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>

diff --git a/mm/mlock.c b/mm/mlock.c
index c59c420fd6e16e81c10d0524dd980f38c09b2afa..192e6eebe4f240e4a8ece7cc0e10e902ff29f308 100644 (file)
--- 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];