+ /*
+ * Page becomes evictable in two ways:
+ * 1) Within LRU lock [munlock_vma_pages() and __munlock_pagevec()].
+ * 2) Before acquiring LRU lock to put the page to correct LRU and then
+ * a) do PageLRU check with lock [check_move_unevictable_pages]
+ * b) do PageLRU check before lock [clear_page_mlock]
+ *
+ * (1) & (2a) are ok as LRU lock will serialize them. For (2b), we need
+ * following strict ordering:
+ *
+ * #0: __pagevec_lru_add_fn #1: clear_page_mlock
+ *
+ * SetPageLRU() TestClearPageMlocked()
+ * smp_mb() // explicit ordering // above provides strict
+ * // ordering
+ * PageMlocked() PageLRU()
+ *
+ *
+ * if '#1' does not observe setting of PG_lru by '#0' and fails
+ * isolation, the explicit barrier will make sure that page_evictable
+ * check will put the page in correct LRU. Without smp_mb(), SetPageLRU
+ * can be reordered after PageMlocked check and can make '#1' to fail
+ * the isolation of the page whose Mlocked bit is cleared (#0 is also
+ * looking at the same page) and the evictable page will be stranded
+ * in an unevictable LRU.
+ */
+ smp_mb();
+
+ if (page_evictable(page)) {
+ lru = page_lru(page);
+ update_page_reclaim_stat(lruvec, page_is_file_cache(page),
+ PageActive(page));
+ if (was_unevictable)
+ count_vm_event(UNEVICTABLE_PGRESCUED);
+ } else {
+ lru = LRU_UNEVICTABLE;
+ ClearPageActive(page);
+ SetPageUnevictable(page);
+ if (!was_unevictable)
+ count_vm_event(UNEVICTABLE_PGCULLED);
+ }
+