// SPDX-License-Identifier: GPL-2.0-only

  /*
   * 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 <linux/sched/mm.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.

   *
   * Returns: %0 on success, error return by @filler otherwise

   */
  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 void read_pages(struct readahead_control *rac, struct list_head *pages,

  		bool skip_page)

  {

  	const struct address_space_operations *aops = rac->mapping->a_ops;

  	struct page *page;

  	struct blk_plug plug;

  	if (!readahead_count(rac))

  		goto out;

  	blk_start_plug(&plug);

  	if (aops->readahead) {
  		aops->readahead(rac);
  		/* Clean up the remaining pages */
  		while ((page = readahead_page(rac))) {
  			unlock_page(page);
  			put_page(page);
  		}
  	} else if (aops->readpages) {

  		aops->readpages(rac->file, rac->mapping, pages,
  				readahead_count(rac));

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

  		rac->_index += rac->_nr_pages;
  		rac->_nr_pages = 0;
  	} else {
  		while ((page = readahead_page(rac))) {

  			aops->readpage(rac->file, page);

  			put_page(page);
  		}

  	}

  	blk_finish_plug(&plug);

  
  	BUG_ON(!list_empty(pages));

  	BUG_ON(readahead_count(rac));
  
  out:
  	if (skip_page)
  		rac->_index++;

  }

  /**

   * page_cache_ra_unbounded - Start unchecked readahead.
   * @ractl: Readahead control.

   * @nr_to_read: The number of pages to read.
   * @lookahead_size: Where to start the next readahead.
   *
   * This function is for filesystems to call when they want to start
   * readahead beyond a file's stated i_size.  This is almost certainly
   * not the function you want to call.  Use page_cache_async_readahead()
   * or page_cache_sync_readahead() instead.
   *
   * Context: File is referenced by caller.  Mutexes may be held by caller.
   * May sleep, but will not reenter filesystem to reclaim memory.

   */

  void page_cache_ra_unbounded(struct readahead_control *ractl,
  		unsigned long nr_to_read, unsigned long lookahead_size)

  {

  	struct address_space *mapping = ractl->mapping;
  	unsigned long index = readahead_index(ractl);

  	LIST_HEAD(page_pool);

  	gfp_t gfp_mask = readahead_gfp_mask(mapping);

  	unsigned long i;

  
  	/*

  	 * Partway through the readahead operation, we will have added
  	 * locked pages to the page cache, but will not yet have submitted
  	 * them for I/O.  Adding another page may need to allocate memory,
  	 * which can trigger memory reclaim.  Telling the VM we're in
  	 * the middle of a filesystem operation will cause it to not
  	 * touch file-backed pages, preventing a deadlock.  Most (all?)
  	 * filesystems already specify __GFP_NOFS in their mapping's
  	 * gfp_mask, but let's be explicit here.
  	 */
  	unsigned int nofs = memalloc_nofs_save();
  
  	/*

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

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

  		struct page *page = xa_load(&mapping->i_pages, index + i);

  		BUG_ON(index + i != ractl->_index + ractl->_nr_pages);

  		if (page && !xa_is_value(page)) {

  			/*

  			 * Page already present?  Kick off the current batch
  			 * of contiguous pages before continuing with the
  			 * next batch.  This page may be the one we would
  			 * have intended to mark as Readahead, but we don't
  			 * have a stable reference to this page, and it's
  			 * not worth getting one just for that.

  			 */

  			read_pages(ractl, &page_pool, true);

  			continue;

  		}

  		page = __page_cache_alloc(gfp_mask);

  		if (!page)
  			break;

  		if (mapping->a_ops->readpages) {
  			page->index = index + i;
  			list_add(&page->lru, &page_pool);
  		} else if (add_to_page_cache_lru(page, mapping, index + i,
  					gfp_mask) < 0) {
  			put_page(page);

  			read_pages(ractl, &page_pool, true);

  			continue;
  		}

  		if (i == nr_to_read - lookahead_size)

  			SetPageReadahead(page);

  		ractl->_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.
  	 */

  	read_pages(ractl, &page_pool, false);

  	memalloc_nofs_restore(nofs);

  }

  EXPORT_SYMBOL_GPL(page_cache_ra_unbounded);

  
  /*

   * do_page_cache_ra() 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.
   */

  void do_page_cache_ra(struct readahead_control *ractl,
  		unsigned long nr_to_read, unsigned long lookahead_size)

  {

  	struct inode *inode = ractl->mapping->host;
  	unsigned long index = readahead_index(ractl);

  	loff_t isize = i_size_read(inode);
  	pgoff_t end_index;	/* The last page we want to read */
  
  	if (isize == 0)
  		return;
  
  	end_index = (isize - 1) >> PAGE_SHIFT;
  	if (index > end_index)
  		return;
  	/* Don't read past the page containing the last byte of the file */
  	if (nr_to_read > end_index - index)
  		nr_to_read = end_index - index + 1;

  	page_cache_ra_unbounded(ractl, nr_to_read, lookahead_size);

