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
  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 }