mm/page_alloc.c: drop dead destroy_compound_page()

The only caller is __free_one_page(). By the time we should have
page->flags to be cleared already:

 - for 0-order pages though PCP list:
	free_hot_cold_page()
		free_pages_prepare()
			free_pages_check()
				page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
		<put the page to PCP list>

	free_pcppages_bulk()
		page = <withdraw pages from PCP list>
		__free_one_page(page)

 - for non-0-order pages:
	__free_pages_ok()
		free_pages_prepare()
			free_pages_check()
				page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
		free_one_page()
			__free_one_page()

So there's no way PageCompound() will return true in __free_one_page().
Let's remove dead destroy_compound_page() and put assert for page->flags
there instead.
Signed-off-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -381,36 +381,6 @@ void prep_compound_page(struct page *page, unsigned long order)
 	}
 }
 
-/* update __split_huge_page_refcount if you change this function */
-static int destroy_compound_page(struct page *page, unsigned long order)
-{
-	int i;
-	int nr_pages = 1 << order;
-	int bad = 0;
-
-	if (unlikely(compound_order(page) != order)) {
-		bad_page(page, "wrong compound order", 0);
-		bad++;
-	}
-
-	__ClearPageHead(page);
-
-	for (i = 1; i < nr_pages; i++) {
-		struct page *p = page + i;
-
-		if (unlikely(!PageTail(p))) {
-			bad_page(page, "PageTail not set", 0);
-			bad++;
-		} else if (unlikely(p->first_page != page)) {
-			bad_page(page, "first_page not consistent", 0);
-			bad++;
-		}
-		__ClearPageTail(p);
-	}
-
-	return bad;
-}
-
 static inline void prep_zero_page(struct page *page, unsigned int order,
 							gfp_t gfp_flags)
 {
@@ -613,10 +583,7 @@ static inline void __free_one_page(struct page *page,
 	int max_order = MAX_ORDER;
 
 	VM_BUG_ON(!zone_is_initialized(zone));
-
-	if (unlikely(PageCompound(page)))
-		if (unlikely(destroy_compound_page(page, order)))
-			return;
+	VM_BUG_ON_PAGE(page->flags & PAGE_FLAGS_CHECK_AT_PREP, page);
 
 	VM_BUG_ON(migratetype == -1);
 	if (is_migrate_isolate(migratetype)) {