Re: [PATCH] A long explanation for a short patch

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 14 Mar 2014 12:53:32 -0400 Josh Boyer <jwboyer@redhat.com> wrote:

> I suppose the
> thing that gave me pause here is that the highlighted example was an
> issue with a proprietary module whereas this one is permissively
> licensed (more permissively than GPL even).

Doesn't really matter much.

a) things used to work, but 309381feaee564 broke it, unintentionally.

b) modules which work OK with CONFIG_DEBUG_VM=n will break with
   CONFIG_DEBUG_VM=y, which makes no sense.


I queued the patch for 3.15-rc1 with a tweaked changelog:


From: John Hubbard <jhubbard@nvidia.com>
Subject: mm/page_alloc.c: change mm debug routines back to EXPORT_SYMBOL

A new dump_page() routine was recently added, and marked
EXPORT_SYMBOL_GPL.  dump_page() was also added to the VM_BUG_ON_PAGE()
macro, and so the end result is that non-GPL code can no longer call
get_page() and a few other routines.

This only happens if the kernel was compiled with CONFIG_DEBUG_VM.

Change dump_page() to be EXPORT_SYMBOL.

Longer explanation:

Prior to 309381feaee564 ("mm: dump page when hitting a VM_BUG_ON using
VM_BUG_ON_PAGE") , it was possible to build MIT-licensed (non-GPL) drivers
on Fedora.  Fedora is semi-unique, in that it sets CONFIG_VM_DEBUG.

Because Fedora sets CONFIG_VM_DEBUG, they end up pulling in dump_page(),
via VM_BUG_ON_PAGE, via get_page().  As one of the authors of NVIDIA's
new, open source, "UVM-Lite" kernel module, I originally choose to use the
kernel's get_page() routine from within nvidia_uvm_page_cache.c, because
get_page() has always seemed to be very clearly intended for use by
non-GPL, driver code.

So I'm hoping that making get_page() widely accessible again will not be
too controversial.  We did check with Fedora first, and they responded
(https://bugzilla.redhat.com/show_bug.cgi?id=1074710#c3) that we should
try to get upstream changed, before asking Fedora to change.  Their
reasoning seems beneficial to Linux: leaving CONFIG_DEBUG_VM set allows
Fedora to help catch mm bugs.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Josh Boyer <jwboyer@redhat.com>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN mm/page_alloc.c~mm-page_allocc-change-mm-debug-routines-back-to-export_symbol mm/page_alloc.c
--- a/mm/page_alloc.c~mm-page_allocc-change-mm-debug-routines-back-to-export_symbol
+++ a/mm/page_alloc.c
@@ -6566,4 +6566,4 @@ void dump_page(struct page *page, const
 {
 	dump_page_badflags(page, reason, 0);
 }
-EXPORT_SYMBOL_GPL(dump_page);
+EXPORT_SYMBOL(dump_page);
_

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