Commit 46fc3e7b authored by Fengguang Wu's avatar Fengguang Wu Committed by Linus Torvalds
Browse files

readahead: add look-ahead support to __do_page_cache_readahead() 



Add look-ahead support to __do_page_cache_readahead().

It works by
	- mark the Nth backwards page with PG_readahead,
	(which instructs the page's first reader to invoke readahead)
	- and only do the marking for newly allocated pages.
	(to prevent blindly doing readahead on already cached pages)

Look-ahead is a technique to achieve I/O pipelining:

While the application is working through a chunk of cached pages, the kernel
reads-ahead the next chunk of pages _before_ time of need.  It effectively
hides low level I/O latencies to high level applications.

Signed-off-by: default avatarFengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Steven Pratt <slpratt@austin.ibm.com>
Cc: Ram Pai <linuxram@us.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent d77c2d7c

mm/readahead.c

+9 −6
Original line number Diff line number Diff line
@@ -265,7 +265,8 @@ out: 
 */

static int

__do_page_cache_readahead(struct address_space *mapping, struct file *filp,

			pgoff_t offset, unsigned long nr_to_read)

			pgoff_t offset, unsigned long nr_to_read,

			unsigned long lookahead_size)

{

	struct inode *inode = mapping->host;

	struct page *page;
@@ -301,6 +302,8 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp, 
			break;

		page->index = page_offset;

		list_add(&page->lru, &page_pool);

		if (page_idx == nr_to_read - lookahead_size)

			SetPageReadahead(page);

		ret++;

	}

	read_unlock_irq(&mapping->tree_lock);
@@ -337,7 +340,7 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp, 
		if (this_chunk > nr_to_read)

			this_chunk = nr_to_read;

		err = __do_page_cache_readahead(mapping, filp,

						offset, this_chunk);

						offset, this_chunk, 0);

		if (err < 0) {

			ret = err;

			break;
@@ -384,7 +387,7 @@ int do_page_cache_readahead(struct address_space *mapping, struct file *filp, 
	if (bdi_read_congested(mapping->backing_dev_info))

		return -1;



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

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

}



/*
@@ -404,7 +407,7 @@ blockable_page_cache_readahead(struct address_space *mapping, struct file *filp, 
	if (!block && bdi_read_congested(mapping->backing_dev_info))

		return 0;



	actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read);

	actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);



	return check_ra_success(ra, nr_to_read, actual);

}
@@ -449,7 +452,7 @@ static int make_ahead_window(struct address_space *mapping, struct file *filp, 
 * @req_size: hint: total size of the read which the caller is performing in

 *            PAGE_CACHE_SIZE units

 *

 * page_cache_readahead() is the main function.  If performs the adaptive

 * page_cache_readahead() is the main function.  It performs the adaptive

 * readahead window size management and submits the readahead I/O.

 *

 * Note that @filp is purely used for passing on to the ->readpage[s]()

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