mm: munlock: fix deadlock in __munlock_pagevec()

Commit 7225522bb429 ("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_page()d immediately, also within the lock.

This can lead to deadlock when __munlock_pagevec() becomes the holder of
the last page pin and put_page() 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_page() 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_page()
operation.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 5 deletions

diff --git a/mm/mlock.c b/mm/mlock.c
index c59c420fd6e1..192e6eebe4f2 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -298,10 +298,12 @@ 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++) {
@@ -330,18 +332,21 @@ 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 */
+        pagevec_release(&pvec_putback);
+
         /* Phase 2: page munlock */
-        pagevec_init(&pvec_putback, 0);
         for (i = 0; i < nr; i++) {
                 struct page *page = pvec->pages[i];