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

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Message-ID: <f3ca533d-7f51-c59e-6a96-4e1fba687c8f@epam.com>
Date:   Thu, 31 Jan 2019 14:02:30 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     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: [Xen-devel] xen/mem-reservation API and out-of-tree kernel modules

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