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@xxxxxxxxxx>
> Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
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
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