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Message-ID: <60f244be-803b-fa70-665e-b5cba15212e@redhat.com>
Date: Wed, 6 Sep 2023 18:01:06 +0200 (CEST)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Christian Brauner <brauner@...nel.org>
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
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
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