From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 References: <4e8c2e0facd46cfaf4ab79e19c9115958ab6f218.1536342881.git.yi.z.zhang@linux.intel.com> <20180920224953.GA53363@tiger-server> In-Reply-To: <20180920224953.GA53363@tiger-server> From: Dan Williams Date: Thu, 20 Sep 2018 14:19:17 -0700 Message-ID: Subject: Re: [PATCH V5 4/4] kvm: add a check if pfn is from NVDIMM pmem. Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org To: David Hildenbrand , KVM list , Linux Kernel Mailing List , linux-nvdimm , Paolo Bonzini , Dave Jiang , "Zhang, Yu C" , Pankaj Gupta , Jan Kara , Christoph Hellwig , Linux MM , rkrcmar@redhat.com, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , "Zhang, Yi Z" List-ID: On Thu, Sep 20, 2018 at 7:11 AM Yi Zhang 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.