[PATCH v2 23/35] ubifs: set/clear MS_RDONLY properly in ubifs_remount

[Dongsheng Yang]

On 08/24/2015 03:02 PM, Artem Bityutskiy wrote:
> On Mon, 2015-08-24 at 09:29 +0800, Dongsheng Yang wrote:
>> On 08/10/2015 10:46 AM, Dongsheng Yang wrote:
>>> On 08/09/2015 05:17 AM, Richard Weinberger wrote:
>>>> Am 30.07.2015 um 07:48 schrieb Dongsheng Yang:
>>>>> We need to set or clear MS_RDONLY in remounting.
>>>>
>>>> Care to explain why?
>>>> Does the quota subsystem need that information?
>>>> If so, where?
>>>
>>> Ha, yes, you are right.
>>>
>>> When we remount to rw from ro, we need to call dquot_resume.
>>> And dquot_resume will call vfs_load_quota_inode() which
>>> will check the MS_RDONLY. If it's ro mode, will return
>>> an -EROFS.
>>>
>>> And I know that we are using ->ro_mount in ubifs
>>> to store the mounted mode. So we don't care about
>>> the MS_RDONLY in ubifs. But why not to tell vfs we are in
>>> ro or rw? Does it cause any problem?
>>
>> Hi Artem,
>> 	I found there is a commit 2ef13294d to introduce
>> ro_mount to ubifs. Yes, I see the reason to use ro_mount
>> rather than MS_RDONLY in ubifs. But why we do not set
>> MS_RDONLY to tell vfs it's in rdonly or not? Current
>> quota depends on it. Does this patch cause some problem?
>
> MS_RDONLY is set at mount time, then it can be changed when we re
> -mount, and then UBIFS can change it when there was a critical error,
> to ask VFS to stop writing more data to the file-system. See
> 'ubifs_ro_mode()'.
>
> The custom ro_mount is the same as MS_RDONLY, but it does not change in
> 'ubifs_ro_mode()'. It preserves the last mount state before the error
> happened - was it R/O or R/W.
>
> Why is this needed?
>
> UBIFS allocate different amount of resources depending on whether we
> are R/O or R/W. We use less memory in R/O mode, for example.
>
> At unmount time, we need to know whether we allocated the "RW" amount
> of resources or the "RO" amount of resources.
>
> We cannot use the MS_RDONLY flag to detect that, because it is changed
> in 'ubifs_ro_mode()' unconditionally, so we lose the information.
> Therefore we use the custom ro_mount flag - it tells us how we were
> mounted, so we know how much resources to free.

Great explanation!!! This makes it much more clear to me.

But about:
 > MS_RDONLY is set at mount time, then it can be changed when we re

Currently, ubifs did not set it at mount time and ubifs did not change
it in re-mounting. So vfs don't know the R/O or R/W mode of ubifs.

That's the reason I sent this patch to set the MS_RDONLY in ubifs
properly. I was concerned that will it cause some problem. But now, from
you explanation, this patch is a correct fix I think.

Thanx
Yang
>
> Artem.
> .
>