Re: [PATCH -next v4] mm/hotplug: silence a lockdep splat with printk()

[David Hildenbrand]

On 17.01.20 10:40, Michal Hocko wrote:
> On Fri 17-01-20 10:25:06, David Hildenbrand wrote:
>> On 17.01.20 09:59, Michal Hocko wrote:
>>> On Fri 17-01-20 09:51:05, David Hildenbrand wrote:
>>>> On 17.01.20 03:21, Qian Cai wrote:
>>> [...]
>>>>> Even though has_unmovable_pages doesn't hold any reference to the
>>>>> returned page this should be reasonably safe for the purpose of
>>>>> reporting the page (dump_page) because it cannot be hotremoved. The
>>>>
>>>> This is only true in the context of memory unplug, but not in the
>>>> context of is_mem_section_removable()-> is_pageblock_removable_nolock().
>>>
>>> Well, the above should hold for that path as well AFAICS. If the page is
>>> unmovable then a racing hotplug cannot remove it, right? Or do you
>>> consider a temporary unmovability to be a problem?
>>
>> Somebody could test /sys/devices/system/memory/memoryX/removable. While
>> returning the unmovable page, it could become movable and
>> offlining+removing could succeed.
> 
> Doesn't this path use device lock or something? If not than the new code
> is not more racy then the existing one. Just look at
> is_pageblock_removable_nolock and how it dereferences struct page
> (page_zonenum in  page_zone.)
> 

AFAIK no device lock, no device hotplug lock, no memory hotplug lock. I
think it holds a reference to the device and to the kernelfs node. But
AFAIK that does not block removal of offlining/memory, just when the
objects get freed.

-- 
Thanks,

David / dhildenb