// SPDX-License-Identifier: GPL-2.0

  /*
   *  linux/mm/swap_state.c
   *
   *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
   *  Swap reorganised 29.12.95, Stephen Tweedie
   *
   *  Rewritten to use page cache, (C) 1998 Stephen Tweedie
   */

  #include <linux/mm.h>

  #include <linux/gfp.h>

  #include <linux/kernel_stat.h>
  #include <linux/swap.h>

  #include <linux/swapops.h>

  #include <linux/init.h>
  #include <linux/pagemap.h>

  #include <linux/backing-dev.h>

  #include <linux/blkdev.h>

  #include <linux/pagevec.h>

  #include <linux/migrate.h>

  #include <linux/vmalloc.h>

  #include <linux/swap_slots.h>

  #include <linux/huge_mm.h>

  
  #include <asm/pgtable.h>
  
  /*
   * swapper_space is a fiction, retained to simplify the path through

   * vmscan's shrink_page_list.

   */

  static const struct address_space_operations swap_aops = {

  	.writepage	= swap_writepage,

  	.set_page_dirty	= swap_set_page_dirty,

  #ifdef CONFIG_MIGRATION

  	.migratepage	= migrate_page,

  #endif

  };

  struct address_space *swapper_spaces[MAX_SWAPFILES] __read_mostly;
  static unsigned int nr_swapper_spaces[MAX_SWAPFILES] __read_mostly;

  static bool enable_vma_readahead __read_mostly = true;

  #define SWAP_RA_WIN_SHIFT	(PAGE_SHIFT / 2)
  #define SWAP_RA_HITS_MASK	((1UL << SWAP_RA_WIN_SHIFT) - 1)
  #define SWAP_RA_HITS_MAX	SWAP_RA_HITS_MASK
  #define SWAP_RA_WIN_MASK	(~PAGE_MASK & ~SWAP_RA_HITS_MASK)
  
  #define SWAP_RA_HITS(v)		((v) & SWAP_RA_HITS_MASK)
  #define SWAP_RA_WIN(v)		(((v) & SWAP_RA_WIN_MASK) >> SWAP_RA_WIN_SHIFT)
  #define SWAP_RA_ADDR(v)		((v) & PAGE_MASK)
  
  #define SWAP_RA_VAL(addr, win, hits)				\
  	(((addr) & PAGE_MASK) |					\
  	 (((win) << SWAP_RA_WIN_SHIFT) & SWAP_RA_WIN_MASK) |	\
  	 ((hits) & SWAP_RA_HITS_MASK))
  
  /* Initial readahead hits is 4 to start up with a small window */
  #define GET_SWAP_RA_VAL(vma)					\
  	(atomic_long_read(&(vma)->swap_readahead_info) ? : 4)

  
  #define INC_CACHE_INFO(x)	do { swap_cache_info.x++; } while (0)

  #define ADD_CACHE_INFO(x, nr)	do { swap_cache_info.x += (nr); } while (0)

  
  static struct {
  	unsigned long add_total;
  	unsigned long del_total;
  	unsigned long find_success;
  	unsigned long find_total;

  } swap_cache_info;

  unsigned long total_swapcache_pages(void)
  {

  	unsigned int i, j, nr;

  	unsigned long ret = 0;

  	struct address_space *spaces;

  	struct swap_info_struct *si;

  	for (i = 0; i < MAX_SWAPFILES; i++) {

  		swp_entry_t entry = swp_entry(i, 1);
  
  		/* Avoid get_swap_device() to warn for bad swap entry */
  		if (!swp_swap_info(entry))
  			continue;
  		/* Prevent swapoff to free swapper_spaces */
  		si = get_swap_device(entry);
  		if (!si)

  			continue;

  		nr = nr_swapper_spaces[i];
  		spaces = swapper_spaces[i];

  		for (j = 0; j < nr; j++)
  			ret += spaces[j].nrpages;

  		put_swap_device(si);

