linux-kernel - Re: [PATCH] vmscan: Remove if statement that will never trigger

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Date:	Fri, 29 Jul 2011 15:27:46 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	jj@...osbits.net
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org, riel@...hat.com,
	minchan.kim@...il.com, mgorman@...e.de, akpm@...ux-foundation.org,
	kanoj@....com, sct@...hat.com
Subject: Re: [PATCH] vmscan: Remove if statement that will never trigger

(2011/07/29 6:05), Jesper Juhl wrote:
> We have this code in mm/vmscan.c:shrink_slab() :
> ...
> 		if (total_scan < 0) {
> 			printk(KERN_ERR "shrink_slab: %pF negative objects to "
> 			       "delete nr=%ld\n",
> 			       shrinker->shrink, total_scan);
> 			total_scan = max_pass;
> 		}
> ...
> but since 'total_scan' is of type 'unsigned long' it will never be
> less than zero, so there is no way we'll ever enter the true branch of
> this if statement - so let's just remove it.
> 
> Signed-off-by: Jesper Juhl <jj@...osbits.net>
> ---
>  mm/vmscan.c |    6 ------
>  1 files changed, 0 insertions(+), 6 deletions(-)
> 
> 	Compile tested only.
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7ef6912..c07d9b1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -271,12 +271,6 @@ unsigned long shrink_slab(struct shrink_control *shrink,
>  		delta *= max_pass;
>  		do_div(delta, lru_pages + 1);
>  		total_scan += delta;
> -		if (total_scan < 0) {
> -			printk(KERN_ERR "shrink_slab: %pF negative objects to "
> -			       "delete nr=%ld\n",
> -			       shrinker->shrink, total_scan);
> -			total_scan = max_pass;
> -		}
>  
>  		/*
>  		 * We need to avoid excessive windup on filesystem shrinkers

Good catch.

However this seems intended to catch a overflow. So, I'd suggest to make proper
overflow check instead.


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