Marcelo Tosatti [off-list ref] wrote:

 static int may_write_to_queue(struct backing_dev_info *bdi)
 {
+	int nr_writeback = read_page_state(nr_writeback);
+
+	if (nr_writeback > (totalram_pages * 25 / 100)) { 
+		blk_congestion_wait(WRITE, HZ/5);
+		return 0;
+	}

That's probably a good way of special-casing this special-place problem.

For a final patch I'd be inclined to take into account /proc/sys/vm/dirty_ratio
and to avoid running the expensive read_page_state() once per writepage.
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