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

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 6 Sep 2023, Christian Brauner wrote:

> > > 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

"umount /opt" doesn't hang. It waits one second (until dmsetup resume is 
called) and then proceeds.

Then, it fails with "rmmod: ERROR: Module brd is in use" because the 
script didn't unmount /mnt/test.

> > 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 lvm may freeze filesystems anytime - so we'd get randomly returned 
errors then.

> 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.

The kernel currently distinguishes between kernel-initiated freeze (that 
is used by the XFS scrub) and userspace-initiated freeze (that is used by 
the FIFREEZE ioctl and by device-mapper initiated freeze through 
freeze_bdev).

Perhaps we could distinguish between FIFREEZE-initiated freezes and 
device-mapper initiated freezes as well. And we could change the logic to 
return -EBUSY if the freeze was initiated by FIFREEZE and to wait for 
unfreeze if it was initiated by the device-mapper.

Mikulas