linux-kernel - Re: [PATCH] oom: fix integer overflow of points in oom

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Date:	Mon, 31 Oct 2011 16:28:33 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frantisek Hrbata <fhrbata@...hat.com>
Cc:	rientjes@...gle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	kosaki.motohiro@...fujitsu.com, minchan.kim@...il.com,
	stable@...nel.org, eteo@...hat.com, pmatouse@...hat.com
Subject: Re: [PATCH] oom: fix integer overflow of points in oom_badness

On 10/31, Frantisek Hrbata wrote:
>
> My understanding is that we may just change the type of points variable from int
> to long and keep the current imho clearer(better readable) computation. There
> should not be an overflow on 32bit and there is a plenty of space for 64bit.
> If you like this solution better I will post the patch as v2.

Up to maintainer, but personally I think the simple s/int/long/ looks better.
Everything like get_mm_*/nr_ptes returns long.

Anyway good catch. Imho stable needs the fix too.

Cosmetic nit,

> -	points = get_mm_rss(p->mm) + p->mm->nr_ptes;
> -	points += get_mm_counter(p->mm, MM_SWAPENTS);
> +	points = (int)((get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) +
                 ^^^^^

Why do we need the explicit typecast? It buys nothing and looks a bit confusing.

And, if you prefer "int", perhaps something like

	- 	points *= 1000;
	-	points /= totalpages;
	+	/* avoid the possible overflow */
	+	points = points * 1000L / totalpages;

looks a bit more readable with the same effect. But I won't insist, this is
up to you and David.

Oleg.

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