Date:   Fri, 20 Aug 2021 10:14:46 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jeff Layton <jlayton@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Matthew Wilcox <willy@...radead.org>,
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Subject: Re: Removing Mandatory Locks

On Fri, Aug 20, 2021 at 9:36 AM Amir Goldstein <amir73il@...il.com> wrote:
>
> On Thu, Aug 19, 2021 at 11:32 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Thu, Aug 19, 2021 at 1:18 PM Jeff Layton <jlayton@...nel.org> wrote:
> > >
> > > Now that I think about it a little more, I actually did get one
> > > complaint a few years ago:
> > >
> > > Someone had upgraded from an earlier distro that supported the -o mand
> > > mount option to a later one that had disabled it, and they had an (old)
> > > fstab entry that specified it.
> >
> > Hmm. We might be able to turn the "return -EINVAL" into just a warning.
> >
> > Yes, yes, currently if you turn off CONFIG_MANDATORY_FILE_LOCKING, we
> > already do that
> >
> >         VFS: "mand" mount option not supported
> >
> > warning print, but then we fail the mount.
> >
> > If CONFIG_MANDATORY_FILE_LOCKING goes away entirely, it might make
> > sense to turn that warning into something bigger, but then let the
> > mount continue - since now that "mand" flag would be purely a legacy
> > thing.
> >
> > And yes, if we do that, we'd want the warning to be a big ugly thing,
> > just to make people very aware of it happening. Right now it's a
> > one-liner that is easy to miss, and the "oh, the mount failed" is the
> > thing that hopefully informs people about the fact that they need to
> > enable CONFIG_MANDATORY_FILE_LOCKING.
> >
> > The logic being that if you can no longer enable mandatory locking in
> > the kernel, the current hard failure seems overly aggressive (and
> > might cause boot failures and inability to fix/report things when it
> > possibly keeps you from using the system at all).
> >
>
> Allow me to play the devil's advocate here - if fstab has '-o mand' we have
> no way of knowing if any application is relying on '-o mand' and adding
> more !!!!! to the warning is mostly good for clearing our conscious ;-)
>
> Not saying we cannot resort to that and not saying there is an easy
> solution, but there is one more solution to consider - force rdonly mount.
> Yes, it could break some systems and possibly fail boot, but then again
> an ext4 fs can already become rdonly due to errors, so it wouldn't
> be the first time that sysadmins/users run into this behavior.
>

Adding an anecdote - this week I got a report from field support
engineers about failure to assemble a RAID0 array, which led to this
warning that *requires* user intervention, in the worse case for boot
device it requires changing kernel boot params:

md/raid0:%s: cannot assemble multi-zone RAID0 with default_layout setting
md/raid0: please set raid.default_layout to 1 or 2

c84a1372df92 md/raid0: avoid RAID0 data corruption due to layout confusion.

There is no way I would have gotten this report from the field if a failure
was not involved...

The rdonly mount is only needed to get the attention of support people
to look the the kernel logs and find the warning - at this point, not too
many !!!!! are needed ;-)

So we could make 'mand' an alias to 'ro' and print a warning that says:
"'mand' mount option is deprecated, please fix your init scripts.
For caution, your filesystem was mounted rdonly, feel free to remount
rw and move on..."

Thanks,
Amir.

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