mm/readahead.c - kernel/msm-4.9 - Gitiles

 /*
  * mm/readahead.c - address_space-level file readahead.
  *
  * Copyright (C) 2002, Linus Torvalds
  *
  * 09Apr2002	akpm@zip.com.au
  *		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)
 		+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
 }

 static int __init readahead_init(void)
 {
 	return bdi_init(&default_backing_dev_info);
 }
 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;

 		read_lock_irq(&mapping->tree_lock);
 		start = radix_tree_next_hole(&mapping->page_tree, offset, max+1);
 		read_unlock_irq(&mapping->tree_lock);

 		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);
	/*
	* mm/readahead.c - address_space-level file readahead.
	*
	* Copyright (C) 2002, Linus Torvalds
	*
	* 09Apr2002 akpm@zip.com.au
	* 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)
	+ node_page_state(numa_node_id(), NR_FREE_PAGES)) / 2);
	}

	static int __init readahead_init(void)
	{
	return bdi_init(&default_backing_dev_info);
	}
	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;

	read_lock_irq(&mapping->tree_lock);
	start = radix_tree_next_hole(&mapping->page_tree, offset, max+1);
	read_unlock_irq(&mapping->tree_lock);

	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);