From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) by kanga.kvack.org (Postfix) with ESMTP id 27D118E0001 for ; Wed, 19 Sep 2018 03:20:32 -0400 (EDT) Received: by mail-qk1-f198.google.com with SMTP id 123-v6so3277731qkl.3 for ; Wed, 19 Sep 2018 00:20:32 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id b37-v6si1106575qkb.397.2018.09.19.00.20.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 19 Sep 2018 00:20:30 -0700 (PDT) Subject: Re: [PATCH V5 4/4] kvm: add a check if pfn is from NVDIMM pmem. References: <4e8c2e0facd46cfaf4ab79e19c9115958ab6f218.1536342881.git.yi.z.zhang@linux.intel.com> From: David Hildenbrand Message-ID: Date: Wed, 19 Sep 2018 09:20:25 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams , Zhang Yi Cc: 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" Am 19.09.18 um 04:53 schrieb Dan Williams: > On Fri, Sep 7, 2018 at 2:25 AM Zhang Yi wrote: >> >> For device specific memory space, when we move these area of pfn to >> memory zone, we will set the page reserved flag at that time, some of >> these reserved for device mmio, and some of these are not, such as >> NVDIMM pmem. >> >> Now, we map these dev_dax or fs_dax pages to kvm for DIMM/NVDIMM >> backend, since these pages are reserved, the check of >> kvm_is_reserved_pfn() misconceives those pages as MMIO. Therefor, we >> introduce 2 page map types, MEMORY_DEVICE_FS_DAX/MEMORY_DEVICE_DEV_DAX, >> to identify these pages are from NVDIMM pmem and let kvm treat these >> as normal pages. >> >> Without this patch, many operations will be missed due to this >> mistreatment to pmem pages, for example, a page may not have chance to >> be unpinned for KVM guest(in kvm_release_pfn_clean), not able to be >> marked as dirty/accessed(in kvm_set_pfn_dirty/accessed) etc. >> >> Signed-off-by: Zhang Yi >> Acked-by: Pankaj Gupta >> --- >> virt/kvm/kvm_main.c | 16 ++++++++++++++-- >> 1 file changed, 14 insertions(+), 2 deletions(-) >> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c >> index c44c406..9c49634 100644 >> --- a/virt/kvm/kvm_main.c >> +++ b/virt/kvm/kvm_main.c >> @@ -147,8 +147,20 @@ __weak void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm, >> >> bool kvm_is_reserved_pfn(kvm_pfn_t pfn) >> { >> - if (pfn_valid(pfn)) >> - return PageReserved(pfn_to_page(pfn)); >> + struct page *page; >> + >> + if (pfn_valid(pfn)) { >> + page = pfn_to_page(pfn); >> + >> + /* >> + * For device specific memory space, there is a case >> + * which we need pass MEMORY_DEVICE_FS[DEV]_DAX pages >> + * to kvm, these pages marked reserved flag as it is a >> + * zone device memory, we need to identify these pages >> + * and let kvm treat these as normal pages >> + */ >> + return PageReserved(page) && !is_dax_page(page); > > 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. > > 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. > > 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