linux-kernel - Re: [PATCH 05/22] mm: export alloc_pages

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Date:   Mon, 24 Jun 2019 11:24:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Ben Skeggs <bskeggs@...hat.com>, Linux MM <linux-mm@...ck.org>,
        nouveau@...ts.freedesktop.org,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/22] mm: export alloc_pages_vma

On Thu, Jun 20, 2019 at 12:17 PM Michal Hocko <mhocko@...nel.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@....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