linux-ext4 - Re: Ideas on unified real-ro mount option across all filesystems

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Message-ID: <20151217140858.GD13224@ws.net.home>
Date:	Thu, 17 Dec 2015 15:08:58 +0100
From:	Karel Zak <kzak@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Qu Wenruo <quwenruo@...fujitsu.com>,
	fsdevel <linux-fsdevel@...r.kernel.org>,
	btrfs <linux-btrfs@...r.kernel.org>, linux-ext4@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: Ideas on unified real-ro mount option across all filesystems

On Wed, Dec 16, 2015 at 09:15:59PM -0600, Eric Sandeen wrote:
> I have always interpreted it as simply "no user changes to the filesystem,"
> and that is clearly what the vfs does with the flag...

Yep,

> Given that nothing in the documentation implies that the block device itself
> must remain unchanged on a read-only mount, I don't see any problem which
> needs fixing.  MS_RDONLY rejects user IO; that's all.

I agree, it's FS specific business to interpret the 'ro'.

And it's already enough complicated, we have three levels of "read-only":

 - read-only device (blockdev --setro ioctl)
 - read-only filesystem (mount -o ro)
 - read-only VFS node (mount -o remount,ro,bind /src /dst)

and for example in /proc/self/mountinfo we distinguish between FS "ro"
and VFS "ro" flag:

# grep test /proc/self/mountinfo
185 59 8:5 / /mnt/test ro,relatime shared:32 - ext4 /dev/sda5 rw,data=ordered
                       ^^                                     ^^


BTW, util-linux 2.27 mount(8) man page:

   -r, --read-only
       Mount the filesystem read-only.  A synonym is -o ro.

       Note  that,  depending  on the filesystem type, state and
       kernel behavior, the system may still write to the device.  For
       example, ext3 and ext4 will replay the journal if the
       filesystem is dirty.  To prevent this kind of write access, you
       may want to mount an ext3 or ext4 filesystem with the ro,noload
       mount options or set the  block  device itself to read-only
       mode, see the blockdev(8) command.


 (maybe we need to copy this note to "ro" description too and add hint
 about btrfs too :-)

     Karel


-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com
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