linux-kernel - Re: [PATCH][RFC] dm: Do not open log and cow device read-write for read-only mappings

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Date:	Mon, 14 Feb 2011 15:23:20 +0100
From:	Milan Broz <mbroz@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Tao Ma <tm@....ma>, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	device-mapper development <dm-devel@...hat.com>
Subject: Re: [PATCH][RFC] dm: Do not open log and cow device read-write for
 read-only mappings

On 02/14/2011 03:09 PM, Tejun Heo wrote:
> On Mon, Feb 14, 2011 at 02:14:57PM +0100, Milan Broz wrote:
>>> But it doesn't fix the read-only snapshot issue and I guess there will be
>>> the same problem with read-only MD code too.
>>> (so the 2) issue here https://lkml.org/lkml/2011/2/12/209).
> 
> So, the problem is caused by dm opening members rw even for ro
> devices, right?

The patch uncover these shortcomings in code.
(Unfortunately quite late in RC...)

>> I am not sure if this is complete fix... note that:
>> - what happens during mirror resync and read-only log?
>> - for COW, it there situation we need to update header in read-oly mode? (invalidated snap?)
> 
> But if the underlying device is marked ro, dm shouldn't update it at
> all.  The device should be opened ro and ro policy should be enforced.

Sure. So we need to check these situations I described.


Btw the same pattern is in MD code in lock_rdev() ...

Milan

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