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Subject: Re: Removing Mandatory Locks
From:   Jeff Layton <jlayton@kernel.org>
To:     Amir Goldstein <amir73il@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Matthew Wilcox <willy@infradead.org>,
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Date:   Fri, 20 Aug 2021 08:27:12 -0400
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On Fri, 2021-08-20 at 10:14 +0300, Amir Goldstein wrote:
> On Fri, Aug 20, 2021 at 9:36 AM Amir Goldstein <amir73il@gmail.com> wrote:
> > 
> > On Thu, Aug 19, 2021 at 11:32 PM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > > 
> > > On Thu, Aug 19, 2021 at 1:18 PM Jeff Layton <jlayton@kernel.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..."

That is a possibility, but I'm not sure it's any better than just
failing the mount. We could also just keep the code around and throw a
big, scary warning about its impending removal for a few releases before
ripping it out completely (like Willy T. was suggesting).

I'm fine with any of these approaches if the consensus is that it's too
risky to just remove it. OTOH, I've yet to ever hear of any application
that uses this feature, even in a historical sense. You have to jump
through so many hoops that nothing can rely on having it available.
-- 
Jeff Layton <jlayton@kernel.org>