[PATCH 1/5] mm/sparse: check size of struct mm_section

[Gavin Shan <shangw@linux.vnet.ibm.com>]

Platforms like PPC might need two level mem_section for SPARSEMEM
with enabled CONFIG_SPARSEMEM_EXTREME. On the other hand, the
memory section descriptor might be allocated from bootmem allocator
with PAGE_SIZE alignment. In order to fully utilize the memory chunk
allocated from bootmem allocator, it'd better to assure memory
sector descriptor won't run across the boundary (PAGE_SIZE).

The patch introduces the check on size of "struct mm_section" to
assure that.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
---
 mm/sparse.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/sparse.c b/mm/sparse.c
index 6a4bf91..afd0998 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -63,6 +63,15 @@ static struct mem_section noinline __init_refok *sparse_index_alloc(int nid)
 	unsigned long array_size = SECTIONS_PER_ROOT *
 				   sizeof(struct mem_section);
 
+	/*
+	 * The root memory section descriptor might be allocated
+	 * from bootmem, which has minimal memory chunk requirement
+	 * of page. In order to fully utilize the memory, the sparse
+	 * memory section descriptor shouldn't run across the boundary
+	 * that bootmem allocator has.
+	 */
+	BUILD_BUG_ON(PAGE_SIZE % sizeof(struct mem_section));
+
 	if (slab_is_available()) {
 		if (node_state(nid, N_HIGH_MEMORY))
 			section = kmalloc_node(array_size, GFP_KERNEL, nid);
-- 
1.7.9.5

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