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>

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 09d381b71fd8a10a68e88ed2b4ab3599b1277b1e..ae19af5ec02c0c4dd6fe149d4d97f9ca8a37cb71 100644 (file)
--- a/include/linux/mmzone.h
+++ b/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

diff --git a/mm/sparse.c b/mm/sparse.c
index 9ac2f743f723c35b0d011e5ad6a57d9c6d3d3fd8..b38400f0fb8d9527f2d4b3b97e6ac77f3fb084d7 100644 (file)
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -480,6 +480,9 @@ 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();