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 16:44:30 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Milan Broz <mbroz@...hat.com>
Cc:	Tao Ma <tm@....ma>, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	device-mapper development <dm-devel@...hat.com>,
	Neil Brown <neilb@...e.de>
Subject: Re: [PATCH][RFC] dm: Do not open log and cow device read-write for
 read-only mappings

Hello,

On Mon, Feb 14, 2011 at 03:23:20PM +0100, Milan Broz wrote:
> >> 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.

Yeap, it seems dm folks are gonna take care of dm part.

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

Indeed, cc'ing Neil.  Hi, the whole thread can be read from the
following URL.

  http://thread.gmane.org/gmane.linux.kernel/1099399/focus=1099735

blkdev_get() now rejects rw open of devices which are marked
read-only.  I think the right thing to do would be opening the member
devices ro if the array is assembled for ro access (similar to Milan's
patch for dm).  How does that sound?

Thanks.

-- 
tejun
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