text: /*
   * mm/readahead.c - address_space-level file readahead.
   *
   * Copyright (C) 2002, Linus Torvalds
   *
   * 09Apr2002	Andrew Morton
   *		Initial version.
   */
  
  #include <linux/kernel.h>
  #include <linux/dax.h>
  #include <linux/gfp.h>
  #include <linux/export.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>
  #include <linux/syscalls.h>
  #include <linux/file.h>
  #include <linux/mm_inline.h>
  #include <linux/blk-cgroup.h>
  #include <linux/fadvise.h>
  
  #include "internal.h"
  
  /*
   * 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 = inode_to_bdi(mapping->host)->ra_pages;
  	ra->prev_pos = -1;
  }
  EXPORT_SYMBOL_GPL(file_ra_state_init);
  
  /*
   * see if a page needs releasing upon read_cache_pages() failure
   * - the caller of read_cache_pages() may have set PG_private or PG_fscache
   *   before calling, such as the NFS fs marking pages that are cached locally
   *   on disk, thus we need to give the fs a chance to clean up in the event of
   *   an error
   */
  static void read_cache_pages_invalidate_page(struct address_space *mapping,
  					     struct page *page)
  {
  	if (page_has_private(page)) {
  		if (!trylock_page(page))
  			BUG();
  		page->mapping = mapping;
  		do_invalidatepage(page, 0, PAGE_SIZE);
  		page->mapping = NULL;
  		unlock_page(page);
  	}
  	put_page(page);
  }
  
  /*
   * release a list of pages, invalidating them first if need be
   */
  static void read_cache_pages_invalidate_pages(struct address_space *mapping,
  					      struct list_head *pages)
  {
  	struct page *victim;
  
  	while (!list_empty(pages)) {
  		victim = lru_to_page(pages);
  		list_del(&victim->lru);
  		read_cache_pages_invalidate_page(mapping, victim);
  	}
  }
  
  /**
   * 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 = lru_to_page(pages);
  		list_del(&page->lru);
  		if (add_to_page_cache_lru(page, mapping, page->index,
  				readahead_gfp_mask(mapping))) {
  			read_cache_pages_invalidate_page(mapping, page);
  			continue;
  		}
  		put_page(page);
  
  		ret = filler(data, page);
  		if (unlikely(ret)) {
  			read_cache_pages_invalidate_pages(mapping, pages);
  			break;
  		}
  		task_io_account_read(PAGE_SIZE);
  	}
  	return ret;
  }
  
  EXPORT_SYMBOL(read_cache_pages);
  
  static int read_pages(struct address_space *mapping, struct file *filp,
  		struct list_head *pages, unsigned int nr_pages, gfp_t gfp)
  {
  	struct blk_plug plug;
  	unsigned page_idx;
  	int ret;
  
  	blk_start_plug(&plug);
  
  	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 = lru_to_page(pages);
  		list_del(&page->lru);
  		if (!add_to_page_cache_lru(page, mapping, page->index, gfp))
  			mapping->a_ops->readpage(filp, page);
  		put_page(page);
  	}
  	ret = 0;
  
  out:
  	blk_finish_plug(&plug);
  
  	return ret;
  }
  
  /*
   * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates
   * the pages first, then submits them 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.
   */
  unsigned 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;
  	unsigned int nr_pages = 0;
  	loff_t isize = i_size_read(inode);
  	gfp_t gfp_mask = readahead_gfp_mask(mapping);
  
  	if (isize == 0)
  		goto out;
  
  	end_index = ((isize - 1) >> PAGE_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->i_pages, page_offset);
  		rcu_read_unlock();
  		if (page && !radix_tree_exceptional_entry(page)) {
  			/*
  			 * Page already present?  Kick off the current batch of
  			 * contiguous pages before continuing with the next
  			 * batch.
  			 */
  			if (nr_pages)
  				read_pages(mapping, filp, &page_pool, nr_pages,
  						gfp_mask);
  			nr_pages = 0;
  			continue;
  		}
  
  		page = __page_cache_alloc(gfp_mask);
  		if (!page)
  			break;
  		page->index = page_offset;
  		list_add(&page->lru, &page_pool);
  		if (page_idx == nr_to_read - lookahead_size)
  			SetPageReadahead(page);
  		nr_pages++;
  	}
  
  	/*
  	 * 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 (nr_pages)
  		read_pages(mapping, filp, &page_pool, nr_pages, gfp_mask);
  	BUG_ON(!list_empty(&page_pool));
  out:
  	return nr_pages;
  }
  
  /*
   * 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)
  {
  	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
  	struct file_ra_state *ra = &filp->f_ra;
  	unsigned long max_pages;
  
  	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
  		return -EINVAL;
  
