[PATCH] loop: remove the balance_dirty_pages() call

The loop thread is getting permanently stuck in balance_dirty_pages()
(nr_writeback is exceeded) because the loop thread itself is responsible for
completing writeback on behalf of higher layers.

So we need to take that out: don't throttle the loop thread.  Throttle the
tasks which are generating all the dirty data instead.

drivers/block/loop.c

@@ -236,7 +236,6 @@ do_lo_send(struct loop_device *lo, struct bio_vec *bvec, int bsize, loff_t pos)
 	up(&mapping->host->i_sem);
 out:
 	kunmap(bvec->bv_page);
-	balance_dirty_pages_ratelimited(mapping);
 	return ret;
 
 unlock:

mm/page-writeback.c

@@ -220,7 +220,6 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
 	}
 	put_cpu();
 }
-EXPORT_SYMBOL_GPL(balance_dirty_pages_ratelimited);
 
 /*
  * writeback at least _min_pages, and keep writing until the amount of dirty