writeback: Fix periodic writeback after fs mount

Code in blkdev.c moves a device inode to default_backing_dev_info when
the last reference to the device is put and moves the device inode back
to its bdi when the first reference is acquired. This includes moving to
wb.b_dirty list if the device inode is dirty. The code however doesn't
setup timer to wake corresponding flusher thread and while wb.b_dirty
list is non-empty __mark_inode_dirty() will not set it up either. Thus
periodic writeback is effectively disabled until a sync(2) call which can
lead to unexpected data loss in case of crash or power failure.

Fix the problem by setting up a timer for periodic writeback in case we
add the first dirty inode to wb.b_dirty list in bdev_inode_switch_bdi().

Reported-by: Bert De Jonghe <Bert.DeJonghe@amplidata.com>
CC: stable@vger.kernel.org # >= 3.0
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 8 insertions, 1 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 2091db8cdd78..85f5c85ec91c 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -58,17 +58,24 @@ static void bdev_inode_switch_bdi(struct inode *inode,
                         struct backing_dev_info *dst)
 {
         struct backing_dev_info *old = inode->i_data.backing_dev_info;
+        bool wakeup_bdi = false;
 
         if (unlikely(dst == old))               /* deadlock avoidance */
                 return;
         bdi_lock_two(&old->wb, &dst->wb);
         spin_lock(&inode->i_lock);
         inode->i_data.backing_dev_info = dst;
-        if (inode->i_state & I_DIRTY)
+        if (inode->i_state & I_DIRTY) {
+                if (bdi_cap_writeback_dirty(dst) && !wb_has_dirty_io(&dst->wb))
+                        wakeup_bdi = true;
                 list_move(&inode->i_wb_list, &dst->wb.b_dirty);
+        }
         spin_unlock(&inode->i_lock);
         spin_unlock(&old->wb.list_lock);
         spin_unlock(&dst->wb.list_lock);
+
+        if (wakeup_bdi)
+                bdi_wakeup_thread_delayed(dst);
 }
 
 /* Kill _all_ buffers and pagecache , dirty or not.. */