linux-kernel - Re: [PATCH] mmzone: Introduce for_each_populated_zone

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Message-Id: <20230424145823.b8e8435dd3242614371be6d5@linux-foundation.org>
Date:   Mon, 24 Apr 2023 14:58:23 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Yajun Deng <yajun.deng@...ux.dev>, david@...hat.com,
        osalvador@...e.de, gregkh@...uxfoundation.org, rafael@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] mmzone: Introduce for_each_populated_zone_pgdat()

On Mon, 24 Apr 2023 04:50:37 +0100 Matthew Wilcox <willy@...radead.org> 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.

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 suspect it was done the original way in order to save a tabstop,
which is no longer needed.