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
 12DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);
 13
 14static bool shuffle_param;
 15
 16static __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
 26static const struct kernel_param_ops shuffle_param_ops = {
 27	.set = shuffle_param_set,
 28	.get = param_get_bool,
 29};
 30module_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 */
 36static 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
 80void __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 */
153void __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
161bool 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}