[PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes

[David Hildenbrand]

On 08.11.19 06:09, Dan Williams wrote:
> On Thu, Nov 7, 2019 at 2:07 PM David Hildenbrand <david at redhat.com> wrote:
>>
>> On 07.11.19 19:22, David Hildenbrand wrote:
>>>
>>>
>>>> Am 07.11.2019 um 16:40 schrieb Dan Williams <dan.j.williams at intel.com>:
>>>>
>>>> On Thu, Oct 24, 2019 at 5:12 AM David Hildenbrand <david at redhat.com> wrote:
>>>>>
>>>>> Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
>>>>> change that.
>>>>>
>>>>> KVM has this weird use case that you can map anything from /dev/mem
>>>>> into the guest. pfn_valid() is not a reliable check whether the memmap
>>>>> was initialized and can be touched. pfn_to_online_page() makes sure
>>>>> that we have an initialized memmap (and don't have ZONE_DEVICE memory).
>>>>>
>>>>> Rewrite is_invalid_reserved_pfn() similar to kvm_is_reserved_pfn() to make
>>>>> sure the function produces the same result once we stop setting ZONE_DEVICE
>>>>> pages PG_reserved.
>>>>>
>>>>> Cc: Alex Williamson <alex.williamson at redhat.com>
>>>>> Cc: Cornelia Huck <cohuck at redhat.com>
>>>>> Signed-off-by: David Hildenbrand <david at redhat.com>
>>>>> ---
>>>>> drivers/vfio/vfio_iommu_type1.c | 10 ++++++++--
>>>>> 1 file changed, 8 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>> index 2ada8e6cdb88..f8ce8c408ba8 100644
>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>> @@ -299,9 +299,15 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>>>>>    */
>>>>> static bool is_invalid_reserved_pfn(unsigned long pfn)
>>>>> {
>>>>> -       if (pfn_valid(pfn))
>>>>> -               return PageReserved(pfn_to_page(pfn));
>>>>> +       struct page *page = pfn_to_online_page(pfn);
>>>>
>>>> Ugh, I just realized this is not a safe conversion until
>>>> pfn_to_online_page() is moved over to subsection granularity. As it
>>>> stands it will return true for any ZONE_DEVICE pages that share a
>>>> section with boot memory.
>>>
>>> That should not happen right now and I commented back when you introduced subsection support that I don’t want to have ZONE_DEVICE mixed with online pages in a section. Having memory block devices that partially span ZONE_DEVICE would be ... really weird. With something like pfn_active() - as discussed - we could at least make this check work - but I am not sure if we really want to go down that path. In the worst case, some MB of RAM are lost ... I guess this needs more thought.
>>>
>>
>> I just realized the "boot memory" part. Is that a real thing? IOW, can
>> we have ZONE_DEVICE falling into a memory block (with holes)? I somewhat
>> have doubts that this would work ...
> 
> One of the real world failure cases that started the subsection effect
> is that Persistent Memory collides with System RAM on a 64MB boundary
> on shipping platforms. System RAM ends on a 64MB boundary and due to a
> lack of memory controller resources PMEM is mapped contiguously at the
> end of that boundary. Some more details in the subsection cover letter
> / changelogs [1] [2]. It's not sufficient to just lose some memory,
> that's the broken implementation that lead to the subsection work
> because the lost memory may change from one boot to the next and
> software can't reliably inject a padding that conforms to the x86
> 128MB section constraint.

Thanks, I thought it was mostly for weird alignment where other parts of 
the section are basically "holes" and not memory.

Yes, it is a real bug that ZONE_DEVICE pages fall into sections that are 
marked SECTION_IS_ONLINE.

> 
> Suffice to say I think we need your pfn_active() to get subsection
> granularity pfn_to_online_page() before PageReserved() can be removed.

I agree that we have to fix this. I don't like ZONE_DEVICE pages falling 
into memory device blocks (e.g., cannot get offlined), but I guess that 
train is gone :) As long as it's not for memory hotplug, I can most 
probably live with this.

Also, I'd like to get Michals opinion on this and the pfn_active() 
approach, but I can understand he's busy.

This patch set can wait, I won't be working next week besides 
reading/writing mails either way.

Is anybody looking into the pfn_active() thingy?

> 
> [1]: https://lore.kernel.org/linux-mm/156092349300.979959.17603710711957735135.stgit@dwillia2-desk3.amr.corp.intel.com/
> [2]: https://lore.kernel.org/linux-mm/156092354368.979959.6232443923440952359.stgit@dwillia2-desk3.amr.corp.intel.com/
> 


-- 

Thanks,

David / dhildenb