[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20230906-aufheben-hagel-9925501b7822@brauner>
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.
Powered by blists - more mailing lists