Hi Ted, Did you see my query about emergency remount of loop mounted ext4? Do you have any insights to share? Thanks, Amir. On Fri, Apr 24, 2015 at 10:54 AM, Amir Goldstein <amir@xxxxxxxxxxx> wrote: > Hi Al, > > This patch is very naive. The fact that it tries to change a behavior > that pre-dates modern git history suggests that I may be missing something. > > I am quite sure that the change of behavior is beneficial to my use case > which involves loop mounts on Android, hence uploaded for Android: > https://android-review.googlesource.com/148047 > > I am not sure about the implications for other use cases, hence this RFC. > Is there a reason to start remount from root fs, because later remounts > may hang or take too long before some watchdog kicks in? > > Please enlighten me. > Thanks, > > Amir. > > ---------- Forwarded message ---------- > > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
