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

From: Christoph Hellwig
Date: Thu May 05 2016 - 10:24:41 EST

On Mon, May 02, 2016 at 06:41:51PM +0300, Boaz Harrosh wrote:
> > All IO in a dax filesystem used to go through dax_do_io, which cannot
> > handle media errors, and thus cannot provide a recovery path that can
> > send a write through the driver to clear errors.
> >
> > Add a new iocb flag for DAX, and set it only for DAX mounts. In the IO
> > path for DAX filesystems, use the same direct_IO path for both DAX and
> > direct_io iocbs, but use the flags to identify when we are in O_DIRECT
> > mode vs non O_DIRECT with DAX, and for O_DIRECT, use the conventional
> > direct_IO path instead of DAX.
> >
> Really? What are your thinking here?
> What about all the current users of O_DIRECT, you have just made them
> 4 times slower and "less concurrent*" then "buffred io" users. Since
> direct_IO path will queue an IO request and all.
> (And if it is not so slow then why do we need dax_do_io at all? [Rhetorical])
> I hate it that you overload the semantics of a known and expected
> O_DIRECT flag, for special pmem quirks. This is an incompatible
> and unrelated overload of the semantics of O_DIRECT.

Agreed - makig O_DIRECT less direct than not having it is plain stupid,
and I somehow missed this initially.

This whole DAX story turns into a major nightmare, and I fear all our
hodge podge tweaks to the semantics aren't helping it.

It seems like we simply need an explicit O_DAX for the read/write
bypass if can't sort out the semantics (error, writer synchronization)
just as we need a special flag for MMAP..