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

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Date:   Tue, 23 Jun 2020 08:02:36 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ian Kent <raven@...maw.net>
Cc:     Tejun Heo <tj@...nel.org>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency
 improvement

On Tue, Jun 23, 2020 at 01:09:08PM +0800, Ian Kent wrote:
> 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)?

First off, this is not my platform, and not my problem, so it's funny
you ask me :)

Anyway, as I have said before, my first guesses would be:
	- increase the granularity size of the "memory chunks", reducing
	  the number of devices you create.
	- delay creating the devices until way after booting, or do it
	  on a totally different path/thread/workqueue/whatever to
	  prevent delay at booting

And then there's always:
	- don't create them at all, only only do so if userspace asks
	  you to.

You all have the userspace tools/users for this interface and know it
best to know what will work for them.  If you don't, then hey, let's
just delete the whole thing and see who screams :)

thanks,

greg k-h