mm/readahead.c - linux - Git at Google

 // 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 "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 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.
  */
 void __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)
 		return;

 	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;

 		page = xa_load(&mapping->i_pages, page_offset);
 		if (page && !xa_is_value(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));
 }

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

 	/*
 	 * 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;
 	}
 }

 /*
  * 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 @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_miss(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 void 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_miss(mapping, offset + 1, max_pages);
 		rcu_read_unlock();

 		if (!start || start - offset > max_pages)
 			return;

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

 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;
 		}
 	}

 	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);
 }
	// 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 "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 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.
	*/
	void __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)
	return;

	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;

	page = xa_load(&mapping->i_pages, page_offset);
	if (page && !xa_is_value(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));
	}

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

	/*
	* 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;
	}
	}

	/*
	* 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 @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_miss(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 void 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_miss(mapping, offset + 1, max_pages);
	rcu_read_unlock();

	if (!start \|\| start - offset > max_pages)
	return;

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

	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;
	}
	}

	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);
	}