[PATCH] ubifs: silence error output if MS_SILENT is set

[Richard Weinberger]

Hi!

Am 18.06.2016 um 15:42 schrieb Daniel Golle:
> Hi Richard,
> 
> first of all, thanks for reviewing my patch!
> 
> On Sat, Jun 18, 2016 at 02:57:22PM +0200, Richard Weinberger wrote:
>> On Sat, Jun 18, 2016 at 11:52 AM, Daniel Golle <daniel at makrotopia.org> wrote:
>>> Signed-off-by: Daniel Golle <daniel at makrotopia.org>
>>> ---
>>>  fs/ubifs/super.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
>>> index 7034995..ae32b3c 100644
>>> --- a/fs/ubifs/super.c
>>> +++ b/fs/ubifs/super.c
>>> @@ -2108,8 +2108,9 @@ static struct dentry *ubifs_mount(struct file_system_type *fs_type, int flags,
>>>          */
>>>         ubi = open_ubi(name, UBI_READONLY);
>>>         if (IS_ERR(ubi)) {
>>> -               pr_err("UBIFS error (pid: %d): cannot open \"%s\", error %d",
>>> -                      current->pid, name, (int)PTR_ERR(ubi));
>>> +               if (flags & MS_SILENT)
>>> +                       pr_err("UBIFS error (pid: %d): cannot open \"%s\", error %d",
>>> +                              current->pid, name, (int)PTR_ERR(ubi));
>>
>> This needs a more detailed explanation.
> 
> Basically this should be seen in context of
> commit 90bea5a3f0bf680b87b90516f3c231997f4b8f3b
> which already implements support for MS_SILENT except for that one
> error message which is still being displayed despite MS_SILENT being
> set.

This should be part of the commit message since it is a fixup for the
said change.

>> Why does UBIFS need this? Only very few filesystems care about MS_SILENT.
> 
> Afaik all filesystems potentially used as rootfs on Linux do support
> MS_SILENT.
> 
>> What problem are you trying to address?
> 
> On OpenWrt / LEDE we are probing whether the UBI volume set to be
> rootfs contains a squashfs and thus a ubiblock device is being
> created and ROOT_DEV is set accordingly. Similarly, we are also
> probing whether the rootfs volume contains a ubifs (which is supported
> so people can use our kernel and e.g. Debian's userspace or to
> waste less space compared to overlay(lower=squashfs,upper=ubifs) when
> frequently updating or heavily modifying the rootfs during runtime).
> Thus, users get to see that error message in their kernel logs
> [    3.060000] UBIFS error (pid: 1): cannot open "ubi0:rootfs", error -19
> even though every thing is fine (rootfs just happends to not be ubifs)
> and that's misleading as people often blame the first error message
> they see to be the cause for anything not working.

While your fix is fine I think you should do better than just trial&error
probing. e.g. Identify the rootfs type from the volume label.

Thanks,
//richard