Re: [PATCH] fix writing to the filesystem after unmount

[Christian Brauner <brauner@kernel.org>]

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.