tmpfs: fix race between swapoff and writepage

Shame on me!  Commit b1dea800ac39 "tmpfs: fix race between umount and
writepage" fixed the advertized race, but introduced another: as even
its comment makes clear, we cannot safely rely on a peek at list_empty()
while holding no lock - until info->swapped is set, shmem_unuse_inode()
may delete any formerly-swapped inode from the shmem_swaplist, which
in this case would leave a swap area impossible to swapoff.

Although I don't relish taking the mutex every time, I don't care much
for the alternatives either; and at least the peek at list_empty() in
shmem_evict_inode() (a hotter path since most inodes would never have
been swapped) remains safe, because we already truncated the whole file.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 6 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 9e755c166cc5..dfc7069102ee 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1037,7 +1037,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
         struct address_space *mapping;
         unsigned long index;
         struct inode *inode;
-        bool unlock_mutex = false;
 
         BUG_ON(!PageLocked(page));
         mapping = page->mapping;
@@ -1072,15 +1071,14 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
          * we've taken the spinlock, because shmem_unuse_inode() will
          * prune a !swapped inode from the swaplist under both locks.
          */
-        if (swap.val && list_empty(&info->swaplist)) {
+        if (swap.val) {
                 mutex_lock(&shmem_swaplist_mutex);
-                /* move instead of add in case we're racing */
-                list_move_tail(&info->swaplist, &shmem_swaplist);
-                unlock_mutex = true;
+                if (list_empty(&info->swaplist))
+                        list_add_tail(&info->swaplist, &shmem_swaplist);
         }
 
         spin_lock(&info->lock);
-        if (unlock_mutex)
+        if (swap.val)
                 mutex_unlock(&shmem_swaplist_mutex);
 
         if (index >= info->next_index) {