  	}

  	return ret;
  }

  static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);

  void show_swap_cache_info(void)
  {

  	printk("%lu pages in swap cache
  ", total_swapcache_pages());

  	printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu
  ",

  		swap_cache_info.add_total, swap_cache_info.del_total,

  		swap_cache_info.find_success, swap_cache_info.find_total);

  	printk("Free swap  = %ldkB
  ",
  		get_nr_swap_pages() << (PAGE_SHIFT - 10));

  	printk("Total swap = %lukB
  ", total_swap_pages << (PAGE_SHIFT - 10));
  }
  
  /*

   * add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,

   * but sets SwapCache flag and private instead of mapping and index.
   */

  int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp)

  {

  	struct address_space *address_space = swap_address_space(entry);

  	pgoff_t idx = swp_offset(entry);

  	XA_STATE_ORDER(xas, &address_space->i_pages, idx, compound_order(page));

  	unsigned long i, nr = compound_nr(page);

  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(PageSwapCache(page), page);
  	VM_BUG_ON_PAGE(!PageSwapBacked(page), page);

  	page_ref_add(page, nr);

  	SetPageSwapCache(page);

  	do {
  		xas_lock_irq(&xas);
  		xas_create_range(&xas);
  		if (xas_error(&xas))
  			goto unlock;
  		for (i = 0; i < nr; i++) {
  			VM_BUG_ON_PAGE(xas.xa_index != idx + i, page);
  			set_page_private(page + i, entry.val + i);

  			xas_store(&xas, page);

  			xas_next(&xas);
  		}

  		address_space->nrpages += nr;
  		__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, nr);
  		ADD_CACHE_INFO(add_total, nr);

  unlock:
  		xas_unlock_irq(&xas);
  	} while (xas_nomem(&xas, gfp));

  	if (!xas_error(&xas))
  		return 0;

  	ClearPageSwapCache(page);
  	page_ref_sub(page, nr);
  	return xas_error(&xas);

  }

  /*
   * This must be called only on pages that have
   * been verified to be in the swap cache.
   */

  void __delete_from_swap_cache(struct page *page, swp_entry_t entry)

  {

  	struct address_space *address_space = swap_address_space(entry);

  	int i, nr = hpage_nr_pages(page);

  	pgoff_t idx = swp_offset(entry);
  	XA_STATE(xas, &address_space->i_pages, idx);

  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
  	VM_BUG_ON_PAGE(PageWriteback(page), page);

  	for (i = 0; i < nr; i++) {

  		void *entry = xas_store(&xas, NULL);

  		VM_BUG_ON_PAGE(entry != page, entry);

  		set_page_private(page + i, 0);

  		xas_next(&xas);

  	}

  	ClearPageSwapCache(page);

  	address_space->nrpages -= nr;
  	__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, -nr);
  	ADD_CACHE_INFO(del_total, nr);

  }
  
  /**
   * add_to_swap - allocate swap space for a page
   * @page: page we want to move to swap
   *
   * Allocate swap space for the page and add the page to the
   * swap cache.  Caller needs to hold the page lock. 
   */

  int add_to_swap(struct page *page)

  {
  	swp_entry_t entry;

  	int err;

  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(!PageUptodate(page), page);

  	entry = get_swap_page(page);

  	if (!entry.val)

  		return 0;

  	/*

  	 * XArray node allocations from PF_MEMALLOC contexts could

  	 * completely exhaust the page allocator. __GFP_NOMEMALLOC
  	 * stops emergency reserves from being allocated.
  	 *
  	 * TODO: this could cause a theoretical memory reclaim
  	 * deadlock in the swap out path.
  	 */
  	/*

  	 * Add it to the swap cache.

  	 */
  	err = add_to_swap_cache(page, entry,
  			__GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);

  	if (err)

  		/*

  		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
  		 * clear SWAP_HAS_CACHE flag.

