mm/page_alloc: restrict and formalize compound_page_dtors[]

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.
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: http://lkml.kernel.org/r/1589795958-19317-1-git-send-email-anshuman.khandual@arm.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -867,7 +867,7 @@ 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)

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
 };