linux-kernel - Re: [Xen-devel] xen/mem-reservation API and out-of-tree kernel modules

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Message-ID: <6deaf110-44a7-59f9-a4d8-52ff9428cd9d@epam.com>
Date:   Fri, 1 Feb 2019 08:39:53 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
CC:     xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "marmarek@...isiblethingslab.com" <marmarek@...isiblethingslab.com>
Subject: Re: [Xen-devel] xen/mem-reservation API and out-of-tree kernel
 modules

On 1/31/19 11:44 PM, Stefano Stabellini wrote:
> On Thu, 31 Jan 2019, Oleksandr Andrushchenko wrote:
>> Hello,
>>
>> I am working on porting an out-of-tree kernel driver to the kernel
>> 5.0 and that driver uses functionality provided by
>> drivers/xen/mem-reservation.c
>> module.  Since commit [1] it is not possible to build a kernel module
>> which uses mem-reservation API as xen_scrub_pages variable, which is
>> checked in
>> xenmem_reservation_scrub_page, became a kernel module parameter and is
>> now only
>> accessible for built-in modules:
>>
>> static inline void xenmem_reservation_scrub_page(struct page *page)
>> ^^^^^^^^^^^^^
>> {
>>       if (xen_scrub_pages)
>>           ^^^^^^^^^^^^^^^
>>           clear_highpage(page);
>> }
>>
>> This results in link-time warning:
>>
>>       WARNING: "xen_scrub_pages" [yourmodule.ko] undefined!
>>
>> and thus not allowing the module to run. At the moment I can only see a
>> possible fix
>> for this by making the following change:
>>
>> diff --git a/drivers/xen/mem-reservation.c b/drivers/xen/mem-reservation.c
>> index 3782cf070338..85fecfec50e1 100644
>> --- a/drivers/xen/mem-reservation.c
>> +++ b/drivers/xen/mem-reservation.c
>> @@ -18,6 +18,7 @@
>>
>>    bool __read_mostly xen_scrub_pages =
>> IS_ENABLED(CONFIG_XEN_SCRUB_PAGES_DEFAULT);
>>    core_param(xen_scrub_pages, xen_scrub_pages, bool, 0);
>> +EXPORT_SYMBOL(xen_scrub_pages);
>>
>> but this looks a bit unusual for the kernel?
>>
>> I am looking for community advice here and help
>>
>> Thank you,
>> Oleksandr
>>
>> [1]
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=197ecb3802c04499d8ff4f8cb28f6efa008067db
> The alternative would be to turn xenmem_reservation_scrub_page into a
> regular function (not a static inline)?
Yes, it seems there is no other reasonable solution to this, but
a regular function. I'll send a patch for that

Thank you,
Oleksandr