[dm-devel] [PATCH] fix writing to the filesystem after unmount

[Mikulas Patocka]

On Wed, 6 Sep 2023, Christian Brauner wrote:

> On Wed, Sep 06, 2023 at 06:01:06PM +0200, Mikulas Patocka wrote:
> > 
> > 
> > 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.
> 
> So unless I'm really misreading the code - entirely possible - the
> umount of the bind-mount now waits until the filesystem is resumed with
> your patch. And if that's the case that's a bug.

Yes.

It can be fixed by changing wait_and_deactivate_super to this:

void wait_and_deactivate_super(struct super_block *s)
{
	down_write(&s->s_umount);
	while (s->s_writers.frozen != SB_UNFROZEN && atomic_read(&s->s_active) == 2) {
		up_write(&s->s_umount);
		msleep(1);
		down_write(&s->s_umount);
	}
	deactivate_locked_super(s);
}

> > > > 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.
> 
> So? Or you might hang at anytime.

lvm doesn't keep logical volumes suspended for a prolonged amount of time. 
It will unfreeze them after it made updates to the dm table and to the 
metadata. So, it won't hang forever.

I think it's better to sleep for a short time in umount than to return an 
error.

Mikulas