I'm curious what is viewed as a backing device. Lets say I have a SAN exporting three volumes via iSCSI to a single server. Does this patch think those volumes share a single BDI, or does it treat them separately?

I think the three volumes will be seen as independent BDIs, because that's the simplest implementation (look at the block device the pages are for, don't attempt to count things at multiple levels). Being too granular isn't a problem, because a process will be slowed down if it writes to any of the three iSCSI volumes that are throttled by the network.

As long as there's a "dirtytop" to watch what's going on this will make a lot of people happy.