  	/*
  	 * If the request exceeds the readahead window, allow the read to
  	 * be up to the optimal hardware IO size
  	 */
  	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
  	nr_to_read = min(nr_to_read, max_pages);
  	while (nr_to_read) {
  		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
  
  		if (this_chunk > nr_to_read)
  			this_chunk = nr_to_read;
  		__do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
  
  		offset += this_chunk;
  		nr_to_read -= this_chunk;
  	}
  	return 0;
  }
  
  /*
   * 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.
   */
  
  /*
   * Count contiguously cached pages from @offset-1 to @offset-@max,
   * this count is a conservative estimation of
   * 	- length of the sequential read sequence, or
   * 	- thrashing threshold in memory tight systems
   */
  static pgoff_t count_history_pages(struct address_space *mapping,
  				   pgoff_t offset, unsigned long max)
  {
  	pgoff_t head;
  
  	rcu_read_lock();
  	head = page_cache_prev_hole(mapping, offset - 1, max);
  	rcu_read_unlock();
  
  	return offset - 1 - head;
  }
  
  /*
   * page cache context based read-ahead
   */
  static int try_context_readahead(struct address_space *mapping,
  				 struct file_ra_state *ra,
  				 pgoff_t offset,
  				 unsigned long req_size,
  				 unsigned long max)
  {
  	pgoff_t size;
  
  	size = count_history_pages(mapping, offset, max);
  
  	/*
  	 * not enough history pages:
  	 * it could be a random read
  	 */
  	if (size <= req_size)
  		return 0;
  
  	/*
  	 * starts from beginning of file:
  	 * it is a strong indication of long-run stream (or whole-file-read)
  	 */
  	if (size >= offset)
  		size *= 2;
  
  	ra->start = offset;
  	ra->size = min(size + req_size, max);
  	ra->async_size = 1;
  
  	return 1;
  }
  
  /*
   * 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)
  {
  	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
  	unsigned long max_pages = ra->ra_pages;
  	unsigned long add_pages;
  	pgoff_t prev_offset;
  
  	/*
  	 * If the request exceeds the readahead window, allow the read to
  	 * be up to the optimal hardware IO size
  	 */
  	if (req_size > max_pages && bdi->io_pages > max_pages)
  		max_pages = min(req_size, bdi->io_pages);
  
  	/*
  	 * start of file
  	 */
  	if (!offset)
  		goto initial_readahead;
  
  	/*
  	 * It's the expected callback offset, assume sequential access.
  	 * Ramp up sizes, and push forward the readahead window.
  	 */
  	if ((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_pages);
  		ra->async_size = ra->size;
  		goto readit;
  	}
  
  	/*
  	 * 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 = page_cache_next_hole(mapping, offset + 1, max_pages);
  		rcu_read_unlock();
  
  		if (!start || start - offset > max_pages)
  			return 0;
  
  		ra->start = start;
  		ra->size = start - offset;	/* old async_size */
  		ra->size += req_size;
  		ra->size = get_next_ra_size(ra, max_pages);
  		ra->async_size = ra->size;
  		goto readit;
  	}
  
  	/*
  	 * oversize read
  	 */
  	if (req_size > max_pages)
  		goto initial_readahead;
  
  	/*
  	 * sequential cache miss
  	 * trivial case: (offset - prev_offset) == 1
  	 * unaligned reads: (offset - prev_offset) == 0
  	 */
  	prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
  	if (offset - prev_offset <= 1UL)
  		goto initial_readahead;
  
  	/*
  	 * Query the page cache and look for the traces(cached history pages)
  	 * that a sequential stream would leave behind.
  	 */
  	if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
  		goto readit;
  
  	/*
  	 * standalone, small random read
  	 * Read as is, and do not pollute the readahead state.
  	 */
  	return __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
  
  initial_readahead:
  	ra->start = offset;
  	ra->size = get_init_ra_size(req_size, max_pages);
  	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
  
  readit:
  	/*
  	 * Will this read hit the readahead marker made by itself?
  	 * If so, trigger the readahead marker hit now, and merge
  	 * the resulted next readahead window into the current one.
  	 * Take care of maximum IO pages as above.
  	 */
  	if (offset == ra->start && ra->size == ra->async_size) {
  		add_pages = get_next_ra_size(ra, max_pages);
  		if (ra->size + add_pages <= max_pages) {
  			ra->async_size = add_pages;
  			ra->size += add_pages;
  		} else {
  			ra->size = max_pages;
  			ra->async_size = max_pages >> 1;
  		}
  	}
  
  	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;
  
  	if (blk_cgroup_congested())
  		return;
  
  	/* be dumb */
  	if (filp && (filp->f_mode & FMODE_RANDOM)) {
  		force_page_cache_readahead(mapping, filp, offset, req_size);
  		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_readahead() 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 (inode_read_congested(mapping->host))
  		return;
  
  	if (blk_cgroup_congested())
  		return;
  
  	/* do read-ahead */
  	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
  }
  EXPORT_SYMBOL_GPL(page_cache_async_readahead);
  
  ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
  {
  	ssize_t ret;
  	struct fd f;
  
  	ret = -EBADF;
  	f = fdget(fd);
  	if (!f.file || !(f.file->f_mode & FMODE_READ))
  		goto out;
  
  	/*
  	 * The readahead() syscall is intended to run only on files
  	 * that can execute readahead. If readahead is not possible
  	 * on this file, then we must return -EINVAL.
  	 */
  	ret = -EINVAL;
  	if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
  	    !S_ISREG(file_inode(f.file)->i_mode))
  		goto out;
  
  	ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
  out:
  	fdput(f);
  	return ret;
  }
  
  SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
  {
  	return ksys_readahead(fd, offset, count);
  }
  