linux-kernel - Re: [PATCH]vmscan: handle underflow for get_scan

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Date:	Tue, 30 Mar 2010 17:26:56 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, fengguang.wu@...el.com,
	kosaki.motohiro@...fujitsu.com
Subject: Re: [PATCH]vmscan: handle underflow for get_scan_ratio

On Tue, Mar 30, 2010 at 11:23 AM, Shaohua Li <shaohua.li@...el.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@...el.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;
> +       }
>  }
>

Can you please post the meminfo before and after the changes (diff
maybe?). Can you also please share the ap and fp data from which the
percent figures are being calculated Is your swappiness set to 60? Can
you please share the OOM/panic message as well.

Balbir
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