// 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 int shuffle_show(char *buffer, const struct kernel_param *kp)

  {

  	return sprintf(buffer, "%c
  ", shuffle_param ? 'Y' : 'N');

  }
  
  static __meminit int shuffle_store(const char *val,
  		const struct kernel_param *kp)
  {
  	int rc = param_set_bool(val, kp);
  
  	if (rc < 0)
  		return rc;
  	if (shuffle_param)

  		static_branch_enable(&page_alloc_shuffle_key);

  	return 0;
  }
  module_param_call(shuffle, shuffle_store, shuffle_show, &shuffle_param, 0400);
  
  /*
   * 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)

  {

  	struct page *page = pfn_to_online_page(pfn);

  
  	/*
  	 * Given we're dealing with randomly selected pfns in a zone we
  	 * need to ask questions like...
  	 */

  	/* ... is the page managed by the buddy? */
  	if (!page)

  		return NULL;

  	/* ... is the page assigned to the same zone? */
  	if (page_zone(page) != zone)

  		return NULL;
  
  	/* ...is the page free and currently on a free_area list? */

  	if (!PageBuddy(page))
  		return NULL;
  
  	/*
  	 * ...is the page on the same list as the page we will
  	 * shuffle it with?
  	 */

  	if (buddy_order(page) != order)

  		return NULL;
  
  	return page;
  }
  
  /*
   * 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 10
  void __meminit __shuffle_zone(struct zone *z)
  {
  	unsigned long i, flags;
  	unsigned long start_pfn = z->zone_start_pfn;
  	unsigned long end_pfn = zone_end_pfn(z);
  	const int order = SHUFFLE_ORDER;
  	const int order_pages = 1 << order;
  
  	spin_lock_irqsave(&z->lock, flags);
  	start_pfn = ALIGN(start_pfn, order_pages);
  	for (i = start_pfn; i < end_pfn; i += order_pages) {
  		unsigned long j;
  		int migratetype, retry;
  		struct page *page_i, *page_j;
  
  		/*
  		 * We expect page_i, in the sub-range of a zone being added
  		 * (@start_pfn to @end_pfn), to more likely be valid compared to
  		 * page_j randomly selected in the span @zone_start_pfn to
  		 * @spanned_pages.
  		 */

  		page_i = shuffle_valid_page(z, i, order);

  		if (!page_i)
  			continue;
  
  		for (retry = 0; retry < SHUFFLE_RETRY; retry++) {
  			/*
  			 * Pick a random order aligned page in the zone span as
  			 * a swap target. If the selected pfn is a hole, retry
  			 * up to SHUFFLE_RETRY attempts find a random valid pfn
  			 * in the zone.
  			 */
  			j = z->zone_start_pfn +
  				ALIGN_DOWN(get_random_long() % z->spanned_pages,
  						order_pages);

  			page_j = shuffle_valid_page(z, j, order);

  			if (page_j && page_j != page_i)
  				break;
  		}
  		if (retry >= SHUFFLE_RETRY) {
  			pr_debug("%s: failed to swap %#lx
  ", __func__, i);
  			continue;
  		}
  
  		/*
  		 * Each migratetype corresponds to its own list, make sure the
  		 * types match otherwise we're moving pages to lists where they
  		 * do not belong.
  		 */
  		migratetype = get_pageblock_migratetype(page_i);
  		if (get_pageblock_migratetype(page_j) != migratetype) {
  			pr_debug("%s: migratetype mismatch %#lx
  ", __func__, i);
  			continue;
  		}
  
  		list_swap(&page_i->lru, &page_j->lru);
  
  		pr_debug("%s: swap: %#lx -> %#lx
  ", __func__, i, j);
  
  		/* take it easy on the zone lock */
  		if ((i % (100 * order_pages)) == 0) {
  			spin_unlock_irqrestore(&z->lock, flags);
  			cond_resched();
  			spin_lock_irqsave(&z->lock, flags);
  		}
  	}
  	spin_unlock_irqrestore(&z->lock, flags);
  }
  
  /**
   * shuffle_free_memory - reduce the predictability of the page allocator
   * @pgdat: node page data
   */
  void __meminit __shuffle_free_memory(pg_data_t *pgdat)
  {
  	struct zone *z;
  
  	for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
  		shuffle_zone(z);
  }

  bool shuffle_pick_tail(void)

  {
  	static u64 rand;
  	static u8 rand_bits;

  	bool ret;

  
  	/*
  	 * The lack of locking is deliberate. If 2 threads race to
  	 * update the rand state it just adds to the entropy.
  	 */
  	if (rand_bits == 0) {
  		rand_bits = 64;
  		rand = get_random_u64();
  	}

  	ret = rand & 1;

  	rand_bits--;
  	rand >>= 1;

  
  	return ret;

  }