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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/mlock.c b/mm/mlock.c
index 4f4f53b..39b3a7d 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -78,6 +78,7 @@ void __clear_page_mlock(struct page *page)
  */
 void mlock_vma_page(struct page *page)
 {
+       /* Serialize with page migration */
        BUG_ON(!PageLocked(page));
 
        if (!TestSetPageMlocked(page)) {
@@ -105,6 +106,7 @@ void mlock_vma_page(struct page *page)
  */
 void munlock_vma_page(struct page *page)
 {
+       /* For try_to_munlock() and to serialize with page migration */
        BUG_ON(!PageLocked(page));
 
        if (TestClearPageMlocked(page)) {

diff --git a/mm/rmap.c b/mm/rmap.c
index 52a2f36..9ac405b 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1385,9 +1385,19 @@ 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 */
                }