Re: [Ksummit-discuss] [Stable kernel] feature backporting collaboration

[Mark Brown <broonie@kernel.org>]

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On Fri, Sep 02, 2016 at 03:16:37PM -0400, Levin, Alexander wrote:

> Look at KASLR and KASan, it has complex interactions with pretty much the rest
> of the kernel. Quite a few things not directly related to either of those had
> to be fixed just because they were found to not integrate right (For example,
> KASLR uncovered a bunch of bugs before it was actually merged in), who says
> that there aren't any similar interactions with the older kernels that no one
> looked into?

Sure, and this sort of thing is one of the reasons we have the ability
to disable things in Kconfig.  It's not risk free but it's very much
mitigated compared to tracking mainline.

> > It's what people are doing for products, they want newer features but
> > they also don't want to rebase their product kernel onto mainline as
> > that's an even bigger integration risk.  People aren't using this kernel

> I'm sorry but just calling a kernel "stable" doesn't mean that suddenly it
> acquires the qualities of a stable kernel that follows the very strict rules
> we have for those.

> Given that you're backporting features into a stable kernel it really inherits
> the code quality of a release candidate kernel; nowhere close to a stable
> kernel.

> This following is just my opinion as an LTS kernel maintainer: if you think
> that the integration risk of a newer stable/LTS is bigger than using these
> frankenstein kernels you are very much mistaken.

I really don't think you understand the environment that this work is
done in.  You may have heard people mention the large amount of out of
tree code that vendors tend to be sitting on.  That interacts with a
*very* large chunk of the kernel, and of course there's also a bunch of
performance stuff that's being looked at beyond pure correctness issues.
Taking a new upstream requires a bunch of work to update the out of tree
code to any new kernel APIs and realistically it's going to trash a huge
chunk of the testing that's been done on the product and require at
least revalidation.  Taking a targeted update, especially one where the
riskier changes are configuration options, isn't free either but the
surface that needs to be looked at is much more known and controlled.

> In your case it's nice if you could share backports betweek multiple users
> (just like we try doing for all the stable/LTS trees), but the coverage and
> testing you're going to get for that isn't anywhere close to what you'll have
> for a more recent stable kernel that already has those features baked into
> that.

If everything were upstream, everyone was working directly upstream and
everyone had their QA focused on upstream what you're saying would be
more true but as everyone is so keen to point out that's just not what's
happening.  There's a bunch of other code in play on the relevant
systems which makes things that little bit more involved.

> > > As an alternative, why not use more recent stable kernels and customize the
> > > config specifically for each user to enable on features that that specific
> > > user wants to have.

> > That's just shipping a kernel - I don't think anyone is silly enough to
> > ship an allmodconfig or similar in production (though I'm sure someone
> > can come up with an example).

> I highly doubt that most shipped kernels actually go through the process of
> auditing every single config option and figuring out if they actually need it
> or not (in part because the kernel's config is quite a mess). I really doubt
> that the kernel is fine-tuned for majority of the released products that run
> linux.

I'm sorry but I really don't follow what you're saying here - I'm not
sure anyone's out of tree code is the result of a failure to understand
Kconfig and I don't really understand the relevance of a detailed study
of configuration to the issues around rebasing.

> > Like I say in this case updating to a newer kernel also means rebasing
> > the out of tree patch stack and taking a bunch of test risk from that -
> > in product development for the sorts of products that end up including
> > the LSK the churn and risk from targeted backports is seen as much safer
> > than updating to an entire new upstream kernel.

> Same as I said before, the risk LSK introduces, IMO, is much greater than
> rebasing and out-of-tree driver stack.

I'm afraid you're very much mistaken if you believe that people are only
working on leaf drivers, or that nothing we do upstream has a meaningful
impact at the system level.

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