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

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Date:   Tue, 25 Apr 2023 06:38:18 +0000
From:   "Yajun Deng" <yajun.deng@...ux.dev>
To:     "Huang, Ying" <ying.huang@...el.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>
Cc:     "Matthew Wilcox" <willy@...radead.org>, 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()

April 25, 2023 1:51 PM, "Huang, Ying" <ying.huang@...el.com> wrote:

> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
>> 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.
> 
> 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");
> 

Thanks Huang, Ying for the example.

Yes, this macros with multiple statements but doesn't have a do - while loop,
It needs if and else together.

> Best Regards,
> Huang, Ying