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

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Date:   Wed, 6 Sep 2023 17:33:27 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Zdenek Kabelac <zkabelac@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] fix writing to the filesystem after unmount

> 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.