linux/mm/shuffle.c

// SPDX-License-Identifier: GPL-2.0
// Copyright(c) 2018 Intel Corporation. All rights reserved.

#include <linux/mm.h>
#include <linux/init.h>
#include <linux/mmzone.h>
#include <linux/random.h>
#include <linux/moduleparam.h>
#include "internal.h"
#include "shuffle.h"

DEFINE_STATIC_KEY_FALSE(page_alloc_shuffle_key);

static bool shuffle_param;

static __meminit int shuffle_param_set(const char *val,
		const struct kernel_param *kp)
{ … }

static const struct kernel_param_ops shuffle_param_ops = …;
module_param_cb(…);

/*
 * For two pages to be swapped in the shuffle, they must be free (on a
 * 'free_area' lru), have the same order, and have the same migratetype.
 */
static struct page * __meminit shuffle_valid_page(struct zone *zone,
						  unsigned long pfn, int order)
{ … }

/*
 * Fisher-Yates shuffle the freelist which prescribes iterating through an
 * array, pfns in this case, and randomly swapping each entry with another in
 * the span, end_pfn - start_pfn.
 *
 * To keep the implementation simple it does not attempt to correct for sources
 * of bias in the distribution, like modulo bias or pseudo-random number
 * generator bias. I.e. the expectation is that this shuffling raises the bar
 * for attacks that exploit the predictability of page allocations, but need not
 * be a perfect shuffle.
 */
#define SHUFFLE_RETRY …
void __meminit __shuffle_zone(struct zone *z)
{ … }

/*
 * __shuffle_free_memory - reduce the predictability of the page allocator
 * @pgdat: node page data
 */
void __meminit __shuffle_free_memory(pg_data_t *pgdat)
{ … }

bool shuffle_pick_tail(void)
{ … }