On Fri 24-01-14 17:03:03, Johannes Weiner wrote:
> The dirty_balance_reserve is an approximation of the fraction of free
> pages that the page allocator does not make available for page cache
> allocations. As a result, it has to be taken into account when
> calculating the amount of "dirtyable memory", the baseline to which
> dirty_background_ratio and dirty_ratio are applied.
>
> However, currently the reserve is subtracted from the sum of free and
> reclaimable pages, which is non-sensical and leads to erroneous
> results when the system is dominated by unreclaimable pages and the
> dirty_balance_reserve is bigger than free+reclaimable. In that case,
> at least the already allocated cache should be considered dirtyable.
>
> Fix the calculation by subtracting the reserve from the amount of free
> pages, then adding the reclaimable pages on top.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Makes sense
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> mm/page-writeback.c | 52 +++++++++++++++++++++++-----------------------------
> 1 file changed, 23 insertions(+), 29 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 63807583d8e8..79cf52b058a7 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -191,6 +191,25 @@ static unsigned long writeout_period_time = 0;
> * global dirtyable memory first.
> */
>
> +/**
> + * zone_dirtyable_memory - number of dirtyable pages in a zone
> + * @zone: the zone
> + *
> + * Returns the zone's number of pages potentially available for dirty
> + * page cache. This is the base value for the per-zone dirty limits.
> + */
> +static unsigned long zone_dirtyable_memory(struct zone *zone)
> +{
> + unsigned long nr_pages;
> +
> + nr_pages = zone_page_state(zone, NR_FREE_PAGES);
> + nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
> +
> + nr_pages += zone_reclaimable_pages(zone);
> +
> + return nr_pages;
> +}
> +
> static unsigned long highmem_dirtyable_memory(unsigned long total)
> {
> #ifdef CONFIG_HIGHMEM
> @@ -201,8 +220,7 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
> struct zone *z =
> &NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
>
> - x += zone_page_state(z, NR_FREE_PAGES) +
> - zone_reclaimable_pages(z) - z->dirty_balance_reserve;
> + x += zone_dirtyable_memory(zone);
> }
> /*
> * Unreclaimable memory (kernel memory or anonymous memory
> @@ -238,9 +256,11 @@ static unsigned long global_dirtyable_memory(void)
> {
> unsigned long x;
>
> - x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> + x = global_page_state(NR_FREE_PAGES);
> x -= min(x, dirty_balance_reserve);
>
> + x += global_reclaimable_pages();
> +
> if (!vm_highmem_is_dirtyable)
> x -= highmem_dirtyable_memory(x);
>
> @@ -289,32 +309,6 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
> }
>
> /**
> - * zone_dirtyable_memory - number of dirtyable pages in a zone
> - * @zone: the zone
> - *
> - * Returns the zone's number of pages potentially available for dirty
> - * page cache. This is the base value for the per-zone dirty limits.
> - */
> -static unsigned long zone_dirtyable_memory(struct zone *zone)
> -{
> - /*
> - * The effective global number of dirtyable pages may exclude
> - * highmem as a big-picture measure to keep the ratio between
> - * dirty memory and lowmem reasonable.
> - *
> - * But this function is purely about the individual zone and a
> - * highmem zone can hold its share of dirty pages, so we don't
> - * care about vm_highmem_is_dirtyable here.
> - */
> - unsigned long nr_pages = zone_page_state(zone, NR_FREE_PAGES) +
> - zone_reclaimable_pages(zone);
> -
> - /* don't allow this to underflow */
> - nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
> - return nr_pages;
> -}
> -
> -/**
> * zone_dirty_limit - maximum number of dirty pages allowed in a zone
> * @zone: the zone
> *
> --
> 1.8.4.2
>
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--
Michal Hocko
SUSE Labs
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