aboutsummaryrefslogtreecommitdiff
path: root/mm/zsmalloc.c
blob: 36b4591a7a2d3b2eca7111b26bee3efa19c7c5c5 (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
/*
 * zsmalloc memory allocator
 *
 * Copyright (C) 2011  Nitin Gupta
 * Copyright (C) 2012, 2013 Minchan Kim
 *
 * This code is released using a dual license strategy: BSD/GPL
 * You can choose the license that better fits your requirements.
 *
 * Released under the terms of 3-clause BSD License
 * Released under the terms of GNU General Public License Version 2.0
 */

/*
 * This allocator is designed for use with zram. Thus, the allocator is
 * supposed to work well under low memory conditions. In particular, it
 * never attempts higher order page allocation which is very likely to
 * fail under memory pressure. On the other hand, if we just use single
 * (0-order) pages, it would suffer from very high fragmentation --
 * any object of size PAGE_SIZE/2 or larger would occupy an entire page.
 * This was one of the major issues with its predecessor (xvmalloc).
 *
 * To overcome these issues, zsmalloc allocates a bunch of 0-order pages
 * and links them together using various 'struct page' fields. These linked
 * pages act as a single higher-order page i.e. an object can span 0-order
 * page boundaries. The code refers to these linked pages as a single entity
 * called zspage.
 *
 * For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
 * since this satisfies the requirements of all its current users (in the
 * worst case, page is incompressible and is thus stored "as-is" i.e. in
 * uncompressed form). For allocation requests larger than this size, failure
 * is returned (see zs_malloc).
 *
 * Additionally, zs_malloc() does not return a dereferenceable pointer.
 * Instead, it returns an opaque handle (unsigned long) which encodes actual
 * location of the allocated object. The reason for this indirection is that
 * zsmalloc does not keep zspages permanently mapped since that would cause
 * issues on 32-bit systems where the VA region for kernel space mappings
 * is very small. So, before using the allocating memory, the object has to
 * be mapped using zs_map_object() to get a usable pointer and subsequently
 * unmapped using zs_unmap_object().
 *
 * Following is how we use various fields and flags of underlying
 * struct page(s) to form a zspage.
 *
 * Usage of struct page fields:
 *	page->first_page: points to the first component (0-order) page
 *	page->index (union with page->freelist): offset of the first object
 *		starting in this page. For the first page, this is
 *		always 0, so we use this field (aka freelist) to point
 *		to the first free object in zspage.
 *	page->lru: links together all component pages (except the first page)
 *		of a zspage
 *
 *	For _first_ page only:
 *
 *	page->private (union with page->first_page): refers to the
 *		component page after the first page
 *	page->freelist: points to the first free object in zspage.
 *		Free objects are linked together using in-place
 *		metadata.
 *	page->objects: maximum number of objects we can store in this
 *		zspage (class->zspage_order * PAGE_SIZE / class->size)
 *	page->lru: links together first pages of various zspages.
 *		Basically forming list of zspages in a fullness group.
 *	page->mapping: class index and fullness group of the zspage
 *
 * Usage of struct page flags:
 *	PG_private: identifies the first component page
 *	PG_private2: identifies the last component page
 *
 */

#ifdef CONFIG_ZSMALLOC_DEBUG
#define DEBUG
#endif

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/bitops.h>
#include <linux/errno.h>
#include <linux/highmem.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <asm/tlbflush.h>
#include <asm/pgtable.h>
#include <linux/cpumask.h>
#include <linux/cpu.h>
#include <linux/vmalloc.h>
#include <linux/hardirq.h>
#include <linux/spinlock.h>
#include <linux/types.h>
#include <linux/zsmalloc.h>

/*
 * This must be power of 2 and greater than of equal to sizeof(link_free).
 * These two conditions ensure that any 'struct link_free' itself doesn't
 * span more than 1 page which avoids complex case of mapping 2 pages simply
 * to restore link_free pointer values.
 */
#define ZS_ALIGN		8

/*
 * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
 * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
 */
#define ZS_MAX_ZSPAGE_ORDER 2
#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)

