linux-kernel - Re: [PATCH 1/2] mm: change free_cma and free

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Message-ID: <20150929102657.GA27621@gmail.com>
Date:	Tue, 29 Sep 2015 13:26:57 +0300
From:	Alexandru Moise <00moses.alexander00@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	akpm@...ux-foundation.org, mgorman@...e.dec, vbabka@...e.cz,
	mhocko@...e.com, js1304@...il.com, hannes@...xchg.org,
	alexander.h.duyck@...hat.com, sasha.levin@...cle.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: change free_cma and free_pages declarations to
 unsigned

On Tue, Sep 29, 2015 at 11:13:27AM +0100, Mel Gorman wrote:
> On Sun, Sep 27, 2015 at 09:04:16PM +0000, Alexandru Moise wrote:
> > Their stored values come from zone_page_state() which returns
> > an unsigned long. To improve code correctness we should avoid
> > mixing signed and unsigned integers.
> > 
> > Signed-off-by: Alexandru Moise <00moses.alexander00@...il.com>
> > ---
> >  mm/page_alloc.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 48aaf7b..f55e3a2 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2242,7 +2242,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> >  	/* free_pages may go negative - that's OK */
> >  	long min = mark;
> >  	int o;
> > -	long free_cma = 0;
> > +	unsigned long free_cma = 0;
> >  
> 
> NAK.
> 
> free_cma is used with free_pages which is explicitly commented as saying
> it can go negative. With your patch, there is a signed/unsigned operation
> where the unsigned type cannot fit into the signed type which casts them
> both to unsigned which is then broken for the comparison.  This patch
> looks broken for very subtle reasons. Please do not do any similar style
> patches to this because they can introduce subtle breakage if issues are
> not caught at review.
> 

Understood, I thought the comment only applied to the "min" variable.
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