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From: Jeff Moyer <jmoyer@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>, linux-kernel@vger.kernel.org,
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Subject: Re: [RFC 00/11] DAX fsynx/msync support
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X-PCLoadLetter: What the f**k does that mean?
Date: Thu, 05 Nov 2015 14:49:21 -0500
In-Reply-To: <20151105083309.GJ19199@dastard> (Dave Chinner's message of "Thu,
	5 Nov 2015 19:33:09 +1100")
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Dave Chinner <david@fromorbit.com> writes:

>> But this part is not.  It is up to the I/O scheduler to decide when to
>> dispatch requests.  It can hold on to them for a variety of reasons.
>> Flush requests, however, do not go through the I/O scheduler.  At the
>
> That's pure REQ_FLUSH bios, right? Aren't data IOs with
> REQ_FLUSH|REQ_FUA sorted like any other IO?

No, they also go through the flush machinery, and so short-circuit the
I/O scheduler.

>> Des xfs rely on this model for correctness?  If so, I'd say we've got a
>> problem
>
> No, it doesn't. The XFS integrity model doesn't trust the IO layers
> to tell the truth about IO ordering and completion or for it's
> developers to fully understand how IO layer ordering works. :P
>
> i.e. we wait for full completions of all dependent IO before issuing
> flushes or log writes that use REQ_FLUSH|REQ_FUA semantics to ensure
> the dependent IOs are fully caught by the cache flushes...

OK, phew!  ;-)

-Jeff