  		 */

  		goto fail;

  	/*
  	 * Normally the page will be dirtied in unmap because its pte should be
  	 * dirty. A special case is MADV_FREE page. The page'e pte could have
  	 * dirty bit cleared but the page's SwapBacked bit is still set because
  	 * clearing the dirty bit and SwapBacked bit has no lock protected. For
  	 * such page, unmap will not set dirty bit for it, so page reclaim will
  	 * not write the page out. This can cause data corruption when the page
  	 * is swap in later. Always setting the dirty bit for the page solves
  	 * the problem.
  	 */
  	set_page_dirty(page);

  
  	return 1;

  fail:

  	put_swap_page(page, entry);

  	return 0;

  }
  
  /*
   * This must be called only on pages that have
   * been verified to be in the swap cache and locked.
   * It will never put the page into the free list,
   * the caller has a reference on the page.
   */
  void delete_from_swap_cache(struct page *page)
  {

  	swp_entry_t entry = { .val = page_private(page) };
  	struct address_space *address_space = swap_address_space(entry);

  	xa_lock_irq(&address_space->i_pages);

  	__delete_from_swap_cache(page, entry);

  	xa_unlock_irq(&address_space->i_pages);

  	put_swap_page(page, entry);

  	page_ref_sub(page, hpage_nr_pages(page));

  }

  /* 
   * If we are the only user, then try to free up the swap cache. 
   * 
   * Its ok to check for PageSwapCache without the page lock

   * here because we are going to recheck again inside
   * try_to_free_swap() _with_ the lock.

   * 					- Marcelo
   */
  static inline void free_swap_cache(struct page *page)
  {

  	if (PageSwapCache(page) && !page_mapped(page) && trylock_page(page)) {
  		try_to_free_swap(page);

  		unlock_page(page);
  	}
  }
  
  /* 
   * Perform a free_page(), also freeing any swap cache associated with

   * this page if it is the last user of the page.

   */
  void free_page_and_swap_cache(struct page *page)
  {
  	free_swap_cache(page);

  	if (!is_huge_zero_page(page))

  		put_page(page);

  }
  
  /*
   * Passed an array of pages, drop them all from swapcache and then release
   * them.  They are removed from the LRU and freed if this is their last use.
   */
  void free_pages_and_swap_cache(struct page **pages, int nr)
  {

  	struct page **pagep = pages;

  	int i;

  
  	lru_add_drain();

  	for (i = 0; i < nr; i++)
  		free_swap_cache(pagep[i]);

  	release_pages(pagep, nr);

  }

  static inline bool swap_use_vma_readahead(void)
  {
  	return READ_ONCE(enable_vma_readahead) && !atomic_read(&nr_rotate_swap);
  }

  /*
   * Lookup a swap entry in the swap cache. A found page will be returned
   * unlocked and with its refcount incremented - we rely on the kernel
   * lock getting page table operations atomic even if we drop the page
   * lock before returning.
   */

  struct page *lookup_swap_cache(swp_entry_t entry, struct vm_area_struct *vma,
  			       unsigned long addr)

  {
  	struct page *page;

  	struct swap_info_struct *si;

  	si = get_swap_device(entry);
  	if (!si)
  		return NULL;

  	page = find_get_page(swap_address_space(entry), swp_offset(entry));

  	put_swap_device(si);

  	INC_CACHE_INFO(find_total);
  	if (page) {

  		bool vma_ra = swap_use_vma_readahead();
  		bool readahead;

  		INC_CACHE_INFO(find_success);

  		/*
  		 * At the moment, we don't support PG_readahead for anon THP
  		 * so let's bail out rather than confusing the readahead stat.
  		 */

  		if (unlikely(PageTransCompound(page)))
  			return page;

  		readahead = TestClearPageReadahead(page);

  		if (vma && vma_ra) {
  			unsigned long ra_val;
  			int win, hits;
  
  			ra_val = GET_SWAP_RA_VAL(vma);
  			win = SWAP_RA_WIN(ra_val);
  			hits = SWAP_RA_HITS(ra_val);

  			if (readahead)
  				hits = min_t(int, hits + 1, SWAP_RA_HITS_MAX);
  			atomic_long_set(&vma->swap_readahead_info,
  					SWAP_RA_VAL(addr, win, hits));
  		}

  		if (readahead) {

  			count_vm_event(SWAP_RA_HIT);

  			if (!vma || !vma_ra)

  				atomic_inc(&swapin_readahead_hits);

