mm: try_to_unmap_cluster() should lock_page() before mlocking

commit 57e68e9cd65b4b8eb4045a1e0d0746458502554c upstream.

A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin
fuzzing with trinity.  The call site try_to_unmap_cluster() does not lock
the pages other than its check_page parameter (which is already locked).

The BUG_ON in mlock_vma_page() is not documented and its purpose is
somewhat unclear, but apparently it serializes against page migration,
which could otherwise fail to transfer the PG_mlocked flag.  This would
not be fatal, as the page would be eventually encountered again, but
NR_MLOCK accounting would become distorted nevertheless.  This patch adds
a comment to the BUG_ON in mlock_vma_page() and munlock_vma_page() to that
effect.

The call site try_to_unmap_cluster() is fixed so that for page !=
check_page, trylock_page() is attempted (to avoid possible deadlocks as we
already have check_page locked) and mlock_vma_page() is performed only
upon success.  If the page lock cannot be obtained, the page is left
without PG_mlocked, which is again not a problem in the whole unevictable
memory design.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 3f6077461aea..fbf0040a7342 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1390,9 +1390,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
                 BUG_ON(!page || PageAnon(page));
 
                 if (locked_vma) {
-                        mlock_vma_page(page);   /* no-op if already mlocked */
-                        if (page == check_page)
+                        if (page == check_page) {
+                                /* we know we have check_page locked */
+                                mlock_vma_page(page);
                                 ret = SWAP_MLOCK;
+                        } else if (trylock_page(page)) {
+                                /*
+                                 * If we can lock the page, perform mlock.
+                                 * Otherwise leave the page alone, it will be
+                                 * eventually encountered again later.
+                                 */
+                                mlock_vma_page(page);
+                                unlock_page(page);
+                        }
                         continue;       /* don't unmap */
                 }