linux-kernel - Re: OpenAFS module libafs.ko uses GPL-only symbol '__put_devmap_managed

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Date:   Fri, 15 Jun 2018 21:43:33 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Joe Gorse <jhgorse@...il.com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, Christoph Hellwig <hch@....de>
Subject: Re: OpenAFS module libafs.ko uses GPL-only symbol
 '__put_devmap_managed_page'

On 06/13/2018 12:51 PM, Dan Williams wrote:
> [ adding Andrew, Christoph, and linux-mm ]
> 
> On Wed, Jun 13, 2018 at 12:33 PM, Joe Gorse <jhgorse@...il.com> wrote:
>> Greetings,
>>
>> Please CC answers & comments to this email. Thanks! =)
>>
>> Our build is breaking as of
>> commit e7638488434415aa478e78435cac8f0365737638
>> Author: Dan Williams <dan.j.williams@...el.com>
>> Date: Wed May 16 11:46:08 2018 -0700
>>
>> mm: introduce MEMORY_DEVICE_FS_DAX and CONFIG_DEV_PAGEMAP_OPS
>> ... snip ...
>> https://patchwork.kernel.org/patch/10412459/
>>
>> We do not directly use the GPL-only symbol '__put_devmap_managed_page'. It
>> appears to be picked up from static-inlining in put_page(), which we need.
>>
>> How shall we proceed? Would it be reasonable to request the change of the
>> GPL-only exports for this commit?
>>
>> Cheers,
>> Joe Gorse
>>
>> P.S. The build failure, for the morbidly curious:
>>> FATAL: modpost: GPL-incompatible module libafs.ko uses GPL-only symbol
>>> '__put_devmap_managed_page'
>>> scripts/Makefile.modpost:92: recipe for target '__modpost' failed
>>> make[6]: *** [__modpost] Error 1
> 
> I think the right answer here is to make __put_devmap_managed_page()
> EXPORT_SYMBOL(), since features like devm_memremap_pages() want to
> change the behavior of all users of put_page(). It again holds that
> devm_memremap_pages() needs to become EXPORT_SYMBOL_GPL() because it,
> not put_page(), is the interface that is leaking control of core
> kernel state/infrastructure to its users.
> 

Hi Dan and all,

It looks like put_page() also picks up one more GPL symbol: 
devmap_managed_key.

put_page
    put_devmap_managed_page
        devmap_managed_key

    __put_devmap_managed_page


So if the goal is to restore put_page() to be effectively EXPORT_SYMBOL
again, then I think there would also need to be either a non-inlined 
wrapper for devmap_managed_key (awkward for a static key), or else make
it EXPORT_SYMBOL, or maybe something else that's less obvious to me at the
moment.


thanks,
-- 
John Hubbard
NVIDIA