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

[Andrew Morton]

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

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> 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;
_