struct kobj_attribute *attr, char *buf)
{
	return sysfs_emit(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,
	&pages_shared_attr.attr,
	&pages_sharing_attr.attr,
	&pages_unshared_attr.attr,
	&pages_volatile_attr.attr,
	&full_scans_attr.attr,
#ifdef CONFIG_NUMA
	&merge_across_nodes_attr.attr,
#endif
	&max_page_sharing_attr.attr,
	&stable_node_chains_attr.attr,
	&stable_node_dups_attr.attr,
	&stable_node_chains_prune_millisecs_attr.attr,
	&use_zero_pages_attr.attr,
	NULL,
};

static const struct attribute_group ksm_attr_group = {
	.attrs = ksm_attrs,
	.name = "ksm",
};
#endif /* CONFIG_SYSFS */

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

	/* The correct value depends on page size and endianness */
	zero_checksum = calc_checksum(ZERO_PAGE(0));
	/* Default to false for backwards compatibility */
	ksm_use_zero_pages = false;

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

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

#ifdef CONFIG_SYSFS
	err = sysfs_create_group(mm_kobj, &ksm_attr_group);
	if (err) {
		pr_err("ksm: register sysfs failed\n");
		kthread_stop(ksm_thread);
		goto out_free;
	}
#else
	ksm_run = KSM_RUN_MERGE;	/* no way for user to start it */

#endif /* CONFIG_SYSFS */

#ifdef CONFIG_MEMORY_HOTREMOVE
	/* There is no significance to this priority 100 */
	hotplug_memory_notifier(ksm_memory_callback, 100);
#endif
	return 0;

out_free:
	ksm_slab_free();
out:
	return err;
}
subsys_initcall(ksm_init);