Re: [PATCH 2/7] writeback: switch to per-bdi threads for flushing data

Chris Mason
2009-03-16T13:31:26Z

On Mon, 2009-03-16 at 06:22 -0400, Christoph Hellwig wrote:
> On Thu, Mar 12, 2009 at 03:33:43PM +0100, Jens Axboe wrote:
> > +static void bdi_kupdated(struct backing_dev_info *bdi)
> > +{
> > + long nr_to_write;
> > + struct writeback_control wbc = {
> > + .bdi = bdi,
> > + .sync_mode = WB_SYNC_NONE,
> > + .nr_to_write = 0,
> > + .for_kupdate = 1,
> > + .range_cyclic = 1,
> > + };
> > +
> > + sync_supers();
>
> Not directly related to your patch, but can someone explain WTF
> sync_supers is doing here or in the old kupdated? We're writing back
> dirty pages from the VM, and for some reason we try to also write back
> superblocks. This doesn't really make any sense.

Some of our poor filesystem cousins don't write the super until kupdate
kicks them (see ext2_write_super). kupdate has always been the periodic
FS thread of last resort.

-chris