On Mon, Apr 24, 2023 at 02:58:23PM -0700, Andrew Morton wrote:
> On Mon, 24 Apr 2023 04:50:37 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> > On Mon, Apr 24, 2023 at 11:07:56AM +0800, Yajun Deng wrote:
> > > Instead of define an index and determining if the zone has memory,
> > > introduce for_each_populated_zone_pgdat() helper that can be used
> > > to iterate over each populated zone in pgdat, and convert the most
> > > obvious users to it.
> >
> > I don't think the complexity of the helper justifies the simplification
> > of the users.
>
> Are you sure?
>
> > > +++ b/include/linux/mmzone.h
> > > @@ -1580,6 +1580,14 @@ extern struct zone *next_zone(struct zone *zone);
> > > ; /* do nothing */ \
> > > else
> > >
> > > +#define for_each_populated_zone_pgdat(zone, pgdat, max) \
> > > + for (zone = pgdat->node_zones; \
> > > + zone < pgdat->node_zones + max; \
> > > + zone++) \
> > > + if (!populated_zone(zone)) \
> > > + ; /* do nothing */ \
> > > + else
> > > +
>
> But each of the call sites is doing this, so at least the complexity is
> now seen in only one place.
But they're not doing _that_. They're doing something normal and
obvious like:
for (zone = pgdat->node_zones; zone < pgdat->node_zones + max; zone++) {
if (!populated_zone(zone)
continue;
...
}
which clearly does what it's supposed to. But with this patch, there's
macro expansion involved, and it's not a nice simple macro, it has a loop
_and_ an if-condition, and there's an else, and now I have to think hard
about whether flow control is going to do the right thing if the body
of the loop isn't simple.
> btw, do we need to do the test that way? Why won't this work?
>
> #define for_each_populated_zone_pgdat(zone, pgdat, max) \
> for (zone = pgdat->node_zones; \
> zone < pgdat->node_zones + max; \
> zone++) \
> if (populated_zone(zone))
I think it will work, except that this is now legal:
for_each_populated_zone_pgdat(zone, pgdat, 3)
else i++;
and really, I think that demonstrates why we don't want macros that are
that darn clever.
