linux-kernel - Re: [PATCH 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non

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Date:	Wed, 6 Feb 2013 10:41:17 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Lin Feng <linfeng@...fujitsu.com>, bcrl@...ck.org,
	viro@...iv.linux.org.uk, khlebnikov@...nvz.org,
	kamezawa.hiroyu@...fujitsu.com, minchan@...nel.org,
	riel@...hat.com, rientjes@...gle.com,
	isimatu.yasuaki@...fujitsu.com, wency@...fujitsu.com,
	laijs@...fujitsu.com, jiang.liu@...wei.com, linux-mm@...ck.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: hotplug: implement non-movable version of
 get_user_pages() called get_user_pages_non_movable()

On Tue, Feb 05, 2013 at 06:26:51PM -0800, Michel Lespinasse wrote:
> Just nitpicking, but:
> 
> On Tue, Feb 5, 2013 at 3:57 AM, Mel Gorman <mgorman@...e.de> wrote:
> > +static inline bool zone_is_idx(struct zone *zone, enum zone_type idx)
> > +{
> > +       /* This mess avoids a potentially expensive pointer subtraction. */
> > +       int zone_off = (char *)zone - (char *)zone->zone_pgdat->node_zones;
> > +       return zone_off == idx * sizeof(*zone);
> > +}
> 
> Maybe:
> return zone == zone->zone_pgdat->node_zones + idx;
> ?
> 

Not a nit at all. Yours is more readable but it generates more code. A
single line function that uses the helper generates 0x3f bytes of code
(mostly function entry/exit) with your version and 0x39 bytes with mine. The
difference in efficiency is marginal as your version uses lea to multiply
by a constant but it's still slightly heavier.

The old code is fractionally better, your version is more readable so it's
up to Andrew really. Right now I think he's gone with his own version with
zone_idx() in the name of readability whatever sparse has to say about
the matter.

-- 
Mel Gorman
SUSE Labs
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