Nouveau - Jun 2019 - [PATCH 05/22] mm: export alloc_pages_vma

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.

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.

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.

Thanks!
-- 
Michal Hocko
SUSE Labs

On Tue, Jun 25, 2019 at 8:01 AM Michal Hocko <mhocko at kernel.org>
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.
> 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.