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Re: [dm-devel] [PATCH 02/35] block: add REQ_OP definitions and bi_op/op

To: Mike Christie <michaelc@xxxxxxxxxxx>
Subject: Re: [dm-devel] [PATCH 02/35] block: add REQ_OP definitions and bi_op/op fields
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Sat, 9 Jan 2016 23:15:58 -0800
Cc: device-mapper development <dm-devel@xxxxxxxxxx>, mchristi@xxxxxxxxxx, linux-bcache@xxxxxxxxxxxxxxx, linux-block@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-scsi@xxxxxxxxxxxxxxx, konrad.wilk@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, philipp.reisner@xxxxxxxxxx, linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx, linux-raid@xxxxxxxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx, target-devel@xxxxxxxxxxxxxxx, linux-mtd@xxxxxxxxxxxxxxxxxxx, osd-dev@xxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, lars.ellenberg@xxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx, drbd-dev@xxxxxxxxxxxxxxxx
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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.

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