linux-kernel - Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

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Date:   Tue, 27 Sep 2016 08:14:27 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Ian Kent <raven@...maw.net>
Cc:     Mateusz Guzik <mguzik@...hat.com>, NeilBrown <neilb@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Omar Sandoval <osandov@...ndov.com>
Subject: Re: [PATCH 3/4] autofs - make mountpoint checks namespace aware

Ian Kent <raven@...maw.net> writes:

> On Mon, 2016-09-26 at 11:05 -0500, Eric W. Biederman wrote:
>> Ian Kent <raven@...maw.net> writes:
>> 
>> > On Fri, 2016-09-23 at 14:15 -0500, Eric W. Biederman wrote:
>> > > Ian Kent <raven@...maw.net> writes:
>> > > 
>> > > 2> On Thu, 2016-09-22 at 20:37 -0500, Eric W. Biederman wrote:
>> > > > > Ian Kent <raven@...maw.net> writes:
>> > > > > 
>> > > > > > On Thu, 2016-09-22 at 10:43 -0500, Eric W. Biederman wrote:
>> > > > > > > Ian Kent <raven@...maw.net> writes:
>> > > > > > > 
>> > > > > > > > Eric, Mateusz, I appreciate your spending time on this and
>> > > > > > > > particularly
>> > > > > > > > pointing
>> > > > > > > > out my embarrassingly stupid is_local_mountpoint() usage
>> > > > > > > > mistake.
>> > > > > > > > 
>> > > > > > > > Please accept my apology for the inconvenience.
>> > > > > > > > 
>> > > > > > > > If all goes well (in testing) I'll have follow up patches to
>> > > > > > > > correct
>> > > > > > > > this
>> > > > > > > > fairly
>> > > > > > > > soon.
>> > > > > > > 
>> > > > > > > Related question.  Do you happen to know how many mounts per mount
>> > > > > > > namespace tend to be used?  It looks like it is going to be wise
>> > > > > > > to
>> > > > > > > put
>> > > > > > > a configurable limit on that number.  And I would like the default
>> > > > > > > to
>> > > > > > > be
>> > > > > > > something high enough most people don't care.  I believe autofs is
>> > > > > > > likely where people tend to use the most mounts.
>> > > > 
>> > > > Yes, I agree, I did want to try and avoid changing the parameters to
>> > > > ->d_mamange() but passing a struct path pointer might be better in the
>> > > > long
>> > > > run
>> > > > anyway.
>> > > 
>> > > Given that there is exactly one implementation of d_manage in the tree I
>> > > don't imagine it will be disruptive to change that.
>> > 
>> > Yes, but it could be used by external modules.
>> > 
>> > And there's also have_submounts().
>> 
>> Good point about have_submounts.
>> 
>> > I can update that using the existing d_walk() infrastructure or take it
>> > (mostly)
>> > into the autofs module and get rid of have_submounts().
>> > 
>> > I'll go with the former to start with and see what people think.
>> 
>> That will be interesting to so.  It is not clear to me that if d_walk
>> needs to be updated, and if d_walk doesn't need to be updated I would
>> be surprised to see it take into autofs.  But I am happy to look at the
>> end result and see what you come up with.
>
> I didn't mean d_walk() itself, just the have_submounts() function that's used
> only by autofs these days. That's all I'll be changing.
>
> To take this functionality into the autofs module shouldn't be a big deal as it
> amounts to a directory traversal with a check at each node.
>
> But I vaguely remember talk of wanting to get rid of have_submounts() and autofs
> being the only remaining user.
>
> So I mentioned it to try and elicit a comment, ;)

>From my perspective the key detail is that d_walk is private to dcache.c

That said you want to look at may_umount_tree, or may_umount that
are already exported from fs/namespace.c, and already used by autofs.
One of those may already do the job you are trying to do.

It looks like at least may_umount needs to be examined to see if it does
the right thing for the tweaked semantics of autofs.

Eric