Re: [PATCH v2] mm/sparse: set section nid for hot-add memory

[David Hildenbrand <david@redhat.com>]

On 19.06.19 11:16, Michal Hocko wrote:
> On Wed 19-06-19 11:07:30, David Hildenbrand wrote:
>> On 19.06.19 11:04, Michal Hocko wrote:
>>> On Wed 19-06-19 10:51:47, David Hildenbrand wrote:
>>>> On 19.06.19 09:53, Oscar Salvador wrote:
>>>>> On Wed, Jun 19, 2019 at 08:23:30AM +0200, Michal Hocko wrote:
>>>>>> On Tue 18-06-19 08:55:37, Wei Yang wrote:
>>>>>>> In case of NODE_NOT_IN_PAGE_FLAGS is set, we store section's node id in
>>>>>>> section_to_node_table[]. While for hot-add memory, this is missed.
>>>>>>> Without this information, page_to_nid() may not give the right node id.
>>>>>>
>>>>>> Which would mean that NODE_NOT_IN_PAGE_FLAGS doesn't really work with
>>>>>> the hotpluged memory, right? Any idea why nobody has noticed this
>>>>>> so far? Is it because NODE_NOT_IN_PAGE_FLAGS is rare and essentially
>>>>>> unused with the hotplug? page_to_nid providing an incorrect result
>>>>>> sounds quite serious to me.
>>>>>
>>>>> The thing is that for NODE_NOT_IN_PAGE_FLAGS to be enabled we need to run out of
>>>>> space in page->flags to store zone, nid and section. 
>>>>> Currently, even with the largest values (with pagetable level 5), that is not
>>>>> possible on x86_64.
>>>>> It is possible though, that somewhere in the future, when the values get larger
>>>>> (e.g: we add more zones, NODE_SHIFT grows, or we need more space to store
>>>>> the section) we finally run out of room for the flags though.
>>>>>
>>>>> I am not sure about the other arches though, we probably should audit them
>>>>> and see which ones can fall in there.
>>>>>
>>>>
>>>> I'd love to see NODE_NOT_IN_PAGE_FLAGS go.
>>>
>>> NODE_NOT_IN_PAGE_FLAGS is an implementation detail on where the
>>> information is stored.
>>
>> Yes and no. Storing it per section clearly doesn't allow storing node
>> information on smaller granularity, like storing in page->flags does.
>>
>> So no, it is not only an implementation detail.
> 
> Let me try to put it differently. NODE_NOT_IN_PAGE_FLAGS is not about
> storing the mapping per section. You can do what ever other data
> structure. NODE_NOT_IN_PAGE_FLAGS is in fact about telling that it is
> not in page->flags.

Okay, I get what you are saying. Storing it differently is problematic,
though, if we want o minimize memory consumption and have a fast lookup.

I was also looking into avoiding to store the section number in
page-flags with CONFIG_SPARSEMEM. Especially, because the
CONFIG_HAVE_ARCH_PFN_VALID hack is really ugly. But it's tricky :(

-- 

Thanks,

David / dhildenb