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    SubjectRe: [PATCH 3/5] writeback: prevent sync livelock with the sync_after timestamp
    On Thu, Jul 29, 2010 at 11:02:41PM +0800, Jan Kara wrote:
    > Hi Fengguang,
    >
    > On Thu 29-07-10 19:51:45, Wu Fengguang wrote:
    > > The start time in writeback_inodes_wb() is not very useful because it
    > > slips at each invocation time. Preferrably one _constant_ time shall be
    > > used at the beginning to cover the whole sync() work.
    > >
    > > The newly dirtied inodes are now guarded at the queue_io() time instead
    > > of the b_io walk time. This is more natural: non-empty b_io/b_more_io
    > > means "more work pending".
    > >
    > > The timestamp is now grabbed the sync work submission time, and may be
    > > further optimized to the initial sync() call time.
    > The patch seems to have some issues...
    >
    > > + if (wbc->for_sync) {
    > For example this is never set. You only set wb->for_sync.

    Ah right.

    > > + expire_interval = 1;
    > > + older_than_this = wbc->sync_after;
    > And sync_after is never set either???

    Sorry I must lose some chunk when rebasing the patch ..

    > > - if (!(wbc->for_kupdate || wbc->for_background) || list_empty(&wb->b_io))
    > > + if (list_empty(&wb->b_io))
    > > queue_io(wb, wbc);
    > And what is the purpose of this? It looks as an unrelated change to me.

    Yes it's not tightly related. It may be simpler to do

    - if (!wbc->for_kupdate || list_empty(&wb->b_io))
    + if (list_empty(&wb->b_io))

    in the previous patch "writeback: sync expired inodes first in
    background writeback".

    Thanks,
    Fengguang


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