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

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Message-ID: <20170524173939.GJ24798@htj.duckdns.org>
Date:   Wed, 24 May 2017 13:39:39 -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 10:38:22AM -0700, Christoph Hellwig wrote:
> On Tue, May 23, 2017 at 02:57:11PM -0400, Tejun Heo wrote:
> > 
> > Yeah, this is one of the repeatedly requested features - a cgroup id
> > which can be looked up in a scalable way.  We probably should have
> > added this earlier too as we already have places where we're passing
> > in full cgroup path into the kernel (ipt_cgroup match).
> 
> I still have no idea how this ties into export ops.  Currently I don't
> see any tracing code using export operations, so maybe a big part of
> the series is missing/

Oh yeah, it's not adding any users yet.

> > IIUC, Shaohua is adding it so that block tracing can be made aware of
> > cgroups but something like this is necessary whenever we try to refer
> > to a cgroup in a race-free / scalable way.
> 
> You can already get i_ino using statx, and i_generation using
> FS_IOC_GETVERSION, so I'm a little lost on why you need the full
> exportfs infrastructure.

But how do you map that back to the cgroup without scanning the cgroup
hierarchy?

Thanks.

-- 
tejun