/*
 * Object location (<PFN>, <obj_idx>) is encoded as
 * as single (unsigned long) handle value.
 *
 * Note that object index <obj_idx> is relative to system
 * page <PFN> it is stored in, so for each sub-page belonging
 * to a zspage, obj_idx starts with 0.
 *
 * This is made more complicated by various memory models and PAE.
 */

#ifndef MAX_PHYSMEM_BITS
#ifdef CONFIG_HIGHMEM64G
#define MAX_PHYSMEM_BITS 36
#else /* !CONFIG_HIGHMEM64G */
/*
 * If this definition of MAX_PHYSMEM_BITS is used, OBJ_INDEX_BITS will just
 * be PAGE_SHIFT
 */
#define MAX_PHYSMEM_BITS BITS_PER_LONG
#endif
#endif
#define _PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
#define OBJ_INDEX_BITS	(BITS_PER_LONG - _PFN_BITS)
#define OBJ_INDEX_MASK	((_AC(1, UL) << OBJ_INDEX_BITS) - 1)

#define MAX(a, b) ((a) >= (b) ? (a) : (b))
/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
#define ZS_MIN_ALLOC_SIZE \
	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
#define ZS_MAX_ALLOC_SIZE	PAGE_SIZE

/*
 * On systems with 4K page size, this gives 254 size classes! There is a
 * trader-off here:
 *  - Large number of size classes is potentially wasteful as free page are
 *    spread across these classes
 *  - Small number of size classes causes large internal fragmentation
 *  - Probably its better to use specific size classes (empirically
 *    determined). NOTE: all those class sizes must be set as multiple of
 *    ZS_ALIGN to make sure link_free itself never has to span 2 pages.
 *
 *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
 *  (reason above)
 */
#define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> 8)
#define ZS_SIZE_CLASSES		((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / \
					ZS_SIZE_CLASS_DELTA + 1)

/*
 * We do not maintain any list for completely empty or full pages
 */
enum fullness_group {
	ZS_ALMOST_FULL,
	ZS_ALMOST_EMPTY,
	_ZS_NR_FULLNESS_GROUPS,

	ZS_EMPTY,
	ZS_FULL
};

/*
 * We assign a page to ZS_ALMOST_EMPTY fullness group when:
 *	n <= N / f, where
 * n = number of allocated objects
 * N = total number of objects zspage can store
 * f = 1/fullness_threshold_frac
 *
 * Similarly, we assign zspage to:
 *	ZS_ALMOST_FULL	when n > N / f
 *	ZS_EMPTY	when n == 0
 *	ZS_FULL		when n == N
 *
 * (see: fix_fullness_group())
 */
static const int fullness_threshold_frac = 4;

struct size_class {
	/*
	 * Size of objects stored in this class. Must be multiple
	 * of ZS_ALIGN.
	 */
	int size;
	unsigned int index;

	/* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
	int pages_per_zspage;

	spinlock_t lock;

	/* stats */
	u64 pages_allocated;

	struct page *fullness_list[_ZS_NR_FULLNESS_GROUPS];
};

/*
 * Placed within free objects to form a singly linked list.
 * For every zspage, first_page->freelist gives head of this list.
 *
 * This must be power of 2 and less than or equal to ZS_ALIGN
 */
struct link_free {
	/* Handle of next free chunk (encodes <PFN, obj_idx>) */
	void *next;
};

struct zs_pool {
	struct size_class size_class[ZS_SIZE_CLASSES];

	gfp_t flags;	/* allocation flags used when growing pool */
};

/*
 * A zspage's class index and fullness group
 * are encoded in its (first)page->mapping
 */
#define CLASS_IDX_BITS	28
#define FULLNESS_BITS	4
#define CLASS_IDX_MASK	((1 << CLASS_IDX_BITS) - 1)
#define FULLNESS_MASK	((1 << FULLNESS_BITS) - 1)

struct mapping_area {
#ifdef CONFIG_PGTABLE_MAPPING
	struct vm_struct *vm; /* vm area for mapping object that span pages */
#else
	char *vm_buf; /* copy buffer for objects that span pages */
#endif
	char *vm_addr; /* address of kmap_atomic()'ed pages */
	enum zs_mapmode vm_mm; /* mapping mode */
};


