Re: [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order

[Theodore Ts'o <tytso@mit.edu>]

On Wed, Apr 29, 2015 at 09:33:45PM +0300, Amir Goldstein wrote:
> Hi Ted,
> 
> Did you see my query about emergency remount of loop mounted ext4?
> Do you have any insights to share?

Your patch makes sense to me.  Doing the remount-ro in reverse order
that file systems were mounted sounds like the right thing to do.

I can imagine trying to make ext4 handle the situation where it is
trying to do an unmount, or remount r/o, and the device has gone
read-only in a similar situation as one where the device has
disappeared altogether, but it's not going to solve all possible
issues.  Trying to unmount or remount r/o the most recently mounted
file system first is simpler and will solve more problems.

Al?

					- Ted

> >
> > This change fixes a problem where reboot on Android panics the kernel
> > almost every time when file systems are mounted over loop devices.
> >
> > Android reboot command does:
> > - sync
> > - echo u > /proc/sysrq-trigger
> > - syscall_reboot
> >
> > The problem is with sysrq emergency remount R/O trying to remount-ro
> > in wrong order.
> > since /data is re-mounted ro before loop devices, loop device
> > remount-ro fails to flush the journal and panics the kernel:
> >
> >   EXT4-fs (loop0): Remounting filesystem read-only
> >   EXT4-fs (loop0): previous I/O error to superblock detected
> >   loop: Write error at byte offset 0, length 4096.
> >   Buffer I/O error on device loop0, logical block 0
> >   lost page write due to I/O error on loop0
> >   Kernel panic - not syncing: EXT4-fs panic from previous error
> >
> > The fix is quite simple. In do_emergency_remount(), use
> > list_for_each_entry_reverse() on sb list instead of list_for_each_entry().
> > It makes a lot of sense to umount the file systems in reverse order in
> > which they were added to sb list.
> >
> > Signed-off-by: Amir Goldstein <amir@cellrox.com>
> > Acked-by: Oren Laadan <orenl@cellrox.com>
> > ---
> >  fs/super.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/super.c b/fs/super.c
> > index 2b7dc90..f1315e0 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -774,7 +774,7 @@ static void do_emergency_remount(struct work_struct
> > *work)
> >         struct super_block *sb, *p = NULL;
> >
> >         spin_lock(&sb_lock);
> > -       list_for_each_entry(sb, &super_blocks, s_list) {
> > +       list_for_each_entry_reverse(sb, &super_blocks, s_list) {
> >                 if (hlist_unhashed(&sb->s_instances))
> >                         continue;
> >                 sb->s_count++;
> > --
> > 1.8.2
> >