/*
   * mm/readahead.c - address_space-level file readahead.
   *
   * Copyright (C) 2002, Linus Torvalds
   *

   * 09Apr2002	Andrew Morton

   *		Initial version.
   */
  
  #include <linux/kernel.h>
  #include <linux/fs.h>
  #include <linux/mm.h>
  #include <linux/module.h>
  #include <linux/blkdev.h>
  #include <linux/backing-dev.h>

  #include <linux/task_io_accounting_ops.h>

  #include <linux/pagevec.h>

  #include <linux/pagemap.h>

  
  void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
  {
  }
  EXPORT_SYMBOL(default_unplug_io_fn);
  
  struct backing_dev_info default_backing_dev_info = {

  	.ra_pages	= VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,

  	.state		= 0,
  	.capabilities	= BDI_CAP_MAP_COPY,
  	.unplug_io_fn	= default_unplug_io_fn,
  };
  EXPORT_SYMBOL_GPL(default_backing_dev_info);
  
  /*
   * Initialise a struct file's readahead state.  Assumes that the caller has
   * memset *ra to zero.
   */
  void
  file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
  {
  	ra->ra_pages = mapping->backing_dev_info->ra_pages;

  	ra->prev_pos = -1;

  }

  EXPORT_SYMBOL_GPL(file_ra_state_init);

  #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
  
  /**

   * read_cache_pages - populate an address space with some pages & start reads against them

   * @mapping: the address_space
   * @pages: The address of a list_head which contains the target pages.  These
   *   pages have their ->index populated and are otherwise uninitialised.
   * @filler: callback routine for filling a single page.
   * @data: private data for the callback routine.
   *
   * Hides the details of the LRU cache etc from the filesystems.
   */
  int read_cache_pages(struct address_space *mapping, struct list_head *pages,
  			int (*filler)(void *, struct page *), void *data)
  {
  	struct page *page;

  	int ret = 0;

  	while (!list_empty(pages)) {
  		page = list_to_page(pages);
  		list_del(&page->lru);

  		if (add_to_page_cache_lru(page, mapping,
  					page->index, GFP_KERNEL)) {

  			page_cache_release(page);
  			continue;
  		}

  		page_cache_release(page);

  		ret = filler(data, page);

  		if (unlikely(ret)) {

  			put_pages_list(pages);

  			break;
  		}

  		task_io_account_read(PAGE_CACHE_SIZE);

  	}

  	return ret;
  }
  
  EXPORT_SYMBOL(read_cache_pages);
  
  static int read_pages(struct address_space *mapping, struct file *filp,
  		struct list_head *pages, unsigned nr_pages)
  {
  	unsigned page_idx;

  	int ret;

  
  	if (mapping->a_ops->readpages) {
  		ret = mapping->a_ops->readpages(filp, mapping, pages, nr_pages);

  		/* Clean up the remaining pages */
  		put_pages_list(pages);

  		goto out;
  	}

  	for (page_idx = 0; page_idx < nr_pages; page_idx++) {
  		struct page *page = list_to_page(pages);
  		list_del(&page->lru);

  		if (!add_to_page_cache_lru(page, mapping,

  					page->index, GFP_KERNEL)) {

  			mapping->a_ops->readpage(filp, page);

  		}
  		page_cache_release(page);

  	}

  	ret = 0;

  out:
  	return ret;
  }
  
  /*

   * do_page_cache_readahead actually reads a chunk of disk.  It allocates all
   * the pages first, then submits them all for I/O. This avoids the very bad
   * behaviour which would occur if page allocations are causing VM writeback.
   * We really don't want to intermingle reads and writes like that.
   *
   * Returns the number of pages requested, or the maximum amount of I/O allowed.
   *
   * do_page_cache_readahead() returns -1 if it encountered request queue
   * congestion.
   */
  static int
  __do_page_cache_readahead(struct address_space *mapping, struct file *filp,

  			pgoff_t offset, unsigned long nr_to_read,
  			unsigned long lookahead_size)

  {
  	struct inode *inode = mapping->host;
  	struct page *page;
  	unsigned long end_index;	/* The last page we want to read */
  	LIST_HEAD(page_pool);
  	int page_idx;
  	int ret = 0;
  	loff_t isize = i_size_read(inode);
  
  	if (isize == 0)
  		goto out;

  	end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);

  
  	/*
  	 * Preallocate as many pages as we will need.
  	 */

  	for (page_idx = 0; page_idx < nr_to_read; page_idx++) {

  		pgoff_t page_offset = offset + page_idx;

  		if (page_offset > end_index)
  			break;

  		rcu_read_lock();

  		page = radix_tree_lookup(&mapping->page_tree, page_offset);

  		rcu_read_unlock();

  		if (page)
  			continue;

  		page = page_cache_alloc_cold(mapping);

  		if (!page)
  			break;
  		page->index = page_offset;
  		list_add(&page->lru, &page_pool);

  		if (page_idx == nr_to_read - lookahead_size)
  			SetPageReadahead(page);

  		ret++;
  	}

  
  	/*
  	 * Now start the IO.  We ignore I/O errors - if the page is not
  	 * uptodate then the caller will launch readpage again, and
  	 * will then handle the error.
  	 */
  	if (ret)
  		read_pages(mapping, filp, &page_pool, ret);
  	BUG_ON(!list_empty(&page_pool));
  out:
  	return ret;
  }
  
