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Date: Wed, 5 Jul 2017 16:00:55 -0700
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: disallow early_pfn_to_nid on configurations which
 do not implement it
Message-Id: <20170705160055.013fa5ff34bdf1f6efa4e6ce@linux-foundation.org>
In-Reply-To: <20170704075803.15979-1-mhocko@kernel.org>
References: <20170704075803.15979-1-mhocko@kernel.org>
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Sender: owner-linux-mm@kvack.org
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To: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>, Joonsoo Kim <js1304@gmail.com>, Yang Shi <yang.shi@linaro.org>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>

On Tue,  4 Jul 2017 09:58:03 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> From: Michal Hocko <mhocko@suse.com>
> 
> early_pfn_to_nid will return node 0 if both HAVE_ARCH_EARLY_PFN_TO_NID
> and HAVE_MEMBLOCK_NODE_MAP are disabled. It seems we are safe now
> because all architectures which support NUMA define one of them (with an
> exception of alpha which however has CONFIG_NUMA marked as broken) so
> this works as expected. It can get silently and subtly broken too
> easily, though. Make sure we fail the compilation if NUMA is enabled and
> there is no proper implementation for this function. If that ever
> happens we know that either the specific configuration is invalid
> and the fix should either disable NUMA or enable one of the above
> configs.
> 
> ...
>
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1055,6 +1055,7 @@ static inline struct zoneref *first_zones_zonelist(struct zonelist *zonelist,
>  	!defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP)
>  static inline unsigned long early_pfn_to_nid(unsigned long pfn)
>  {
> +	BUILD_BUG_ON(IS_ENABLED(CONFIG_NUMA));
>  	return 0;
>  }
>  #endif

Wouldn't this be more conventional?

--- a/include/linux/mmzone.h~a
+++ a/include/linux/mmzone.h
@@ -1052,7 +1052,8 @@ static inline struct zoneref *first_zone
 #endif
 
 #if !defined(CONFIG_HAVE_ARCH_EARLY_PFN_TO_NID) && \
-	!defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP)
+	!defined(CONFIG_HAVE_MEMBLOCK_NODE_MAP) && \
+	!defined(CONFIG_NUMA)
 static inline unsigned long early_pfn_to_nid(unsigned long pfn)
 {
 	return 0;
_

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