Re: [PATCH 1/2] mm: hotplug: implement non-movable version ofget_user_pages() called get_user_pages_non_movable()

[Mel Gorman]

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@xxxxxxx> 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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