#include <linux/kernel.h>

  #include <linux/string.h>

  #include <linux/mm.h>

  #include <linux/highmem.h>

  #include <linux/page-debug-flags.h>
  #include <linux/poison.h>

  #include <linux/ratelimit.h>

  
  static inline void set_page_poison(struct page *page)
  {
  	__set_bit(PAGE_DEBUG_FLAG_POISON, &page->debug_flags);
  }
  
  static inline void clear_page_poison(struct page *page)
  {
  	__clear_bit(PAGE_DEBUG_FLAG_POISON, &page->debug_flags);
  }
  
  static inline bool page_poison(struct page *page)
  {
  	return test_bit(PAGE_DEBUG_FLAG_POISON, &page->debug_flags);
  }

  static void poison_page(struct page *page)
  {

  	void *addr = kmap_atomic(page);

  	set_page_poison(page);

  	memset(addr, PAGE_POISON, PAGE_SIZE);

  	kunmap_atomic(addr);

  }
  
  static void poison_pages(struct page *page, int n)
  {
  	int i;
  
  	for (i = 0; i < n; i++)
  		poison_page(page + i);
  }
  
  static bool single_bit_flip(unsigned char a, unsigned char b)
  {
  	unsigned char error = a ^ b;
  
  	return error && !(error & (error - 1));
  }
  
  static void check_poison_mem(unsigned char *mem, size_t bytes)
  {

  	static DEFINE_RATELIMIT_STATE(ratelimit, 5 * HZ, 10);

  	unsigned char *start;
  	unsigned char *end;

  	start = memchr_inv(mem, PAGE_POISON, bytes);
  	if (!start)

  		return;
  
  	for (end = mem + bytes - 1; end > start; end--) {
  		if (*end != PAGE_POISON)
  			break;
  	}

  	if (!__ratelimit(&ratelimit))

  		return;
  	else if (start == end && single_bit_flip(*start, PAGE_POISON))
  		printk(KERN_ERR "pagealloc: single bit error
  ");
  	else
  		printk(KERN_ERR "pagealloc: memory corruption
  ");
  
  	print_hex_dump(KERN_ERR, "", DUMP_PREFIX_ADDRESS, 16, 1, start,
  			end - start + 1, 1);
  	dump_stack();
  }

  static void unpoison_page(struct page *page)
  {

  	void *addr;
  
  	if (!page_poison(page))

  		return;

  	addr = kmap_atomic(page);
  	check_poison_mem(addr, PAGE_SIZE);
  	clear_page_poison(page);
  	kunmap_atomic(addr);

  }
  
  static void unpoison_pages(struct page *page, int n)
  {
  	int i;
  
  	for (i = 0; i < n; i++)
  		unpoison_page(page + i);
  }
  
  void kernel_map_pages(struct page *page, int numpages, int enable)
  {

  	if (enable)
  		unpoison_pages(page, numpages);
  	else
  		poison_pages(page, numpages);
  }