[PATCH] Introduce new Kconfig option for NUMA or DISCONTIG

There is some confusion that arose when working on SPARSEMEM patch between
what is needed for DISCONTIG vs. NUMA.

Multiple pg_data_t's are needed for DISCONTIGMEM or NUMA, independently.
All of the current NUMA implementations require an implementation of
DISCONTIG.  Because of this, quite a lot of code which is really needed for
NUMA is actually under DISCONTIG #ifdefs.  For SPARSEMEM, we changed some
of these #ifdefs to CONFIG_NUMA, but that broke the DISCONTIG=y and NUMA=n
case.

Introducing this new NEED_MULTIPLE_NODES config option allows code that is
needed for both NUMA or DISCONTIG to be separated out from code that is
specific to DISCONTIG.

One great advantage of this approach is that it doesn't require every
architecture to be converted over.  All of the current implementations
should "just work", only the ones implementing SPARSEMEM will have to be
fixed up.

The change to free_area_init() makes it work inside, or out of the new
config option.

Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/mmzone.h

@@ -402,7 +402,7 @@ int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *, int, struct file *,
 /* Returns the number of the current Node. */
 #define numa_node_id()		(cpu_to_node(raw_smp_processor_id()))
 
-#ifndef CONFIG_DISCONTIGMEM
+#ifndef CONFIG_NEED_MULTIPLE_NODES
 
 extern struct pglist_data contig_page_data;
 #define NODE_DATA(nid)		(&contig_page_data)
@@ -410,11 +410,11 @@ extern struct pglist_data contig_page_data;
 #define MAX_NODES_SHIFT		1
 #define pfn_to_nid(pfn)		(0)
 
-#else /* CONFIG_DISCONTIGMEM */
+#else /* CONFIG_NEED_MULTIPLE_NODES */
 
 #include <asm/mmzone.h>
 
-#endif /* !CONFIG_DISCONTIGMEM */
+#endif /* !CONFIG_NEED_MULTIPLE_NODES */
 
 #if BITS_PER_LONG == 32 || defined(ARCH_HAS_ATOMIC_UNSIGNED)
 /*

mm/Kconfig

@@ -23,3 +23,11 @@ config DISCONTIGMEM
 
 endchoice
 
+#
+# Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
+# to represent different areas of memory.  This variable allows
+# those dependencies to exist individually.
+#
+config NEED_MULTIPLE_NODES
+	def_bool y
+	depends on DISCONTIGMEM || NUMA

mm/page_alloc.c

@@ -1972,18 +1972,18 @@ void __init free_area_init_node(int nid, struct pglist_data *pgdat,
 	free_area_init_core(pgdat, zones_size, zholes_size);
 }
 
-#ifndef CONFIG_DISCONTIGMEM
+#ifndef CONFIG_NEED_MULTIPLE_NODES
 static bootmem_data_t contig_bootmem_data;
 struct pglist_data contig_page_data = { .bdata = &contig_bootmem_data };
 
 EXPORT_SYMBOL(contig_page_data);
+#endif
 
 void __init free_area_init(unsigned long *zones_size)
 {
-	free_area_init_node(0, &contig_page_data, zones_size,
+	free_area_init_node(0, NODE_DATA(0), zones_size,
 			__pa(PAGE_OFFSET) >> PAGE_SHIFT, NULL);
 }
-#endif
 
 #ifdef CONFIG_PROC_FS