/* per-cpu VM mapping areas for zspage accesses that cross page boundaries */
static DEFINE_PER_CPU(struct mapping_area, zs_map_area);

static int is_first_page(struct page *page)
{
	return PagePrivate(page);
}

static int is_last_page(struct page *page)
{
	return PagePrivate2(page);
}

static void get_zspage_mapping(struct page *page, unsigned int *class_idx,
				enum fullness_group *fullness)
{
	unsigned long m;
	BUG_ON(!is_first_page(page));

	m = (unsigned long)page->mapping;
	*fullness = m & FULLNESS_MASK;
	*class_idx = (m >> FULLNESS_BITS) & CLASS_IDX_MASK;
}

static void set_zspage_mapping(struct page *page, unsigned int class_idx,
				enum fullness_group fullness)
{
	unsigned long m;
	BUG_ON(!is_first_page(page));

	m = ((class_idx & CLASS_IDX_MASK) << FULLNESS_BITS) |
			(fullness & FULLNESS_MASK);
	page->mapping = (struct address_space *)m;
}

/*
 * zsmalloc divides the pool into various size classes where each
 * class maintains a list of zspages where each zspage is divided
 * into equal sized chunks. Each allocation falls into one of these
 * classes depending on its size. This function returns index of the
 * size class which has chunk size big enough to hold the give size.
 */
static int get_size_class_index(int size)
{
	int idx = 0;

	if (likely(size > ZS_MIN_ALLOC_SIZE))
		idx = DIV_ROUND_UP(size - ZS_MIN_ALLOC_SIZE,
				ZS_SIZE_CLASS_DELTA);

	return idx;
}

/*
 * For each size class, zspages are divided into different groups
 * depending on how "full" they are. This was done so that we could
 * easily find empty or nearly empty zspages when we try to shrink
 * the pool (not yet implemented). This function returns fullness
 * status of the given page.
 */
static enum fullness_group get_fullness_group(struct page *page)
{
	int inuse, max_objects;
	enum fullness_group fg;
	BUG_ON(!is_first_page(page));

	inuse = page->inuse;
	max_objects = page->objects;

	if (inuse == 0)
		fg = ZS_EMPTY;
	else if (inuse == max_objects)
		fg = ZS_FULL;
	else if (inuse <= max_objects / fullness_threshold_frac)
		fg = ZS_ALMOST_EMPTY;
	else
		fg = ZS_ALMOST_FULL;

	return fg;
}

/*
 * Each size class maintains various freelists and zspages are assigned
 * to one of these freelists based on the number of live objects they
 * have. This functions inserts the given zspage into the freelist
 * identified by <class, fullness_group>.
 */
static void insert_zspage(struct page *page, struct size_class *class,
				enum fullness_group fullness)
{
	struct page **head;

	BUG_ON(!is_first_page(page));

	if (fullness >= _ZS_NR_FULLNESS_GROUPS)
		return;

	head = &class->fullness_list[fullness];
	if (*head)
		list_add_tail(&page->lru, &(*head)->lru);

	*head = page;
}

/*
 * This function removes the given zspage from the freelist identified
 * by <class, fullness_group>.
 */
static void remove_zspage(struct page *page, struct size_class *class,
				enum fullness_group fullness)
{
	struct page **head;

	BUG_ON(!is_first_page(page));

	if (fullness >= _ZS_NR_FULLNESS_GROUPS)
		return;

	head = &class->fullness_list[fullness];
	BUG_ON(!*head);
	if (list_empty(&(*head)->lru))
		*head = NULL;
	else if (*head == page)
		*head = (struct page *)list_entry((*head)->lru.next,
					struct page, lru);

	list_del_init(&page->lru);
}

/*
 * Each size class maintains zspages in different fullness groups depending
 * on the number of live objects they contain. When allocating or freeing
 * objects, the fullness status of the page can change, say, from ALMOST_FULL
 * to ALMOST_EMPTY when freeing an object. This function checks if such
 * a status change has occurred for the given page and accordingly moves the
 * page from the freelist of the old fullness group to that of the new
 * fullness group.
 */
static enum fullness_group fix_fullness_group(struct zs_pool *pool,
						struct page *page)
{
	int class_idx;
	struct size_class *class;
	enum fullness_group currfg, newfg;

