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

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Message-Id: <2CF5220A-5452-4913-AFCB-41E1C642E521@redhat.com>
Date:   Thu, 28 Jan 2021 23:30:33 +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 23:28 schrieb David Rientjes <rientjes@...gle.com>:
> 
> On Thu, 28 Jan 2021, David Hildenbrand wrote:
> 
>>> 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.
>> 
> 
> Ah, I certainly think your original intuition was correct.
> 
>> 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?
>> 
> 
> Yeah, that's probably undue risk, the ship has sailed and there's no 
> significant upside.
> 
> I still think "cma 0" in /proc/zoneinfo carries value, though, especially 
> for NUMA and it looks like this is how it's done in linux-next.  With a 
> single read of the file, userspace can make the determination what CMA 
> pages exist on this node.
> 
> In general, I think the rule-of-thumb is that the fewer ifdefs in 
> /proc/zoneinfo, the easier it is for userspace to parse it.

Makes sense, I‘ll send an updated version tomorrow - thanks!


> 
> (I made that change to /proc/zoneinfo to even print non-existant zones for 
> each node because otherwise you cannot determine what the indices of 
> things like vm.lowmem_reserve_ratio represent.)