mm: add __clear_page_lru_flags() to replace page_off_lru()

Similar to page_off_lru(), the new function does non-atomic clearing
of PageLRU() in addition to PageActive() and PageUnevictable(), on a
page that has no references left.

If PageActive() and PageUnevictable() are both set, refuse to clear
either and leave them to bad_page(). This is a behavior change that
is meant to help debug.

Link: https://lore.kernel.org/linux-mm/20201207220949.830352-7-yuzhao@google.com/
Signed-off-by: Yu Zhao <yuzhao@google.com>

include/linux/mm_inline.h

@@ -61,27 +61,19 @@ static inline enum lru_list page_lru_base_type(struct page *page)
 }
 
 /**
- * page_off_lru - which LRU list was page on? clearing its lru flags.
- * @page: the page to test
- *
- * Returns the LRU list a page was on, as an index into the array of LRU
- * lists; and clears its Unevictable or Active flags, ready for freeing.
+ * __clear_page_lru_flags - clear page lru flags before releasing a page
+ * @page: the page that was on lru and now has a zero reference
  */
-static __always_inline enum lru_list page_off_lru(struct page *page)
+static __always_inline void __clear_page_lru_flags(struct page *page)
 {
-	enum lru_list lru;
+	__ClearPageLRU(page);
 
-	if (PageUnevictable(page)) {
-		__ClearPageUnevictable(page);
-		lru = LRU_UNEVICTABLE;
-	} else {
-		lru = page_lru_base_type(page);
-		if (PageActive(page)) {
-			__ClearPageActive(page);
-			lru += LRU_ACTIVE;
-		}
-	}
-	return lru;
+	/* this shouldn't happen, so leave the flags to bad_page() */
+	if (PageActive(page) && PageUnevictable(page))
+		return;
+
+	__ClearPageActive(page);
+	__ClearPageUnevictable(page);
 }
 
 /**

mm/swap.c

@@ -84,9 +84,8 @@ static void __page_cache_release(struct page *page)
 
 		lruvec = lock_page_lruvec_irqsave(page, &flags);
 		VM_BUG_ON_PAGE(!PageLRU(page), page);
-		__ClearPageLRU(page);
 		del_page_from_lru_list(page, lruvec);
-		page_off_lru(page);
+		__clear_page_lru_flags(page);
 		unlock_page_lruvec_irqrestore(lruvec, flags);
 	}
 	__ClearPageWaiters(page);
@@ -911,9 +910,8 @@ void release_pages(struct page **pages, int nr)
 				lock_batch = 0;
 
 			VM_BUG_ON_PAGE(!PageLRU(page), page);
-			__ClearPageLRU(page);
 			del_page_from_lru_list(page, lruvec);
-			page_off_lru(page);
+			__clear_page_lru_flags(page);
 		}
 
 		__ClearPageWaiters(page);

mm/vmscan.c

@@ -1855,8 +1855,7 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,
 		SetPageLRU(page);
 
 		if (unlikely(put_page_testzero(page))) {
-			__ClearPageLRU(page);
-			__ClearPageActive(page);
+			__clear_page_lru_flags(page);
 
 			if (unlikely(PageCompound(page))) {
 				spin_unlock_irq(&lruvec->lru_lock);