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Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Tejun Heo <tj@kernel.org>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Fri, 26 Jun 2020 08:19:50 +0800
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On Thu, 2020-06-25 at 11:43 +0200, Greg Kroah-Hartman wrote:
> On Thu, Jun 25, 2020 at 04:15:19PM +0800, Ian Kent wrote:
> > On Tue, 2020-06-23 at 19:13 -0400, Tejun Heo wrote:
> > > Hello, Rick.
> > > 
> > > On Mon, Jun 22, 2020 at 02:22:34PM -0700, Rick Lindsley wrote:
> > > > > I don't know. The above highlights the absurdity of the
> > > > > approach
> > > > > itself to
> > > > > me. You seem to be aware of it too in writing: 250,000
> > > > > "devices".
> > > > 
> > > > Just because it is absurd doesn't mean it wasn't built that way
> > > > :)
> > > > 
> > > > I agree, and I'm trying to influence the next hardware design.
> > > > However,
> > > 
> > > I'm not saying that the hardware should not segment things into
> > > however many
> > > pieces that it wants / needs to. That part is fine.
> > > 
> > > > what's already out there is memory units that must be accessed
> > > > in
> > > > 256MB
> > > > blocks. If you want to remove/add a GB, that's really 4 blocks
> > > > of
> > > > memory
> > > > you're manipulating, to the hardware. Those blocks have to be
> > > > registered
> > > > and recognized by the kernel for that to work.
> > > 
> > > The problem is fitting that into an interface which wholly
> > > doesn't
> > > fit that
> > > particular requirement. It's not that difficult to imagine
> > > different
> > > ways to
> > > represent however many memory slots, right? It'd take work to
> > > make
> > > sure that
> > > integrates well with whatever tooling or use cases but once done
> > > this
> > > particular problem will be resolved permanently and the whole
> > > thing
> > > will
> > > look a lot less silly. Wouldn't that be better?
> > 
> > Well, no, I am finding it difficult to imagine different ways to
> > represent this but perhaps that's because I'm blinker eyed on what
> > a solution might look like because of my file system focus.
> > 
> > Can "anyone" throw out some ideas with a little more detail than we
> > have had so far so we can maybe start to formulate an actual plan
> > of
> > what needs to be done.
> 
> I think both Tejun and I have provided a number of alternatives for
> you
> all to look into, and yet you all keep saying that those are
> impossible
> for some unknown reason.

Yes, those comments are a starting point to be sure.
And continuing on that path isn't helping anyone.

That's why I'm asking for your input on what a solution you
would see as adequate might look like to you (and Tejun).

> 
> It's not up to me to tell you what to do to fix your broken
> interfaces
> as only you all know who is using this and how to handle those
> changes.

But it would be useful to go into a little more detail, based on
your own experience, about what you think a suitable solution might
be.

That surely needs to be taken into account and used to guide the
direction of our investigation of what we do.

> 
> It is up to me to say "don't do that!" and to refuse patches that
> don't
> solve the root problem here.  I'll review these later on (I have
> 1500+
> patches to review at the moment) as these are a nice
> micro-optimization...

Sure, and I get the "I don't want another post and run set of
patches that I have to maintain forever that don't fully solve
the problem" view and any ideas and perhaps a little more detail
on where we might go with this would be very much appreciated.

> 
> And as this conversation seems to just going in circles, I think this
> is
> going to be my last response to it...

Which is why I'm asking this, I really would like to see this
discussion change course and become useful.

Ian