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

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Message-ID: <CAPM=9twm=17t=2=M27ELB=vZWzpqM7GuwCUsC891jJ0t3JM4vg@mail.gmail.com>
Date:   Wed, 30 May 2018 09:00:25 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Dan Williams <dan.j.williams@...el.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 30 May 2018 at 08:31, Dan Williams <dan.j.williams@...el.com> wrote:
> 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

So you wrote some code in 2015 (3 years ago) and you've now decided
to change the EXPORT marker on it? what changed in 3 years, and why
would changing that marker 3 years later have any effect on your original
statement that it was an EXPORT_SYMBOL.

Think what EXPORT_SYMBOL vs GPL means, it isn't a bit stick that magically
makes things into derived works. If something wasn't a derived work for 3 years
using that API, then it isn't a derived work now 3 years later because you
changed the marker. Retrospectively changing the markers doesn't really
make any sense legally or otherwise.

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

Just looking in my current tree hmm_devmem_pages_create and
devm_memremap_pages don't look like duplicates, they might have
code but they definitely aren't one for one copies. I'm not sure you can
just say Jerome copied that code in, you've now refactored the code
so HMM can use it and are changing the symbol exports underneath it,

Again if Christoph believes all uses of this are a derived work he didn't
indicate it 3 years ago, but neither does the mark make any legal difference
in this case, since everything in the kernel is GPL, and if you
consider something
a derived work or not is well into legal land.

I'd rather anyways the original author of HMM wishes were respected
on his code, or at least you wait until he gets back from holidays before
pushing to merge this.

Dave.