	BUG_ON(!is_first_page(page));

	get_zspage_mapping(page, &class_idx, &currfg);
	newfg = get_fullness_group(page);
	if (newfg == currfg)
		goto out;

	class = &pool->size_class[class_idx];
	remove_zspage(page, class, currfg);
	insert_zspage(page, class, newfg);
	set_zspage_mapping(page, class_idx, newfg);

out:
	return newfg;
}

/*
 * We have to decide on how many pages to link together
 * to form a zspage for each size class. This is important
 * to reduce wastage due to unusable space left at end of
 * each zspage which is given as:
 *	wastage = Zp - Zp % size_class
 * where Zp = zspage size = k * PAGE_SIZE where k = 1, 2, ...
 *
 * For example, for size class of 3/8 * PAGE_SIZE, we should
 * link together 3 PAGE_SIZE sized pages to form a zspage
 * since then we can perfectly fit in 8 such objects.
 */
static int get_pages_per_zspage(int class_size)
{
	int i, max_usedpc = 0;
	/* zspage order which gives maximum used size per KB */
	int max_usedpc_order = 1;

	for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
		int zspage_size;
		int waste, usedpc;

		zspage_size = i * PAGE_SIZE;
		waste = zspage_size % class_size;
		usedpc = (zspage_size - waste) * 100 / zspage_size;

		if (usedpc > max_usedpc) {
			max_usedpc = usedpc;
			max_usedpc_order = i;
		}
	}

	return max_usedpc_order;
}

/*
 * A single 'zspage' is composed of many system pages which are
 * linked together using fields in struct page. This function finds
 * the first/head page, given any component page of a zspage.
 */
static struct page *get_first_page(struct page *page)
{
	if (is_first_page(page))
		return page;
	else
		return page->first_page;
}

static struct page *get_next_page(struct page *page)
{
	struct page *next;

	if (is_last_page(page))
		next = NULL;
	else if (is_first_page(page))
		next = (struct page *)page_private(page);
	else
		next = list_entry(page->lru.next, struct page, lru);

	return next;
}

/*
 * Encode <page, obj_idx> as a single handle value.
 * On hardware platforms with physical memory starting at 0x0 the pfn
 * could be 0 so we ensure that the handle will never be 0 by adjusting the
 * encoded obj_idx value before encoding.
 */
static void *obj_location_to_handle(struct page *page, unsigned long obj_idx)
{
	unsigned long handle;

	if (!page) {
		BUG_ON(obj_idx);
		return NULL;
	}

	handle = page_to_pfn(page) << OBJ_INDEX_BITS;
	handle |= ((obj_idx + 1) & OBJ_INDEX_MASK);

	return (void *)handle;
}

/*
 * Decode <page, obj_idx> pair from the given object handle. We adjust the
 * decoded obj_idx back to its original value since it was adjusted in
 * obj_location_to_handle().
 */
static void obj_handle_to_location(unsigned long handle, struct page **page,
				unsigned long *obj_idx)
{
	*page = pfn_to_page(handle >> OBJ_INDEX_BITS);
	*obj_idx = (handle & OBJ_INDEX_MASK) - 1;
}

static unsigned long obj_idx_to_offset(struct page *page,
				unsigned long obj_idx, int class_size)
{
	unsigned long off = 0;

	if (!is_first_page(page))
		off = page->index;

	return off + obj_idx * class_size;
}

static void reset_page(struct page *page)
{
	clear_bit(PG_private, &page->flags);
	clear_bit(PG_private_2, &page->flags);
	set_page_private(page, 0);
	page->mapping = NULL;
	page->freelist = NULL;
	page_mapcount_reset(page);
}

static void free_zspage(struct page *first_page)
{
	struct page *nextp, *tmp, *head_extra;

	BUG_ON(!is_first_page(first_page));
	BUG_ON(first_page->inuse);

	head_extra = (struct page *)page_private(first_page);

	reset_page(first_page);
	__free_page(first_page);

