linux-kernel - Re: [PATCH] mm: disallow early_pfn_to_nid on configurations which do not implement it

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Message-ID: <61b4422c-34d7-f46b-2566-779f6a86d917@suse.cz>
Date:   Tue, 4 Jul 2017 11:28:42 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Joonsoo Kim <js1304@...il.com>, Yang Shi <yang.shi@...aro.org>,
        Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm: disallow early_pfn_to_nid on configurations which do
 not implement it

On 07/04/2017 09:58 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.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.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
> Hi,
> I have brought this up earlier [1] because I thought the deferred
> initialization might be broken but then found out that this is not the
> case right now. This is an attempt to prevent any subtly broken users in
> future.
> 
> [1] http://lkml.kernel.org/r/20170630141847.GN22917@dhcp22.suse.cz
> 
>  include/linux/mmzone.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 16532fa0bb64..fc14b8b3f6ce 100644
> --- 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
>