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Subject[PATCH 8/8] readahead: dont do start-of-file readahead after lseek()
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@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
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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