This sounds like a good idea, but suppose a file had several pages that were mapped into some processes' memory with PROT_WRITE access. Wouldn't this require the filesystem to do the following?:
(1) temporarily mark those pages as write only while writeback was in progress
(2) catch any page faults during that time
(3) either duplicate the page using a copy on write strategy or hold the faulting process until writeback on that page was complete?