netdev - Re: RFC: Ioctl v2

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Date:   Wed, 25 May 2022 13:02:33 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Andrew Lunn <andrew@...n.ch>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        netdev@...r.kernel.org, mcgrof@...nel.org, tytso@....edu
Subject: Re: RFC: Ioctl v2

On Sat, May 21, 2022 at 12:45:59PM -0700, Stephen Hemminger wrote:
> On Sat, 21 May 2022 12:45:46 -0400
> Kent Overstreet <kent.overstreet@...il.com> wrote:
> 
> > On Fri, May 20, 2022 at 10:31:02PM +0200, Andrew Lunn wrote:
> > > > I want to circulate this and get some comments and feedback, and if
> > > > no one raises any serious objections - I'd love to get collaborators
> > > > to work on this with me. Flame away!  
> > > 
> > > Hi Kent
> > > 
> > > I doubt you will get much interest from netdev. netdev already
> > > considers ioctl as legacy, and mostly uses netlink and a message
> > > passing structure, which is easy to extend in a backwards compatible
> > > manor.  
> > 
> > The more I look at netlink the more I wonder what on earth it's targeted at or
> > was trying to solve. It must exist for a reason, but I've written a few ioctls
> > myself and I can't fathom a situation where I'd actually want any of the stuff
> > netlink provides.
> 
> Netlink was built for networking operations, you want to set something like a route with a large
> number of varying parameters in one transaction. And you don't want to have to invent
> a new system call every time a new option is added.
> 
> Also, you want to monitor changes and see these events for a userspace control
> application such as a routing daemon.

That makes sense - perhaps the new mount API could've been done as a netlink
interface :)

But perhaps it makes sense to have both - netlink for the big complicated
stateful operations, ioctl v2 for the simpler ones. I haven't looked at netlink
usage at all, but most of the filesystem ioctls I've looked at fall into the the
simple bucket, for me.

Actually, I have one in bcachefs that might fit better into the netlink bucket -
maybe while I've got your attention you could tell me what this is like in
netlink land.

In bcachefs, we have "data jobs", where userspace asks us to do something that
requires walking data and performing some operation on them - this is used for
manual rebalance, evacuating data off a device, scrub (when that gets
implemented), etc.

The way I did this was with an ioctl that takes as a parameter the job to
perform, then it kicks off a kernel thread to do the work and returns a file
descriptor, which userspace reads from to find out the current status of the job
(which it uses to implement a progress indicator). We kill off the kthread if
the file descriptor is closed, meaning ctrl-c works as expected.

I really like how this turned out, it's not much code and super slick - I was
considering abstracting it out as generic functionality. But this definitely
sounds like what netlink is targeted at - thoughts?