[PATCH 12/16] 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    |    3 +++
 include/linux/fs.h |    1 +
 mm/readahead.c     |    5 +++++
 3 files changed, 9 insertions(+)

--- linux.orig/mm/readahead.c	2010-03-01 13:23:45.000000000 +0800
+++ linux/mm/readahead.c	2010-03-01 13:23:45.000000000 +0800
@@ -670,6 +670,11 @@ ondemand_readahead(struct address_space 
 	if (!offset) {
 		ra_set_pattern(ra, RA_PATTERN_INITIAL);
 		ra->start = offset;
+		if ((ra->ra_flags & READAHEAD_LSEEK) && req_size <= max) {
+			ra->size = req_size;
+			ra->async_size = 0;
+			goto readit;
+		}
 		ra->size = get_init_ra_size(req_size, max);
 		ra->async_size = ra->size > req_size ?
 				 ra->size - req_size : ra->size;
--- linux.orig/fs/read_write.c	2010-03-01 13:21:43.000000000 +0800
+++ linux/fs/read_write.c	2010-03-01 13:23:45.000000000 +0800
@@ -71,6 +71,9 @@ generic_file_llseek_unlocked(struct file
 		file->f_version = 0;
 	}
 
+	if (!(file->f_ra.ra_flags & READAHEAD_LSEEK))
+		file->f_ra.ra_flags |= READAHEAD_LSEEK;
+
 	return offset;
 }
 EXPORT_SYMBOL(generic_file_llseek_unlocked);
--- linux.orig/include/linux/fs.h	2010-03-01 13:23:42.000000000 +0800
+++ linux/include/linux/fs.h	2010-03-01 13:23:45.000000000 +0800
@@ -899,6 +899,7 @@ struct file_ra_state {
 #define	READAHEAD_MMAP_MISS	0x00000fff /* cache misses for mmap access */
 #define READAHEAD_THRASHED	0x10000000
 #define	READAHEAD_MMAP		0x20000000
+#define	READAHEAD_LSEEK		0x40000000 /* be conservative after lseek() */
 
 /*
  * Which policy makes decision to do the current read-ahead IO?


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