	/* zspage with only 1 system page */
	if (!head_extra)
		return;

	list_for_each_entry_safe(nextp, tmp, &head_extra->lru, lru) {
		list_del(&nextp->lru);
		reset_page(nextp);
		__free_page(nextp);
	}
	reset_page(head_extra);
	__free_page(head_extra);
}

/* Initialize a newly allocated zspage */
static void init_zspage(struct page *first_page, struct size_class *class)
{
	unsigned long off = 0;
	struct page *page = first_page;

	BUG_ON(!is_first_page(first_page));
	while (page) {
		struct page *next_page;
		struct link_free *link;
		unsigned int i, objs_on_page;

		/*
		 * page->index stores offset of first object starting
		 * in the page. For the first page, this is always 0,
		 * so we use first_page->index (aka ->freelist) to store
		 * head of corresponding zspage's freelist.
		 */
		if (page != first_page)
			page->index = off;

		link = (struct link_free *)kmap_atomic(page) +
						off / sizeof(*link);
		objs_on_page = (PAGE_SIZE - off) / class->size;

		for (i = 1; i <= objs_on_page; i++) {
			off += class->size;
			if (off < PAGE_SIZE) {
				link->next = obj_location_to_handle(page, i);
				link += class->size / sizeof(*link);
			}
		}

		/*
		 * We now come to the last (full or partial) object on this
		 * page, which must point to the first object on the next
		 * page (if present)
		 */
		next_page = get_next_page(page);
		link->next = obj_location_to_handle(next_page, 0);
		kunmap_atomic(link);
		page = next_page;
		off = (off + class->size) % PAGE_SIZE;
	}
}

/*
 * Allocate a zspage for the given size class
 */
static struct page *alloc_zspage(struct size_class *class, gfp_t flags)
{
	int i, error;
	struct page *first_page = NULL, *uninitialized_var(prev_page);

	/*
	 * Allocate individual pages and link them together as:
	 * 1. first page->private = first sub-page
	 * 2. all sub-pages are linked together using page->lru
	 * 3. each sub-page is linked to the first page using page->first_page
	 *
	 * For each size class, First/Head pages are linked together using
	 * page->lru. Also, we set PG_private to identify the first page
	 * (i.e. no other sub-page has this flag set) and PG_private_2 to
	 * identify the last page.
	 */
	error = -ENOMEM;
	for (i = 0; i < class->pages_per_zspage; i++) {
		struct page *page;

		page = alloc_page(flags);
		if (!page)
			goto cleanup;

		INIT_LIST_HEAD(&page->lru);
		if (i == 0) {	/* first page */
			SetPagePrivate(page);
			set_page_private(page, 0);
			first_page = page;
			first_page->inuse = 0;
		}
		if (i == 1)
			set_page_private(first_page, (unsigned long)page);
		if (i >= 1)
			page->first_page = first_page;
		if (i >= 2)
			list_add(&page->lru, &prev_page->lru);
		if (i == class->pages_per_zspage - 1)	/* last page */
			SetPagePrivate2(page);
		prev_page = page;
	}

	init_zspage(first_page, class);

	first_page->freelist = obj_location_to_handle(first_page, 0);
	/* Maximum number of objects we can store in this zspage */
	first_page->objects = class->pages_per_zspage * PAGE_SIZE / class->size;

	error = 0; /* Success */

cleanup:
	if (unlikely(error) && first_page) {
		free_zspage(first_page);
		first_page = NULL;
	}

	return first_page;
}

static struct page *find_get_zspage(struct size_class *class)
{
	int i;
	struct page *page;

	for (i = 0; i < _ZS_NR_FULLNESS_GROUPS; i++) {
		page = class->fullness_list[i];
		if (page)
			break;
	}

	return page;
}

#ifdef CONFIG_PGTABLE_MAPPING
static inline int __zs_cpu_up(struct mapping_area *area)
{
	/*
	 * Make sure we don't leak memory if a cpu UP notification
	 * and zs_init() race and both call zs_cpu_up() on the same cpu
	 */
	if (area->vm)
		return 0;
	area->vm = alloc_vm_area(PAGE_SIZE * 2, NULL);
	if (!area->vm)
		return -ENOMEM;
	return 0;
}

