mm: try_to_unmap_cluster() should lock_page() before mlocking

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 <[email protected]>
Signed-off-by: Bob Liu <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Cc: Wanpeng Li <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/mlock.c b/mm/mlock.c
index 4e1a68162285688ad1f1158681a6908e7e237924..b1eb53634005606298d115ac5cdeb90cac923957 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -79,6 +79,7 @@ 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)) {
@@ -174,6 +175,7 @@ unsigned int munlock_vma_page(struct page *page)
        unsigned int nr_pages;
        struct zone *zone = page_zone(page);
 
+       /* For try_to_munlock() and to serialize with page migration */
        BUG_ON(!PageLocked(page));
 
        /*

diff --git a/mm/rmap.c b/mm/rmap.c
index 11cf322f8133d7048d414c6f0eed227f6b12d3ac..9c3e77396d1a118edb8f2de7d16ff835ed214f0a 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1332,9 +1332,19 @@ 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 */
                }