sparsemem: add BUILD_BUG_ON when sizeof mem_section is non-power-of-2

Instead of leaving a hidden trap for the next person who comes along and
wants to add something to mem_section, add a big fat warning about it
needing to be a power-of-2, and insert a BUILD_BUG_ON() in sparse_init()
to catch mistakes.

Right now non-power-of-2 mem_sections cause a number of WARNs at boot
(which don't clearly point to the size of mem_section as an issue), but
the system limps on (temporarily, at least).

This is based upon Dave Hansen's earlier RFC where he ran into the same
issue:
	"sparsemem: fix boot when SECTIONS_PER_ROOT is not power-of-2"
	http://lkml.indiana.edu/hypermail/linux/kernel/1205.2/03077.html

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Jiang Liu <liuj97@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -1137,6 +1137,10 @@ struct mem_section {
 	struct page_cgroup *page_cgroup;
 	unsigned long pad;
 #endif
+	/*
+	 * WARNING: mem_section must be a power-of-2 in size for the
+	 * calculation and use of SECTION_ROOT_MASK to make sense.
+	 */
 };
 
 #ifdef CONFIG_SPARSEMEM_EXTREME

mm/sparse.c

@@ -481,6 +481,9 @@ void __init sparse_init(void)
 	struct page **map_map;
 #endif
 
+	/* see include/linux/mmzone.h 'struct mem_section' definition */
+	BUILD_BUG_ON(!is_power_of_2(sizeof(struct mem_section)));
+
 	/* Setup pageblock_order for HUGETLB_PAGE_SIZE_VARIABLE */
 	set_pageblock_order();