[PATCH] mm, vmscan: unlock page while waiting on writeback

[Hugh Dickins <hughd@google.com>]

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 <hughd@google.com>
---
I remembered this old patch when we were discussing the more important
ecf5fc6e9654 "mm, vmscan: Do not wait for page writeback for GFP_NOFS
allocations", and now retested it against mmotm.

 mm/vmscan.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- mmotm/mm/vmscan.c	2015-08-17 18:46:26.601521575 -0700
+++ linux/mm/vmscan.c	2015-08-17 18:53:41.335108240 -0700
@@ -991,7 +991,7 @@ static unsigned long shrink_page_list(st
 		 *    __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
@@ -1021,12 +1021,15 @@ static unsigned long shrink_page_list(st
 				 */
 				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;
 			}
 		}
 

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