Re: [PATCH v1] mm: inititalize struct pages when adding a section

[David Hildenbrand]

On 30.07.2018 16:10, Michal Hocko wrote:
> On Mon 30-07-18 15:51:45, David Hildenbrand wrote:
>> On 30.07.2018 15:30, Pavel Tatashin wrote:
> [...]
>>> Hi David,
>>>
>>> Have you figured out why we access struct pages during hot-unplug for
>>> offlined memory? Also, a panic trace would be useful in the patch.
>>
>> __remove_pages() needs a zone as of now (e.g. to recalculate if the zone
>> is contiguous). This zone is taken from the first page of memory to be
>> removed. If the struct pages are uninitialized that value is random and
>> we might even get an invalid zone.
>>
>> The zone is also used to locate pgdat.
>>
>> No stack trace available so far, I'm just reading the code and try to
>> understand how this whole memory hotplug/unplug machinery works.
> 
> Yes this is a mess (evolution of the code called otherwise ;) [1].

So I guess I should not feel bad if I am having problems understanding
all the details? ;)

> Functionality has been just added on top of not very well thought
> through bases. This is a nice example of it. We are trying to get a zone
> to 1) special case zone_device 2) recalculate zone state. The first
> shouldn't be really needed because we should simply rely on altmap.
> Whether it is used for zone device or not. 2) shouldn't be really needed
> if the section is offline and we can check that trivially.
> 

About 2, I am not sure if this is the case and that easy. To me it looks
more like remove_pages() fixes up things that should be done in
offline_pages(). Especially, if the same memory was onlined/offlined to
different zones we might be in trouble (looking at code on a very high
level view).

"if the section is offline and we can check that trivially" is not was
is being used here. It is "of the section was online and is now offline".

Accessing a zone when removing memory sounds very wrong. offline_pages()
should cleanup everything that online_pages() did.

Removing memory with pages that are still online should not be allowed.
And I think this is already enforced via check_memblock_offlined_cb().

> [1] on the other hand I can see why people were reluctant to understand
> the mess and rather tweak their tiny thing on top...
> 


-- 

Thanks,

David / dhildenb