writeback: check for registered bdi in flusher add and inode dirty

Also a debugging aid. We want to catch dirty inodes being added to
backing devices that don't do writeback.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/fs-writeback.c

@@ -1046,6 +1046,14 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 		 */
 		if (!was_dirty) {
 			struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
+			struct backing_dev_info *bdi = wb->bdi;
+
+			if (bdi_cap_writeback_dirty(bdi) &&
+			    !test_bit(BDI_registered, &bdi->state)) {
+				WARN_ON(1);
+				printk(KERN_ERR "bdi-%s not registered\n",
+								bdi->name);
+			}
 
 			inode->dirtied_when = jiffies;
 			list_move(&inode->i_list, &wb->b_dirty);

include/linux/backing-dev.h

@@ -29,6 +29,7 @@ enum bdi_state {
 	BDI_wb_alloc,		/* Default embedded wb allocated */
 	BDI_async_congested,	/* The async (write) queue is getting full */
 	BDI_sync_congested,	/* The sync queue is getting full */
+	BDI_registered,		/* bdi_register() was done */
 	BDI_unused,		/* Available bits start here */
 };
 

mm/backing-dev.c

@@ -465,6 +465,12 @@ void static bdi_add_default_flusher_task(struct backing_dev_info *bdi)
 	if (!bdi_cap_writeback_dirty(bdi))
 		return;
 
+	if (WARN_ON(!test_bit(BDI_registered, &bdi->state))) {
+		printk(KERN_ERR "bdi %p/%s is not registered!\n",
+							bdi, bdi->name);
+		return;
+	}
+
 	/*
 	 * Check with the helper whether to proceed adding a task. Will only
 	 * abort if we two or more simultanous calls to
@@ -528,6 +534,7 @@ int bdi_register(struct backing_dev_info *bdi, struct device *parent,
 	}
 
 	bdi_debug_register(bdi, dev_name(dev));
+	set_bit(BDI_registered, &bdi->state);
 exit:
 	return ret;
 }