linux-kernel - Re: [PATCH 2/7] proc: Implement support for automounts in task directories

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Date:	Fri, 07 Nov 2008 07:51:32 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, adobriyan@...il.com,
	viro@...IV.linux.org.uk, containers@...ts.osdl.org
Subject: Re: [PATCH 2/7] proc: Implement support for automounts in task directories

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Thu, 06 Nov 2008 19:51:23 -0800 ebiederm@...ssion.com (Eric W. Biederman)
> wrote:
>
>> If we could do all of this with reference counting so that the
>> mount would persist exactly until the last user of it has gone
>> away without a periodic poll I would love it.  But the infrastructure
>> doesn't support that today,
>
> Well that sucks.  The free-on-last-put idiom occurs in so many places
> and serves us so well.  I wonder what went wrong here?

> I guess it has interactions with dentry and inode cache aging which
> could get tricky.

At least in part.  If you just have the dentry you can't  easily
find what is mounted on it.  

>> and where this is at least partially
>> a bug fix I would rather not have the change depend on enhancing
>> the VFS.
>> 
>> The algorithm is actually very aggressive and in practice you don't
>> see any /proc/<pid>/net showing up as a mount point.
>
> Do you think it has failure modes?  Most particularly: obscure usage
> patterns which can cause memory exhaustion?

I don't think we can pin anything that way that we can't
pin right now.

You might be able to pin more if you happen to mount something
on top of /proc/<pid>/net/  but that is an unprivileged operation.

>> > Obviously, that becomes clearer as one spends more time with the code,
>> > but I wonder whether this has all been made as maintainble as it
>> > possibly could be.
>> 
>> Good question.
>> 
>> In the sense of will we have to go through and futz with the code all
>> of the time.  The abstraction seems good.   You put a mount on
>> the proc_automounts list with do_add_mounts and it goes away eventually
>> with all of the vfs rules maintained.
>> 
>> In the sense of can the code be read?    Perhaps it could be better.
>> I expect it helps to have run the code and see /proc/net as a filesystem.
>> that is magically mounted.
>
> 'twould be a useful contribution if you were to enshrine your
> discoveries in /*these things*/.  You knew I was working up to that :)

Short of a big fat comment I'm not certain if there is something I can do
better.

Eric

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