  /*
   * Chunk the readahead into 2 megabyte units, so that we don't pin too much
   * memory at once.
   */
  int force_page_cache_readahead(struct address_space *mapping, struct file *filp,

  		pgoff_t offset, unsigned long nr_to_read)

  {
  	int ret = 0;
  
  	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
  		return -EINVAL;
  
  	while (nr_to_read) {
  		int err;
  
  		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE;
  
  		if (this_chunk > nr_to_read)
  			this_chunk = nr_to_read;
  		err = __do_page_cache_readahead(mapping, filp,

  						offset, this_chunk, 0);

  		if (err < 0) {
  			ret = err;
  			break;
  		}
  		ret += err;
  		offset += this_chunk;
  		nr_to_read -= this_chunk;
  	}
  	return ret;
  }
  
  /*

   * This version skips the IO if the queue is read-congested, and will tell the
   * block layer to abandon the readahead if request allocation would block.
   *
   * force_page_cache_readahead() will ignore queue congestion and will block on
   * request queues.
   */
  int do_page_cache_readahead(struct address_space *mapping, struct file *filp,

  			pgoff_t offset, unsigned long nr_to_read)

  {
  	if (bdi_read_congested(mapping->backing_dev_info))
  		return -1;

  	return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);

  }
  
  /*

   * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a
   * sensible upper limit.
   */
  unsigned long max_sane_readahead(unsigned long nr)
  {

  	return min(nr, (node_page_state(numa_node_id(), NR_INACTIVE_FILE)

  		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);

  }

  static int __init readahead_init(void)
  {

  	int err;
  
  	err = bdi_init(&default_backing_dev_info);
  	if (!err)
  		bdi_register(&default_backing_dev_info, NULL, "default");
  
  	return err;

  }
  subsys_initcall(readahead_init);

  /*
   * Submit IO for the read-ahead request in file_ra_state.
   */

  static unsigned long ra_submit(struct file_ra_state *ra,

  		       struct address_space *mapping, struct file *filp)
  {

  	int actual;

  	actual = __do_page_cache_readahead(mapping, filp,

  					ra->start, ra->size, ra->async_size);

  
  	return actual;
  }

  
  /*

   * Set the initial window size, round to next power of 2 and square
   * for small size, x 4 for medium, and x 2 for large
   * for 128k (32 page) max ra
   * 1-8 page = 32k initial, > 8 page = 128k initial
   */
  static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
  {
  	unsigned long newsize = roundup_pow_of_two(size);
  
  	if (newsize <= max / 32)
  		newsize = newsize * 4;
  	else if (newsize <= max / 4)
  		newsize = newsize * 2;
  	else
  		newsize = max;
  
  	return newsize;
  }
  
  /*

   *  Get the previous window size, ramp it up, and
   *  return it as the new window size.
   */

  static unsigned long get_next_ra_size(struct file_ra_state *ra,

  						unsigned long max)
  {

  	unsigned long cur = ra->size;

  	unsigned long newsize;
  
  	if (cur < max / 16)

  		newsize = 4 * cur;

  	else

  		newsize = 2 * cur;

  
  	return min(newsize, max);
  }
  
  /*
   * On-demand readahead design.
   *
   * The fields in struct file_ra_state represent the most-recently-executed
   * readahead attempt:
   *

   *                        |<----- async_size ---------|
   *     |------------------- size -------------------->|
   *     |==================#===========================|
   *     ^start             ^page marked with PG_readahead

   *
   * To overlap application thinking time and disk I/O time, we do
   * `readahead pipelining': Do not wait until the application consumed all
   * readahead pages and stalled on the missing page at readahead_index;

   * Instead, submit an asynchronous readahead I/O as soon as there are
   * only async_size pages left in the readahead window. Normally async_size
   * will be equal to size, for maximum pipelining.

   *
   * In interleaved sequential reads, concurrent streams on the same fd can
   * be invalidating each other's readahead state. So we flag the new readahead

   * page at (start+size-async_size) with PG_readahead, and use it as readahead

   * indicator. The flag won't be set on already cached pages, to avoid the
   * readahead-for-nothing fuss, saving pointless page cache lookups.
   *

   * prev_pos tracks the last visited byte in the _previous_ read request.

