Actually, there is right now another problem with the synchronous waiting,
which is completely different: because bdflush can be waited on
synchronously by various entities that hold various IO locks, bdflush
itself cannot do certain kinds of IO at all. In particular, it has to use
GFP_BUFFER when it calls down to page_launder(), because it cannot afford
to write out dirty pages which might deadlock on the locks that are held
by people waiting for bdflush..
The deadlock issue is the one I dislike the most: bdflush being
synchronously waited on is fundamentally always going to cripple it. In
comparison, the automatic rebalancing is just a latency issue (but the
automatic balancing _is_ the thing that brings on the fact that we call
rebalance with locks held, so they are certainly related).
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