mm, page_alloc: use check_pages_enabled static key to check tail pages

Commit 700d2e9a36b9 ("mm, page_alloc: reduce page alloc/free sanity
checks") has introduced a new static key check_pages_enabled to control
when struct pages are sanity checked during allocation and freeing. Mel
Gorman suggested that free_tail_pages_check() could use this static key as
well, instead of relying on CONFIG_DEBUG_VM. That makes sense, so do
that. Also rename the function to free_tail_page_prepare() because it
works on a single tail page and has a struct page preparation component as
well as the optional checking component.

Also remove some unnecessary unlikely() within static_branch_unlikely()
statements that Mel pointed out for commit 700d2e9a36b9.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Vlastimil Babka <[e-mail]>
Suggested-by: Mel Gorman <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Cc: Alexander Halbuer <[e-mail]>
Cc: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/hugetlb_vmemmap.c

···
  * How many struct page structs need to be reset. When we reuse the head
  * struct page, the special metadata (e.g. page->flags or page->mapping)
  * cannot copy to the tail struct page structs. The invalid value will be
- * checked in the free_tail_pages_check(). In order to avoid the message
+ * checked in the free_tail_page_prepare(). In order to avoid the message
  * of "corrupted mapping in tail page". We need to reset at least 3 (one
  * head struct page struct and two tail struct page structs) struct page
  * structs.

mm/page_alloc.c

···
 	return true;
 }
 
-static int free_tail_pages_check(struct page *head_page, struct page *page)
+static int free_tail_page_prepare(struct page *head_page, struct page *page)
 {
 	struct folio *folio = (struct folio *)head_page;
 	int ret = 1;
···
 	 */
 	BUILD_BUG_ON((unsigned long)LIST_POISON1 & 1);
 
-	if (!IS_ENABLED(CONFIG_DEBUG_VM)) {
+	if (!static_branch_unlikely(&check_pages_enabled)) {
 		ret = 0;
 		goto out;
 	}
···
 			ClearPageHasHWPoisoned(page);
 		for (i = 1; i < (1 << order); i++) {
 			if (compound)
-				bad += free_tail_pages_check(page, page + i);
+				bad += free_tail_page_prepare(page, page + i);
 			if (is_check_pages_enabled()) {
-				if (unlikely(free_page_is_bad(page + i))) {
+				if (free_page_is_bad(page + i)) {
 					bad++;
 					continue;
 				}
···
 		for (int i = 0; i < (1 << order); i++) {
 			struct page *p = page + i;
 
-			if (unlikely(check_new_page(p)))
+			if (check_new_page(p))
 				return true;
 		}
 	}