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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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