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
