Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized memmaps in memory_failure()

[David Hildenbrand]

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@xxxxxxxxxxxxx>
>>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>>>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>>>> ---
>>>>  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.

-- 

Thanks,

David / dhildenb