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

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Message-ID: <20200622174845.GB13061@mtj.duckdns.org>
Date:   Mon, 22 Jun 2020 13:48:45 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Ian Kent <raven@...maw.net>
Cc:     Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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

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.

> Maybe your talking about the underlying notifications system where
> a notification is sent for every event.
> 
> There's nothing new about that problem and it's becoming increasingly
> clear that existing kernel notification sub-systems don't scale well.
> 
> Mount handling is a current example which is one of the areas David
> Howells is trying to improve and that's taken years now to get as
> far as it has.

There sure are scalability issues everywhere that needs to be improved but
there also are cases where the issue is the approach itself rather than
scalability and I'm wondering whether this is the latter.

Thanks.

-- 
tejun