Re: [PATCHv2] mm: Fix calculation of dirtyable memory

[Fengguang Wu <fengguang.wu@intel.com>]

On Thu, Nov 08, 2012 at 04:52:33PM -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).
> 
> Fix is to ensure there is no underflow while doing the math.

Good catch, thanks!

> Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
> Signed-off-by: Puneet Kumar <puneetster@chromium.org>
> ---
>  v2: added apkm's suggestion to make the highmem calculation better
>  mm/page-writeback.c |   17 +++++++++++++++--
>  1 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 830893b..ce62442 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 +234,9 @@ 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();
> +	if (x >= dirty_balance_reserve)
> +		x -= dirty_balance_reserve;

That can be converted to "if ((long)x < 0) x = 0;", too.

And I suspect zone_dirtyable_memory() needs similar fix, too.

Thanks,
Fengguang

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