We're currently just setting the referenced bit when modifying pagecache in
write().

Consequently overwritten (and redirtied) pages are remaining on the inactive
list.  The net result is that a lot of dirty pages are reaching the tail of
the LRU in page reclaim and are getting written via the writepage() in there.

But a core design objective is to minimise the amount of IO via that path,
and to maximise the amount of IO via balance_dirty_pages().  Because the
latter has better IO patterns.

This may explain the bad IO patterns which Gerrit talked about at KS.