static inline void __zs_cpu_down(struct mapping_area *area)
{
	if (area->vm)
		free_vm_area(area->vm);
	area->vm = NULL;
}

static inline void *__zs_map_object(struct mapping_area *area,
				struct page *pages[2], int off, int size)
{
	BUG_ON(map_vm_area(area->vm, PAGE_KERNEL, &pages));
	area->vm_addr = area->vm->addr;
	return area->vm_addr + off;
}

static inline void __zs_unmap_object(struct mapping_area *area,
				struct page *pages[2], int off, int size)
{
	unsigned long addr = (unsigned long)area->vm_addr;

	unmap_kernel_range(addr, PAGE_SIZE * 2);
}

#else /* CONFIG_PGTABLE_MAPPING */

static inline int __zs_cpu_up(struct mapping_area *area)
{
	/*
	 * Make sure we don't leak memory if a cpu UP notification
	 * and zs_init() race and both call zs_cpu_up() on the same cpu
	 */
	if (area->vm_buf)
		return 0;
	area->vm_buf = (char *)__get_free_page(GFP_KERNEL);
	if (!area->vm_buf)
		return -ENOMEM;
	return 0;
}

static inline void __zs_cpu_down(struct mapping_area *area)
{
	if (area->vm_buf)
		free_page((unsigned long)area->vm_buf);
	area->vm_buf = NULL;
}

static void *__zs_map_object(struct mapping_area *area,
			struct page *pages[2], int off, int size)
{
	int sizes[2];
	void *addr;
	char *buf = area->vm_buf;

	/* disable page faults to match kmap_atomic() return conditions */
	pagefault_disable();

	/* no read fastpath */
	if (area->vm_mm == ZS_MM_WO)
		goto out;

	sizes[0] = PAGE_SIZE - off;
	sizes[1] = size - sizes[0];

	/* copy object to per-cpu buffer */
	addr = kmap_atomic(pages[0]);
	memcpy(buf, addr + off, sizes[0]);
	kunmap_atomic(addr);
	addr = kmap_atomic(pages[1]);
	memcpy(buf + sizes[0], addr, sizes[1]);
	kunmap_atomic(addr);
out:
	return area->vm_buf;
}

static void __zs_unmap_object(struct mapping_area *area,
			struct page *pages[2], int off, int size)
{
	int sizes[2];
	void *addr;
	char *buf = area->vm_buf;

	/* no write fastpath */
	if (area->vm_mm == ZS_MM_RO)
		goto out;

	sizes[0] = PAGE_SIZE - off;
	sizes[1] = size - sizes[0];

	/* copy per-cpu buffer to object */
	addr = kmap_atomic(pages[0]);
	memcpy(addr + off, buf, sizes[0]);
	kunmap_atomic(addr);
	addr = kmap_atomic(pages[1]);
	memcpy(addr, buf + sizes[0], sizes[1]);
	kunmap_atomic(addr);

out:
	/* enable page faults to match kunmap_atomic() return conditions */
	pagefault_enable();
}

#endif /* CONFIG_PGTABLE_MAPPING */

static int zs_cpu_notifier(struct notifier_block *nb, unsigned long action,
				void *pcpu)
{
	int ret, cpu = (long)pcpu;
	struct mapping_area *area;

	switch (action) {
	case CPU_UP_PREPARE:
		area = &per_cpu(zs_map_area, cpu);
		ret = __zs_cpu_up(area);
		if (ret)
			return notifier_from_errno(ret);
		break;
	case CPU_DEAD:
	case CPU_UP_CANCELED:
		area = &per_cpu(zs_map_area, cpu);
		__zs_cpu_down(area);
		break;
	}

	return NOTIFY_OK;
}

static struct notifier_block zs_cpu_nb = {
	.notifier_call = zs_cpu_notifier
};

static void zs_exit(void)
{
	int cpu;

	cpu_notifier_register_begin();

	for_each_online_cpu(cpu)
		zs_cpu_notifier(NULL, CPU_DEAD, (void *)(long)cpu);
	__unregister_cpu_notifier(&zs_cpu_nb);

	cpu_notifier_register_done();
}

static int zs_init(void)
{
	int cpu, ret;

	cpu_notifier_register_begin();

