linux-kernel - Re: [PATCH 3/3] autofs: add ignore mount option

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Message-ID: <804b86df6fd678147da816be038b11c87333b23f.camel@themaw.net>
Date:   Wed, 30 Jan 2019 10:44:15 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     autofs mailing list <autofs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] autofs: add ignore mount option

On Wed, 2019-01-30 at 10:07 +0800, Ian Kent wrote:
> On Tue, 2019-01-29 at 17:16 -0800, Andrew Morton wrote:
> > On Sat, 12 Jan 2019 08:00:40 +0800 Ian Kent <raven@...maw.net> wrote:
> > 
> > > Add an autofs file system mount option that can be used to provide
> > > a generic indicator to applications that the mount entry should be
> > > ignored when displaying mount information.
> > 
> > What is the reason for adding this feature?
> 
> In other OSes that provide autofs and that provide a mount list
> to user space based on the kernel mount list a no-op mount option
> ("ignore" is the one use on the most common OS) is allowed so that
> autofs file system users can optionally use it.
> 
> The idea is that it be used by user space programs to exclude
> autofs mounts from consideration when reading the mounts list.
> 
> Prior to the change to link /etc/mtab to /proc/self/mounts all
> I needed to do to achieve this was to use mount(2) and not update
> the mtab but now that no longer works.
> 
> I know the symlinking happened a long time ago and I considered
> doing this then but, at the time I couldn't remember the commonly
> used option name and thought persuading the various utility
> maintainers would be too hard.
> 
> But now I have a RHEL request to do this for compatibility for a
> widely used product so I want to go ahead with it and try and
> enlist the help of some utility package maintainers.

Al,

On a different note the above request also raised another
question about statvfs(3) automount behaviour.

In glibc statvfs(3) uses statfs(2) and translates the return
to a statvfs structure.

I wasn't aware but apparently statvfs(3) (and presumably statfs(2))
doesn't trigger an automount on Solaris whereas we do. I think
statfs() is probably the only exception to the convention that
stat family system calls don't trigger an automount.

So far I've said that this is a long standing behaviour in the
Linux kernel and changing it could lead to unpleasant surprises
for those that have come to expect this behaviour so such a change
would not be well received.

But I do need to ask your opinion, so what are your thoughts about
changing this?

Ian