Re: [PATCH]vmscan: handle underflow for get_scan_ratio

Minchan Kim
2010-03-30T10:17:19Z

On Tue, Mar 30, 2010 at 2:53 PM, Shaohua Li <shaohua.li@intel.com> wrote:
> Commit 84b18490d1f1bc7ed5095c929f78bc002eb70f26 introduces a regression.
> With it, our tmpfs test always oom. The test has a lot of rotated anon
> pages and cause percent[0] zero. Actually the percent[0] is a very small
> value, but our calculation round it to zero. The commit makes vmscan
> completely skip anon pages and cause oops.
> An option is if percent[x] is zero in get_scan_ratio(), forces it
> to 1. See below patch.
> But the offending commit still changes behavior. Without the commit, we scan
> all pages if priority is zero, below patch doesn't fix this. Don't know if
> It's required to fix this too.
>
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 79c8098..d5cc34e 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1604,6 +1604,18 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
>        /* Normalize to percentages */
>        percent[0] = 100 * ap / (ap + fp + 1);
>        percent[1] = 100 - percent[0];
> +       /*
> +        * if percent[x] is small and rounded to 0, this case doesn't mean we
> +        * should skip scan. Give it at least 1% share.
> +        */
> +       if (percent[0] == 0) {
> +               percent[0] = 1;
> +               percent[1] = 99;
> +       }
> +       if (percent[1] == 0) {
> +               percent[0] = 99;
> +               percent[1] = 1;
> +       }
>  }
>
>  /*
>

Yes. It made subtle change.
But we should not depend that change.
Current logic seems to be good and clear than old.
I think you were lucky at that time by not-good and not-clear logic.

BTW, How about this?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 79c8098..f0df563 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1646,11 +1646,6 @@ static void shrink_zone(int priority, struct zone *zone,
int file = is_file_lru(l);
unsigned long scan;

- if (percent[file] == 0) {
- nr[l] = 0;
- continue;
- }
-
scan = zone_nr_lru_pages(zone, sc, l);
if (priority) {
scan >>= priority;

--
Kind regards,
Minchan Kim