linux-kernel - Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing free

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Message-ID: <4EE0E4B9.4050903@gmail.com>
Date:	Thu, 08 Dec 2011 11:24:25 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Wang Sheng-Hui <shhuiw@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Question about __zone_watermark_ok: why there is a "+ 1" in computing
 free_pages?

(12/5/11 11:14 AM), Michal Hocko wrote:
> On Fri 25-11-11 09:21:35, Wang Sheng-Hui wrote:
>> In line 1459, we have "free_pages -= (1<<  order) + 1;".
>> Suppose allocating one 0-order page, here we'll get
>>      free_pages -= 1 + 1
>> I wonder why there is a "+ 1"?
>
> Good spot. Check the patch bellow.
> ---
>  From 38a1cf351b111e8791d2db538c8b0b912f5df8b8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko<mhocko@...e.cz>
> Date: Mon, 5 Dec 2011 17:04:23 +0100
> Subject: [PATCH] mm: fix off-by-two in __zone_watermark_ok
>
> 88f5acf8 [mm: page allocator: adjust the per-cpu counter threshold when
> memory is low] changed the form how free_pages is calculated but it
> forgot that we used to do free_pages - ((1<<  order) - 1) so we ended up
> with off-by-two when calculating free_pages.
>
> Spotted-by: Wang Sheng-Hui<shhuiw@...il.com>
> Signed-off-by: Michal Hocko<mhocko@...e.cz>
> ---
>   mm/page_alloc.c |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9dd443d..8a2f1b6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1457,7 +1457,7 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>   	long min = mark;
>   	int o;
>
> -	free_pages -= (1<<  order) + 1;
> +	free_pages -= (1<<  order) - 1;
>   	if (alloc_flags&  ALLOC_HIGH)
>   		min -= min / 2;
>   	if (alloc_flags&  ALLOC_HARDER)

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>


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