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    SubjectRe: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io
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    On Thu, 2016-05-05 at 07:24 -0700, Christoph Hellwig wrote:
    > 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.

    How is it any 'less direct'? All it does now is follow the blockdev
    O_DIRECT path. There still isn't any page cache involved..

    >
    > 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..
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