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    SubjectRe: dirty balancing deadlock
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    Date
    > > How about this?
    >
    > I still don't understand this bug.
    >
    > > Solves the FUSE deadlock, but not the throttle_vm_writeout() one.
    > > I'll try to tackle that one as well.
    > >
    > > If the per-bdi dirty counter goes below 16, balance_dirty_pages()
    > > returns.
    > >
    > > Does the constant need to tunable? If it's too large, then the global
    > > threshold is more easily exceeded. If it's too small, then in a tight
    > > situation progress will be slower.
    > >
    > > Thanks,
    > > Miklos
    > >
    > > Index: linux/mm/page-writeback.c
    > > ===================================================================
    > > --- linux.orig/mm/page-writeback.c 2007-02-19 17:32:41.000000000 +0100
    > > +++ linux/mm/page-writeback.c 2007-02-19 18:05:28.000000000 +0100
    > > @@ -198,6 +198,25 @@ static void balance_dirty_pages(struct a
    > > dirty_thresh)
    > > break;
    > >
    > > + /*
    > > + * Acquit this producer if there's little or nothing
    > > + * to write back to this particular queue
    > > + *
    > > + * Without this check a deadlock is possible in the
    > > + * following case:
    > > + *
    > > + * - filesystem A writes data through filesystem B
    > > + * - filesystem A has dirty pages over dirty_thresh
    > > + * - writeback is started, this triggers a write in B
    > > + * - balance_dirty_pages() is called synchronously
    > > + * - the write to B blocks
    > > + * - the writeback completes, but dirty is still over threshold
    > > + * - the blocking write prevents futher writes from happening
    > > + */
    > > + if (atomic_long_read(&bdi->nr_dirty) +
    > > + atomic_long_read(&bdi->nr_writeback) < 16)
    > > + break;
    > > +
    >
    > The problem seems to that little "- the write to B blocks".
    >
    > How come it blocks? I mean, if we cannot retire writes to that filesystem
    > then we're screwed anyway.

    Sorry about the sloppy description. I mean, it's not the lowlevel
    write that will block, but rather the VFS one
    (generic_file_aio_write). It will block (or rather loop forever with
    0.1 second sleeps) in balance_dirty_pages(). That means, that for
    this inode, i_mutex is held and no other writer can continue the work.

    Miklos
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