On 12/17/15 8:01 PM, Christoph Anton Mitterer wrote:

On Fri, 2015-12-18 at 09:29 +0800, Qu Wenruo wrote:

quoted

Given that nothing in the documentation implies that the block

quoted

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:

quoted

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

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