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@xxxxxxxxxxx>
> > Acked-by: Oren Laadan <orenl@xxxxxxxxxxx>
> > ---
> > 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
> >
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