mm/shuffle.c

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