linux-kernel - Re: [dm-devel] [PATCH 02/35] block: add REQ_OP definitions and bi

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Date:	Sat, 9 Jan 2016 23:15:58 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Mike Christie <michaelc@...wisc.edu>
Cc:	device-mapper development <dm-devel@...hat.com>,
	mchristi@...hat.com, linux-bcache@...r.kernel.org,
	linux-block@...r.kernel.org, xfs@....sgi.com,
	linux-scsi@...r.kernel.org, konrad.wilk@...cle.com,
	linux-kernel@...r.kernel.org, philipp.reisner@...bit.com,
	linux-f2fs-devel@...ts.sourceforge.net, linux-raid@...r.kernel.org,
	ocfs2-devel@....oracle.com, target-devel@...r.kernel.org,
	linux-mtd@...ts.infradead.org, osd-dev@...n-osd.org,
	linux-fsdevel@...r.kernel.org, lars.ellenberg@...bit.com,
	linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org,
	drbd-dev@...ts.linbit.com
Subject: Re: [dm-devel] [PATCH 02/35] block: add REQ_OP definitions and
 bi_op/op fields

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.