[PATCH 8/8] readahead: dont do start-of-file readahead after lseek()

[Wu Fengguang]

Some applications (eg. blkid, id3tool etc.) seek around the file
to get information. For example, blkid does

	     seek to	0
	     read	1024
	     seek to	1536
	     read	16384

The start-of-file readahead heuristic is wrong for them, whose
access pattern can be identified by lseek() calls.

So test-and-set a READAHEAD_LSEEK flag on lseek() and don't
do start-of-file readahead on seeing it. Proposed by Linus.

Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/read_write.c    |    4 ++++
 include/linux/fs.h |    1 +
 mm/readahead.c     |    3 +++
 3 files changed, 8 insertions(+)

--- linux-next.orig/mm/readahead.c	2011-11-20 22:02:01.000000000 +0800
+++ linux-next/mm/readahead.c	2011-11-20 22:02:03.000000000 +0800
@@ -629,6 +629,8 @@ ondemand_readahead(struct address_space 
 	 * start of file
 	 */
 	if (!offset) {
+		if ((ra->ra_flags & READAHEAD_LSEEK) && req_size < max)
+			goto random_read;
 		ra_set_pattern(ra, RA_PATTERN_INITIAL);
 		goto initial_readahead;
 	}
@@ -707,6 +709,7 @@ ondemand_readahead(struct address_space 
 	if (try_context_readahead(mapping, ra, offset, req_size, max))
 		goto readit;
 
+random_read:
 	/*
 	 * standalone, small random read
 	 */
--- linux-next.orig/fs/read_write.c	2011-11-20 22:02:01.000000000 +0800
+++ linux-next/fs/read_write.c	2011-11-20 22:02:03.000000000 +0800
@@ -47,6 +47,10 @@ static loff_t lseek_execute(struct file 
 		file->f_pos = offset;
 		file->f_version = 0;
 	}
+
+	if (!(file->f_ra.ra_flags & READAHEAD_LSEEK))
+		file->f_ra.ra_flags |= READAHEAD_LSEEK;
+
 	return offset;
 }
 
--- linux-next.orig/include/linux/fs.h	2011-11-20 22:02:01.000000000 +0800
+++ linux-next/include/linux/fs.h	2011-11-20 22:02:03.000000000 +0800
@@ -952,6 +952,7 @@ struct file_ra_state {
 /* ra_flags bits */
 #define	READAHEAD_MMAP_MISS	0x000003ff /* cache misses for mmap access */
 #define	READAHEAD_MMAP		0x00010000
+#define	READAHEAD_LSEEK		0x00020000 /* be conservative after lseek() */
 
 #define READAHEAD_PATTERN_SHIFT	28
 #define READAHEAD_PATTERN	0xf0000000


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