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Date: Wed, 31 Aug 2016 14:10:33 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm: Move definition of 'zone_names' array into
 mmzone.h
Message-Id: <20160831141033.8f617b6000bf129bbc40bda7@linux-foundation.org>
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On Wed, 31 Aug 2016 08:55:49 +0530 Anshuman Khandual <khandual@linux.vnet.ibm.com> wrote:

> zone_names[] is used to identify any zone given it's index which
> can be used in many other places. So moving the definition into
> include/linux/mmzone.h for broader access.
> 
> ...
>
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -341,6 +341,23 @@ enum zone_type {
>  
>  };
>  
> +static char * const zone_names[__MAX_NR_ZONES] = {
> +#ifdef CONFIG_ZONE_DMA
> +	 "DMA",
> +#endif
> +#ifdef CONFIG_ZONE_DMA32
> +	 "DMA32",
> +#endif
> +	 "Normal",
> +#ifdef CONFIG_HIGHMEM
> +	 "HighMem",
> +#endif
> +	 "Movable",
> +#ifdef CONFIG_ZONE_DEVICE
> +	 "Device",
> +#endif
> +};
> +
>  #ifndef __GENERATING_BOUNDS_H
>  
>  struct zone {
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3fbe73a..8e2261c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -207,23 +207,6 @@ int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1] = {
>  
>  EXPORT_SYMBOL(totalram_pages);
>  
> -static char * const zone_names[MAX_NR_ZONES] = {
> -#ifdef CONFIG_ZONE_DMA
> -	 "DMA",
> -#endif
> -#ifdef CONFIG_ZONE_DMA32
> -	 "DMA32",
> -#endif
> -	 "Normal",
> -#ifdef CONFIG_HIGHMEM
> -	 "HighMem",
> -#endif
> -	 "Movable",
> -#ifdef CONFIG_ZONE_DEVICE
> -	 "Device",
> -#endif
> -};
> -
>  char * const migratetype_names[MIGRATE_TYPES] = {
>  	"Unmovable",
>  	"Movable",

This is worrisome.  On some (ancient) compilers, this will produce a
copy of that array into each compilation unit which includes mmzone.h.

On smarter compilers, it will produce a copy of the array in each
compilation unit which *uses* zone_names[].

On even smarter compilers (and linkers!), only one copy of zone_names[]
will exist in vmlinux.

I don't know if gcc is an "even smarter compiler" and I didn't check,
and I didn't check which gcc versions are even smarter.  I'd rather not
have to ;) It is risky.

So, let's just make it non-static and add a declaration into mmzone.h,
please.