linux-kernel - Re: [PATCH v2] mm/page_alloc: count CMA pages per zone and print them in /proc/zoneinfo

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Date:   Thu, 28 Jan 2021 23:03:44 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...nel.org>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_alloc: count CMA pages per zone and print them in /proc/zoneinfo


> Am 28.01.2021 um 22:54 schrieb David Rientjes <rientjes@...gle.com>:
> 
> On Thu, 28 Jan 2021, David Hildenbrand wrote:
> 
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index 7758486097f9..957680db41fa 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -1650,6 +1650,11 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
>>           zone->spanned_pages,
>>           zone->present_pages,
>>           zone_managed_pages(zone));
>> +#ifdef CONFIG_CMA
>> +    seq_printf(m,
>> +           "\n        cma      %lu",
>> +           zone->cma_pages);
>> +#endif
>> 
>>    seq_printf(m,
>>           "\n        protection: (%ld",
> 
> Hmm, not sure about this.  If cma is only printed for CONFIG_CMA, we can't 
> distinguish between (1) a kernel without your patch without including some 
> version checking and (2) a kernel without CONFIG_CMA enabled.  IOW, 
> "cma 0" carries value: we know immediately that we do not have any CMA 
> pages on this zone, period.
> 
> /proc/zoneinfo is also not known for its conciseness so I think printing 
> "cma 0" even for !CONFIG_CMA is helpful :)
> 
> I think this #ifdef should be removed and it should call into a 
> zone_cma_pages(struct zone *zone) which returns 0UL if disabled.
> 

Yeah, that’s also what I proposed in a sub-thread here.

The last option would be going the full mile and not printing nr_free_cma. Code might get a bit uglier though, but we could also remove that stats counter ;)

I don‘t particularly care, while printing „0“ might be easier, removing nr_free_cma might be cleaner.

But then, maybe there are tools that expect that value to be around on any kernel?

Thoughts?

Thanks