vmscan: let GFP_NOFS go to swap again

In the past, GFP_NOFS (but of course not GFP_NOIO) was allowed to reclaim
by writing to swap.  That got partially broken in 2.6.23, when may_enter_fs
initialization was moved up before the allocation of swap, so its
PageSwapCache test was failing the first time around,

Fix it by setting may_enter_fs when add_to_swap() succeeds with
__GFP_IO.  In fact, check __GFP_IO before calling add_to_swap():
allocating swap we're not ready to use just increases disk seeking.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: 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/vmscan.c

@@ -623,6 +623,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		 * Try to allocate it some swap space here.
 		 */
 		if (PageAnon(page) && !PageSwapCache(page)) {
+			if (!(sc->gfp_mask & __GFP_IO))
+				goto keep_locked;
 			switch (try_to_munlock(page)) {
 			case SWAP_FAIL:		/* shouldn't happen */
 			case SWAP_AGAIN:
@@ -634,6 +636,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			}
 			if (!add_to_swap(page, GFP_ATOMIC))
 				goto activate_locked;
+			may_enter_fs = 1;
 		}
 #endif /* CONFIG_SWAP */