On 2015-12-02 11:54, Eric Sandeen wrote:
> On 12/2/15 3:23 AM, Qu Wenruo wrote:
>>
>>
>> Qu Wenruo wrote on 2015/12/02 17:06 +0800:
>>>
>>>
>>> Russell Coker wrote on 2015/12/02 17:25 +1100:
>>>> On Wed, 2 Dec 2015 06:05:09 AM Eric Sandeen wrote:
>>>>> yes, xfs does; we have "-o norecovery" if you don't want that, or need
>>>>> to mount a filesystem with a dirty log on a readonly device.
>>>>
>>>> That option also works with Ext3/4 so it seems to be a standard way of
>>>> dealing
>>>> with this.  I think that BTRFS should do what Ext3/4 and XFS do in this
>>>> regard.
>>>>
>>> BTW, does -o norecovery implies -o ro?
>>>
>>> If not, how does it keep the filesystem consistent?
>>>
>>> I'd like to follow that ext2/xfs behavior, but I'm not familiar with
>>> those filesystems.
>>>
>>> Thanks,
>>> Qu
>>>
>>
>> OK, norecovery implies ro.
>
> For XFS, it doesn't imply it, it requires it; i.e. both must be stated explicitly:
>
>          /*
>           * no recovery flag requires a read-only mount
>           */
>          if ((mp->m_flags & XFS_MOUNT_NORECOVERY) &&
>              !(mp->m_flags & XFS_MOUNT_RDONLY)) {
>                  xfs_warn(mp, "no-recovery mounts must be read-only.");
>                  return -EINVAL;
>          }
>
> ext4 is the same, I believe:
>
>          } else if (test_opt(sb, NOLOAD) && !(sb->s_flags & MS_RDONLY) &&
>                     ext4_has_feature_journal_needs_recovery(sb)) {
>                  ext4_msg(sb, KERN_ERR, "required journal recovery "
>                         "suppressed and not mounted read-only");
>                  goto failed_mount_wq;
>
> so if you'd like btrfs to be consistent with these, I would not make
> norecovery imply ro; rather, make I would make it require an explicit ro, i.e.
>
> mount -o ro,norecovery
Agreed, with something like that, it should as blatantly obvious as 
possible that you can't write to the FS.

On a side note, do either XFS or ext4 support removing the norecovery 
option from the mount flags through mount -o remount?  Even if they 
don't, that might be a nice feature to have in BTRFS if we can safely 
support it.