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

[Yi Zhang]

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?
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