linux-kernel - Re: API break, sysfs "capability" file

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Message-ID: <Zh6J75OrcMY3dAjY@gardel-login>
Date: Tue, 16 Apr 2024 16:23:43 +0200
From: Lennart Poettering <mzxreary@...inter.de>
To: Christoph Hellwig <hch@....de>
Cc: Keith Busch <kbusch@...nel.org>,
	Linux regressions mailing list <regressions@...ts.linux.dev>,
	linux-block@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: API break, sysfs "capability" file

On Di, 09.04.24 16:15, Christoph Hellwig (hch@....de) wrote:
11;rgb:1717/1414/2121
> On Tue, Apr 09, 2024 at 10:19:09AM +0200, Lennart Poettering wrote:
> > All I am looking for is a very simple test that returns me a boolean:
> > is there kernel-level partition scanning enabled on this device or
> > not.
>
> And we can add a trivial sysfs attribute for that.
>
> > At this point it's not clear to me if I can write this at all in
> > a way that works reasonably correctly on any kernel since let's say
> > 4.15 (which is systemd's "recommended baseline" right now).
> >
> > I am really not sure how to salvage this mess at all. AFAICS there's
> > currently no way to write such a test correctly.
>
> You can't.  Maybe that's a lesson to not depend on undocumented internal
> flags exposed by accident by a weirdo interface.  Just talk to
> people.

Undocumented? Internal?

It's was actually one of the *best* documented kernel *public* APIs I
ever came across:

   https://www.kernel.org/doc/html/v5.16/block/capability.html

So much detail, I love it!

I mean, you did good work here, documented it, with all flags in all
details. I think that's great work! You should take pride in this, not
try to deny its existance!

> > I think it would be nice if the "capabilities" thing would be brought
> > back in a limited form. For example, if it would be changed to start
> > to return 0x200|0x1000 for part scanning is off, 0x1000 when it is on.
> >
> > That would then mean we return to compatibility with Linux <= 5.15,
> > but the new 0x1000 bit would tell us that the information is
> > reliable. i.e. if userspace sees 0x1000 being set we know that the
> > 0x200 bit is definitely correct. That would then just mean that
> > kernels >= 5.16 until today are left in the cold...
>
> At this point we're just better off with a clean new interface.
> And you can use the old hack for < 5.15 if you care strongly enough
> or just talk distros into backporting it to make their lives easier.

I'll take what I can get. If API compatibility is not coming back,
then sure, a new sysattr is better than nothing.

Lennart

--
Lennart Poettering, Berlin