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

[David Hildenbrand]

On 30.07.2018 14:05, Michal Hocko wrote:
> On Mon 30-07-18 13:53:06, David Hildenbrand wrote:
>> On 30.07.2018 13:30, Michal Hocko wrote:
>>> On Fri 27-07-18 18:54:54, David Hildenbrand wrote:
>>>> Right now, struct pages are inititalized when memory is onlined, not
>>>> when it is added (since commit d0dc12e86b31 ("mm/memory_hotplug: optimize
>>>> memory hotplug")).
>>>>
>>>> remove_memory() will call arch_remove_memory(). Here, we usually access
>>>> the struct page to get the zone of the pages.
>>>>
>>>> So effectively, we access stale struct pages in case we remove memory that
>>>> was never onlined. So let's simply inititalize them earlier, when the
>>>> memory is added. We only have to take care of updating the zone once we
>>>> know it. We can use a dummy zone for that purpose.
>>>
>>> I have considered something like this when I was reworking memory
>>> hotplug to not associate struct pages with zone before onlining and I
>>> considered this to be rather fragile. I would really not like to get
>>> back to that again if possible.
>>>
>>>> So effectively, all pages will already be initialized and set to
>>>> reserved after memory was added but before it was onlined (and even the
>>>> memblock is added). We only inititalize pages once, to not degrade
>>>> performance.
>>>
>>> To be honest, I would rather see d0dc12e86b31 reverted. It is late in
>>> the release cycle and if the patch is buggy then it should be reverted
>>> rather than worked around. I found the optimization not really
>>> convincing back then and this is still the case TBH.
>>>
>>
>> If I am not wrong, that's already broken in 4.17, no? What about that?
> 
> Ohh, I thought this was merged in 4.18.
> $ git describe --contains d0dc12e86b31 --match="v*"
> v4.17-rc1~99^2~44
> 
> proves me wrong. This means that the fix is not so urgent as I thought.
> If you can figure out a reasonable fix then it should be preferable to
> the revert.
> 
> Fake zone sounds too hackish to me though.
> 

If I am not wrong, that's the same we had before d0dc12e86b31 but now it
is explicit and only one single value for all kernel configs
("ZONE_NORMAL").

Before d0dc12e86b31, struct pages were initialized to 0. So it was
(depending on the config) ZONE_DMA, ZONE_DMA32 or ZONE_NORMAL.

Now the value is random and might not even be a valid zone.

-- 

Thanks,

David / dhildenb