[PATCH] shrink_list(): skip anon pages if not may_swap

Martin Hicks' page cache reclaim patch added the 'may_swap' flag to the
scan_control struct; and modified shrink_list() not to add anon pages to
the swap cache if may_swap is not asserted.

Ref:  http://marc.theaimsgroup.com/?l=linux-mm&m=111461480725322&w=4

However, further down, if the page is mapped, shrink_list() calls
try_to_unmap() which will call try_to_unmap_one() via try_to_unmap_anon ().
 try_to_unmap_one() will BUG_ON() an anon page that is NOT in the swap
cache.  Martin says he never encountered this path in his testing, but
agrees that it might happen.

This patch modifies shrink_list() to skip anon pages that are not already
in the swap cache when !may_swap, rather than just not adding them to the
cache.
Signed-off-by: NLee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/vmscan.c

@@ -417,7 +417,9 @@ static int shrink_list(struct list_head *page_list, struct scan_control *sc)
 		 * Anonymous process memory has backing store?
 		 * Try to allocate it some swap space here.
 		 */
-		if (PageAnon(page) && !PageSwapCache(page) && sc->may_swap) {
+		if (PageAnon(page) && !PageSwapCache(page)) {
+			if (!sc->may_swap)
+				goto keep_locked;
 			if (!add_to_swap(page))
 				goto activate_locked;
 		}