Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

[Christoph Hellwig]

On Thu, May 05, 2016 at 08:15:32AM -0700, Dan Williams wrote:
> > Agreed - makig O_DIRECT less direct than not having it is plain stupid,
> > and I somehow missed this initially.
> 
> Of course I disagree because like Dave argues in the msync case we
> should do the correct thing first and make it fast later, but also
> like Dave this arguing in circles is getting tiresome.

We should do the right thing first, and make it fast later.  But this
proposal is not getting it right - it still does not handle errors
for the fast path, but magically makes it work for direct I/O by
in general using a less optional path for O_DIRECT.  It's getting the
worst of all choices.

As far as I can tell the only sensible option is to:

 - always try dax-like I/O first
 - have a custom get_user_pages + rw_bytes fallback handles bad blocks
   when hitting EIO

And then we need to sort out the concurrent write synchronization.
Again there I think we absolutely have to obey Posix for the !O_DIRECT
case and can avoid it for O_DIRECT, similar to the existing non-DAX
semantics.  If we want any special additional semantics we _will_ need
a special O_DAX flag.