On Wed, Sep 06, 2023 at 05:33:32PM +0200, Christian Brauner wrote: > > Currently, if we freeze a filesystem with "fsfreeze" and unmount it, the > > mount point is removed, but the filesystem stays active and it is leaked. > > You can't unfreeze it with "fsfreeze --unfreeze" because the mount point > > is gone. (the only way how to recover it is "echo j>/proc/sysrq-trigger"). > > You can of course always remount and unfreeze it. > > > > IOW, you'd also hang on any umount of a bind-mount. IOW, every > > > single container making use of this filesystems via bind-mounts would > > > hang on umount and shutdown. > > > > bind-mount doesn't modify "s->s_writers.frozen", so the patch does nothing > > in this case. I tried unmounting bind-mounts and there was no deadlock. > > With your patch what happens if you do the following? > > #!/bin/sh -ex > modprobe brd rd_size=4194304 > vgcreate vg /dev/ram0 > lvcreate -L 16M -n lv vg > mkfs.ext4 /dev/vg/lv > > mount -t ext4 /dev/vg/lv /mnt/test > mount --bind /mnt/test /opt > mount --make-private /opt > > dmsetup suspend /dev/vg/lv > (sleep 1; dmsetup resume /dev/vg/lv) & > > umount /opt # I'd expect this to hang > > md5sum /dev/vg/lv > md5sum /dev/vg/lv > dmsetup remove_all > rmmod brd > > > BTW. what do you think that unmount of a frozen filesystem should properly > > do? Fail with -EBUSY? Or, unfreeze the filesystem and unmount it? Or > > something else? > > In my opinion we should refuse to unmount frozen filesystems and log an > error that the filesystem is frozen. Waiting forever isn't a good idea > in my opinion. > > But this is a significant uapi change afaict so this would need to be > hidden behind a config option, a sysctl, or it would have to be a new > flag to umount2() MNT_UNFROZEN which would allow an administrator to use > this flag to not unmount a frozen filesystems. That's probably too careful. I think we could risk starting to return an error when trying to unmount a frozen filesystem. And if that causes regressions we could go and look at another option like MNT_UNFROZEN or whatever.
