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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  8 Nov 2012 15:25:35 -0800
Sonny Rao <sonnyrao@chromium.org> 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@chromium.org>
> Signed-off-by: Puneet Kumar <puneetster@chromium.org>
> ---
>  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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