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    SubjectRe: [PATCH] bdi_sync_writeback should WB_SYNC_NONE first
    On Sun, Sep 27 2009, Andrew Morton wrote:
    > On Sun, 27 Sep 2009 18:44:32 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:
    >
    > > On Sun, Sep 27 2009, Andrew Morton wrote:
    > > > On Fri, 25 Sep 2009 10:10:14 -0400 Chris Mason <chris.mason@oracle.com> wrote:
    > > >
    > > > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    > > > > index 8e1e5e1..27f8e0e 100644
    > > > > --- a/fs/fs-writeback.c
    > > > > +++ b/fs/fs-writeback.c
    > > > > @@ -225,7 +225,7 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
    > > > > {
    > > > > struct wb_writeback_args args = {
    > > > > .sb = sb,
    > > > > - .sync_mode = WB_SYNC_ALL,
    > > > > + .sync_mode = WB_SYNC_NONE,
    > > > > .nr_pages = LONG_MAX,
    > > > > .range_cyclic = 0,
    > > > > };
    > > > > @@ -236,6 +236,13 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
    > > > >
    > > > > bdi_queue_work(bdi, &work);
    > > > > bdi_wait_on_work_clear(&work);
    > > > > +
    > > > > + args.sync_mode = WB_SYNC_ALL;
    > > > > + args.nr_pages = LONG_MAX;
    > > > > +
    > > > > + work.state = WS_USED | WS_ONSTACK;
    > > > > + bdi_queue_work(bdi, &work);
    > > > > + bdi_wait_on_work_clear(&work);
    > > > > }
    > > >
    > > > Those LONG_MAX's are a worry. What prevents a very long
    > > > almost-livelock from occurring if userspace is concurrently dirtying
    > > > pagecache at a high rate?
    > >
    > > Not sure whether Chris' system is back up again, but I discussed this
    > > with him on irc. Since the WB_SYNC_ALL writeback should be queued behind
    > > the WB_SYNC_NONE that the non-wait sync already issued, not sure why
    > > this patch makes a difference. It's definitely not the right approach.
    > >
    >
    > I wasn't referring to this patch actually. The code as it stands in
    > Linus's tree right now attempts to write back up to 2^63 pages...

    I agree, it could make the fs sync take a looong time. This is not a new
    issue, though.

    --
    Jens Axboe



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