[PATCH] mm: vmscan: unlock_page page when forcing reclaim

[Richard Yao]

A small userland program I wrote to assist me in drive forensic
operations soft deadlocked on Linux 3.14.4. The stack trace from /proc
was:

[<ffffffff8112968e>] sleep_on_page_killable+0xe/0x40
[<ffffffff81129829>] wait_on_page_bit_killable+0x79/0x80
[<ffffffff811299a5>] __lock_page_or_retry+0x95/0xc0
[<ffffffff8112a95b>] filemap_fault+0x21b/0x420
[<ffffffff8115685e>] __do_fault+0x6e/0x520
[<ffffffff81156de3>] handle_pte_fault+0xd3/0x1f0
[<ffffffff81157073>] __handle_mm_fault+0x173/0x290
[<ffffffff811571d2>] handle_mm_fault+0x42/0xb0
[<ffffffff81587a11>] __do_page_fault+0x191/0x490
[<ffffffff81587dec>] do_page_fault+0xc/0x10
[<ffffffff81584622>] page_fault+0x22/0x30
[<ffffffffffffffff>] 0xffffffffffffffff

The program used mmap() to do a linear scan of the device on 64-bit
hardware. The block device in question was 200GB in size and the system
had only 8GB of RAM. All IO operations stopped following pageout.

shrink_page_list() seemed to have raced with filemap_fault() by evicting
a page when we had an active fault handler. This is possible only
because 02c6de8d757cb32c0829a45d81c3dfcbcafd998b altered the behavior of
shrink_page_list() to ignore references. Consequently, we must call
unlock_page() instead of __clear_page_locked() when doing this so that
waiters are notified. unlock_page() here will cause active page fault
handlers to retry (depending on the architecture), which avoids the soft
deadlock.

Signed-off-by: Richard Yao <ryao@xxxxxxxxxx>
---
 mm/vmscan.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3f56c8d..c07c635 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1083,13 +1083,16 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			goto keep_locked;
 
 		/*
-		 * At this point, we have no other references and there is
-		 * no way to pick any more up (removed from LRU, removed
-		 * from pagecache). Can use non-atomic bitops now (and
+		 * Unless we force reclaim, we have no other references and
+		 * there is no way to pick any more up (removed from LRU,
+		 * removed from pagecache). Can use non-atomic bitops now (and
 		 * we obviously don't have to worry about waking up a process
 		 * waiting on the page lock, because there are no references.
 		 */
-		__clear_page_locked(page);
+		if (force_reclaim)
+			unlock_page(page);
+		else
+			__clear_page_locked(page);
 free_it:
 		nr_reclaimed++;
 
-- 
1.8.3.2

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