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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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