  		}

  	}

  	return page;
  }

  struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
  			struct vm_area_struct *vma, unsigned long addr,
  			bool *new_page_allocated)

  {

  	struct page *found_page = NULL, *new_page = NULL;
  	struct swap_info_struct *si;

  	int err;

  	*new_page_allocated = false;

  
  	do {
  		/*
  		 * First check the swap cache.  Since this is normally
  		 * called after lookup_swap_cache() failed, re-calling
  		 * that would confuse statistics.
  		 */

  		si = get_swap_device(entry);
  		if (!si)
  			break;
  		found_page = find_get_page(swap_address_space(entry),
  					   swp_offset(entry));
  		put_swap_device(si);

  		if (found_page)
  			break;

  		/*
  		 * Just skip read ahead for unused swap slot.
  		 * During swap_off when swap_slot_cache is disabled,
  		 * we have to handle the race between putting
  		 * swap entry in swap cache and marking swap slot
  		 * as SWAP_HAS_CACHE.  That's done in later part of code or
  		 * else swap_off will be aborted if we return NULL.
  		 */
  		if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
  			break;

  		/*
  		 * Get a new page to read into from swap.
  		 */
  		if (!new_page) {

  			new_page = alloc_page_vma(gfp_mask, vma, addr);

  			if (!new_page)
  				break;		/* Out of memory */
  		}
  
  		/*

  		 * Swap entry may have been freed since our caller observed it.
  		 */

  		err = swapcache_prepare(entry);

  		if (err == -EEXIST) {

  			/*
  			 * We might race against get_swap_page() and stumble
  			 * across a SWAP_HAS_CACHE swap_map entry whose page

  			 * has not been brought into the swapcache yet.

  			 */
  			cond_resched();

  			continue;

  		} else if (err)		/* swp entry is obsolete ? */

  			break;

  		/* May fail (-ENOMEM) if XArray node allocation failed. */

  		__SetPageLocked(new_page);

  		__SetPageSwapBacked(new_page);

  		err = add_to_swap_cache(new_page, entry, gfp_mask & GFP_KERNEL);

  		if (likely(!err)) {

  			/* Initiate read into locked page */

  			SetPageWorkingset(new_page);

  			lru_cache_add_anon(new_page);

  			*new_page_allocated = true;

  			return new_page;
  		}

  		__ClearPageLocked(new_page);

  		/*
  		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
  		 * clear SWAP_HAS_CACHE flag.
  		 */

  		put_swap_page(new_page, entry);

  	} while (err != -ENOMEM);

  
  	if (new_page)

  		put_page(new_page);

  	return found_page;
  }

  /*
   * Locate a page of swap in physical memory, reserving swap cache space
   * and reading the disk if it is not already cached.
   * A failure return means that either the page allocation failed or that
   * the swap entry is no longer in use.
   */
  struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,

  		struct vm_area_struct *vma, unsigned long addr, bool do_poll)

  {
  	bool page_was_allocated;
  	struct page *retpage = __read_swap_cache_async(entry, gfp_mask,
  			vma, addr, &page_was_allocated);
  
  	if (page_was_allocated)

  		swap_readpage(retpage, do_poll);

  
  	return retpage;
  }

  static unsigned int __swapin_nr_pages(unsigned long prev_offset,
  				      unsigned long offset,
  				      int hits,
  				      int max_pages,
  				      int prev_win)

  {

  	unsigned int pages, last_ra;

  
  	/*
  	 * This heuristic has been found to work well on both sequential and
  	 * random loads, swapping to hard disk or to SSD: please don't ask
  	 * what the "+ 2" means, it just happens to work well, that's all.
  	 */

  	pages = hits + 2;

  	if (pages == 2) {
  		/*
  		 * We can have no readahead hits to judge by: but must not get
  		 * stuck here forever, so check for an adjacent offset instead
  		 * (and don't even bother to check whether swap type is same).
  		 */
  		if (offset != prev_offset + 1 && offset != prev_offset - 1)
  			pages = 1;

  	} else {
  		unsigned int roundup = 4;
  		while (roundup < pages)
  			roundup <<= 1;
  		pages = roundup;
  	}
  
  	if (pages > max_pages)
  		pages = max_pages;
  
