Re: [PATCH 1/2 v2] mm: vmpressure: fix sending wrong events on underflow

[Michal Hocko <mhocko@kernel.org>]

On Thu 09-02-17 17:29:36, Vinayak Menon wrote:
> At the end of a window period, if the reclaimed pages
> is greater than scanned, an unsigned underflow can
> result in a huge pressure value and thus a critical event.
> Reclaimed pages is found to go higher than scanned because
> of the addition of reclaimed slab pages to reclaimed in
> shrink_node without a corresponding increment to scanned
> pages. Minchan Kim mentioned that this can also happen in
> the case of a THP page where the scanned is 1 and reclaimed
> could be 512.
> 
> Acked-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>

Acked-by: Michal Hocko <mhocko@suse.com>

I would prefer the fixup in vmpressure() as already mentioned but this
should work as well.

> ---
> v2: Adding a comment and reordering the patches
>     as per Michal's suggestion
> 
>  mm/vmpressure.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index 149fdf6..6063581 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -112,9 +112,16 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
>  						    unsigned long reclaimed)
>  {
>  	unsigned long scale = scanned + reclaimed;
> -	unsigned long pressure;
> +	unsigned long pressure = 0;
>  
>  	/*
> +	 * reclaimed can be greater than scanned in cases
> +	 * like THP, where the scanned is 1 and reclaimed
> +	 * could be 512
> +	 */
> +	if (reclaimed >= scanned)
> +		goto out;
> +	/*
>  	 * We calculate the ratio (in percents) of how many pages were
>  	 * scanned vs. reclaimed in a given time frame (window). Note that
>  	 * time is in VM reclaimer's "ticks", i.e. number of pages
> @@ -124,6 +131,7 @@ static enum vmpressure_levels vmpressure_calc_level(unsigned long scanned,
>  	pressure = scale - (reclaimed * scale / scanned);
>  	pressure = pressure * 100 / scale;
>  
> +out:
>  	pr_debug("%s: %3lu  (s: %lu  r: %lu)\n", __func__, pressure,
>  		 scanned, reclaimed);
>  
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
> member of the Code Aurora Forum, hosted by The Linux Foundation
> 

-- 
Michal Hocko
SUSE Labs