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
