Re: xfs: untangle the direct I/O and DAX path, fix DAX locking

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Fri, Jun 24, 2016 at 09:24:46AM +1000, Dave Chinner wrote:
> Except we did that *intentionally* - by definition there is no
> cache to bypass with DAX and so all IO is "direct". That, combined
> with the fact that all Linux filesystems except XFS break the POSIX
> exclusive writer rule you are quoting to begin with, it seemed
> pointless to enforce it for DAX....

No file system breaks the exclusive writer rule - most filesystem
don't make writers atomic vs readers.

More importantly every other filesystem (well there only are ext2
and ext4..) exludes DAX writers against other DAX writers.

> So, before taking any patches to change that behaviour in XFS, a
> wider discussion about the policy needs to be had. I don't think
> we should care about POSIX here - if you have an application that
> needs this serialisation, turn off DAX. That's why I made it a
> per-inode inheritable flag and why the mount option will go away
> over time.

Sorry, but this is simply broken - allowing apps to opt-in behavior
(e.g. like we're using O_DIRECT) is always fine.  Requriring
filesystem-specific tuning that has affect outside the app to get
existing documented behavior is not how to design APIs.

Maybe we'll need to opt-in to use DAX for mmap, but giving the same
existing behavior for read and write and avoiding a copy to the pagecache
is an obvious win.