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Date:   Thu, 28 Jan 2021 13:54:04 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     David Hildenbrand <david@redhat.com>
cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Mike Rapoport <rppt@kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@kernel.org>,
        Wei Yang <richard.weiyang@linux.alibaba.com>,
        linux-api@vger.kernel.org
Subject: Re: [PATCH v2] mm/page_alloc: count CMA pages per zone and print
 them in /proc/zoneinfo
In-Reply-To: <20210128164533.18566-1-david@redhat.com>
Message-ID: <6d9cff33-39c1-ef8e-a18e-c865b95240b6@google.com>
References: <20210127101813.6370-3-david@redhat.com> <20210128164533.18566-1-david@redhat.com>
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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.