Re: [PATCH] [RFC] iomap: Use FUA for pure data O_DSYNC DIO writes

[Dave Chinner <david@fromorbit.com>]

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@fromorbit.com