Date:   Thu, 28 Jan 2021 15:01:46 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     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>,
        Michal Hocko <mhocko@...nel.org>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>
Subject: Re: [PATCH v1 2/2] mm/page_alloc: count CMA pages per zone and print
 them in /proc/zoneinfo

On 28.01.21 14:44, Oscar Salvador wrote:
> On Thu, Jan 28, 2021 at 11:43:41AM +0100, David Hildenbrand wrote:
>>> My knowledge of CMA tends to be quite low, actually I though that CMA
>>> was somehow tied to ZONE_MOVABLE.
>>
>> CMA is often placed into one of the kernel zones, but can also end up in the movable zone.
> 
> Ok good to know.
> 
>>> I see how tracking CMA pages per zona might give you a clue, but what do
>>> you mean by "might behave differently - even after some of these pages might
>>> already have been allocated"
>>
>> Assume you have 4GB in ZONE_NORMAL but 1GB is assigned for CMA. You actually only have 3GB available for random kernel allocations, not 4GB.
>>
>> Currently, you can only observe the free CMA pages, excluding any pages that are already allocated. Having that information how many CMA pages we have can be helpful - similar to what we already have in /proc/meminfo.
> 
> I see, I agree that it can provide some guidance.
> 
>>> I see that NR_FREE_CMA_PAGES is there even without CONFIG_CMA, as you
>>> said, but I am not sure about adding size to a zone unconditionally.
>>> I mean, it is not terrible as IIRC, the maximum MAX_NUMNODES can get
>>> is 1024, and on x86_64 that would be (1024 * 4 zones) * 8 = 32K.
>>> So not a big deal, but still.
>>
>> I'm asking myself how many such systems will run without
>> CONFIG_CMA in the future.
> 
> I am not sure, my comment was just to point out that even the added size might
> not be that large, hiding it under CONFIG_CMA seemed the right thing to
> do.
> 
>>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>>> index 8ba0870ecddd..5757df4bfd45 100644
>>> --- a/mm/vmstat.c
>>> +++ b/mm/vmstat.c
>>> @@ -1559,13 +1559,15 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
>>>                      "\n        spanned  %lu"
>>>                      "\n        present  %lu"
>>>                      "\n        managed  %lu",
>>> +                  "\n        cma      %lu",
>>>                      zone_page_state(zone, NR_FREE_PAGES),
>>>                      min_wmark_pages(zone),
>>>                      low_wmark_pages(zone),
>>>                      high_wmark_pages(zone),
>>>                      zone->spanned_pages,
>>>                      zone->present_pages,
>>> -                  zone->managed_pages);
>>> +                  zone->managed_pages,
>>> +                  IS_ENABLED(CONFIG_CMA) ? zone->cma_pages : 0);
>>>           seq_printf(m,
>>>                      "\n        protection: (%ld",
>>>
>>>
>>> I do not see it that ugly, but just my taste.
>>
>> IIRC, that does not work. The compiler will still complain
>> about a missing struct members. We would have to provide a
>> zone_cma_pages() helper with some ifdefery.
> 
> Of course, it seems I switched off my brain.
> 
>> We could do something like this on top
>>
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -530,7 +530,9 @@ struct zone {
>>          atomic_long_t           managed_pages;
>>          unsigned long           spanned_pages;
>>          unsigned long           present_pages;
>> +#ifdef CONFIG_CMA
>>          unsigned long           cma_pages;
>> +#endif
>>          const char              *name;
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index 97fc32a53320..b753a64f099f 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -1643,7 +1643,10 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
>>                     "\n        spanned  %lu"
>>                     "\n        present  %lu"
>>                     "\n        managed  %lu"
>> -                  "\n        cma      %lu",
>> +#ifdef CONFIG_CMA
>> +                  "\n        cma      %lu"
>> +#endif
>> +                  "%s",
>>                     zone_page_state(zone, NR_FREE_PAGES),
>>                     min_wmark_pages(zone),
>>                     low_wmark_pages(zone),
>> @@ -1651,7 +1654,10 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
>>                     zone->spanned_pages,
>>                     zone->present_pages,
>>                     zone_managed_pages(zone),
>> -                  zone->cma_pages);
>> +#ifdef CONFIG_CMA
>> +                  zone->cma_pages,
>> +#endif
>> +                  "");
>>          seq_printf(m,
>>                     "\n        protection: (%ld",
> 
> Looks good to me, but I can see how those #ifdef can raise some
> eyebrows.

We could print it further above to avoid the "%s" ... "", or print it 
separately below. Then we'd only need a single ifdef. Might make sense

> Let us see what other thinks as well.
> 
> Btw, should linux-uapi be CCed, as /proc/vmstat layout will change?

Is there a linux-uapi@ list? I know linux-api@ ("forum to discuss 
changes that affect the Linux programming interface (API or ABI)".

Good question, I can certainly cc linux-api@, although I doubt it's 
strictly necessary when adding something here.

Thanks!

-- 
Thanks,

David / dhildenb

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