UBIFS is not remounted read-only from within do_emergency_remount

UBIFS is not remounted read-only from within do_emergency_remount

[Alexander Stein]

Hello,

upon testing do_emergency_remount from a power fail interrupt (using the 
workqueue of course), we noticed UBIFS is not remounted read-only afterwards.
The current code in do_emergency_remount checks if the kernel actually needs 
to remount a filesystem using the following code:
> if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&
>     !(sb->s_flags & MS_RDONLY)) {
>         /*
>          * What lock protects sb->s_flags??
>          */
>         do_remount_sb(sb, MS_RDONLY, NULL, 1);
> }

I'm not in the details of this part of the kernel, but I suspect that sb-
>s_bdev is NULL for UBIFS as it has no block device, but the character device 
/dev/ubiX_Y instead. I have the information from a collegue that removing the 
check for testing purposes for sb->s_bdev mounts UBIFS read-only.
Any comments/ideas how to remount UBIFS as read-only from 
do_emergency_remount?

Best regards,
Alexander

Re: UBIFS is not remounted read-only from within do_emergency_remount

[Artem Bityutskiy]

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On Tue, 2012-07-31 at 13:24 +0200, Alexander Stein wrote:
> upon testing do_emergency_remount from a power fail interrupt (using the 
> workqueue of course), we noticed UBIFS is not remounted read-only afterwards.
> The current code in do_emergency_remount checks if the kernel actually needs 
> to remount a filesystem using the following code:
> > if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&
> >     !(sb->s_flags & MS_RDONLY)) {
> >         /*
> >          * What lock protects sb->s_flags??
> >          */
> >         do_remount_sb(sb, MS_RDONLY, NULL, 1);
> > }
> 
> I'm not in the details of this part of the kernel, but I suspect that sb-
> >s_bdev is NULL for UBIFS as it has no block device, but the character device 
> /dev/ubiX_Y instead. I have the information from a collegue that removing the 
> check for testing purposes for sb->s_bdev mounts UBIFS read-only.
> Any comments/ideas how to remount UBIFS as read-only from 
> do_emergency_remount?

I think this check is there to prevent file-systems like sysfs, tmpfs,
procfs, cgroup and debugfs from re-mounting R/O. If we just remove the
'sb->s_bdev' check - they all become R/O as well. For me it sounds OK,
but I've never used this functionality, so not sure.


-- 
Best Regards,
Artem Bityutskiy

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Re: UBIFS is not remounted read-only from within do_emergency_remount

[Alexander Stein]

Hello,

On Friday 24 August 2012 10:22:50, Artem Bityutskiy wrote:
> On Tue, 2012-07-31 at 13:24 +0200, Alexander Stein wrote:
> > upon testing do_emergency_remount from a power fail interrupt (using the 
> > workqueue of course), we noticed UBIFS is not remounted read-only afterwards.
> > The current code in do_emergency_remount checks if the kernel actually needs 
> > to remount a filesystem using the following code:
> > > if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&
> > >     !(sb->s_flags & MS_RDONLY)) {
> > >         /*
> > >          * What lock protects sb->s_flags??
> > >          */
> > >         do_remount_sb(sb, MS_RDONLY, NULL, 1);
> > > }
> > 
> > I'm not in the details of this part of the kernel, but I suspect that sb-
> > >s_bdev is NULL for UBIFS as it has no block device, but the character device 
> > /dev/ubiX_Y instead. I have the information from a collegue that removing the 
> > check for testing purposes for sb->s_bdev mounts UBIFS read-only.
> > Any comments/ideas how to remount UBIFS as read-only from 
> > do_emergency_remount?
> 
> I think this check is there to prevent file-systems like sysfs, tmpfs,
> procfs, cgroup and debugfs from re-mounting R/O. If we just remove the
> 'sb->s_bdev' check - they all become R/O as well. For me it sounds OK,
> but I've never used this functionality, so not sure.

I agree this check filters out file system without a backing storage (block device), which is fine. It just doesn't make a difference if those are remounted R/O.
I'm just wondering how to make the emergeny_remount usable with ubifs which has a backing storage, obviously, but no block device.

Regards,
Alexander