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Subject[PATCH 7/9] 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 | 3 +++
include/linux/fs.h | 1 +
mm/readahead.c | 4 ++++
3 files changed, 8 insertions(+)
--- linux-next.orig/mm/readahead.c 2012-02-11 12:30:59.000000000 +0800
+++ linux-next/mm/readahead.c 2012-02-11 12:31:05.000000000 +0800
@@ -485,6 +485,7 @@ unsigned long ra_submit(struct file_ra_s
ra->pattern, ra->start, ra->size, ra->async_size,
actual);

+ ra->lseek = 0;
ra->for_mmap = 0;
ra->for_metadata = 0;
return actual;
@@ -636,6 +637,8 @@ ondemand_readahead(struct address_space
* start of file
*/
if (!offset) {
+ if (ra->lseek && req_size < max)
+ goto random_read;
ra->pattern = RA_PATTERN_INITIAL;
goto initial_readahead;
}
@@ -701,6 +704,7 @@ 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 2012-02-11 12:30:59.000000000 +0800
+++ linux-next/fs/read_write.c 2012-02-11 12:31:05.000000000 +0800
@@ -47,6 +47,9 @@ static loff_t lseek_execute(struct file
file->f_pos = offset;
file->f_version = 0;
}
+
+ file->f_ra.lseek = 1;
+
return offset;
}

--- linux-next.orig/include/linux/fs.h 2012-02-11 12:30:59.000000000 +0800
+++ linux-next/include/linux/fs.h 2012-02-11 12:31:05.000000000 +0800
@@ -956,6 +956,7 @@ struct file_ra_state {
u8 pattern; /* one of RA_PATTERN_* */
unsigned int for_mmap:1; /* readahead for mmap accesses */
unsigned int for_metadata:1; /* readahead for meta data */
+ unsigned int lseek:1; /* this read has a leading lseek */

loff_t prev_pos; /* Cache last read() position */
};



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