Re: [PATCH 09/10] mm/hmm: allow to mirror vma of a file on a DAX backed filesystem

[Dan Williams]

On Thu, Mar 7, 2019 at 10:56 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>
> On Thu, Mar 07, 2019 at 09:46:54AM -0800, Andrew Morton wrote:
> > On Tue, 5 Mar 2019 20:20:10 -0800 Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > > My hesitation would be drastically reduced if there was a plan to
> > > avoid dangling unconsumed symbols and functionality. Specifically one
> > > or more of the following suggestions:
> > >
> > > * EXPORT_SYMBOL_GPL on all exports to avoid a growing liability
> > > surface for out-of-tree consumers to come grumble at us when we
> > > continue to refactor the kernel as we are wont to do.
> >
> > The existing patches use EXPORT_SYMBOL() so that's a sticking point.
> > Jerome, what would happen is we made these EXPORT_SYMBOL_GPL()?
>
> So Dan argue that GPL export solve the problem of out of tree user and
> my personnal experience is that it does not. The GPU sub-system has tons
> of GPL drivers that are not upstream and we never felt that we were bound
> to support them in anyway. We always were very clear that if you are not
> upstream that you do not have any voice on changes we do.
>
> So my exeperience is that GPL does not help here. It is just about being
> clear and ignoring anyone who does not have an upstream driver ie we have
> free hands to update HMM in anyway as long as we keep supporting the
> upstream user.
>
> That being said if the GPL aspect is that much important to some then
> fine let switch all HMM symbol to GPL.

I should add that I would not be opposed to moving symbols to
non-GPL-only over time, but that should be based on our experience
with the stability and utility of the implementation. For brand new
symbols there's just no data to argue that we can / should keep the
interface stable, or that the interface exposes something fragile that
we'd rather not export at all. That experience gathering and thrash is
best constrained to upstream GPL-only drivers that are signing up to
participate in that maturation process.

So I think it is important from a practical perspective and is a lower
risk way to run this HMM experiment of "merge infrastructure way in
advance of an upstream user".

> > > * A commitment to consume newly exported symbols in the same merge
> > > window, or the following merge window. When that goal is missed revert
> > > the functionality until such time that it can be consumed, or
> > > otherwise abandoned.
> >
> > It sounds like we can tick this box.
>
> I wouldn't be too strick either, when adding something in release N
> the driver change in N+1 can miss N+1 because of bug or regression
> and be push to N+2.
>
> I think a better stance here is that if we do not get any sign-off
> on the feature from driver maintainer for which the feature is intended
> then we just do not merge.

Agree, no driver maintainer sign-off then no merge.

> If after few release we still can not get
> the driver to use it then we revert.

As long as it is made clear to the driver maintainer that they have
one cycle to consume it then we can have a conversation if it is too
early to merge the infrastructure. If no one has time to consume the
feature, why rush dead code into the kernel? Also, waiting 2 cycles
means the infrastructure that was hard to review without a user is now
even harder to review because any review momentum has been lost by the
time the user show up, so we're better off keeping them close together
in time.


> It just feels dumb to revert at N+1 just to get it back in N+2 as
> the driver bit get fix.

No, I think it just means the infrastructure went in too early if a
driver can't consume it in a development cycle. Lets revisit if it
becomes a problem in practice.

> > > * No new symbol exports and functionality while existing symbols go unconsumed.
> >
> > Unsure about this one?
>
> With nouveau upstream now everything is use. ODP will use some of the
> symbol too. PPC has patchset posted to use lot of HMM too. I have been
> working with other vendor that have patchset being work on to use HMM
> too.
>
> I have not done all those function just for the fun of it :) They do
> have real use and user. It took a longtime to get nouveau because of
> userspace we had a lot of catchup to do in mesa and llvm and we are
> still very rough there.

Sure, this one is less of a concern if we can stick to tighter
timelines between infrastructure and driver consumer merge.