Re: [RFC PATCH v2] mm: initialize struct pages reserved by ZONE_DEVICE driver.

[Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>]

On 2019/09/17 19:20, David Hildenbrand wrote:
> On 17.09.19 11:32, Toshiki Fukasawa wrote:
>> On 2019/09/17 16:13, David Hildenbrand wrote:
>>> On 17.09.19 04:34, Toshiki Fukasawa wrote:
>>>> On 2019/09/09 16:46, David Hildenbrand wrote:
>>>>> Let's take a step back here to understand the issues I am aware of. I
>>>>> think we should solve this for good now:
>>>>>
>>>>> A PFN walker takes a look at a random PFN at a random point in time. It
>>>>> finds a PFN with SECTION_MARKED_PRESENT && !SECTION_IS_ONLINE. The
>>>>> options are:
>>>>>
>>>>> 1. It is buddy memory (add_memory()) that has not been online yet. The
>>>>> memmap contains garbage. Don't access.
>>>>>
>>>>> 2. It is ZONE_DEVICE memory with a valid memmap. Access it.
>>>>>
>>>>> 3. It is ZONE_DEVICE memory with an invalid memmap, because the section
>>>>> is only partially present: E.g., device starts at offset 64MB within a
>>>>> section or the device ends at offset 64MB within a section. Don't access it.
>>>>
>>>> I don't agree with case #3. In the case, struct page area is not allocated on
>>>> ZONE_DEVICE, but is allocated on system memory. So I think we can access the
>>>> struct pages. What do you mean "invalid memmap"?
>>> No, that's not the case. There is no memory, especially not system
>>> memory. We only allow partially present sections (sub-section memory
>>> hotplug) for ZONE_DEVICE.
>>
>> Let me clear my thoughts. If I read correctly, the struct pages for sections
>> (including partially present sections) on ZONE_DEVICE are allocated by
>> vmemmap_populate(). And all the struct pages except (altmap->base_pfn) to
>> (altmap->base_pfn + altmap->reserve) are initialized by memmap_init_zone()
>> and memmap_init_zone_device().
>>
>> Do struct pages for partially present sections go through a different process?
> 
> No. However, the memmap is initialized via move_pfn_range_to_zone(). So
> partially present sections will have partially uninitialized memmaps.
> 
Thank you for explanation.
To my understanding, depending on architecture, the situation is possible
that the struct pages for entire section is allocated, but only pages
in the zone are initialized.

Thanks,
Toshiki Fukasawa