text: /*
   *  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 <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] = {
  	[0 ... MAX_SWAPFILES - 1] = {
  		.page_tree	= RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
  		.i_mmap_writable = ATOMIC_INIT(0),
  		.a_ops		= &swap_aops,
  		/* swap cache doesn't use writeback related tags */
  		.flags		= 1 << AS_NO_WRITEBACK_TAGS,
  	}
  };
  
  #define INC_CACHE_INFO(x)	do { swap_cache_info.x++; } 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)
  {
  	int i;
  	unsigned long ret = 0;
  
  	for (i = 0; i < MAX_SWAPFILES; i++)
  		ret += swapper_spaces[i].nrpages;
  	return ret;
  }
  
  static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);
  
  void show_swap_cache_info(void)
  {
  	printk("%lu pages in swap cache\n", total_swapcache_pages());
  	printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
  		swap_cache_info.add_total, swap_cache_info.del_total,
  		swap_cache_info.find_success, swap_cache_info.find_total);
  	printk("Free swap  = %ldkB\n",
  		get_nr_swap_pages() << (PAGE_SHIFT - 10));
  	printk("Total swap = %lukB\n", 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)
  {
  	int error;
  	struct address_space *address_space;
  
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(PageSwapCache(page), page);
  	VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
  
  	get_page(page);
  	SetPageSwapCache(page);
  	set_page_private(page, entry.val);
  
  	address_space = swap_address_space(entry);
  	spin_lock_irq(&address_space->tree_lock);
  	error = radix_tree_insert(&address_space->page_tree,
  				  swp_offset(entry), page);
  	if (likely(!error)) {
  		address_space->nrpages++;
  		__inc_node_page_state(page, NR_FILE_PAGES);
  		INC_CACHE_INFO(add_total);
  	}
  	spin_unlock_irq(&address_space->tree_lock);
  
  	if (unlikely(error)) {
  		/*
  		 * Only the context which have set SWAP_HAS_CACHE flag
  		 * would call add_to_swap_cache().
  		 * So add_to_swap_cache() doesn't returns -EEXIST.
  		 */
  		VM_BUG_ON(error == -EEXIST);
  		set_page_private(page, 0UL);
  		ClearPageSwapCache(page);
  		put_page(page);
  	}
  
  	return error;
  }
  
  
  int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
  {
  	int error;
  
  	error = radix_tree_maybe_preload(gfp_mask);
  	if (!error) {
  		error = __add_to_swap_cache(page, entry);
  		radix_tree_preload_end();
  	}
  	return error;
  }
  
  /*
   * 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;
  
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
  	VM_BUG_ON_PAGE(PageWriteback(page), page);
  
  	entry.val = page_private(page);
  	address_space = swap_address_space(entry);
  	radix_tree_delete(&address_space->page_tree, swp_offset(entry));
  	set_page_private(page, 0);
  	ClearPageSwapCache(page);
  	address_space->nrpages--;
  	__dec_node_page_state(page, NR_FILE_PAGES);
  	INC_CACHE_INFO(del_total);
  }
  
  /**
   * 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, struct list_head *list)
  {
  	swp_entry_t entry;
  	int err;
  
  	VM_BUG_ON_PAGE(!PageLocked(page), page);
  	VM_BUG_ON_PAGE(!PageUptodate(page), page);
  
  	entry = get_swap_page();
  	if (!entry.val)
  		return 0;
  
  	if (mem_cgroup_try_charge_swap(page, entry)) {
  		swapcache_free(entry);
  		return 0;
  	}
  
  	if (unlikely(PageTransHuge(page)))
  		if (unlikely(split_huge_page_to_list(page, list))) {
  			swapcache_free(entry);
  			return 0;
  		}
  
  	/*
  	 * Radix-tree 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) {
  		return 1;
  	} else {	/* -ENOMEM radix-tree allocation failure */
  		/*
  		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
  		 * clear SWAP_HAS_CACHE flag.
  		 */
  		swapcache_free(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;
  	struct address_space *address_space;
  
  	entry.val = page_private(page);
  
  	address_space = swap_address_space(entry);
  	spin_lock_irq(&address_space->tree_lock);
  	__delete_from_swap_cache(page);
  	spin_unlock_irq(&address_space->tree_lock);
  
  	swapcache_free(entry);
  	put_page(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, false);
  }
  
