Re: [PATCH] mmzone: Introduce for_each_populated_zone_pgdat()

["Yajun Deng" <yajun.deng@linux.dev>]

April 25, 2023 11:23 AM, "Matthew Wilcox" <willy@infradead.org> wrote:

> 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@infradead.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.
> 
> 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;
> ...
> }
>

They will be like:

for (zone = pgdat->node_zones; zone < pgdat->node_zones + max; zone++)
        if (!populated_zone(zone))
                ;
        else {

                ...
        }
     
 
> 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.