linux-ext4 - Re: [PATCH] vfs: freeze filesystems just prior to reboot

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Message-Id: <1495211669.1931975.982263184.0F641F32@webmail.messagingengine.com>
Date:   Fri, 19 May 2017 12:34:29 -0400
From:   Colin Walters <walters@...bum.org>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        xfs <linux-xfs@...r.kernel.org>,
        "linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "linux-ext4" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] vfs: freeze filesystems just prior to reboot

On Fri, May 19, 2017, at 11:27 AM, Theodore Ts'o wrote:
> 
> One of the things that came up when Darrick and I discussed this on
> the weekly ext4 developer's conference call was our mutual wonderment
> that none of the userspace tools implemented a reboot by created a
> tmpfs chroot, pivoting into the chroot, and then unmounting all of the
> remaining file systems.

On general purpose systems we have a tmpfs chroot already: the initramfs.
Although IIRC, systemd will only switch back to it on shutdown I think only
if you have a root storage daemon enabled:
https://www.freedesktop.org/wiki/Software/systemd/RootStorageDaemons/

That said I'd like to focus on the harder case: supporting powerloss/system lockup on
single-partition systems.  IMO, the shutdown case is just a special variant
of that where the user asked nicely for the system to halt =)
(See also https://en.wikipedia.org/wiki/Crash-only_software)

I was thinking about this a bit, and I think if userspace tools (like ostree)
*delayed* their updates to /boot until shutdown, then we could ensure
that on powerloss, the system is unchanged.  (In a traditional dpkg/rpm
scenario where you only have one userspace root, you'd end up with
old kernel + new rootfs, but that's exactly the problem ostree solves)

That narrows the problem down to keeping `/boot` consistent at
shutdown time.  AIUI, a problem here is that XFS doesn't flush the
journal on `syncfs`, only on unmount?  And from what I can tell,
even the `XFS_IOC_FREEZE` ioctl won't do that either.

So as far as I can see, a userspace API to ensure the journal is
flushed on a mounted filesystem is going to be necessary for
the general case.  I don't have a strong opinion on whether or not
that's `syncfs()` - if it's e.g. a `XFS_IOC_FREEZE` `_THAW` pair
that seems OK to me too.