linux-kernel - Re: [PATCH 3/3] autofs - fix AT_NO

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Date:   Thu, 23 Nov 2017 13:21:41 +1100
From:   NeilBrown <neilb@...e.com>
To:     Ian Kent <raven@...maw.net>, Al Viro <viro@...IV.linux.org.uk>
Cc:     Colin Walters <walters@...hat.com>, Ondrej Holy <oholy@...hat.com>,
        autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 3/3] autofs - fix AT_NO_AUTOMOUNT not being honored

On Thu, Nov 23 2017, Ian Kent wrote:

>
> Hey Neil, I'm looking at this again because RH QE have complained about
> a regression test failing with a kernel that has this change.
>
> Maybe I'm just dumb but I though a "find <base directory> <options>"
> would, well, just look at the contents below <base directory> but an
> strace shows that it reads and calls fstatat() on "every entry in the
> mount table" regardless of the path.

weird ... I can only get find to look at the mount table if given the
-fstyp option, and even then it doesn't fstatat anything that isn't in
the tree it is searching.


>
> And with the move of userspace to use /proc based mount tables (one
> example being the symlink of /etc/mtab into /proc) even modest sized
> direct mount maps will be a problem with every entry getting mounted.

But the patch in question is only about indirect mount maps, isn't it?
How is it relevant to direct mount maps?

>
> Systems will cope with this fine but larger systems not so much.
>
> If find does this then the user space changes needed to accommodate
> this sort of change are almost certainly far more than I expected.
>
> I think this is an example of the larger problem I'm faced with and
> this change was was meant to be a starting point for resolution.
>
> The most obvious symptom of the problem is auto-mounts no longer able
> to be expired due to being re-mounted immediately after expire. Another
> symptom is unwanted (by the user) accesses causing unexpected auto-mount
> attempts.
>
> I believe this monitoring of the mount table is what leads to excessive
> CPU consumption I've seen, usually around six processes, under heavy
> mount activity. And following this, when the mount table is large and
> there is "no mount activity" two of the six processes continue to consume
> excessive CPU, until the mount table shrinks.
>
> So now I'm coming around to the idea of reverting this change ..... and
> going back to the drawing board.

I can well imaging that a large mount table could cause problems for
applications that are written to expect one, and I can imagine that
autofs could cause extra issues for such a program as it might change
the mount table more often.  But I haven't yet worked out how this is
related to the patch in question....

Thanks,
NeilBrown

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