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

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Date:   Thu, 31 Jan 2019 13:44:15 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
cc:     xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "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 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)?