Re: [PATCHv4] mm: Fix calculation of dirtyable memory

[Johannes Weiner <hannes@cmpxchg.org>]

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@chromium.org>
> Signed-off-by: Puneet Kumar <puneetster@chromium.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> CC: stable@vger.kernel.org
> ---
>  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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