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

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