On Fri, May 19, 2017 at 11:29:04AM +0300, Amir Goldstein wrote: > On Fri, May 19, 2017 at 3:20 AM, Darrick J. Wong > <darrick.wong@xxxxxxxxxx> wrote: > > > > > Therefore, add a reboot hook to freeze all filesystems (which in general > > will induce ext4/xfs/btrfs to checkpoint the log) just prior to reboot. > > This is an unfortunate and insufficient workaround for multiple layers > > of inadequate external software, but at least it will reduce boot time > > surprises for the "OS updater failed to disengage the filesystem before > > rebooting" case. > > > > Darrick, > > Did you consider how many support calls this will generate for a stuck > reboot command? > > I can think of at least one situation where this is guarantied to hang. > See this patch for the details: > https://patchwork.kernel.org/patch/6266791/ > > The referenced patch was applied to Android kernel to prevent > system crash on emergency remount-ro via sysrq trigger. Hmmm, I agree that we ought to avoid hanging on loopmounted filesystems, and that iterating superblocks backwards is one (rough) way to do that. > I don't know if it was even seriously considered by Al, because > I got no comment, but I do realize that the change of behavior > could generate support calls, so it's scary to make that change > in mainline. > > I know it's not going to work around broken system software update, > but how about providing sysrq trigger for emergency_freeze_all()? > like emergency_remount(), but stronger. > And this time, iterate supers in reverse order like I suggested to > avoid loop mounted fs freeze dependencies. > > There is one little tiny problem though. Eric used up the last sysrq trigger > key for emergency_thaw_all(). Do you see the irony in that? ;) LOL. > I am wondering how many people know about or use the emergency > thaw trigger, but one dodgy option is to use the 't' trigger to toggle > thaw_all/freeze_all. > > Another perhaps slightly less dodgy option is to trigger freeze_all > on a sequence of sysrq "emergency" triggers where it makes sense > and is least likely to change any existing behavior, for example: > > echo u > /proc/sysrq-trigger > > # Remember if do_emergency_remount() completed with failures > > echo u > /proc/sysrq-trigger > > # Escalate to emergency freeze Or maybe it's simpler just to have a counter -- three sysrq-u in a row and we freeze all? > OR > > echo u > /proc/sysrq-trigger > > # Remember if do_emergency_remount() completed with failures > > echo s > /proc/sysrq-trigger > > # Sync *after* remount r/o? That must mean emergency freeze > > I bet that system software that is already aware of and is issuing > emergency remount r/o trigger prior to reboot, won't see any harm > in adding an extra u/s trigger for good luck. > > Do you know if the gnarly system software in question is issuing > emergency remount r/o prior to reboot? It does not. --D > > Amir. > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