  /*
   * 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 page *page;
  
  	page = find_get_page(swap_address_space(entry), swp_offset(entry));
  
  	if (page) {
  		INC_CACHE_INFO(find_success);
  		if (TestClearPageReadahead(page))
  			atomic_inc(&swapin_readahead_hits);
  	}
  
  	INC_CACHE_INFO(find_total);
  	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, *new_page = NULL;
  	struct address_space *swapper_space = swap_address_space(entry);
  	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.
  		 */
  		found_page = find_get_page(swapper_space, swp_offset(entry));
  		if (found_page)
  			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 */
  		}
  
  		/*
  		 * call radix_tree_preload() while we can wait.
  		 */
  		err = radix_tree_maybe_preload(gfp_mask & GFP_KERNEL);
  		if (err)
  			break;
  
  		/*
  		 * Swap entry may have been freed since our caller observed it.
  		 */
  		err = swapcache_prepare(entry);
  		if (err == -EEXIST) {
  			radix_tree_preload_end();
  			/*
  			 * 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, while
  			 * the other end is scheduled away waiting on discard
  			 * I/O completion at scan_swap_map().
  			 *
  			 * In order to avoid turning this transitory state
  			 * into a permanent loop around this -EEXIST case
  			 * if !CONFIG_PREEMPT and the I/O completion happens
  			 * to be waiting on the CPU waitqueue where we are now
  			 * busy looping, we just conditionally invoke the
  			 * scheduler here, if there are some more important
  			 * tasks to run.
  			 */
  			cond_resched();
  			continue;
  		}
  		if (err) {		/* swp entry is obsolete ? */
  			radix_tree_preload_end();
  			break;
  		}
  
  		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
  		__SetPageLocked(new_page);
  		__SetPageSwapBacked(new_page);
  		err = __add_to_swap_cache(new_page, entry);
  		if (likely(!err)) {
  			radix_tree_preload_end();
  			/*
  			 * Initiate read into locked page and return.
  			 */
  			lru_cache_add_anon(new_page);
  			*new_page_allocated = true;
  			return new_page;
  		}
  		radix_tree_preload_end();
  		__ClearPageLocked(new_page);
  		/*
  		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
  		 * clear SWAP_HAS_CACHE flag.
  		 */
  		swapcache_free(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 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);
  
  	return retpage;
  }
  
  static unsigned long swapin_nr_pages(unsigned long offset)
  {
  	static unsigned long prev_offset;
  	unsigned int pages, max_pages, last_ra;
  	static atomic_t last_readahead_pages;
  
  	max_pages = 1 << READ_ONCE(page_cluster);
  	if (max_pages <= 1)
  		return 1;
  
  	/*
  	 * 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 = atomic_xchg(&swapin_readahead_hits, 0) + 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;
  		prev_offset = offset;
  	} 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 = atomic_read(&last_readahead_pages) / 2;
  	if (pages < last_ra)
  		pages = last_ra;
  	atomic_set(&last_readahead_pages, pages);
  
  	return pages;
  }
  
  /**
   * swapin_readahead - swap in pages in hope we need them soon
   * @entry: swap entry of this memory
   * @gfp_mask: memory allocation flags
   * @vma: user vma this address belongs to
   * @addr: target address for mempolicy
   *
   * 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 down_read on the vma->vm_mm if vma is not NULL.
   */
  struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
  			struct vm_area_struct *vma, unsigned long addr)
  {
  	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 blk_plug plug;
  
  	mask = swapin_nr_pages(offset) - 1;
  	if (!mask)
  		goto skip;
  
  	/* 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++;
  
  	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);
  		if (!page)
  			continue;
  		if (offset != entry_offset)
  			SetPageReadahead(page);
  		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);
  }
  