Re: [PATCH 3/5] mm: Implement IO-less balance_dirty_pages()

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2011-02-04 at 02:38 +0100, Jan Kara wrote:
> +static int check_dirty_limits(struct backing_dev_info *bdi,
> +                             struct dirty_limit_state *pst)
> +{
> +       struct dirty_limit_state st;
> +       unsigned long bdi_thresh;
> +       unsigned long min_bdi_thresh;
> +       int ret = DIRTY_OK;
>  
> +       get_global_dirty_limit_state(&st);
> +       /*
> +        * Throttle it only when the background writeback cannot catch-up. This
> +        * avoids (excessively) small writeouts when the bdi limits are ramping
> +        * up.
> +        */
> +       if (st.nr_reclaimable + st.nr_writeback <=
> +                       (st.background_thresh + st.dirty_thresh) / 2)
> +               goto out;
>  
> +       get_bdi_dirty_limit_state(bdi, &st);
> +       min_bdi_thresh = task_min_dirty_limit(st.bdi_thresh);
> +       bdi_thresh = task_dirty_limit(current, st.bdi_thresh);
> +
> +       /*
> +        * The bdi thresh is somehow "soft" limit derived from the global
> +        * "hard" limit. The former helps to prevent heavy IO bdi or process
> +        * from holding back light ones; The latter is the last resort
> +        * safeguard.
> +        */
> +       if ((st.bdi_nr_reclaimable + st.bdi_nr_writeback > bdi_thresh)
> +           || (st.nr_reclaimable + st.nr_writeback > st.dirty_thresh)) {
> +               ret = DIRTY_EXCEED_LIMIT;
> +               goto out;
> +       }
> +       if (st.bdi_nr_reclaimable + st.bdi_nr_writeback > min_bdi_thresh) {
> +               ret = DIRTY_MAY_EXCEED_LIMIT;
> +               goto out;
> +       }
> +       if (st.nr_reclaimable > st.background_thresh)
> +               ret = DIRTY_EXCEED_BACKGROUND;
> +out:
> +       if (pst)
> +               *pst = st;

By mandating pst is always provided you can reduce the total stack
footprint, avoid the memcopy and clean up the control flow ;-)

> +       return ret;
> +} 

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