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    Subject[PATCH 5/6] writeback: try more writeback as long as something was written
    writeback_inodes_wb()/__writeback_inodes_sb() are not aggressive in that
    they only populate possibly a subset of elegible inodes into b_io at
    entrance time. When the queued set of inodes are all synced, they just
    return, possibly with all queued inode pages written but still
    wbc.nr_to_write > 0.

    For kupdate and background writeback, there may be more eligible inodes
    sitting in b_dirty when the current set of b_io inodes are completed. So
    it is necessary to try another round of writeback as long as we made some
    progress in this round. When there are no more eligible inodes, no more
    inodes will be enqueued in queue_io(), hence nothing could/will be
    synced and we may safely bail.

    Jan raised the concern

    I'm just afraid that in some pathological cases this could
    result in bad writeback pattern - like if there is some process
    which manages to dirty just a few pages while we are doing
    writeout, this looping could result in writing just a few pages
    in each round which is bad for fragmentation etc.

    However it requires really strong timing to make that to (continuously)
    happen. In practice it's very hard to produce such a pattern even if
    it's possible in theory. I actually tried to write 1 page per 1ms with
    this command

    write-and-fsync -n10000 -S 1000 -c 4096 /fs/test

    and do sync(1) at the same time. The sync completes quickly on ext4,
    xfs, btrfs. The readers could try other write-and-sleep patterns and
    check if it can block sync for longer time.

    CC: Jan Kara <>
    Signed-off-by: Wu Fengguang <>
    fs/fs-writeback.c | 16 ++++++++--------
    1 file changed, 8 insertions(+), 8 deletions(-)

    --- linux-next.orig/fs/fs-writeback.c 2011-04-19 10:18:30.000000000 +0800
    +++ linux-next/fs/fs-writeback.c 2011-04-19 10:18:31.000000000 +0800
    @@ -750,23 +750,23 @@ static long wb_writeback(struct bdi_writ
    wrote += write_chunk - wbc.nr_to_write;

    - * If we consumed everything, see if we have more
    + * Did we write something? Try for more
    + *
    + * Dirty inodes are moved to b_io for writeback in batches.
    + * The completion of the current batch does not necessarily
    + * mean the overall work is done. So we keep looping as long
    + * as made some progress on cleaning pages or inodes.
    - if (wbc.nr_to_write <= 0)
    + if (wbc.nr_to_write < write_chunk)
    if (wbc.inodes_cleaned)
    - * Didn't write everything and we don't have more IO, bail
    + * No more inodes for IO, bail
    if (!wbc.more_io)
    - * Did we write something? Try for more
    - */
    - if (wbc.nr_to_write < write_chunk)
    - continue;
    - /*
    * Nothing written. Wait for some inode to
    * become available for writeback. Otherwise
    * we'll just busyloop.

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      Last update: 2011-04-19 05:13    [W:0.037 / U:85.816 seconds]
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