1 // SPDX-License-Identifier: GPL-2.0
2 // Copyright(c) 2018 Intel Corporation. All rights reserved.
3
4 #include <linux/mm.h>
5 #include <linux/init.h>
6 #include <linux/mmzone.h>
7 #include <linux/random.h>
8 #include <linux/moduleparam.h>
9 #include "internal.h"
10 #include "shuffle.h"
11
12 DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);
13
14 static bool shuffle_param;
15
16 static __meminit int shuffle_param_set(const char *val,
17 const struct kernel_param *kp)
18 {
19 if (param_set_bool(val, kp))
20 return -EINVAL;
21 if (*(bool *)kp->arg)
22 static_branch_enable(&page_alloc_shuffle_key);
23 return 0;
24 }
25
26 static const struct kernel_param_ops shuffle_param_ops = {
27 .set = shuffle_param_set,
28 .get = param_get_bool,
29 };
30 module_param_cb(shuffle, &shuffle_param_ops, &shuffle_param, 0400);
31
32 /*
33 * For two pages to be swapped in the shuffle, they must be free (on a
34 * 'free_area' lru), have the same order, and have the same migratetype.
35 */
36 static struct page * __meminit shuffle_valid_page(struct zone *zone,
37 unsigned long pfn, int order)
38 {
39 struct page *page = pfn_to_online_page(pfn);
40
41 /*
42 * Given we're dealing with randomly selected pfns in a zone we
43 * need to ask questions like...
44 */
45
46 /* ... is the page managed by the buddy? */
47 if (!page)
48 return NULL;
49
50 /* ... is the page assigned to the same zone? */
51 if (page_zone(page) != zone)
52 return NULL;
53
54 /* ...is the page free and currently on a free_area list? */
55 if (!PageBuddy(page))
56 return NULL;
57
58 /*
59 * ...is the page on the same list as the page we will
60 * shuffle it with?
61 */
62 if (buddy_order(page) != order)
63 return NULL;
64
65 return page;
66 }
67
68 /*
69 * Fisher-Yates shuffle the freelist which prescribes iterating through an
70 * array, pfns in this case, and randomly swapping each entry with another in
71 * the span, end_pfn - start_pfn.
72 *
73 * To keep the implementation simple it does not attempt to correct for sources
74 * of bias in the distribution, like modulo bias or pseudo-random number
75 * generator bias. I.e. the expectation is that this shuffling raises the bar
76 * for attacks that exploit the predictability of page allocations, but need not
77 * be a perfect shuffle.
78 */
79 #define SHUFFLE_RETRY 10
80 void __meminit __shuffle_zone(struct zone *z)
81 {
82 unsigned long i, flags;
83 unsigned long start_pfn = z->zone_start_pfn;
84 unsigned long end_pfn = zone_end_pfn(z);
85 const int order = SHUFFLE_ORDER;
86 const int order_pages = 1 << order;
87
88 spin_lock_irqsave(&z->lock, flags);
89 start_pfn = ALIGN(start_pfn, order_pages);
90 for (i = start_pfn; i < end_pfn; i += order_pages) {
91 unsigned long j;
92 int migratetype, retry;
93 struct page *page_i, *page_j;
94
95 /*
96 * We expect page_i, in the sub-range of a zone being added
97 * (@start_pfn to @end_pfn), to more likely be valid compared to
98 * page_j randomly selected in the span @zone_start_pfn to
99 * @spanned_pages.
100 */
101 page_i = shuffle_valid_page(z, i, order);
102 if (!page_i)
103 continue;
104
105 for (retry = 0; retry < SHUFFLE_RETRY; retry++) {
106 /*
107 * Pick a random order aligned page in the zone span as
108 * a swap target. If the selected pfn is a hole, retry
109 * up to SHUFFLE_RETRY attempts find a random valid pfn
110 * in the zone.
111 */
112 j = z->zone_start_pfn +
113 ALIGN_DOWN(get_random_long() % z->spanned_pages,
114 order_pages);
115 page_j = shuffle_valid_page(z, j, order);
116 if (page_j && page_j != page_i)
117 break;
118 }
119 if (retry >= SHUFFLE_RETRY) {
120 pr_debug("%s: failed to swap %#lx\n", __func__, i);
121 continue;
122 }
123
124 /*
125 * Each migratetype corresponds to its own list, make sure the
126 * types match otherwise we're moving pages to lists where they
127 * do not belong.
128 */
129 migratetype = get_pageblock_migratetype(page_i);
130 if (get_pageblock_migratetype(page_j) != migratetype) {
131 pr_debug("%s: migratetype mismatch %#lx\n", __func__, i);
132 continue;
133 }
134
135 list_swap(&page_i->lru, &page_j->lru);
136
137 pr_debug("%s: swap: %#lx -> %#lx\n", __func__, i, j);
138
139 /* take it easy on the zone lock */
140 if ((i % (100 * order_pages)) == 0) {
141 spin_unlock_irqrestore(&z->lock, flags);
142 cond_resched();
143 spin_lock_irqsave(&z->lock, flags);
144 }
145 }
146 spin_unlock_irqrestore(&z->lock, flags);
147 }
148
149 /*
150 * __shuffle_free_memory - reduce the predictability of the page allocator
151 * @pgdat: node page data
152 */
153 void __meminit __shuffle_free_memory(pg_data_t *pgdat)
154 {
155 struct zone *z;
156
157 for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
158 shuffle_zone(z);
159 }
160
161 bool shuffle_pick_tail(void)
162 {
163 static u64 rand;
164 static u8 rand_bits;
165 bool ret;
166
167 /*
168 * The lack of locking is deliberate. If 2 threads race to
169 * update the rand state it just adds to the entropy.
170 */
171 if (rand_bits == 0) {
172 rand_bits = 64;
173 rand = get_random_u64();
174 }
175
176 ret = rand & 1;
177
178 rand_bits--;
179 rand >>= 1;
180
181 return ret;
182 }
183
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