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:

quoted

quoted

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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