linux-kernel - Re: [PATCH] mm/highmem: simplify is

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Message-Id: <20160331153806.960c2299698d40a625809e91@linux-foundation.org>
Date:	Thu, 31 Mar 2016 15:38:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Chanho Min <chanho.min@....com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Vlastimil Babka <vbabka@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Zhang Zhen <zhenzhang.zhang@...wei.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Gunho Lee <gunho.lee@....com>
Subject: Re: [PATCH] mm/highmem: simplify is_highmem()

On Wed, 30 Mar 2016 11:24:38 +0200 Michal Hocko <mhocko@...nel.org> wrote:

> On Wed 30-03-16 13:43:42, Chanho Min wrote:
> > The is_highmem() is can be simplified by use of is_highmem_idx().
> > This patch removes redundant code and will make it easier to maintain
> > if the zone policy is changed or a new zone is added.
> > 
> > Signed-off-by: Chanho Min <chanho.min@....com>
> > ---
> >  include/linux/mmzone.h |    5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index e23a9e7..9ac90c3 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -817,10 +817,7 @@ static inline int is_highmem_idx(enum zone_type idx)
> >  static inline int is_highmem(struct zone *zone)
> >  {
> >  #ifdef CONFIG_HIGHMEM
> > -	int zone_off = (char *)zone - (char *)zone->zone_pgdat->node_zones;
> > -	return zone_off == ZONE_HIGHMEM * sizeof(*zone) ||
> > -	       (zone_off == ZONE_MOVABLE * sizeof(*zone) &&
> > -		zone_movable_is_highmem());
> > +	return is_highmem_idx(zone_idx(zone));
> 
> This will reintroduce the pointer arithmetic removed by ddc81ed2c5d4
> ("remove sparse warning for mmzone.h") AFAICS. I have no idea how much
> that matters though. The mentioned commit doesn't tell much about saves
> except for
> "
> 	On X86_32 this saves a sar, but code size increases by one byte per
>         is_highmem() use due to 32-bit cmps rather than 16 bit cmps.
> "

The patch shrinks my i386 allmodconfig page_alloc.o by 50 bytes, and
that has just two is_highmem() callsites.  So I think it's OK from a
code-size and performance piont of view