[PATCH 2/3] mm: munlock: fix deadlock in __munlock_pagevec()

[Vlastimil Babka]

Commit 7225522bb ("mm: munlock: batch non-THP page isolation and
munlock+putback using pagevec" introduced __munlock_pagevec() to speed up
munlock by holding lru_lock over multiple isolated pages. Pages that fail to
be isolated are put_back() immediately, also within the lock.

This can lead to deadlock when __munlock_pagevec() becomes the holder of the
last page pin and put_back() leads to __page_cache_release() which also locks
lru_lock. The deadlock has been observed by Sasha Levin using trinity.

This patch avoids the deadlock by deferring put_back() operations until
lru_lock is released. Another pagevec (which is also used by later phases
of the function is reused to gather the pages for put_back() operation.

Cc: stable@xxxxxxxxxx
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/mlock.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 3847b13..31383d5 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -295,10 +295,12 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
 {
 	int i;
 	int nr = pagevec_count(pvec);
-	int delta_munlocked = -nr;
+	int delta_munlocked;
 	struct pagevec pvec_putback;
 	int pgrescued = 0;
 
+	pagevec_init(&pvec_putback, 0);
+
 	/* Phase 1: page isolation */
 	spin_lock_irq(&zone->lru_lock);
 	for (i = 0; i < nr; i++) {
@@ -327,16 +329,22 @@ skip_munlock:
 			/*
 			 * We won't be munlocking this page in the next phase
 			 * but we still need to release the follow_page_mask()
-			 * pin.
+			 * pin. We cannot do it under lru_lock however. If it's
+			 * the last pin, __page_cache_release would deadlock.
 			 */
+			pagevec_add(&pvec_putback, pvec->pages[i]);
 			pvec->pages[i] = NULL;
-			put_page(page);
-			delta_munlocked++;
 		}
 	}
+	delta_munlocked = -nr + pagevec_count(&pvec_putback);
 	__mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);
 	spin_unlock_irq(&zone->lru_lock);
 
+	/* Now we can release pins of pages that we are not munlocking */
+	for (i = 0; i < pagevec_count(&pvec_putback); i++) {
+		put_page(pvec_putback.pages[i]);
+	}
+
 	/* Phase 2: page munlock */
 	pagevec_init(&pvec_putback, 0);
 	for (i = 0; i < nr; i++) {
-- 
1.8.4

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