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

[Dan Williams]

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.