Re: [PATCH V5 4/4] kvm: add a check if pfn is from NVDIMM pmem.

[David Hildenbrand]

On 22/09/2018 00:47, Yi Zhang wrote:
> On 2018-09-20 at 14:19:17 -0700, Dan Williams wrote:
>> On Thu, Sep 20, 2018 at 7:11 AM Yi Zhang <yi.z.zhang@xxxxxxxxxxxxxxx> wrote:
>>>
>>> On 2018-09-19 at 09:20:25 +0200, David Hildenbrand wrote:
>>>> Am 19.09.18 um 04:53 schrieb Dan Williams:
>>>>>
>>>>> Should we consider just not setting PageReserved for
>>>>> devm_memremap_pages()? Perhaps kvm is not be the only component making
>>>>> these assumptions about this flag?
>>>>
>>>> I was asking the exact same question in v3 or so.
>>>>
>>>> I was recently going through all PageReserved users, trying to clean up
>>>> and document how it is used.
>>>>
>>>> PG_reserved used to be a marker "not available for the page allocator".
>>>> This is only partially true and not really helpful I think. My current
>>>> understanding:
>>>>
>>>> "
>>>> PG_reserved is set for special pages, struct pages of such pages should
>>>> in general not be touched except by their owner. Pages marked as
>>>> reserved include:
>>>> - Kernel image (including vDSO) and similar (e.g. BIOS, initrd)
>>>> - Pages allocated early during boot (bootmem, memblock)
>>>> - Zero pages
>>>> - Pages that have been associated with a zone but were not onlined
>>>>   (e.g. NVDIMM/pmem, online_page_callback used by XEN)
>>>> - Pages to exclude from the hibernation image (e.g. loaded kexec images)
>>>> - MCA (memory error) pages on ia64
>>>> - Offline pages
>>>> Some architectures don't allow to ioremap RAM pages that are not marked
>>>> as reserved. Allocated pages might have to be set reserved to allow for
>>>> that - if there is a good reason to enforce this. Consequently,
>>>> PG_reserved part of a user space table might be the indicator for the
>>>> zero page, pmem or MMIO pages.
>>>> "
>>>>
>>>> Swapping code does not care about PageReserved at all as far as I
>>>> remember. This seems to be fine as it only looks at the way pages have
>>>> been mapped into user space.
>>>>
>>>> I don't really see a good reason to set pmem pages as reserved. One
>>>> question would be, how/if to exclude them from the hibernation image.
>>>> But that could also be solved differently (we would have to double check
>>>> how they are handled in hibernation code).
>>>>
>>>>
>>>> A similar user of PageReserved to look at is:
>>>>
>>>> drivers/vfio/vfio_iommu_type1.c:is_invalid_reserved_pfn()
>>>>
>>>> It will not mark pages dirty if they are reserved. Similar to KVM code.
>>> Yes, kvm is not the only one user of the dax reserved page.
>>>>
>>>>>
>>>>> Why is MEMORY_DEVICE_PUBLIC memory specifically excluded?
>>>>>
>>>>> This has less to do with "dax" pages and more to do with
>>>>> devm_memremap_pages() established ranges. P2PDMA is another producer
>>>>> of these pages. If either MEMORY_DEVICE_PUBLIC or P2PDMA pages can be
>>>>> used in these kvm paths then I think this points to consider clearing
>>>>> the Reserved flag.
>>>
>>> Thanks Dan/David's comments.
>>> for MEMORY_DEVICE_PUBLIC memory, since host driver could manager the
>>> memory resource to share to guest, Jerome says we could ignore it at
>>> this time.
>>>
>>> And p2pmem, it seems mapped in a PCI bar space which should most likely
>>> a mmio. I think kvm should treated as a reserved page.
>>
>> Ok, but the question you left unanswered is whether it would be better
>> for devm_memremap_pages() to clear the PageReserved flag for
>> MEMORY_DEVICE_{FS,DEV}_DAX rather than introduce a local kvm-only hack
>> for what looks like a global problem.
> 
> Remove the PageReserved flag sounds more reasonable. 
> And Could we still have a flag to identify it is a device private memory, or
> where these pages coming from?

We could use a page type for that or what you proposed. (as I said, we
might have to change hibernation code to skip the pages once we drop the
reserved flag).

-- 

Thanks,

David / dhildenb