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Subject: Re: [lkp] [mm] 7d2eba0557: BUG: unable to handle kernel NULL pointer
 dereference at (null)
To: Cyrill Gorcunov <gorcunov@gmail.com>, David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <8737tue3mn.fsf@yhuang-dev.intel.com>
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From: Vlastimil Babka <vbabka@suse.cz>
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Date: Wed, 20 Jan 2016 00:07:46 +0100
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On 01/19/2016 11:45 PM, Cyrill Gorcunov wrote:
> On Tue, Jan 19, 2016 at 02:30:45PM -0800, David Rientjes wrote:
>> On Tue, 19 Jan 2016, Cyrill Gorcunov wrote:
>> 
>> > Seems it should be something like that (untested at moment, please
>> > take a look).
>> > ---
>> > From: Cyrill Gorcunov <gorcunov@openvz.org>
>> > Subject: [PATCH] mm: Fix tracepoint for scanning pages
>> > 
>> > The page might be NULL so use page_to_pfn in conditional call.
>> > 
>> > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
>> 
>> I think this is obviously correct, so
>> 
>> Acked-by: David Rientjes <rientjes@google.com>
> 
> Thank you. I've compile tested it, but didn't run.

This has been in mmotm since november:
http://ozlabs.org/~akpm/mmotm/broken-out/mm-fix-kernel-crash-in-khugepaged-thread.patch

Although your patch does use -1UL instead of -1 (AFAICS the only difference, but
it's too late here), which Steven Rostedt suggested, so it's probably better.