  	/* Don't shrink readahead too fast */

  	last_ra = prev_win / 2;

  	if (pages < last_ra)
  		pages = last_ra;

  
  	return pages;
  }
  
  static unsigned long swapin_nr_pages(unsigned long offset)
  {
  	static unsigned long prev_offset;
  	unsigned int hits, pages, max_pages;
  	static atomic_t last_readahead_pages;
  
  	max_pages = 1 << READ_ONCE(page_cluster);
  	if (max_pages <= 1)
  		return 1;
  
  	hits = atomic_xchg(&swapin_readahead_hits, 0);
  	pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages,
  				  atomic_read(&last_readahead_pages));
  	if (!hits)
  		prev_offset = offset;

  	atomic_set(&last_readahead_pages, pages);
  
  	return pages;
  }

  /**

   * swap_cluster_readahead - swap in pages in hope we need them soon

   * @entry: swap entry of this memory

   * @gfp_mask: memory allocation flags

   * @vmf: fault information

   *
   * Returns the struct page for entry and addr, after queueing swapin.
   *
   * Primitive swap readahead code. We simply read an aligned block of
   * (1 << page_cluster) entries in the swap area. This method is chosen
   * because it doesn't cost us any seek time.  We also make sure to queue
   * the 'original' request together with the readahead ones...
   *
   * This has been extended to use the NUMA policies from the mm triggering
   * the readahead.
   *

   * Caller must hold read mmap_sem if vmf->vma is not NULL.

   */

  struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
  				struct vm_fault *vmf)

  {

  	struct page *page;

  	unsigned long entry_offset = swp_offset(entry);
  	unsigned long offset = entry_offset;

  	unsigned long start_offset, end_offset;

  	unsigned long mask;

  	struct swap_info_struct *si = swp_swap_info(entry);

  	struct blk_plug plug;

  	bool do_poll = true, page_allocated;

  	struct vm_area_struct *vma = vmf->vma;
  	unsigned long addr = vmf->address;

  	mask = swapin_nr_pages(offset) - 1;
  	if (!mask)
  		goto skip;

  	/* Test swap type to make sure the dereference is safe */
  	if (likely(si->flags & (SWP_BLKDEV | SWP_FS))) {
  		struct inode *inode = si->swap_file->f_mapping->host;
  		if (inode_read_congested(inode))
  			goto skip;
  	}

  	do_poll = false;

  	/* Read a page_cluster sized and aligned cluster around offset. */
  	start_offset = offset & ~mask;
  	end_offset = offset | mask;
  	if (!start_offset)	/* First page is swap header. */
  		start_offset++;

  	if (end_offset >= si->max)
  		end_offset = si->max - 1;

  	blk_start_plug(&plug);

  	for (offset = start_offset; offset <= end_offset ; offset++) {

  		/* Ok, do the async read-ahead now */

  		page = __read_swap_cache_async(
  			swp_entry(swp_type(entry), offset),
  			gfp_mask, vma, addr, &page_allocated);

  		if (!page)

  			continue;

  		if (page_allocated) {
  			swap_readpage(page, false);

  			if (offset != entry_offset) {

  				SetPageReadahead(page);
  				count_vm_event(SWAP_RA);
  			}

  		}

  		put_page(page);

  	}

  	blk_finish_plug(&plug);

  	lru_add_drain();	/* Push any new pages onto the LRU now */

  skip:

  	return read_swap_cache_async(entry, gfp_mask, vma, addr, do_poll);

  }

  
  int init_swap_address_space(unsigned int type, unsigned long nr_pages)
  {
  	struct address_space *spaces, *space;
  	unsigned int i, nr;
  
  	nr = DIV_ROUND_UP(nr_pages, SWAP_ADDRESS_SPACE_PAGES);

  	spaces = kvcalloc(nr, sizeof(struct address_space), GFP_KERNEL);

  	if (!spaces)
  		return -ENOMEM;
  	for (i = 0; i < nr; i++) {
  		space = spaces + i;

  		xa_init_flags(&space->i_pages, XA_FLAGS_LOCK_IRQ);

  		atomic_set(&space->i_mmap_writable, 0);
  		space->a_ops = &swap_aops;
  		/* swap cache doesn't use writeback related tags */
  		mapping_set_no_writeback_tags(space);

