Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement

From: Ian Kent <raven@themaw.net>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Tejun Heo <tj@kernel.org>
Cc: 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>
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement
Date: Tue, 23 Jun 2020 13:09:08 +0800	[thread overview]
Message-ID: <2ead27912e2a852bffb1477e8720bdadb591628d.camel@themaw.net> (raw)
In-Reply-To: <20200622180306.GA1917323@kroah.com>

On Mon, 2020-06-22 at 20:03 +0200, Greg Kroah-Hartman wrote:
> On Mon, Jun 22, 2020 at 01:48:45PM -0400, Tejun Heo wrote:
> > Hello, Ian.
> > 
> > On Sun, Jun 21, 2020 at 12:55:33PM +0800, Ian Kent wrote:
> > > > > They are used for hotplugging and partitioning memory. The
> > > > > size of
> > > > > the
> > > > > segments (and thus the number of them) is dictated by the
> > > > > underlying
> > > > > hardware.
> > > > 
> > > > This sounds so bad. There gotta be a better interface for that,
> > > > right?
> > > 
> > > I'm still struggling a bit to grasp what your getting at but ...
> > 
> > I was more trying to say that the sysfs device interface with per-
> > object
> > directory isn't the right interface for this sort of usage at all.
> > Are these
> > even real hardware pieces which can be plugged in and out? While
> > being a
> > discrete piece of hardware isn't a requirement to be a device model
> > device,
> > the whole thing is designed with such use cases on mind. It
> > definitely isn't
> > the right design for representing six digit number of logical
> > entities.
> > 
> > It should be obvious that representing each consecutive memory
> > range with a
> > separate directory entry is far from an optimal way of representing
> > something like this. It's outright silly.
> 
> I agree.  And again, Ian, you are just "kicking the problem down the
> road" if we accept these patches.  Please fix this up properly so
> that
> this interface is correctly fixed to not do looney things like this.

Fine, mitigating this problem isn't the end of the story, and you
don't want to do accept a change to mitigate it because that could
mean no further discussion on it and no further work toward solving
it.

But it seems to me a "proper" solution to this will cross a number
of areas so this isn't just "my" problem and, as you point out, it's
likely to become increasingly problematic over time.

So what are your ideas and recommendations on how to handle hotplug
memory at this granularity for this much RAM (and larger amounts)?

Ian