	__register_cpu_notifier(&zs_cpu_nb);
	for_each_online_cpu(cpu) {
		ret = zs_cpu_notifier(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
		if (notifier_to_errno(ret)) {
			cpu_notifier_register_done();
			goto fail;
		}
	}

	cpu_notifier_register_done();

	return 0;
fail:
	zs_exit();
	return notifier_to_errno(ret);
}

/**
 * zs_create_pool - Creates an allocation pool to work from.
 * @flags: allocation flags used to allocate pool metadata
 *
 * This function must be called before anything when using
 * the zsmalloc allocator.
 *
 * On success, a pointer to the newly created pool is returned,
 * otherwise NULL.
 */
struct zs_pool *zs_create_pool(gfp_t flags)
{
	int i, ovhd_size;
	struct zs_pool *pool;

	ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
	pool = kzalloc(ovhd_size, GFP_KERNEL);
	if (!pool)
		return NULL;

	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
		int size;
		struct size_class *class;

		size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
		if (size > ZS_MAX_ALLOC_SIZE)
			size = ZS_MAX_ALLOC_SIZE;

		class = &pool->size_class[i];
		class->size = size;
		class->index = i;
		spin_lock_init(&class->lock);
		class->pages_per_zspage = get_pages_per_zspage(size);

	}

	pool->flags = flags;

	return pool;
}
EXPORT_SYMBOL_GPL(zs_create_pool);

void zs_destroy_pool(struct zs_pool *pool)
{
	int i;

	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
		int fg;
		struct size_class *class = &pool->size_class[i];

		for (fg = 0; fg < _ZS_NR_FULLNESS_GROUPS; fg++) {
			if (class->fullness_list[fg]) {
				pr_info("Freeing non-empty class with size %db, fullness group %d\n",
					class->size, fg);
			}
		}
	}
	kfree(pool);
}
EXPORT_SYMBOL_GPL(zs_destroy_pool);

/**
 * zs_malloc - Allocate block of given size from pool.
 * @pool: pool to allocate from
 * @size: size of block to allocate
 *
 * On success, handle to the allocated object is returned,
 * otherwise 0.
 * Allocation requests with size > ZS_MAX_ALLOC_SIZE will fail.
 */
unsigned long zs_malloc(struct zs_pool *pool, size_t size)
{
	unsigned long obj;
	struct link_free *link;
	int class_idx;
	struct size_class *class;

	struct page *first_page, *m_page;
	unsigned long m_objidx, m_offset;

	if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
		return 0;

	class_idx = get_size_class_index(size);
	class = &pool->size_class[class_idx];
	BUG_ON(class_idx != class->index);

	spin_lock(&class->lock);
	first_page = find_get_zspage(class);

	if (!first_page) {
		spin_unlock(&class->lock);
		first_page = alloc_zspage(class, pool->flags);
		if (unlikely(!first_page))
			return 0;

		set_zspage_mapping(first_page, class->index, ZS_EMPTY);
		spin_lock(&class->lock);
		class->pages_allocated += class->pages_per_zspage;
	}

	obj = (unsigned long)first_page->freelist;
	obj_handle_to_location(obj, &m_page, &m_objidx);
	m_offset = obj_idx_to_offset(m_page, m_objidx, class->size);

	link = (struct link_free *)kmap_atomic(m_page) +
					m_offset / sizeof(*link);
	first_page->freelist = link->next;
	memset(link, POISON_INUSE, sizeof(*link));
	kunmap_atomic(link);

	first_page->inuse++;
	/* Now move the zspage to another fullness group, if required */
	fix_fullness_group(pool, first_page);
	spin_unlock(&class->lock);

	return obj;
}
EXPORT_SYMBOL_GPL(zs_malloc);

void zs_free(struct zs_pool *pool, unsigned long obj)
{
	struct link_free *link;
	struct page *first_page, *f_page;
	unsigned long f_objidx, f_offset;

	int class_idx;
	struct size_class *class;
	enum fullness_group fullness;

	if (unlikely(!obj))
		return;

	obj_handle_to_location(obj, &f_page, &f_objidx);
	first_page = get_first_page(f_page);

	get_zspage_mapping(first_page, &class_idx, &fullness);
	class = &pool->size_class[class_idx];
	f_offset = obj_idx_to_offset(f_page, f_objidx, class->size);

	spin_lock(&class->lock);

