linux-kernel - Re: [PATCH 5/5] kernfs: add exportfs operations

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Message-ID: <20170524174604.GK24798@htj.duckdns.org>
Date:   Wed, 24 May 2017 13:46:04 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Shaohua Li <shli@...com>, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        Kernel-team@...com
Subject: Re: [PATCH 5/5] kernfs: add exportfs operations

Hello, Christoph.

On Wed, May 24, 2017 at 10:41:38AM -0700, Christoph Hellwig wrote:
> > But how do you map that back to the cgroup without scanning the cgroup
> > hierarchy?
> 
> I'm totally lost on why you would do that.  So maybe you just need
> to send the full patch so that reviewers get the full picture.

Here's a simple scenario.  Let's say blktrace now exposes the cgroup
inode and generation numbers per trace.  Userland tool now wants to
show that in a human readable format but it can only map back the
inode and generation numbers to the path by scanning the cgroup tree.
So, the goal is having a token which is not path which uniquely
identifies a cgroup and the ability to map that back to cgroup path.
We can add a dedicated interface to cgroup root, for example, and
allow querying by echoing inode and generation numbers into it but
that's kinda clumsy.

Thanks.

-- 
tejun