From: NeilBrown <neilb@suse.de>
To: Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>
Cc: linux-raid@vger.kernel.org
Subject: Re: md device is read only mode
Date: Mon, 9 Jul 2012 11:40:22 +1000 [thread overview]
Message-ID: <20120709114022.58f5d52b@notabene.brown> (raw)
In-Reply-To: <20120705172748.GA6464@lazy.lzy>
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On Thu, 5 Jul 2012 19:27:48 +0200 Piergiorgio Sartor
<piergiorgio.sartor@nexgo.de> wrote:
> Hi all,
>
> following the comment of Neil about the fact that
> an md device, in r/o mode, will BUG() in case of
> write attempt, I would like to ask for some more
> information on the topic.
>
> The problem is the following, mounting a filesystem
> in r/o does not, correctly, prevent writes to the
> underlying device.
This is only half the story.
If the mount command requests read-only, the filesystem may still write to
the device.
However if the device is marked read-only then the filesystem should not
write to the disk - if it finds that it needs to, it should fail the
mount request.
I believe that filesystems behave correctly in this manner, though there
could still be bugs lurking.
>
> Now, assuming a filesystem is mounted r/w over an
> md device, being r/w too, it is pretty clear that
> switching the md device to r/o will create problems.
Not quite. When the filesystem is mounted, md will refuse to switch the
device to read-only, so that scenario is not possible.
>
> What if the md device is started in r/o mode and
> than, later, the filesystem is mounted *without*
> specifying r/o mode?
The mount system call will notice that the device is read-only, and will make
assume the read-only mount option.
>
> The filesystem might try to write to the device,
> but it will not work, of course.
> Does this will cause a BUG() in md?
Only if there are bugs.
>
> Because using a r/o memory card, there are no
> problems, the mount recognize the device is r/o
> and it sets the filesystems r/o too.
> I do not know if it anyway tries to write to the
> r/o device, ignoring the errors reported.
>
> I would say that, if md BUG() in any case, it
> could be a bug... :-)
>
> Any comments?
>
> Thanks,
>
> bye,
>
NeilBrown
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next prev parent reply other threads:[~2012-07-09 1:40 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-05 17:27 md device is read only mode Piergiorgio Sartor
2012-07-09 1:40 ` NeilBrown [this message]
2012-07-09 17:34 ` Piergiorgio Sartor
2012-07-09 21:42 ` NeilBrown
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