	/* Insert this object in containing zspage's freelist */
	link = (struct link_free *)((unsigned char *)kmap_atomic(f_page)
							+ f_offset);
	link->next = first_page->freelist;
	kunmap_atomic(link);
	first_page->freelist = (void *)obj;

	first_page->inuse--;
	fullness = fix_fullness_group(pool, first_page);

	if (fullness == ZS_EMPTY)
		class->pages_allocated -= class->pages_per_zspage;

	spin_unlock(&class->lock);

	if (fullness == ZS_EMPTY)
		free_zspage(first_page);
}
EXPORT_SYMBOL_GPL(zs_free);

/**
 * zs_map_object - get address of allocated object from handle.
 * @pool: pool from which the object was allocated
 * @handle: handle returned from zs_malloc
 *
 * Before using an object allocated from zs_malloc, it must be mapped using
 * this function. When done with the object, it must be unmapped using
 * zs_unmap_object.
 *
 * Only one object can be mapped per cpu at a time. There is no protection
 * against nested mappings.
 *
 * This function returns with preemption and page faults disabled.
 */
void *zs_map_object(struct zs_pool *pool, unsigned long handle,
			enum zs_mapmode mm)
{
	struct page *page;
	unsigned long obj_idx, off;

	unsigned int class_idx;
	enum fullness_group fg;
	struct size_class *class;
	struct mapping_area *area;
	struct page *pages[2];

	BUG_ON(!handle);

	/*
	 * Because we use per-cpu mapping areas shared among the
	 * pools/users, we can't allow mapping in interrupt context
	 * because it can corrupt another users mappings.
	 */
	BUG_ON(in_interrupt());

	obj_handle_to_location(handle, &page, &obj_idx);
	get_zspage_mapping(get_first_page(page), &class_idx, &fg);
	class = &pool->size_class[class_idx];
	off = obj_idx_to_offset(page, obj_idx, class->size);

	area = &get_cpu_var(zs_map_area);
	area->vm_mm = mm;
	if (off + class->size <= PAGE_SIZE) {
		/* this object is contained entirely within a page */
		area->vm_addr = kmap_atomic(page);
		return area->vm_addr + off;
	}

	/* this object spans two pages */
	pages[0] = page;
	pages[1] = get_next_page(page);
	BUG_ON(!pages[1]);

	return __zs_map_object(area, pages, off, class->size);
}
EXPORT_SYMBOL_GPL(zs_map_object);

void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
{
	struct page *page;
	unsigned long obj_idx, off;

	unsigned int class_idx;
	enum fullness_group fg;
	struct size_class *class;
	struct mapping_area *area;

	BUG_ON(!handle);

	obj_handle_to_location(handle, &page, &obj_idx);
	get_zspage_mapping(get_first_page(page), &class_idx, &fg);
	class = &pool->size_class[class_idx];
	off = obj_idx_to_offset(page, obj_idx, class->size);

	area = &__get_cpu_var(zs_map_area);
	if (off + class->size <= PAGE_SIZE)
		kunmap_atomic(area->vm_addr);
	else {
		struct page *pages[2];

		pages[0] = page;
		pages[1] = get_next_page(page);
		BUG_ON(!pages[1]);

		__zs_unmap_object(area, pages, off, class->size);
	}
	put_cpu_var(zs_map_area);
}
EXPORT_SYMBOL_GPL(zs_unmap_object);

u64 zs_get_total_size_bytes(struct zs_pool *pool)
{
	int i;
	u64 npages = 0;

	for (i = 0; i < ZS_SIZE_CLASSES; i++)
		npages += pool->size_class[i].pages_allocated;

	return npages << PAGE_SHIFT;
}
EXPORT_SYMBOL_GPL(zs_get_total_size_bytes);

module_init(zs_init);
module_exit(zs_exit);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Nitin Gupta <ngupta@vflare.org>");