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