  	}
  	nr_swapper_spaces[type] = nr;

  	swapper_spaces[type] = spaces;

  
  	return 0;
  }
  
  void exit_swap_address_space(unsigned int type)
  {

  	kvfree(swapper_spaces[type]);

  	nr_swapper_spaces[type] = 0;

  	swapper_spaces[type] = NULL;

  }

  
  static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
  				     unsigned long faddr,
  				     unsigned long lpfn,
  				     unsigned long rpfn,
  				     unsigned long *start,
  				     unsigned long *end)
  {
  	*start = max3(lpfn, PFN_DOWN(vma->vm_start),
  		      PFN_DOWN(faddr & PMD_MASK));
  	*end = min3(rpfn, PFN_DOWN(vma->vm_end),
  		    PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
  }

  static void swap_ra_info(struct vm_fault *vmf,
  			struct vma_swap_readahead *ra_info)

  {
  	struct vm_area_struct *vma = vmf->vma;

  	unsigned long ra_val;

  	swp_entry_t entry;
  	unsigned long faddr, pfn, fpfn;
  	unsigned long start, end;

  	pte_t *pte, *orig_pte;

  	unsigned int max_win, hits, prev_win, win, left;
  #ifndef CONFIG_64BIT
  	pte_t *tpte;
  #endif

  	max_win = 1 << min_t(unsigned int, READ_ONCE(page_cluster),
  			     SWAP_RA_ORDER_CEILING);
  	if (max_win == 1) {

  		ra_info->win = 1;
  		return;

  	}

  	faddr = vmf->address;

  	orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
  	entry = pte_to_swp_entry(*pte);
  	if ((unlikely(non_swap_entry(entry)))) {
  		pte_unmap(orig_pte);
  		return;
  	}

  	fpfn = PFN_DOWN(faddr);

  	ra_val = GET_SWAP_RA_VAL(vma);
  	pfn = PFN_DOWN(SWAP_RA_ADDR(ra_val));
  	prev_win = SWAP_RA_WIN(ra_val);
  	hits = SWAP_RA_HITS(ra_val);
  	ra_info->win = win = __swapin_nr_pages(pfn, fpfn, hits,

  					       max_win, prev_win);
  	atomic_long_set(&vma->swap_readahead_info,
  			SWAP_RA_VAL(faddr, win, 0));

  	if (win == 1) {
  		pte_unmap(orig_pte);
  		return;
  	}

  
  	/* Copy the PTEs because the page table may be unmapped */
  	if (fpfn == pfn + 1)
  		swap_ra_clamp_pfn(vma, faddr, fpfn, fpfn + win, &start, &end);
  	else if (pfn == fpfn + 1)
  		swap_ra_clamp_pfn(vma, faddr, fpfn - win + 1, fpfn + 1,
  				  &start, &end);
  	else {
  		left = (win - 1) / 2;
  		swap_ra_clamp_pfn(vma, faddr, fpfn - left, fpfn + win - left,
  				  &start, &end);
  	}

  	ra_info->nr_pte = end - start;
  	ra_info->offset = fpfn - start;
  	pte -= ra_info->offset;

  #ifdef CONFIG_64BIT

  	ra_info->ptes = pte;

  #else

  	tpte = ra_info->ptes;

  	for (pfn = start; pfn != end; pfn++)
  		*tpte++ = *pte++;
  #endif

  	pte_unmap(orig_pte);

  }

  /**
   * swap_vma_readahead - swap in pages in hope we need them soon
   * @entry: swap entry of this memory
   * @gfp_mask: memory allocation flags
   * @vmf: fault information
   *
   * Returns the struct page for entry and addr, after queueing swapin.
   *
   * Primitive swap readahead code. We simply read in a few pages whoes
   * virtual addresses are around the fault address in the same vma.
   *
   * Caller must hold read mmap_sem if vmf->vma is not NULL.
   *
   */

  static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
  				       struct vm_fault *vmf)

