mm: fix for infinite churning of mlocked pages

An mlocked page might lose the isolatation race.  This causes the page to
clear PG_mlocked while it remains in a VM_LOCKED vma.  This means it can
be put onto the [in]active list.  We can rescue it by using try_to_unmap()
in shrink_page_list().

But now, As Wu Fengguang pointed out, vmscan has a bug.  If the page has
PG_referenced, it can't reach try_to_unmap() in shrink_page_list() but is
put into the active list.  If the page is referenced repeatedly, it can
remain on the [in]active list without being moving to the unevictable
list.

This patch fixes it.

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KOSAKI Motohiro <<kosaki.motohiro@jp.fujitsu.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -358,6 +358,7 @@ static int page_referenced_one(struct page *page,
 	 */
 	if (vma->vm_flags & VM_LOCKED) {
 		*mapcount = 1;	/* break early from loop */
+		*vm_flags |= VM_LOCKED;
 		goto out_unmap;
 	}
 

mm/vmscan.c

@@ -630,9 +630,14 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 		referenced = page_referenced(page, 1,
 						sc->mem_cgroup, &vm_flags);
-		/* In active use or really unfreeable?  Activate it. */
+		/*
+		 * In active use or really unfreeable?  Activate it.
+		 * If page which have PG_mlocked lost isoltation race,
+		 * try_to_unmap moves it to unevictable list
+		 */
 		if (sc->order <= PAGE_ALLOC_COSTLY_ORDER &&
-					referenced && page_mapping_inuse(page))
+					referenced && page_mapping_inuse(page)
+					&& !(vm_flags & VM_LOCKED))
 			goto activate_locked;
 
 		/*