[PATCH] sparsemem: BUILD_BUG_ON when sizeof mem_section is non-power-of-2

[Cody P Schafer <cody@linux.vnet.ibm.com>]

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>
---

Dave: Consider it resurrected.

---

 include/linux/mmzone.h | 4 ++++
 mm/sparse.c            | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 131989a..88e23f3 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1127,6 +1127,10 @@ 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 1c91f0d3..3194ec4 100644
--- a/mm/sparse.c
+++ b/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();
 
-- 
1.8.3

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