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

Eric Sandeen
2015-12-18T02:51:26Z

On 12/17/15 8:01 PM, Christoph Anton Mitterer wrote:
> On Fri, 2015-12-18 at 09:29 +0800, Qu Wenruo wrote:
>> 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.
>>
>> And thanks for the info provided by Karel, it's clear that at least
>> mount(8) itself already has explain on what ro will do and what it
>> won't do.
>
> I wouldn't really agree, here. At least not from the non-developer side
> (and one should hope filesystems and their manpages aren't only made
> for fs-devlopers).
>
> The manpage says:
>> ro Mount the filesystem read-only.
>> rw Mount the filesystem read-write.
>
> IMHO, that leaves absolutely unclear, what this actually means,
> especially given that most end-users will probably consider the
> filesystem and its device being basically "the same".

<lots of words snipped>

Karel pointed out that recent mount(8) says:

> -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.

which should leave nothing to the imagination.

-Eric