[05/22] mm: export alloc_pages_vma

Submitted by Christoph Hellwig on June 13, 2019, 9:43 a.m.

Details

Message ID 20190613094326.24093-6-hch@lst.de
State New
Headers show
Series "Series without cover letter" ( rev: 3 2 1 ) in Nouveau

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Commit Message

Christoph Hellwig June 13, 2019, 9:43 a.m.
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 <hch@lst.de>
---
 mm/mempolicy.c | 1 +
 1 file changed, 1 insertion(+)

Patch hide | download patch | download mbox

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);
 
 /**
  * 	alloc_pages_current - Allocate pages.

Comments

On 6/13/19 2:43 AM, Christoph Hellwig wrote:
> noveau is currently using this through an odd hmm wrapper, and I plan

  "nouveau"

> to switch it to the real thing later in this series.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---

Reviewed-by: John Hubbard <jhubbard@nvidia.com> 

thanks,
On Thu, Jun 13, 2019 at 06:47:57PM -0700, John Hubbard wrote:
> On 6/13/19 2:43 AM, Christoph Hellwig wrote:
> > noveau is currently using this through an odd hmm wrapper, and I plan
> 
>   "nouveau"

Meh, I keep misspelling that name.  I've already fixed it up a few times
for this series along.
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 <hch@lst.de>
> ---
>  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?
On Thu, Jun 20, 2019 at 12:17 PM Michal Hocko <mhocko@kernel.org> 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 <hch@lst.de>
> > ---
> >  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
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!
On Tue, Jun 25, 2019 at 8:01 AM Michal Hocko <mhocko@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.
On Tue 25-06-19 11:03:53, Dan Williams wrote:
> On Tue, Jun 25, 2019 at 8:01 AM Michal Hocko <mhocko@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.

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
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.
On Tue, Jun 25, 2019 at 12:01 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Tue 25-06-19 11:03:53, Dan Williams wrote:
> > On Tue, Jun 25, 2019 at 8:01 AM Michal Hocko <mhocko@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.
>
> 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.
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?
On Tue, Jun 25, 2019 at 10:46 PM Michal Hocko <mhocko@kernel.org> 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]. I referenced that when debating _GPL
symbol policy with Jérôme [2].

[1]: https://lore.kernel.org/lkml/20190522100959.GA15390@kroah.com/
[2]: https://lore.kernel.org/lkml/CAPcyv4gb+r==riKFXkVZ7gGdnKe62yBmZ7xOa4uBBByhnK9Tzg@mail.gmail.com/
On Wed 26-06-19 09:14:32, Dan Williams wrote:
> On Tue, Jun 25, 2019 at 10:46 PM Michal Hocko <mhocko@kernel.org> 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.GA15390@kroah.com/
> [2]: https://lore.kernel.org/lkml/CAPcyv4gb+r==riKFXkVZ7gGdnKe62yBmZ7xOa4uBBByhnK9Tzg@mail.gmail.com/