Re: [PATCH 05/22] mm: export alloc_pages_vma
On Wed 26-06-19 09:14:32, Dan Williams wrote:
> On Tue, Jun 25, 2019 at 10:46 PM Michal Hocko wrote:
> >
> > On Tue 25-06-19 12:52:18, Dan Williams wrote:
> > [...]
> > > > 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.
> >
> > Can you point me to any specific example where this would be the case
> > for the core kernel symbols please?
>
> The most recent example that comes to mind was the thrash around
> __kernel_fpu_{begin,end} [1].
Well, this seems more like a disagreement over a functionality that has
reduced its visibility rather than enforcement of a specific API. And I
do agree that the above document states that this is perfectly
legitimate and no out-of-tree code can rely on _any_ functionality to be
preserved.
On the other hand, I am not really surprised about the discussion
because d63e79b114c02 is a mere clean up not explaining why the
functionality should be restricted to GPL only code. So there certainly
is a room for clarification. Especially when the code has been exported
without this restriction in the past (see 8546c008924d5). So to me this
sounds more like a usual EXPORT_SYMBOL{_GPL} mess.
In any case I really do not see any relation to the maintenance cost
here. GPL symbols are not in any sense more stable than any other
exported symbol. They can change at any time. The only maintenance
burden is to update all _in_kernel_ users of the said symbol. Any
out-of-tree code is on its own to deal with this. Full stop.
GPL or non-GPL symbols are solely to define a scope of the usage.
Nothing less and nothing more.
> I referenced that when debating _GPL symbol policy with Jérôme [2].
>
> [1]: https://lore.kernel.org/lkml/20190522100959.ga15...@kroah.com/
> [2]:
> https://lore.kernel.org/lkml/CAPcyv4gb+r==rikfxkvz7ggdnke62ybmz7xoa4ubbbyhnk9...@mail.gmail.com/
--
Michal Hocko
SUSE Labs
Re: [PATCH 05/22] mm: export alloc_pages_vma
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
Re: [PATCH 05/22] mm: export alloc_pages_vma
On Thu, Jun 20, 2019 at 12:17 PM Michal Hocko wrote: > > On Thu 13-06-19 11:43:08, Christoph Hellwig wrote: > > noveau is currently using this through an odd hmm wrapper, and I plan > > to switch it to the real thing later in this series. > > > > Signed-off-by: Christoph Hellwig > > --- > > mm/mempolicy.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > > index 01600d80ae01..f9023b5fba37 100644 > > --- a/mm/mempolicy.c > > +++ b/mm/mempolicy.c > > @@ -2098,6 +2098,7 @@ alloc_pages_vma(gfp_t gfp, int order, struct > > vm_area_struct *vma, > > out: > > return page; > > } > > +EXPORT_SYMBOL_GPL(alloc_pages_vma); > > All allocator exported symbols are EXPORT_SYMBOL, what is a reason to > have this one special? 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. [1]: https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2018-September/005688.html
Re: [PATCH 05/22] mm: export alloc_pages_vma
On Thu 13-06-19 11:43:08, Christoph Hellwig wrote: > noveau is currently using this through an odd hmm wrapper, and I plan > to switch it to the real thing later in this series. > > Signed-off-by: Christoph Hellwig > --- > mm/mempolicy.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c > index 01600d80ae01..f9023b5fba37 100644 > --- a/mm/mempolicy.c > +++ b/mm/mempolicy.c > @@ -2098,6 +2098,7 @@ alloc_pages_vma(gfp_t gfp, int order, struct > vm_area_struct *vma, > out: > return page; > } > +EXPORT_SYMBOL_GPL(alloc_pages_vma); All allocator exported symbols are EXPORT_SYMBOL, what is a reason to have this one special? -- Michal Hocko SUSE Labs
