Eww. The block bounc code really needs to go away, so a new user makes me very unhappy. But independent of that I don't think this is enough anyway. Just copying the data out into a new page in the block layer doesn't solve the problem that this page needs to be tracked as dirtied for fs accounting. e.g. every time we write this copy it needs space allocated for COW file systems. Which brings me back to if and when we do writeback for pinned page. I don't think doing any I/O for short term pins like direct I/O make sense. These pins are defined to be unpinned after I/O completes, so we might as well just wait for the unpin instead of doing anything complicated. Long term pins are more troublesome, but I really wonder what the defined semantics for data integrity writeback like fsync on them is to start with as the content is very much undefined. Should an fsync on a (partially) long term pinned file simplfy fail? It's not like we can win in that scenario.
