[PATCH] mm/page_alloc: Restrict and formalize compound_page_dtors[]

[Anshuman Khandual]

Restrict elements in compound_page_dtors[] array per NR_COMPOUND_DTORS and
explicitly position them according to enum compound_dtor_id. This improves
protection against possible misalignment between compound_page_dtors[] and
enum compound_dtor_id later on.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
---
 include/linux/mm.h |  2 +-
 mm/page_alloc.c    | 10 +++++-----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 688558c57751..d1bd7736a5e5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -856,7 +856,7 @@ enum compound_dtor_id {
 #endif
 	NR_COMPOUND_DTORS,
 };
-extern compound_page_dtor * const compound_page_dtors[];
+extern compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS];
 
 static inline void set_compound_page_dtor(struct page *page,
 		enum compound_dtor_id compound_dtor)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cef05d3a23f5..2e4c23b34940 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -302,14 +302,14 @@ const char * const migratetype_names[MIGRATE_TYPES] = {
 #endif
 };
 
-compound_page_dtor * const compound_page_dtors[] = {
-	NULL,
-	free_compound_page,
+compound_page_dtor * const compound_page_dtors[NR_COMPOUND_DTORS] = {
+	[NULL_COMPOUND_DTOR] = NULL,
+	[COMPOUND_PAGE_DTOR] = free_compound_page,
 #ifdef CONFIG_HUGETLB_PAGE
-	free_huge_page,
+	[HUGETLB_PAGE_DTOR] = free_huge_page,
 #endif
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-	free_transhuge_page,
+	[TRANSHUGE_PAGE_DTOR] = free_transhuge_page,
 #endif
 };
 
-- 
2.20.1