quoted

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).

Why? The memory is continuous, right?

Yes, the memory is conginous and the capacity of specific entry
in mem_section[NR_SECTION_ROOTS] has been defined as follows:


#define SECTIONS_PER_ROOT       (PAGE_SIZE / sizeof (struct mem_section))

Also, the memory is prone to be allocated from bootmem by function
alloc_bootmem_node(), which has PAGE_SIZE alignment. So I think it's
reasonable to introduce the extra check here from my personal view :-)

Thanks,
Gavin

quoted

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

Signed-off-by: Gavin Shan <redacted>
---
 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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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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