[PATCH 2/9] readahead: snap readahead request to EOF

[Wu Fengguang <fengguang.wu@intel.com>]

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If the file size is 20kb and readahead request is [0, 16kb),
it's better to expand the readahead request to [0, 20kb), which will
likely save one followup I/O for [16kb, 20kb).

If the readahead request already covers EOF, trimm it down to EOF.
Also don't set the PG_readahead mark to avoid an unnecessary future
invocation of the readahead code.

This special handling looks worthwhile because small to medium sized
files are pretty common.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/readahead.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- linux-next.orig/mm/readahead.c	2011-11-29 11:28:56.000000000 +0800
+++ linux-next/mm/readahead.c	2011-11-29 11:29:05.000000000 +0800
@@ -251,8 +251,16 @@ unsigned long max_sane_readahead(unsigne
 unsigned long ra_submit(struct file_ra_state *ra,
 		       struct address_space *mapping, struct file *filp)
 {
+	pgoff_t eof = ((i_size_read(mapping->host)-1) >> PAGE_CACHE_SHIFT) + 1;
+	pgoff_t start = ra->start;
 	int actual;
 
+	/* snap to EOF */
+	if (start + ra->size + ra->size / 2 > eof) {
+		ra->size = eof - start;
+		ra->async_size = 0;
+	}
+
 	actual = __do_page_cache_readahead(mapping, filp,
 					ra->start, ra->size, ra->async_size);
 


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