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

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Message-ID: <alpine.LNX.2.00.1107291115080.22532@swampdragon.chaosbits.net>
Date:	Fri, 29 Jul 2011 11:16:25 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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

On Fri, 29 Jul 2011, KOSAKI Motohiro wrote:

> (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.
> 
Right. We probably shouldn't just remove it.

I'll cook a new version of the patch tonight that properly checks for 
overflow.


-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
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