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

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Message-ID: <4D5AB0C5.9030304@redhat.com>
Date:	Tue, 15 Feb 2011 17:58:45 +0100
From:	Milan Broz <mbroz@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Alasdair G Kergon <agk@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Tao Ma <tm@....ma>,
	linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH][RFC] dm: Do not open log and cow device read-write
 for read-only mappings


On 02/15/2011 05:41 PM, Tejun Heo wrote:
> Hello,
> 
> On Tue, Feb 15, 2011 at 05:36:31PM +0100, Milan Broz wrote:
>> Well, I am also not sure about that.
>>
>> But the problem is that read-write open fails now while it worked before.
>> (TBH I have no idea when that EROFS fallback worked - because the code
>> opened device RW, issued EROGET ioctl and set read-only... for years.)
>>
>> Anyway I think EROFS is used on block devices, just grep kernel source.
> 
> Ah, okay, so the fallback was there just in case.  It didn't really
> trigger and right it wouldn't have triggered until now, so your
> assertion about how many programs would break is kinda bogus.  You
> just have single isolated case which hasn't been excercised till now.
> There may as well be code pieces which check against EACCES or what
> not.

If you want another example, here is MD one.

# blockdev --setrw /dev/sd[bcde]
# mdadm -A /dev/md0 /dev/sd[bcde]
mdadm: /dev/md0 has been started with 4 drives.
# mdadm --stop /dev/md0
mdadm: stopped /dev/md0
# blockdev --setro /dev/sd[bcde]
# mdadm -A /dev/md0 /dev/sd[bcde]
mdadm: cannot re-read metadata from /dev/sdb - aborting

Works on 2.6.36.

Thanks,
Milan
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