On Mon, Nov 12, 2012 at 01:35:46PM -0800, Sonny Rao wrote:
> The system uses global_dirtyable_memory() to calculate
> number of dirtyable pages/pages that can be allocated
> to the page cache. A bug causes an underflow thus making
> the page count look like a big unsigned number. This in turn
> confuses the dirty writeback throttling to aggressively write
> back pages as they become dirty (usually 1 page at a time).
> This generally only affects systems with highmem because the
> underflowed count gets subtracted from the global count of
> dirtyable memory.
>
> The problem was introduced with v3.2-4896-gab8fabd
>
> Fix is to ensure we don't get an underflowed total of either highmem
> or global dirtyable memory.
>
> Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> Signed-off-by: Puneet Kumar <puneetster@xxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> CC: stable@xxxxxxxxxxxxxxx
> ---
> v2: added apkm's suggestion to make the highmem calculation better
> v3: added Fengguang Wu's suggestions fix zone_dirtyable_memory() and
> (offlist mail) to use max() in global_dirtyable_memory()
> v4: Added suggestions to description clarifying the role of highmem
> and the commit which originally caused the problem
> mm/page-writeback.c | 25 ++++++++++++++++++++-----
> 1 files changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 830893b..f9efbe8 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -201,6 +201,18 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
> zone_reclaimable_pages(z) - z->dirty_balance_reserve;
> }
> /*
> + * Unreclaimable memory (kernel memory or anonymous memory
> + * without swap) can bring down the dirtyable pages below
> + * the zone's dirty balance reserve and the above calculation
> + * will underflow. However we still want to add in nodes
> + * which are below threshold (negative values) to get a more
> + * accurate calculation but make sure that the total never
> + * underflows.
> + */
> + if ((long)x < 0)
> + x = 0;
> +
> + /*
> * Make sure that the number of highmem pages is never larger
> * than the number of the total dirtyable memory. This can only
> * occur in very strange VM situations but we want to make sure
> @@ -222,8 +234,8 @@ static unsigned long global_dirtyable_memory(void)
> {
> unsigned long x;
>
> - x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -
> - dirty_balance_reserve;
> + x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> + x -= max(x, dirty_balance_reserve);
This unconditionally zeroes out x, except when it underflows it...
min().
> if (!vm_highmem_is_dirtyable)
> x -= highmem_dirtyable_memory(x);
> @@ -290,9 +302,12 @@ static unsigned long zone_dirtyable_memory(struct zone *zone)
> * highmem zone can hold its share of dirty pages, so we don't
> * care about vm_highmem_is_dirtyable here.
> */
> - return zone_page_state(zone, NR_FREE_PAGES) +
> - zone_reclaimable_pages(zone) -
> - zone->dirty_balance_reserve;
> + unsigned long nr_pages = zone_page_state(zone, NR_FREE_PAGES) +
> + zone_reclaimable_pages(zone);
> +
> + /* don't allow this to underflow */
> + nr_pages -= max(nr_pages, zone->dirty_balance_reserve);
> + return nr_pages;
min().
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