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Subject[PATCH 11/15] 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: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
fs/read_write.c | 3 +++
include/linux/fs.h | 1 +
mm/readahead.c | 5 +++++
3 files changed, 9 insertions(+)
--- linux.orig/mm/readahead.c 2010-02-24 10:44:47.000000000 +0800
+++ linux/mm/readahead.c 2010-02-24 10:44:48.000000000 +0800
@@ -672,6 +672,11 @@ 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 2010-02-24 10:44:30.000000000 +0800
+++ linux/fs/read_write.c 2010-02-24 10:44:48.000000000 +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 2010-02-24 10:44:45.000000000 +0800
+++ linux/include/linux/fs.h 2010-02-24 10:44:48.000000000 +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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