Re: [PATCH 05/22] mm: export alloc_pages_vma

[Dan Williams]

On Tue, Jun 25, 2019 at 12:01 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Tue 25-06-19 11:03:53, Dan Williams wrote:
> > On Tue, Jun 25, 2019 at 8:01 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > On Tue 25-06-19 09:23:17, Christoph Hellwig wrote:
> > > > On Mon, Jun 24, 2019 at 11:24:48AM -0700, Dan Williams wrote:
> > > > > I asked for this simply because it was not exported historically. In
> > > > > general I want to establish explicit export-type criteria so the
> > > > > community can spend less time debating when to use EXPORT_SYMBOL_GPL
> > > > > [1].
> > > > >
> > > > > The thought in this instance is that it is not historically exported
> > > > > to modules and it is safer from a maintenance perspective to start
> > > > > with GPL-only for new symbols in case we don't want to maintain that
> > > > > interface long-term for out-of-tree modules.
> > > > >
> > > > > Yes, we always reserve the right to remove / change interfaces
> > > > > regardless of the export type, but history has shown that external
> > > > > pressure to keep an interface stable (contrary to
> > > > > Documentation/process/stable-api-nonsense.rst) tends to be less for
> > > > > GPL-only exports.
> > > >
> > > > Fully agreed.  In the end the decision is with the MM maintainers,
> > > > though, although I'd prefer to keep it as in this series.
> > >
> > > I am sorry but I am not really convinced by the above reasoning wrt. to
> > > the allocator API and it has been a subject of many changes over time. I
> > > do not remember a single case where we would be bending the allocator
> > > API because of external modules and I am pretty sure we will push back
> > > heavily if that was the case in the future.
> >
> > This seems to say that you have no direct experience of dealing with
> > changing symbols that that a prominent out-of-tree module needs? GPU
> > drivers and the core-mm are on a path to increase their cooperation on
> > memory management mechanisms over time, and symbol export changes for
> > out-of-tree GPU drivers have been a significant source of friction in
> > the past.
>
> I have an experience e.g. to rework semantic of some gfp flags and that is
> something that users usualy get wrong and never heard that an out of
> tree code would insist on an old semantic and pushing us to the corner.
>
> > > So in this particular case I would go with consistency and export the
> > > same way we do with other functions. Also we do not want people to
> > > reinvent this API and screw that like we have seen in other cases when
> > > external modules try reimplement core functionality themselves.
> >
> > Consistency is a weak argument when the cost to the upstream community
> > is negligible. If the same functionality was available via another /
> > already exported interface *that* would be an argument to maintain the
> > existing export policy. "Consistency" in and of itself is not a
> > precedent we can use more widely in default export-type decisions.
> >
> > Effectively I'm arguing EXPORT_SYMBOL_GPL by default with a later
> > decision to drop the _GPL. Similar to how we are careful to mark sysfs
> > interfaces in Documentation/ABI/ that we are not fully committed to
> > maintaining over time, or are otherwise so new that there is not yet a
> > good read on whether they can be made permanent.
>
> Documentation/process/stable-api-nonsense.rst

That document has failed to preclude symbol export fights in the past
and there is a reasonable argument to try not to retract functionality
that had been previously exported regardless of that document.

> Really. If you want to play with GPL vs. EXPORT_SYMBOL else this is up
> to you but I do not see any technical argument to make this particular
> interface to the page allocator any different from all others that are
> exported to modules.

I'm failing to find any practical substance to your argument, but in
the end I agree with Chrishoph, it's up to MM maintainers.