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 <c...@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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