mm, vmscan: unlock page while waiting on writeback

This is merely a politeness: I've not found that shrink_page_list()
leads to deadlock with the page it holds locked across
wait_on_page_writeback(); but nevertheless, why hold others off by
keeping the page locked there?

And while we're at it: remove the mistaken "not " from the commentary on
this Case 3 (and a distracting blank line from Case 2, if I may).

Signed-off-by: Hugh Dickins <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmscan.c

···
 		 *    __GFP_IO|__GFP_FS for this reason); but more thought
 		 *    would probably show more reasons.
 		 *
-		 * 3) Legacy memcg encounters a page that is not already marked
+		 * 3) Legacy memcg encounters a page that is already marked
 		 *    PageReclaim. memcg does not have any dirty pages
 		 *    throttling so we could easily OOM just because too many
 		 *    pages are in writeback and there is nothing else to
···
 				 */
 				SetPageReclaim(page);
 				nr_writeback++;
-
 				goto keep_locked;
 
 			/* Case 3 above */
 			} else {
+				unlock_page(page);
 				wait_on_page_writeback(page);
+				/* then go back and try same page again */
+				list_add_tail(&page->lru, page_list);
+				continue;
 			}
 		}