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

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Date:   Wed, 29 Nov 2017 13:48:40 +1100
From:   NeilBrown <neilb@...e.com>
To:     Ian Kent <raven@...maw.net>, Mike Marion <mmarion@...lcomm.com>
Cc:     autofs mailing list <autofs@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 3/3] autofs - fix AT_NO_AUTOMOUNT not being honored

On Wed, Nov 29 2017, Ian Kent wrote:

> On 29/11/17 10:13, Mike Marion wrote:
>> On Wed, Nov 29, 2017 at 12:17:27PM +1100, NeilBrown wrote:
>> 
>>> How big do people see /proc/self/mount* getting?  What size reads
>>> does 'strace' show the various programs using to read it?
>> 
>> We already have line counts into 5 figures.  This wasn't an issue until 
>> the change of /etc/mtab to a link.  The large count is due to our large
>> direct automount maps.
>> 

So .... 90,000 lines with a length of may 120 chars or about 10Meg.
Presumably these machines would have so many gigabytes of RAM that
caching a 10M mountinfo file would go unnoticed?

Reading that in 128K chunks without generating bits on the fly will help
a lot I suspect.

We could probably ensure proper alignment by searching backwards for
'\n' when deciding how much to return for a read.

>
> And, admittedly, the testing I was doing was with 15k+ size maps.
>
> Of course it's necessary to have this number of mounts to see serious
> problems which is easiest to do with large direct mount maps.
>
> The thing that's different now is that before applications started
> using /proc directly for mount table information using mount(2)
> instead of mount(8) was enough to prevent the mount entries from
> being added to the table seen by applications.

I wonder who would notice if untriggered direct mounts quietly disappeared from
/proc/mounts...  I suspect systemd would, but there is a good chance it
would fail-safe: assume that the mount worked.
Alternately we could introduce /proc/self/mountinfo2 which doesn't list
direct automounts and encourage problematic programs to use that where
available.

NeilBrown

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