Re: Filesystem configuration parsers - (was: Re: [RESEND][PATCH] xfs: add online uevent for mount operation)

[Dave Chinner <david@fromorbit.com>]

On Fri, Aug 25, 2017 at 03:20:01AM +0200, Luis R. Rodriguez wrote:
> On Fri, Aug 25, 2017 at 11:01:55AM +1000, Dave Chinner wrote:
> > On Thu, Aug 24, 2017 at 08:04:41PM +0200, Luis R. Rodriguez wrote:
> > > While all this is nice, I'm sure we're all aware of the dangers of setting
> > > things in stone through sysfs, its likely already decided for the above
> > > tunables, but adding a uevent *is* yet another layer of user interface
> > > which userspace can *expect* and we should be certain we want this so
> > > we won't regress userspace later.
> > > 
> > > Just saying, we better damn be sure this is the way we want to go.
> > 
> > I'm not sure what you are trying to warn us about? :/
> > 
> > These are events on an XFS specific kobj, it's not a generic VFS
> > filesystem event we are generating here. It's no different from a
> > hardware device generating it's own uevents to tell userspace
> > something changed in the device.
> 
> I meant that once its sent even if it is XFS specific, it will be
> something that some userspace can expect and then we'd always have
> to send it later.

Well, yes. Just like we have to support ioctls, /proc and /sysfs
interfaces and the quota event interface essentially forever.
There's nothing new or difficult about that.

> > A quick grep would have told you that GFS2 already has it's own
> > online/offline uevents (e.g. gfs2_online_uevent()), as does the
> > DLM code. Orangefs seems to use quite a few of uevent notifications,
> > too. So it's not like we're doing something controversial or unique
> > here, nor something that locks us out of a non-existent VFS event
> > notification mechanism.
> 
> I don't think its controversial *at all*. Just that a mount uevent,
> with uuid, sounds like something we could at least agree is pretty generic.

Yeah, right. Call me cynical, but every single time this has been
brought up it devolves into a paint-fest where everyone wants
something to be added before anything can be done because generic
filesystem events is a deep, dark complex hole. e.g. think of bind
mounts: go and develop arguments for and against whether we should
send generic mount notifications for bind mounts. Generic doesn't
mean simple.

IOWs, what you describe as "pretty generic", I see as "a great big
can of worms".

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com