  }

  
  /*
   * Chunk the readahead into 2 megabyte units, so that we don't pin too much
   * memory at once.
   */

  void force_page_cache_ra(struct readahead_control *ractl,

  		struct file_ra_state *ra, unsigned long nr_to_read)

  {

  	struct address_space *mapping = ractl->mapping;

  	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);

  	unsigned long max_pages, index;

  	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages &&
  			!mapping->a_ops->readahead))

  		return;

  	/*
  	 * If the request exceeds the readahead window, allow the read to
  	 * be up to the optimal hardware IO size
  	 */

  	index = readahead_index(ractl);

  	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);

  	nr_to_read = min_t(unsigned long, 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;

  		ractl->_index = index;
  		do_page_cache_ra(ractl, this_chunk, 0);

  		index += this_chunk;

  		nr_to_read -= this_chunk;
  	}

  }

  /*

   * 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;

  
  	if (cur < max / 16)

  		return 4 * cur;
  	if (cur <= max / 2)
  		return 2 * cur;
  	return 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 @index-1 to @index-@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 index, unsigned long max)

  {
  	pgoff_t head;
  
  	rcu_read_lock();

  	head = page_cache_prev_miss(mapping, index - 1, max);

  	rcu_read_unlock();

  	return index - 1 - head;

  }
  
  /*
   * page cache context based read-ahead
   */
  static int try_context_readahead(struct address_space *mapping,
  				 struct file_ra_state *ra,

  				 pgoff_t index,

  				 unsigned long req_size,
  				 unsigned long max)
  {
  	pgoff_t size;

  	size = count_history_pages(mapping, index, 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 >= index)

  		size *= 2;

  	ra->start = index;

  	ra->size = min(size + req_size, max);
  	ra->async_size = 1;

  
  	return 1;
  }
  
  /*

   * A minimal readahead algorithm for trivial sequential/random reads.
   */

  static void ondemand_readahead(struct readahead_control *ractl,
  		struct file_ra_state *ra, bool hit_readahead_marker,

  		unsigned long req_size)

  {

  	struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);

  	unsigned long max_pages = ra->ra_pages;

  	unsigned long add_pages;

  	unsigned long index = readahead_index(ractl);

  	pgoff_t prev_index;

  
  	/*

  	 * 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 (!index)

  		goto initial_readahead;

  
  	/*

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

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

  	if ((index == (ra->start + ra->size - ra->async_size) ||
  	     index == (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_miss(ractl->mapping, index + 1,
  				max_pages);

  		rcu_read_unlock();

  		if (!start || start - index > max_pages)

  			return;

  
  		ra->start = start;

  		ra->size = start - index;	/* 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: (index - prev_index) == 1
  	 * unaligned reads: (index - prev_index) == 0

  	 */

  	prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
  	if (index - prev_index <= 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(ractl->mapping, ra, index, req_size,
  			max_pages))

  		goto readit;
  
  	/*

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

  	do_page_cache_ra(ractl, req_size, 0);

  	return;

  
  initial_readahead:

  	ra->start = index;

  	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 (index == 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;
  		}

  	}

  	ractl->_index = ra->start;
  	do_page_cache_ra(ractl, ra->size, ra->async_size);

  }

  void page_cache_sync_ra(struct readahead_control *ractl,
  		struct file_ra_state *ra, unsigned long req_count)

  {

  	bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);

  	/*
  	 * Even if read-ahead is disabled, issue this request as read-ahead
  	 * as we'll need it to satisfy the requested range. The forced
  	 * read-ahead will do the right thing and limit the read to just the
  	 * requested range, which we'll set to 1 page for this case.
  	 */
  	if (!ra->ra_pages || blk_cgroup_congested()) {
  		if (!ractl->file)
  			return;
  		req_count = 1;
  		do_forced_ra = true;
  	}

  	/* be dumb */

  	if (do_forced_ra) {

  		force_page_cache_ra(ractl, ra, req_count);

  		return;
  	}

  	/* do read-ahead */

  	ondemand_readahead(ractl, ra, false, req_count);

  }

  EXPORT_SYMBOL_GPL(page_cache_sync_ra);

  void page_cache_async_ra(struct readahead_control *ractl,
  		struct file_ra_state *ra, struct page *page,
  		unsigned long req_count)

  {
  	/* 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(ractl->mapping->host))

  		return;

  	if (blk_cgroup_congested())
  		return;

  	/* do read-ahead */

  	ondemand_readahead(ractl, ra, true, req_count);

  }

  EXPORT_SYMBOL_GPL(page_cache_async_ra);

  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);
  }