Re: [PATCH] powerpc: Perform a bounds check in arch_add_memory

[David Hildenbrand <david@redhat.com>]

On 27.08.19 08:39, Alastair D'Silva wrote:
> On Tue, 2019-08-27 at 08:28 +0200, Michal Hocko wrote:
>> On Tue 27-08-19 15:20:46, Alastair D'Silva wrote:
>>> From: Alastair D'Silva <alastair@d-silva.org>
>>>
>>> It is possible for firmware to allocate memory ranges outside
>>> the range of physical memory that we support (MAX_PHYSMEM_BITS).
>>
>> Doesn't that count as a FW bug? Do you have any evidence of that in
>> the
>> field? Just wondering...
>>
> 
> Not outside our lab, but OpenCAPI attached LPC memory is assigned
> addresses based on the slot/NPU it is connected to. These addresses
> prior to:
> 4ffe713b7587 ("powerpc/mm: Increase the max addressable memory to 2PB")
> were inaccessible and resulted in bogus sections - see our discussion
> on 'mm: Trigger bug on if a section is not found in __section_nr'.
> Doing this check here was your suggestion :)
> 
> It's entirely possible that a similar problem will occur in the future,
> and it's cheap to guard against, which is why I've added this.
> 

If you keep it here, I guess this should be wrapped by a WARN_ON_ONCE().

If we move it to common code (e.g., __add_pages() or add_memory()), then
probably not. I can see that s390x allows to configure MAX_PHYSMEM_BITS,
so the check could actually make sense.

-- 

Thanks,

David / dhildenb