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: sta...@kernel.org
Reported-by: Sasha Levin <sasha.le...@oracle.com>
Signed-off-by: Vlastimil Babka <vba...@suse.cz>
---
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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