linux-kernel - Re: [PATCH 0/5] mm: rework hmm to use devm_memremap

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Message-ID: <CAPcyv4g2XQtuYGPu8HMbPj6wXqGwxiL5jDRznf5fmW4WgC2DTw@mail.gmail.com>
Date:   Tue, 29 May 2018 15:31:54 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Airlie <airlied@...il.com>
Cc:     Jerome Glisse <jglisse@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>, Michal Hocko <mhocko@...e.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/5] mm: rework hmm to use devm_memremap_pages

On Tue, May 29, 2018 at 3:22 PM, Dave Airlie <airlied@...il.com> wrote:
>
> On 24 May 2018 at 13:18, Dan Williams <dan.j.williams@...el.com> wrote:
> > On Wed, May 23, 2018 at 5:10 PM, Jerome Glisse <jglisse@...hat.com> wrote:
> >> On Mon, May 21, 2018 at 03:35:14PM -0700, Dan Williams wrote:
> >>> Hi Andrew, please consider this series for 4.18.
> >>>
> >>> For maintainability, as ZONE_DEVICE continues to attract new users,
> >>> it is useful to keep all users consolidated on devm_memremap_pages() as
> >>> the interface for create "device pages".
> >>>
> >>> The devm_memremap_pages() implementation was recently reworked to make
> >>> it more generic for arbitrary users, like the proposed peer-to-peer
> >>> PCI-E enabling. HMM pre-dated this rework and opted to duplicate
> >>> devm_memremap_pages() as hmm_devmem_pages_create().
> >>>
> >>> Rework HMM to be a consumer of devm_memremap_pages() directly and fix up
> >>> the licensing on the exports given the deep dependencies on the mm.
> >>
> >> I am on PTO right now so i won't be able to quickly review it all
> >> but forcing GPL export is problematic for me now. I rather have
> >> device driver using "sane" common helpers than creating their own
> >> crazy thing.
> >
> > Sane drivers that need this level of deep integration with Linux
> > memory management need to be upstream. Otherwise, HMM is an
> > unprecedented departure from the norms of Linux kernel development.
>
> Isn't it the author of code choice what EXPORT_SYMBOL to use? and
> isn't the agreement that if something is EXPORT_SYMBOL now, changing
> underlying exports isn't considered a good idea. We've seen this before
> with the refcount fun,
>
> See d557d1b58b3546bab2c5bc2d624c5709840e6b10
>
> Not commenting on the legality or what derived works are considered,
> since really the markings are just an indication of the authors opinion,
> and at this stage I think are actually meaningless, since we've diverged
> considerably from the advice given to Linus back when this started.

Yes, and in this case devm_memremap_pages() was originally written by
Christoph and I:

    41e94a851304 add devm_memremap_pages

These patches correct the export type on that routine in patch1.

HMM started off by duplicating devm_memremap_pages() which is fixed up
by this series:

    5 files changed, 77 insertions(+), 265 deletions(-)

...and these patches take the extra step of marking any exports that
call devm_memremap_pages() also match the same base export.