  {
  	struct blk_plug plug;
  	struct vm_area_struct *vma = vmf->vma;
  	struct page *page;
  	pte_t *pte, pentry;
  	swp_entry_t entry;
  	unsigned int i;
  	bool page_allocated;

  	struct vma_swap_readahead ra_info = {0,};

  	swap_ra_info(vmf, &ra_info);
  	if (ra_info.win == 1)

  		goto skip;
  
  	blk_start_plug(&plug);

  	for (i = 0, pte = ra_info.ptes; i < ra_info.nr_pte;

  	     i++, pte++) {
  		pentry = *pte;
  		if (pte_none(pentry))
  			continue;
  		if (pte_present(pentry))
  			continue;
  		entry = pte_to_swp_entry(pentry);
  		if (unlikely(non_swap_entry(entry)))
  			continue;
  		page = __read_swap_cache_async(entry, gfp_mask, vma,
  					       vmf->address, &page_allocated);
  		if (!page)
  			continue;
  		if (page_allocated) {
  			swap_readpage(page, false);

  			if (i != ra_info.offset) {

  				SetPageReadahead(page);
  				count_vm_event(SWAP_RA);
  			}
  		}
  		put_page(page);
  	}
  	blk_finish_plug(&plug);
  	lru_add_drain();
  skip:
  	return read_swap_cache_async(fentry, gfp_mask, vma, vmf->address,

  				     ra_info.win == 1);

  }

  /**
   * swapin_readahead - swap in pages in hope we need them soon
   * @entry: swap entry of this memory
   * @gfp_mask: memory allocation flags
   * @vmf: fault information
   *
   * Returns the struct page for entry and addr, after queueing swapin.
   *
   * It's a main entry function for swap readahead. By the configuration,
   * it will read ahead blocks by cluster-based(ie, physical disk based)
   * or vma-based(ie, virtual address based on faulty address) readahead.
   */
  struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
  				struct vm_fault *vmf)
  {
  	return swap_use_vma_readahead() ?
  			swap_vma_readahead(entry, gfp_mask, vmf) :
  			swap_cluster_readahead(entry, gfp_mask, vmf);
  }

  #ifdef CONFIG_SYSFS
  static ssize_t vma_ra_enabled_show(struct kobject *kobj,
  				     struct kobj_attribute *attr, char *buf)
  {

  	return sprintf(buf, "%s
  ", enable_vma_readahead ? "true" : "false");

  }
  static ssize_t vma_ra_enabled_store(struct kobject *kobj,
  				      struct kobj_attribute *attr,
  				      const char *buf, size_t count)
  {
  	if (!strncmp(buf, "true", 4) || !strncmp(buf, "1", 1))

  		enable_vma_readahead = true;

  	else if (!strncmp(buf, "false", 5) || !strncmp(buf, "0", 1))

  		enable_vma_readahead = false;

  	else
  		return -EINVAL;
  
  	return count;
  }
  static struct kobj_attribute vma_ra_enabled_attr =
  	__ATTR(vma_ra_enabled, 0644, vma_ra_enabled_show,
  	       vma_ra_enabled_store);

  static struct attribute *swap_attrs[] = {
  	&vma_ra_enabled_attr.attr,

  	NULL,
  };
  
  static struct attribute_group swap_attr_group = {
  	.attrs = swap_attrs,
  };
  
  static int __init swap_init_sysfs(void)
  {
  	int err;
  	struct kobject *swap_kobj;
  
  	swap_kobj = kobject_create_and_add("swap", mm_kobj);
  	if (!swap_kobj) {
  		pr_err("failed to create swap kobject
  ");
  		return -ENOMEM;
  	}
  	err = sysfs_create_group(swap_kobj, &swap_attr_group);
  	if (err) {
  		pr_err("failed to register swap group
  ");
  		goto delete_obj;
  	}
  	return 0;
  
  delete_obj:
  	kobject_put(swap_kobj);
  	return err;
  }
  subsys_initcall(swap_init_sysfs);
  #endif