On Thu, Apr 30, 2015 at 6:31 PM, Theodore Ts'o [off-list ref] wrote:

On Wed, Apr 29, 2015 at 09:33:45PM +0300, Amir Goldstein wrote:

quoted

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?

Al,
Any comment?
Anybody else who dares to touch this code?
Anybody who benefits from this change?
Anybody who might be harmed from it?

                                        - Ted

quoted

quoted

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 <redacted>
Acked-by: Oren Laadan <redacted>
---
 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