linux-kernel - Re: [PATCH v2] dm ioctl: allow change device target type to error

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Date:	Wed, 21 Aug 2013 11:06:42 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	dm-devel@...hat.com, joe.jin@...cle.com, agk@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] dm ioctl: allow change device target type to error

On Wed, Aug 21 2013 at 10:48am -0400,
Mikulas Patocka <mpatocka@...hat.com> wrote:

> 
> 
> On Wed, 21 Aug 2013, Joe Jin wrote:
> 
> > commit a5664da "dm ioctl: make bio or request based device type immutable"
> > prevented "dmsetup wape_table" change the target type to "error".
> 
> That commit a5664da is there for a reason (it is not possible to change 
> bio-based device to request-based and vice versa) and I don't really see 
> how this patch is supposed to work.
> 
> If there are bios that are in flight and that already passed through 
> blk_queue_bio, and you change the device from request-based to bio-based, 
> what are you going to do with them? - The patch doesn't do anything about 
> it.
> 
> A better approach would be to create a new request-based target "error-rq" 
> and change the multipath target to "error-rq" target. That way, you don't 
> have to change device type from request based to bio based.

My thoughts _exactly_.  This patch is very confused.

Joe, what are you looking to be able to do?  Switch a dm-multipath
device to error?  Or allowing switching a target that has
DM_TARGET_IMMUTABLE flag set to be switched to error target?

The latter restriction was introduced with commit 36a0456fb ("dm table:
add immutable feature").

Mike
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