On Fri, Mar 02, 2018 at 11:26:41PM +0100, Christoph Hellwig wrote: > While we're at it we should probably > also skil the generic_write_sync call for DAX pure overwrites while > we're at it. Now that I've looked at it, DAX is a friggin' mess. We can't push the generic_write_sync() call into dax_iomap_rw() because dax_iomap_rw is called with the inode_lock() held, and both ext2's and ext4's ->fsync implementation can call __generic_file_fsync() which takes the inode_lock(). Deadlock central right there - someone is going to have to screw with ext4's indoe sync code to before we can do this. Further, XFS has post-write metadata updates to do in the case of extending writes, and hence we'd still need the call to generic_write_sync() after we have returned from dax_iomap_rw(). I'm thinking we'd really need a dax write IO completion callback (say in the struct iomap_ops) to run the filesystem specific IO completions (like the DIO path) before we can start to think about optimisations like this for DAX for XFS.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
