[PATCH] memcg, vmscan: Do not wait for writeback if killed

[Michal Hocko <mhocko@kernel.org>]

From: Michal Hocko <mhocko@suse.com>

Legacy memcg reclaim waits for pages under writeback to prevent from a
premature oom killer invocation because there was no memcg dirty limit
throttling implemented back then.

This heuristic might complicate situation when the writeback cannot make
forward progress because of the global OOM situation. E.g. filesystem
backed by the loop device relies on the underlying filesystem hosting
the image to make forward progress which cannot be guaranteed and so
we might end up triggering OOM killer to resolve the situation. If the
oom victim happens to be the task stuck in wait_on_page_writeback in the
memcg reclaim then we are basically deadlocked.

Introduce wait_on_page_writeback_killable and use it in this path to
prevent from the issue. shrink_page_list will back off if the wait
was interrupted.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
Hi,
this came out from the discussion with Vladimir [1]. I haven't seen
a deadlock presented in the above description but I fail to see why
it would be impossible either. wait_on_page_writeback_killable is an
utterly long name but wait_on_page_bit_killable is quite long already
and I felt it is better to use the helper.

Thoughts?

[1] http://lkml.kernel.org/r/1448465801-3280-1-git-send-email-vdavydov@virtuozzo.com

 include/linux/pagemap.h |  8 ++++++++
 mm/vmscan.c             | 11 ++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 4d08b6c33557..d3bb8963f8f0 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -532,6 +532,14 @@ static inline void wait_on_page_writeback(struct page *page)
 		wait_on_page_bit(page, PG_writeback);
 }
 
+static inline int wait_on_page_writeback_killable(struct page *page)
+{
+	if (PageWriteback(page))
+		return wait_on_page_bit_killable(page, PG_writeback);
+
+	return 0;
+}
+
 extern void end_page_writeback(struct page *page);
 void wait_for_stable_page(struct page *page);
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4589cfdbe405..98a1934493af 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1021,10 +1021,19 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 			/* Case 3 above */
 			} else {
+				int ret;
+
 				unlock_page(page);
-				wait_on_page_writeback(page);
+				ret = wait_on_page_writeback_killable(page);
 				/* then go back and try same page again */
 				list_add_tail(&page->lru, page_list);
+
+				/*
+				 * We've got killed while waiting here so
+				 * expedite our way out from the reclaim
+				 */
+				if (ret)
+					break;
 				continue;
 			}
 		}
-- 
2.6.2

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