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

[Yi Zhang]

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.
> > 
> > That said I haven't audited all the locations that test PageReserved().
> > 
> > Sorry for not responding sooner I was on extended leave.
> > 
> 
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb