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    Subject[PATCH] fix readahead pipeline break caused by block plug
    Herbert Poetzl reported a performance regression since 2.6.39. The test
    is a simple dd read, but with big block size. The reason is:

    T1: ra (A, A+128k), (A+128k, A+256k)
    T2: lock_page for page A, submit the 256k
    T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
    because of plug and there isn't any lock_page till we hit page A+256k
    because all pages from A to A+256k is in memory
    T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't
    submitted again.
    T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is
    waitting for (A+256k, A+512k) finish.

    There is no request to disk in T3 and T4, so readahead pipeline breaks.

    We really don't need block plug for generic_file_aio_read() for buffered
    I/O. The readahead already has plug and has fine grained control when I/O
    should be submitted. Deleting plug for buffered I/O fixes the regression.

    One side effect is plug makes the request size 256k, the size is 128k
    without it. This is because default ra size is 128k and not a reason we
    need plug here.

    Signed-off-by: Shaohua Li <>
    Tested-by: Herbert Poetzl <>
    Tested-by: Eric Dumazet <>
    Signed-off-by: Wu Fengguang <>

    diff --git a/mm/filemap.c b/mm/filemap.c
    index 97f49ed..b662757 100644
    --- a/mm/filemap.c
    +++ b/mm/filemap.c
    @@ -1400,15 +1400,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
    unsigned long seg = 0;
    size_t count;
    loff_t *ppos = &iocb->ki_pos;
    - struct blk_plug plug;

    count = 0;
    retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
    if (retval)
    return retval;

    - blk_start_plug(&plug);
    /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
    if (filp->f_flags & O_DIRECT) {
    loff_t size;
    @@ -1424,8 +1421,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
    retval = filemap_write_and_wait_range(mapping, pos,
    pos + iov_length(iov, nr_segs) - 1);
    if (!retval) {
    + struct blk_plug plug;
    + blk_start_plug(&plug);
    retval = mapping->a_ops->direct_IO(READ, iocb,
    iov, pos, nr_segs);
    + blk_finish_plug(&plug);
    if (retval > 0) {
    *ppos = pos + retval;
    @@ -1481,7 +1482,6 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
    - blk_finish_plug(&plug);
    return retval;

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      Last update: 2012-01-31 09:03    [W:0.021 / U:47.368 seconds]
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