Re: [PATCH] mmzone: Introduce for_each_populated_zone_pgdat()

[Huang, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> 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.
>
> 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.

This may cause unexpected effect when used with "if" statement.  For
example,

        if (something)
                for_each_populated_zone_pgdat(zone, pgdat, max)
                        total += zone->present_pages;
        else
                pr_info("something is false!\n");

Best Regards,
Huang, Ying