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 <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8276a3a615ca..2d978b28a410 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -985,7 +985,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                  *    __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
@@ -1015,12 +1015,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                                  */
                                 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;
                         }
                 }