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 14:28:23 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     David Hildenbrand <david@...hat.com>
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>,
        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

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.

(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.)