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    Subject[PATCH 27/32] readahead: loop case
    Disable look-ahead for loop file.

    Loopback files normally contain filesystems, in which case there are already
    proper look-aheads in the upper layer, more look-aheads on the loopback file
    only ruins the read-ahead hit rate.

    Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
    ---

    I'd like to thank Tero Grundstr?m for uncovering the loopback problem.

    drivers/block/loop.c | 6 ++++++
    1 files changed, 6 insertions(+)

    --- linux-2.6.17-rc4-mm3.orig/drivers/block/loop.c
    +++ linux-2.6.17-rc4-mm3/drivers/block/loop.c
    @@ -779,6 +779,12 @@ static int loop_set_fd(struct loop_devic
    mapping = file->f_mapping;
    inode = mapping->host;

    + /*
    + * The upper layer should already do proper look-ahead,
    + * one more look-ahead here only ruins the cache hit rate.
    + */
    + file->f_ra.flags |= RA_FLAG_NO_LOOKAHEAD;
    +
    if (!(file->f_mode & FMODE_WRITE))
    lo_flags |= LO_FLAGS_READ_ONLY;

    --
    -
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