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@xxxxxxxxx>
---
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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