On Thu, 8 Nov 2012 15:25:35 -0800
Sonny Rao <sonnyrao@xxxxxxxxxxxx> 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).
>
> Fix is to ensure there is no underflow while doing the math.
>
> Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> Signed-off-by: Puneet Kumar <puneetster@xxxxxxxxxxxx>
> ---
> mm/page-writeback.c | 17 +++++++++++++----
> 1 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 830893b..2a6356c 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -194,11 +194,19 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
> unsigned long x = 0;
>
> for_each_node_state(node, N_HIGH_MEMORY) {
> + unsigned long nr_pages;
> 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;
> + nr_pages = zone_page_state(z, NR_FREE_PAGES) +
> + zone_reclaimable_pages(z);
> + /*
> + * Unreclaimable memory (kernel memory or anonymous memory
> + * without swap) can bring down the dirtyable pages below
> + * the zone's dirty balance reserve.
> + */
> + if (nr_pages >= z->dirty_balance_reserve)
> + x += nr_pages - z->dirty_balance_reserve;
If the system has two nodes and one is below its dirty_balance_reserve,
we could end up with something like:
x = 0;
...
x += 1000;
...
x += -100;
In this case, your fix would cause highmem_dirtyable_memory() to return
1000. Would it be better to instead return 900?
IOW, we instead add logic along the lines of
if ((long)x < 0)
x = 0;
return x;
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