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Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()
From:   David Hildenbrand <david@redhat.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Andrew Morton <akpm@linux-foundation.org>
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Organization: Red Hat GmbH
Message-ID: <52e81b85-c460-5b99-a297-e065caab3a16@redhat.com>
Date:   Thu, 10 Oct 2019 09:58:40 +0200
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On 10.10.19 09:52, David Hildenbrand wrote:
> On 10.10.19 09:35, Michal Hocko wrote:
>> On Thu 10-10-19 09:27:32, David Hildenbrand wrote:
>>> On 09.10.19 16:43, Michal Hocko wrote:
>>>> On Wed 09-10-19 16:24:35, David Hildenbrand wrote:
>>>>> We should check for pfn_to_online_page() to not access uninitialized
>>>>> memmaps. Reshuffle the code so we don't have to duplicate the error
>>>>> message.
>>>>>
>>>>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>>> Cc: Michal Hocko <mhocko@kernel.org>
>>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>>> ---
>>>>>  mm/memory-failure.c | 14 ++++++++------
>>>>>  1 file changed, 8 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>>> index 7ef849da8278..e866e6e5660b 100644
>>>>> --- a/mm/memory-failure.c
>>>>> +++ b/mm/memory-failure.c
>>>>> @@ -1253,17 +1253,19 @@ int memory_failure(unsigned long pfn, int flags)
>>>>>  	if (!sysctl_memory_failure_recovery)
>>>>>  		panic("Memory failure on page %lx", pfn);
>>>>>  
>>>>> -	if (!pfn_valid(pfn)) {
>>>>> +	p = pfn_to_online_page(pfn);
>>>>> +	if (!p) {
>>>>> +		if (pfn_valid(pfn)) {
>>>>> +			pgmap = get_dev_pagemap(pfn, NULL);
>>>>> +			if (pgmap)
>>>>> +				return memory_failure_dev_pagemap(pfn, flags,
>>>>> +								  pgmap);
>>>>> +		}
>>>>>  		pr_err("Memory failure: %#lx: memory outside kernel control\n",
>>>>>  			pfn);
>>>>>  		return -ENXIO;
>>>>
>>>> Don't we need that earlier at hwpoison_inject level?
>>>>
>>>
>>> Theoretically yes, this is another instance. But pfn_to_online_page(pfn)
>>> alone would not be sufficient as discussed. We would, again, have to
>>> special-case ZONE_DEVICE via things like get_dev_pagemap() ...
>>>
>>> But mm/hwpoison-inject.c:hwpoison_inject() is a pure debug feature either way:
>>>
>>> 	/*
>>> 	 * Note that the below poison/unpoison interfaces do not involve
>>> 	 * hardware status change, hence do not require hardware support.
>>> 	 * They are mainly for testing hwpoison in software level.
>>> 	 */
>>>
>>> So it's not that bad compared to memory_failure() called from real HW or
>>> drivers/base/memory.c:soft_offline_page_store()/hard_offline_page_store()
>>
>> Yes, this is just a toy. And yes we need to handle zone device pages
>> here because a) people likely want to test MCE behavior even on these
>> pages and b) HW can really trigger MCEs there as well. I was just
>> pointing that the patch is likely incomplete.
>>
> 
> I rather think this deserves a separate patch as it is a separate
> interface :)
> 
> I do wonder why hwpoison_inject() has to perform so much extra work
> compared to other memory_failure() users. This smells like legacy
> leftovers to me, but I might be wrong. The interface is fairly old,
> though. Does anybody know why we need this magic? I can spot quite some
> duplicate checks/things getting performed.
> 
> Naiive me would just make the interface perform the same as
> hard_offline_page_store(). But most probably I am not getting the real
> purpose of both different interfaces.
> 
> HWPOISON_INJECT is only selected for DEBUG_KERNEL, so I would have
> guessed that fixing this is not urgent.
> 
> BTW: mm/memory-failure.c:soft_offline_page() also looks wrong and needs
> fixing to make sure we access initialized memmaps.
> 

To be more precise, soft_offline_page_store() needs a
pfn_to_online_page() check. Will send a patch.

-- 

Thanks,

David / dhildenb