   * It should be maintained by the caller, and will be used for detecting
   * small random reads. Note that the readahead algorithm checks loosely
   * for sequential patterns. Hence interleaved reads might be served as
   * sequential ones.
   *
   * There is a special-case: if the first page which the application tries to
   * read happens to be the first page of the file, it is assumed that a linear
   * read is about to happen and the window is immediately set to the initial size
   * based on I/O request size and the max_readahead.
   *
   * The code ramps up the readahead size aggressively at first, but slow down as
   * it approaches max_readhead.
   */
  
  /*
   * A minimal readahead algorithm for trivial sequential/random reads.
   */
  static unsigned long
  ondemand_readahead(struct address_space *mapping,
  		   struct file_ra_state *ra, struct file *filp,

  		   bool hit_readahead_marker, pgoff_t offset,

  		   unsigned long req_size)
  {

  	int	max = ra->ra_pages;	/* max readahead pages */
  	pgoff_t prev_offset;
  	int	sequential;

  
  	/*

  	 * It's the expected callback offset, assume sequential access.

  	 * Ramp up sizes, and push forward the readahead window.
  	 */

  	if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
  			offset == (ra->start + ra->size))) {
  		ra->start += ra->size;
  		ra->size = get_next_ra_size(ra, max);
  		ra->async_size = ra->size;
  		goto readit;

  	}

  	prev_offset = ra->prev_pos >> PAGE_CACHE_SHIFT;
  	sequential = offset - prev_offset <= 1UL || req_size > max;

  	/*
  	 * Standalone, small read.
  	 * Read as is, and do not pollute the readahead state.
  	 */

  	if (!hit_readahead_marker && !sequential) {

  		return __do_page_cache_readahead(mapping, filp,
  						offset, req_size, 0);
  	}
  
  	/*

  	 * Hit a marked page without valid readahead state.
  	 * E.g. interleaved reads.
  	 * Query the pagecache for async_size, which normally equals to
  	 * readahead size. Ramp it up and use it as the new readahead size.
  	 */
  	if (hit_readahead_marker) {
  		pgoff_t start;

  		rcu_read_lock();
  		start = radix_tree_next_hole(&mapping->page_tree, offset,max+1);
  		rcu_read_unlock();

  
  		if (!start || start - offset > max)
  			return 0;
  
  		ra->start = start;
  		ra->size = start - offset;	/* old async_size */
  		ra->size = get_next_ra_size(ra, max);
  		ra->async_size = ra->size;
  		goto readit;
  	}
  
  	/*

  	 * It may be one of
  	 * 	- first read on start of file
  	 * 	- sequential cache miss
  	 * 	- oversize random read
  	 * Start readahead for it.
  	 */

  	ra->start = offset;
  	ra->size = get_init_ra_size(req_size, max);
  	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;

  readit:

  	return ra_submit(ra, mapping, filp);
  }
  
  /**

   * page_cache_sync_readahead - generic file readahead

   * @mapping: address_space which holds the pagecache and I/O vectors
   * @ra: file_ra_state which holds the readahead state
   * @filp: passed on to ->readpage() and ->readpages()

   * @offset: start offset into @mapping, in pagecache page-sized units

   * @req_size: hint: total size of the read which the caller is performing in

   *            pagecache pages

   *

   * page_cache_sync_readahead() should be called when a cache miss happened:
   * it will submit the read.  The readahead logic may decide to piggyback more
   * pages onto the read request if access patterns suggest it will improve
   * performance.

   */

  void page_cache_sync_readahead(struct address_space *mapping,
  			       struct file_ra_state *ra, struct file *filp,
  			       pgoff_t offset, unsigned long req_size)

  {
  	/* no read-ahead */
  	if (!ra->ra_pages)

  		return;
  
  	/* do read-ahead */
  	ondemand_readahead(mapping, ra, filp, false, offset, req_size);
  }
  EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
  
  /**
   * page_cache_async_readahead - file readahead for marked pages
   * @mapping: address_space which holds the pagecache and I/O vectors
   * @ra: file_ra_state which holds the readahead state
   * @filp: passed on to ->readpage() and ->readpages()
   * @page: the page at @offset which has the PG_readahead flag set
   * @offset: start offset into @mapping, in pagecache page-sized units
   * @req_size: hint: total size of the read which the caller is performing in
   *            pagecache pages
   *
   * page_cache_async_ondemand() should be called when a page is used which

   * has the PG_readahead flag; this is a marker to suggest that the application

   * has used up enough of the readahead window that we should start pulling in

   * more pages.
   */

  void
  page_cache_async_readahead(struct address_space *mapping,
  			   struct file_ra_state *ra, struct file *filp,
  			   struct page *page, pgoff_t offset,
  			   unsigned long req_size)
  {
  	/* no read-ahead */
  	if (!ra->ra_pages)
  		return;
  
  	/*
  	 * Same bit is used for PG_readahead and PG_reclaim.
  	 */
  	if (PageWriteback(page))
  		return;
  
  	ClearPageReadahead(page);
  
  	/*
  	 * Defer asynchronous read-ahead on IO congestion.
  	 */
  	if (bdi_read_congested(mapping->backing_dev_info))
  		return;

  
  	/* do read-ahead */

  	ondemand_readahead(mapping, ra, filp, true, offset, req_size);

  }

  EXPORT_SYMBOL_GPL(page_cache_async_readahead);