From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [dm-devel] [PATCH 02/35] block: add REQ_OP definitions and
 bi_op/op fields
Date: Sat, 9 Jan 2016 23:15:58 -0800
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To: Mike Christie <michaelc@cs.wisc.edu>
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On Sat, Jan 09, 2016 at 07:21:12PM -0600, Mike Christie wrote:
> Oh yeah, to answer the second part of your question, REQ_OP_FLUSH is
> only a flush operation like what request_fn drivers wanted.

And that's the odd part that trips me up.

> 
> REQ_PREFLUSH can be set with a REQ_OP_WRITE bio when filesystems want to
> do both.
> 
> There is then the case where filesystems and blkdev_issue_flush could
> just want to request a flush. I left them as a REQ_PREFLUSH with
> REQ_OP_WRITE set, so there would be a single code path.

But the pure flush without data transfer case is pretty different,
so it seems rather odd to handle it like that.  But I suspec we could
just fix that up later.